1134 Silver Crown - Siratik

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Figure 1644. Silver Crown (U.S.N.M. no. 522). A taenite body in the hea t-affected c- 2 zone. Upon reheating carbon diffused outwards and, upon cooling , formed bainitic reaction nms (black). Etched. Scale bar 40 J.L.

through it reveal an exterior magnetite-wiistite layer, generally 50-100 J1 thick, underlain by a laminated sheet of dendritic metallic melts. Each of the metallic layers is 20-50 J1 thick; they cover the surface irregularly, wedging out and reappearing, frequently with tiny 1-25 J1 nodules of oxides embedded in them. Some late terrestrial corrosion products are also present in the fusion crust.

Under the fusion crust follows the heat-affected o:2 zone which ranges from 1.5-2.5 mm in thickness; the zone is relatively narrow at the bottom of the regmaglypts but may increase to 6 mm under protruding knobs where the heat influx simultaneously carne from different sides. The phosphides are micromelted in the exterior half of the o: zone and rhabdite pools here are often interconnected

2 • . by 1-5 J1 wide, intercrystalline fissures filled w1th phosph1de melts. The tarnished taenite has lost its color and appears pure yellow; its solid solution-carbon is now found in the surrounding kamacite where it has created 10-30 J1 wide, dark-etching bainitic rims. Neumann bands are not visible in the o:2 zone; otherwise the zone very much resembles that of the fresh fall Yardymly. The hardness of the o:2 phase is 195±15. In the recovered transition zone from o:2 to the unaffected interior, the hardness drops to 170±1 0 (hardness curve type IV).

Etched sections display a coarse Widmanstatten struc­ture of short, bulky(~- 8) kamacite lamellae with a width of 2.1±0.4 mm. In addition, local grain growth has created almost equiaxial karnacite grains, 10-20 mrn across. The karnacite is rich in subboundaries decorated with 0.5-2 J1 rhabdites. Neumann bands are common (undecorated), but additional cold-work with folding and shearing is also present. Thus, many rhabdites are shear-displaced their o':n thickness, 5-20 Jl. The microhardness of the karnac1te ranges from 185 to 235, reflecting the different degrees of deformation.

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Figure 1645. Silver Crown (U.S.N.M. no. 522) . Horizontal sets of Neumann bands displace a subboundary and a large rhabdite crystal. Etched. Scale bar 40 J.L.

Taenite and plessite cover 2-5% by area. The taenite has tarnished rims and is very hard, due to deformation. Typical, 40 J1 wide taenite ribbons are 425±15. Acicular and comb plessite fields are common, whereas pearlitic and spheroidized fields were not detected. However, they may be present in cohenite-bearing specimens.

Schreibersite occurs as 0.5 mm thick rims around troilite, and occasionally as 0.2-1 mm thick and centimeter­long lamellae in the kamacite. It is also common as 20-100 J1 wide grain boundary precipitates. Most schreibers­ite crystals are brecciated and frequently the fissures are impregnated by terrestrial corrosion products. Rhabdites occur in profusion as 5-25 J1 thick tetragonal prisms and as smaller units on the subboundaries.

Troilite nodules are present in some sections as oval inclusions 5-20 mrn across. Unfortunately, none were avail­able for a detailed examination. Carlsbergite, the chromium nitride CrN, occurs a~ scattered, oriented platelets, 20 x 1 J1 in size, in the kamacite. Cohenite, graphite and silicates were not detected in the U.S. National Museum specimen but may be present elsewhere.

Silver Crown is a well-preserved coarse octahedrite which is related to Campo del Cielo, Seelasgen and Canyon Diablo. It is no doubt a member of the chemical group I.

[J.T. Wasson (personal communication 1974): Group I with 6.98% Ni, 81.6 ppm Ga, 320 ppm Ge and 1.7 ppm Ir.]

Specimens in the U.S. National Museum in Washington:

36 g part slice (no. 522, 6 x 2 x 0.4 em) 133 g part slice (no. 3071, 7.5 x 5 x 0.5 em)

Siratik, Senegal, now Mali , Africa Approximately 14%0 N, 11 %oW

Previously classified as a stony-iron (F letcher 1888: 71), or a nickel-poor ataxite (Brezina 1896; Cohen 1905; Hey 196~).

Numerous specimens examined by the author are , however, artificial

cast irons and decarburized cast irons; only a very small group of specimens are altered hex ahedrites.

HISTORY

Only little is known of the origin of this iron, and what has been printed in the last two hundred years amounts to a small hill of conflicting information.

Most reference works quote Compagnon (1716) as the original source. His report, accompanied by maps, forms Chapter 13 in Volume two of the, now rare, large compila­tion of geographical descriptions: "Allgemeine Historie der Reisen zu Wasser und Lande" printed in Leipzig, by Arkstee and Merkus, in 1748. On page 510 Compagnon, after having discussed the abundant occurrences of gold, briefly states that iron and magnetic ore are common in Bambuk and in the whole of Galam. In the kingdom of Siratik, near the Sanaga (Senegal) River, iron is extracted in large quantities, and it is so ductile that the natives forge vessels and pots from it and hardly need buy any of the wrought iron imported by the French. Wallerius (I 778: Vol­ume 2: 233), in the new and revised Latin edition of his previous German edition from 1763, quotes Compagnon, significantly adding that the iron occurs as native, cubic iron.

The occurrence is next mentioned by Forster & Sprengel (1781 : Volume I: 61) who reported native iron in Bambuk and evidently of such large quantity that it could be exported via Galam to the British plantations. "It looked as if it had been molten, and been cast in sand. Mr. D. Schott (the physician in Fort Louis) possesses a mass of about 30 pounds." There are a few other, old reports (see e.g., Cohen 1899: 127; or Wi.ilfing 1897: 319), but the most important primary source is probably that of Golberry (1802: Volume 1) who gave a full geographic­ethnologic description of Senegal and surrounding areas. He noted that near the right side of the Senegal River there occurred numerous black rocks which were rich in iron, and he suggested that they might be native iron of the same kind as Professor Chladni had recently recognized as meteoritic from other places on the Earth. Unfortunately , no firsthand information or descriptions were given, but his idea was eagerly supported by Ende (1804), Morogues (1812: 339) and Chladni (1819: 333 f.), who used the Senegal irons as one of many examples collected in order to prove the meteoritic origin of the native iron masses. This example was never fully examined but migrated from textbook to textbook on the authority of Chladni. If we examine Golberry's work (1802) we will note that he, in addition to reporting the native iron, described two important professions in the land of Bambuk (page 413) ­that of the ironsmith and that of the coppersmith.

The ironsmith was able to produce chains, rings, hoes, spears, scissors, swords and other items and evidently knew well how to produce cast iron from available iron ore and

· how to reduce this to malleable iron, or perhaps even steel. Four gold mines were worked since about 1100 and formed the economic backbone of Bambuk's trade. Copper and

Siratik 1135

gold were worked into various items and into delicate ornaments and jewelery (page 419). The name "Siratik", under which the "meteoritic" specimens are generally known, is not a geographic location but rather the title given to the kings of Bambuk (pages 419, 424). From this report we learn that the people of Bambuk had a long tradition as metal craftsmen.

Chladni (1891: 333-336) commented upon some of the earlier reports and noted that the specimen he had seen in Paris had forged edges. This must be one of the specimens also mentioned by Meunier (1893a: 285 ; 1898: 63). A 17 g specimen was brought to Paris by Adanson, the botanist for whom the Baobab tree (Adan­sonia) is named. Adanson had acquired the metal near the mouth of the Senegal River; the metal was compact and crystalline but contained several internal cavities and appeared to have been cast.

The two specimens in London (BM. nos. 90236 and 90237), of 354 g and 30 g, were brought from Senegal by General O'Hara and analyzed by Howard (1802), who found about. 5% Ni - in fact, one of the very first nickel determinations on meteoric iron.

Four of the five specimens in Vienna, of 223 g, 44 g, 68 g, and 17 g, were purchased in 1840 from the French mineral dealer, F. Marguier, who had acquired them from a naturalist who was returning to Bordeaux from Senegal. The fifth specimen, of 162 g, was purchased from Dr. Bondi in Dresden, in 1843 (Partsch 1843: 130). In 1853, 1855, 1868 and 1873 more material was acquired, and some specimens were exchanged (Brezina 1885: 269), but the sources, sizes and types of material are unknown. It was a 138 g specimen from Vienna that Cohen examined (1899: 131; 1905: 28). He found 5.21% Ni, 0.77% Co and 0.26 P, an analysis which undoubtedly reflects a meteoritic composition - probably that of an altered hexahedrite.

The largest specimen in New York, No. 179 of 190 g, was once a part of the Bement Collection (Bement 1894), but it is not known from where he got it. The 23 g specimen in Harvard was once No. 200 of J .L. Smith's collection (Smith 1876b), but again, it is not known from where it came. The same is true of No. 9142222of212 gin Ti.ibingen, once a part of Reichenbach's collection and of other, smaller specimens in various collections.

The reason that the sources of the specimens are of interest is that at least two different kinds of material are presently in collections labeled "Siratik." A third type , a coarse octahedrite (No. 9142221 of 60 g in Ti.ibingen), (Machatschki 1940: 15), could be eliminated, since I was able to prove that this particular specimen was a mislabeled fragment of Oscuro Mountains.

DESCRIPTION

In order to come to a conclusion as to the nature of the various specimens which belong to the oldest stock of "meteoritic" material, known since 1716 - and in collec­tions since about 1800 - I examined as many fragments as possible by kind cooperation of several curators. The

1136 Siratik

material may roughly be divided into two groups; Cavern­ous, graphite-rich specimens and massive, metallic specimens.

To the first group belong: New York No. 179 (190 g), No . 218 (36 g), No . 84 (25 g) ; Washington No. 1366 (67 g), previously New York No . 755; Harvard No. 15 (23 g); Amherst, no number (36 g); and perhaps London No. 90236 (354 g) and No . 90237 (30 g).

The New York fragment No. 218 is typical of the first group. It is a 3 x 3 x 0.7 em slice through an individual which weighed, at the most, 1/4 kg. It contains numerous gasholes, 14 mm in diameter and displays signs of hammer­ing. Its structure is that of an unrefined, primitive cast iron in which areas rich in lamellar graphite, e.g., 200 x 8 Jl in size, alternate with areas rich in phosphide and cementite . These are often developed as needles, extending from side to side of the high temperature austenite grains which are 100-500 Jl across. A check under the electron microprobe showed less than 0.1 % nickel in the metal.

New York No. 179 is an endpiece, 6 x 5 x 2 em, with the same general structure but with up to 10 rnrn gasholes, and, in addition , showing a very rough surface which is indented, fractured and beaten in a way that is completely unknown in meteorites . The full individual may have weighed 1 kg. The specimen under the microprobe was found to contain less than 0.1 % Ni. New York No. 84 is a 2 x 2 x 1 em part slice with the same structure as No. 179. J.T . Wasson found less than 0.1 % Ni in this specimen (personal communication). In the U.S. National Museum No. 1366 is a 4 x 3 x 0.8 em slice through a coarsely crystalline individual which may have weighed about

Figure 1646. Pseud o me teo rite, previously labeled Siratik (New York, no. 84 ; 2 x 2 x 1 em of 25 g). A cavernous graphite-rich specimen with less than 0.1 % nickel. The large and small irregular dark spots are holes or oxide-filled cavities. Graphite-rich zones cross the sample. Etched. Scale bar 5 mm.

1/2 kg. It is a cast iron with bayonet-graphite , ASTM type C, with several flakes attaining 5-10 mm length. A phosphorus- and carbon-containing steadite eutectic is also present_ This particular specimen had few gasholes. A spectrographical check indicated less than 1% Ni, no cobalt, and 0.2-0.5% Mn.

The specimens show a large structural range; some have well developed, primary austenite dendrites - later decom­posed to fme or coarse pearlite; while others have grain boundary cementite and rays of cementite, and still others have local decarburized areas, with ferritic grain boundaries and Widmanstiitten development of the pearlite on the 0.3-0.5% C level.

Group I specimens thus represent a variety of cast iron products, not very refined or homogeneous, but still witnesses of the industry of the Bambuk natives in the eighteenth century . There is so little nickel present that it is likely that the sources were plain, local ores with no admixture of any mythical iron meteorite.

To the second group belong: Vienna No. A 299 (35 g), and probably the remaining Vienna specimens; Tubingen No. 9142222 (212 g) ; and Berlin 64 g (No. 479 in Klein's 1906 Catalog), 4 x 3 x 2 em.

Numbers A 299 and 9142222 are so similar that they may have been cut from the same larger specimen which, however, hardly exceeded a few kilograms weight. No. 9142222 is an irregular, wedge-shaped slice, roughly 6 x 3 x 3 em, through a hammered, partly forged mass, with several tool markings on the surface. It is partly opened alone fine fractures which contain 10-30 Jl wide sulfide-oxygen-iron eutectics known to make any iron brittle at forging temperatures. It exhibits a coarsely serrated, ferrite structure, but a previous austenite grain size of I 00-500 Jl may still be seen. Each of these grains have ghost-lines, diffuse structures which indicate the previous location of 10 Jl wide phosphide lamellae which were not

Figure 1647. Pseudo meteorite, the same as Figure 1646. A highly unusual structure showing a mixture of pearlitic and ledeburitic areas with graphite flakes scattered all over. Probably a primitive cast iron . Etched. Sca le bar 200 1-l-

completely dissolved and homogenized. They probably represent almost resorbed rhabdites. Number A 299 is a part slice, 3.5 x 2.5 x 0.7 em in size, which shows the same structures and, in particular, displays the anisotropic grain boundary sulfides well.

A microprobe examination of No. 9142222 indicated an average composition of 5.6% Ni and 0 .2% P, with cobalt between 0.5 and 1%. This composition is clearly that of a hexahedrite, and with this knowledge it also appears possible to interpret the present, severely altered structure as being that of an altered hexahedrite single crystal with rhabdites and scattered inclusions. If the specimens exam­ined by Cohen (1905) could be identified today these would probably turn out to be similar, reheated and hammered specimens of an original hexahedrite.

It thus appears that some original hexahedrite mass -or perhaps even masses (compare Coahuila) - was originally found in Senegal, but we will probably never learn how much. The meteoritic material was used along with the native iron ores for the production of vessels and weapons, but the travelers who visited the region were unable to distinguish between the meteoritic iron and the cast iron and brought both types back to Europe. Only a small number of the specimens labeled Siratik are thus of genuinely cosmic origin, and they are all severely altered by reheating and forging.

Skookum Gulch, Yukon, Canada

63°56'N, 139°20'W; 575 m

Ataxite, with numerous l 011 wide 01-spindles, D. Artificially reheated. HV 260±25 .

Group lVB. About 18% Ni, 0.19% P, 0.30 ppm Ga, 0.05 ppm Ge, 18 ppm Jr.

This 16 kg mass and the 483 g Gay Gulch mass are independent falls, and should be separated in catalogs. Skookum Gulch appears to have been artificially reheated to about 900° C, although no written evidence is preserved.

HISTORY

A mass of 15.9 kg was found by W. Kast in 1905 on claim No. 7 , Skookum Gulch, the auriferous gravels of which were being worked for gold. The iron was encoun­tered in white channel gravels, possibly of Pliocene age, 20m below the surface of the ground and less than 1 m above bedrock. The meteorite was exhibited at the Alaska­Yukon-Pacific Exposition at Seattle in 1909. After this it was secured by the Museum of the Geological Survey in Ottawa where it was described by Johnston (1915). From

Siratik - Skookum Gulch 1137

the microstructure - of which some poor pictures were presented - and the chemical composition, it was inferred that it was identical to the small Gay Gulch meteorite which had been found four years earlier 5 km farther southeast. In most later treatises this conclusion has been accepted and the meteorites have been united under the heading "Klondike." Dawson (1963) , Douglas (1971) and Wasson & Schaudy (1971) rightly concluded, however, that the two masses were independent falls. Owen & Burns (1939) and Owen (1940) examined the structure by X-rays and found only a 2 present. After eight days of annealing at 300° C they could identify lines from a and 'Y · Their data appear to have been obtained upon material from the interior, not from any possible heat-affected rim zone.

Perry (1944: plate 27) showed four photomicrographs that may be interpreted as showing effects of general reheating. Vilcsek & Wanke (1963) found a cosmic ray exposure age of 5,600 million years, but Chang & Wanke (1969) later corrected this to 600±100 million years C6Ar / 10Se ), stating that the first value was falsified by the high terrestrial age of Klondike . By using their 10Se/36Cl method , they estimated the terrestrial age to be about 1 ,000,000 years, which seems to be in harmony with the geological records of the locality of find. Begemann (1965) and Hintenberger et al. (1967) determined the amount of noble gases. Voshage (1967) estimated the exposure age to be 915±90 million years; as usually is the case, the ~j41K method yields a value different from the 36Ar/ 10Se method.

COLLECTIONS

Ottawa (7.8 kg), New York (788 g), Chicago (715 g), London (343 g), Amherst (263 g), Washington (165 g) , Copenhagen (23 g).

DESCRIPTION

The meteorite is a relatively smooth, flat block with the overall dimensions of 29 x 23 x 7 em. It is corroded and covered by a thin crust of terrestrial oxides; no fusion crust or heat-affected zone is present.

Etched sections display a matte structure in which small scattered troilite nodules are the only visible inclu­sions. Tilting the surface at different angles to the light reveals two overlapping kinds of matte structure in the metallic matrix. One, the primary, is in the form of 1-10 mm wide, fringed bands which apparently are oriented in normal Widmanstatten directions and which fully correspond to those seen on Hoba, Tlacotepec and other ataxites of group IVB - except that the bands of Skookum Gulch are narrower. However, these bands are severely obscured by a secondary phenomenon of irregular

SKOOKUM GULCH - SELECTED CHEMICAL ANALYSES

percentage ppm References Ni Co p c s Cr Cu Zn Ga Ge Ir Pt

Whitfield in Johnston 1915 18.2 0.91 0.191 150 20 20

Wasson 1966 17 0.30 0.05 18

1138 Skookum Gulch - Slaghek's Iron

Figure 1648. Skookum Gulch (U.S.N.M. no. 536). Artificially reheated ataxite of group IVB. The kamacite spindle has a central schreibersite lamella. Reheating to above 900° C created unequili­brated a 2 and fuzzy phosphide edges, and the plessitic matrix was altered. Etched. Scale bar 80 1-1· (Perry 1944: plate 27.)

matte patches, the character of which is best compared to those observed on artificially reheated specimens of Babb's Mill, Troost's Iron, see Figures 287-289.

High magnification shows that the meteorite is - or rather was - a normal ataxite in which octahedrally oriented kamacite spindles, typically 10-15 11 wide and I 00 11 long, occur with a frequency of about 100 per mm2

in a plessitic matrix. The larger kamacite spindles are developed around irregular schreibersite crystals which are 5-15 11 across. The kamacite is transformed to serrated aggregates of a 2 grains, and the schreibersite is enveloped in 1-5 11 wide taenite halos that merge with the taenite frames around the kamacite spindles. The schreibersite itself is recrystallized to 2-5 11 units, which are faintly anistropic under crossed nicols. Thorny projections from the taenite are common. Although it is not possible to determine the cause from these observations alone, it is quite clear that, for a short time (less than an hour?), the mass has been exposed to a temperature of about 900° C. This was sufficient to recrystallize the schreibersite without actually melting it and to create a 2 upon cooling.

The plessitic matrix is blurred. Individual, original taenite bodies appear spheroidized on an almost submicro­scopic scale, and it is clear that the matrix is unequilibrated, as Owen's X-ray data also showed. The microhardness of the matrix is 260±25; it is hard because the reheating led to dissolution of Ni and P in the austenite which, upon cooling, formed a distorted a 2 lattice with essential amounts of Ni and P in solid solution. Compare the hardness bands in Buchwald (1966: 18).

Fortunately enough it is possible to find a clue, in the corroded rim zone, to the original structure. The long exposure to terrestrial weathering has selectively trans­formed the nickel-poor alpha phase to oxides and here we observe the structure of a fine paraeutectoid, identical to that found in Hoba, Kokomo and Tlacotepec - only on a

still finer scale. The individual, winding taenite lamellae are hardly more than 0.5 11 wide and separated on the average by 111 (corroded) kamacite. Since the "fossilized" micro­structure in the crust is different from the interior, it may be concluded that the reheating that transformed the structural elements occurred after considerable corrosion had taken place and, therefore, was artificial.

In other words, Skookum Gulch, when it fell perhaps a million years ago, was a normal ataxite, almost similar to Tawallah Valley, Hoba, Kokomo , Cape of Good Hope, and Tlacotepec. When it was found only 65 years ago it was exposed to red heat, possibly in an attempt to discover the nature of the mass; in any event, the reheating went unre­corded. By the heating, the a-spindles transformed to a2 , the taenite lamellae spheroidized and started to dissolve; and the schreibersite rejected some nickel and recrystallized. While some high temperature reactions did start in the oxidized rim zone, sufficient details of the original structure are still preserved to allow the above conclusion.

Skookum Gulch is not quite similar to other irons of group IVB. It contains a little more nickel and significantly more phosphorus, which manifests itself in the structure as numerous schreibersite inclusions. Partly because of the nucleating action of the schreibersite crystals, the fre­quency with which alpha spindles occur is 10-100 times higher per unit of area than in, e .g., Hoba and Iquique. The closest resemblance, before reheating, was probably to Tawallah Valley.

Specimen in the U.S. National Museum in Washington:

158 g part slice (no. 536,7.5 x 5 x 0.6 em)

Slaghek's Iron, Atacama, Chile

Unknown coordinates

A mass of 1.90 kg was acquired (about 1900 ?) by Gino H. Slaghek-Fabbri when he visited the Atacama region of Chile. In 1916 he donated it to the Technical Institute of Livorno, Italy, where the main mass is still preserved.

The material was described, with photographs of the exterior and of etched sections, by Bertolani (I 950). A critical examination of this report follows. The mass measures 13 x 9 x 5 em and weighs 1.90 kg. Densely spaced pits entirely cover the top surface as 3-5 mm depressions with sharp ridges in between. The fusion crust is apparently not preserved. The pits are the hallmark of the corrosive environment of the Atacama desert and prove that the iron meteorite really does come from this part of the world, although little is otherwise known of its origin.

Etched sections display a medium Widmanstiitten structure of straight, long (W- 25) kamacite lamellae with a width of 1.3±0.2 mm. All kamacite is recrystallized to 25-1001-1 irregular to equiaxial units. Taenite and plessite cover about 30% by area, and the plessite is partly spheroidized.

Apparently schreibersite occurs only as insignificant grain boundary veinlets. Troilite and other meteoritic minerals were not reported.

Bertolani (1950) reported the following analysis: 7.89% Ni, 0.67% Co, 0.70% S, 0.34% P, 0.46% MgO, 0.05% Si02 and 0.78% FeO. The analysis was evidently performed on material rich in troilite, schreibersite and terrestrial oxidation products. Whether the magnesium comes from a mineral in the troilite or is an analytical error is unknown. The combination of a relatively low

nickel value with a relatively high phosphorus value appears to be incorrect and prevents us from using either of them as definitive for classification purposes.

Slaghek's Iron is a medium octahedrite probably of group IliA, which is significantly recrystallized. If it is not an independent fall -which cannot be wholly excluded - it may be a fragment of one of the other medium octahedrites from North Chile. In this case, because of its recrystallized structure, it can only be associated with Cachiyual and Joe l's Iron, both of which are small irons with the right bandwidths, primary and secondary structures. It would be interesting to have a sample of Slaghek's Iron reexamined and also reanalyzed for the trace elements. A metallographical examination alone would, however, suffice to determine the relationship with other Chilean octahedrites.

Smithland, Kentucky, U.S.A.

37°8'N, 88°24'W; 100m

A taxite , D. Originally presumably a fine octahedrite, Of. Bandwidth about 0.35 mm.

Now displaying a completely altered structure due to thorough reheating to about 1000° C. HV 220±25.

Group IVA. 9.23% Ni, about 0.15% P, 2.3 ppm Ga, 0.14 ppm Ge, 0.89 ppm Jr.

HISTORY

In 1845, a certain Colonel Player of Nashville was offered a tract of land which was highly prized because it was believed to be rich in iron ore of an excellent quality, requiring no preparation whatever. Player discussed this unusual ore with Troost (1846) who was able to prove that the ore was nothing other than meteoritic iron, so the purchase of the land fell through. In fact, several times before Troost had been shown pieces of the same iron by various people who maintained that they were silver-bearing ores or iron ores. The exact circumstances of find were never learned, but it appears that the original "pretty large" mass had been found near Smithland, Livingston County in about 1840. It had been cut up and worked by a blacksmith, who, among other things, had produced a chisel from it. Eight or ten pounds were still in existence when Troost heard of the iron, and he succeeded in purchasing 4* pounds and the chisel for his collection. He noted that the iron had a fine granular fracture - similar to that of steel - was very compact and showed no crystalline nature.

· Figure 1649. Smithland (Brit. Mus. no. 20465). An ataxite which, however , judging from the analysis, was originally a fine octahedrite similar to, e.g., Boogaldi and Duchesne. Black spots are shock­melted troilite nodules. Etched. Scale bar approximately J em.

Slaghek's Iron -Smithland 1139

Similar observations were published by Reichenbach (1859: 177), Greg (1862), and Brezina (1885: 219; 1896: 297). Rose (1864a: 24, note) believed that the absence of Widmanstiitten structure in this case proved that Smithland was a pseudometeorite. Cohen (1905 : 101) gave a new description; and since his analyst (Sjostrom) found 16.4% Ni, he was convinced that Smithland was an ataxite closely related to Babb's Mill and Deep Springs. The older literature was reviewed by Wi.ilfing (1897: 332) and Farrington (1915: 417).

Owen & Burns (1939) made a detailed study by X-ray analysis. From their X-ray spectra they correctly concluded that the material (which had been annealed at 350° C for 8 weeks) consisted of almost pure a-iron-nickel with 8-9% nickel, and that the previously adopted value of 16.4% was incorrect. Perry (1944: plate 26), however, accepted the high nickel value and gave two photomicrographs which showed a highly unusual structure, rich in diffuse phantom patterns. Somehow it never occurred to the early investi­gators that the peculiar "ataxite" structure could be a result of an artificial reheating by the blacksmith (Buchwald 1965; Hey 1966: 455).

Fisher & Schaeffer (1960) estimated the cosmic ray exposure age to be 90 million years. Bauer (1963) determined the 3 He and 4 He quantities and deduced a cosmic age of 110 million years.

COLLECTWNS

London (2.50 kg), Harvard (1.78 kg), Washington ( 132 g), Vienna (118 g), Chicago (91 g), Paris (72 g), Stockholm (44 g), New York (40 g), Calcutta (35 g), Amherst (26 g), Berlin (14 g), Gottingen (8 g), Yale(< 1 g). The original weight is unknown; the preserved samples add up to 4.86 kg.

DESCRIPTION

The following is based upon a macroscopic and microscopic reexamination of the major specimens in

..

Figure 1650. Smithland (Brit. Mus. no. 20465). Edge of a shock­melted troilite nodule. Black pits are due to imperfect polishing. Etched. Scale bar 300 J.l..

1140 Smithland

London and Harvard, and of several minor specimens in Washington, Chicago and New York. All the samples are identical in exterior appearance and in the microstructure, and any mislabeling can be ruled out.

Specimen No. 20645 in the British Museum is an endpiece of 2.4 kg which was purchased from Mr. H. Reuland in October 1846. It measures 12 x 10 x 4 em and appears at first sight to be in good condition since some regmaglypts 2-3 em in diameter are preserved. In one place there is a 30 mm wide cavity from a burnt out troilite nodule. A close inspection reveals, however, several chisel marks and flat hammered areas. Two flat surfaces, each of about 3 x 6 em in area, are situated exactly opposite each other and are mutually parallel. They represent regions which were flattened by the sledge hammer when the meteorite was reheated to forging temperatures.

Specimen No. 148b in Harvard is an irregular endpiece of 1.78 kg. It was previously part of J. Lawrence Smith's collection, as evidenced by the old catalog number, 175. Smith used to chisel his catalog numbers into the iron meteorites (Smith 1876b: 4 and 5), and this is also the case here. The specimen measures 7 x 5 x 4 em and is violently hammered in various places. The whole surface is covered with oxides of the type and color which occur in forged

Figure 1651. Smithland. Detail of Figure 1650, polished only. Dendritic metal and interdendritic sulfide veins. Scale bar 100 J.l.

iron. Local overfolding, faulting and ear-forming from the forging action are also visible.

Polished sections are homogeneous with a few inclu­sions and gasholes. Upon etching there is no Widmanstiitten pattern observed but rather a uniform "ataxite" structure. The metal is a fine poly crystalline aggregate of nickel-poor a:2 phase and nickel-rich -y-phase. A network of grain boundaries indicates an earlier equiaxial austenite structure with a grain size of 20-50 11. Scattered throughout grain interiors and grain boundaries the residual -y-phase occurs as diffuse rounded particles which range in size from 5 to 10 11 and cover approximately 15% by area. The microhardness is quite variable, 220±25, probably due to an incomplete homogenization by the artificial reheating.

Schreibersite was not revealed as such during the meta!lographical examination. Since related fine octahe­drites, such as Boogaldi and Mart, do contain 0.15%P and exhibit numerous 10-5011 wide grain boundary phosphides, it could be possible that Smithland's original phosphides were redissolved in the metallic matrix by the artificial heating.

Figure 1652. Smithland. Detail of Figure 1650. In the sulfide phase there are numerous tiny chromite fragments (black). The metal is unequilibrated a 2 with imperfect high-nickel taenite rims (white). Etched. Scale bar 40 J.l.

SMITHLAND - SELECfED CHEMICAL ANALYSES

These two analyses were performed upon the British Museum sample and the Harvard sample, respectively. Since they tally very well, there is no doubt that these two major specimens are fragments of the same original mass. As noted above, Owen & Burns also expected 8-9% Ni to be present. The old analysis by Sjostrom, quoted in Cohen (1905: 101), was adopted for two generations as the

References

Smales et al. 1967

Schaudy et a!. 1972

percentage Ni Co P c

9.23

correct one for Smithland: 16.42% Ni, 0.94% Co, 0.09% P, 0.17% S, 0.06% Cr. Sjostrom was a very clever analyst, so the analytical values are no doubt correct as such. It appears, however, that what he really analyzed was one of the group IVB ataxites, probably a Cape of Good Hope sample, which unfortunately had been mislabeled Smithland.

s Cr

12.1

Cu

101

ppm Zn

0.28

Ga Ge Ir Pt

2.29 0.15 2.38 0.133 0.89

Troilite occurs as nodules and lenses up to 3 em in diameter. Under the microscope the troilite appears re­melted and with diffuse edges. The troilite melt has redissolved the surrounding kamacite and schreibersite and , upon solidification, formed Fe-Ni-S-P eutectics with den­dritic-cellular networks. The Fe-Ni cells are 10-50 J1 in diameter and are separated by concave Fe-S-P melts which are partly segregated into sulfides and phosphides them­selves. Tiny subangular fragments of chromite, and several gasholes are also present - incorporated in the melts . The sulfide melts penetrate up to 1 mm into the austenite grain boundaries of the surrounding matrix. The state of the troilite and its associated minerals indicates an artificial reheating to about 1000° C.

Chromite is present as 50-500 fJ. subangular crystals. They are slightly altered by the reheating. The chromite is evidently able to dissolve small amounts of iron at high temperature , and this has reprecipitated, upon cooling, as 1-2 J1 particles in a 10 J1 wide rim zone of the chromite. The metallic matrix, on the other hand, may have dissolved some chromite also. Upon subsequent cooling, this chrom­ite reprecipitated as 1-10 J1 angular crystals which are only found in a 40 J1 wide zone around the chromite.

The surface shales consist of thermally altered limonite and additional high temperature oxidation products, some of which penetrate intergranularly about 100 f.J. . In oxides there are numerous 1-10 fJ. metallic particles, a phenomenon which is typical for iron meteorite oxides artificially re­heated in the atmosphere. Intergranular lace-works of metal and oxides are common along preexisting corroded cracks.

The description above clearly proves that Smithland is an artificially reheated iron meteorite. Maximum tempera­ture reached seems to have been 1000° C; this temperature must have been maintained for a while - probably for some hours - since the structure is thoroughly altered by dif­fusion and partial melting. The evidence from the literature tallies very well with the results of the metallographical ex­amination and with the worked surfaces of the large Smith-

Figure 1653. Smithland (U.S.N.M. no. 2575). Duplex ataxite matrix which shows signs of artificial reheating but is not homogenized. Etched. Scale bar 80 JJ. (Perry 1944: plate 26.)

Smithland 1141

land samples. Evidently Smithland was not very much cor­roded when it was acquired by the blacksmith.

The present structure is too altered to give indication of the structural type to which Smithland belonged before the blacksmith damaged the meteorite . The modern anal­yses by Smales and Wasson do , however, suggest that it was a normal member of group IV A, closely releated to Boogaldi, Duchesne, Mart and Hill City. It is possible that Smithland was already significantly altered in a cosmic event to resemble Maria Elena or Santiago Papasq uiaro struc­turally. If so, the treatment of the blacksmith only served to further eliminate any trace of the original structure.

Figure 1654. Smithland (Brit. Mus. no. 20465). A chromite inclu­sion which is shattered and shows reaction rims against the metal. The "tail," above, contains minute chromite, daubreelite and troilite particles. Lightly etched. Scale bar I 00 JJ.

' ., ..

c ..: .....

'i ' ~· ~

L----1

""""'-

.. ·~ ,.q· 11).;~- ~~ e ) ~

' ~

Figure 1655. Smithland. Detail of Figure 1654 showing small bluish chromite particles that have been separated (rom the central crystal. In other parts of the meteorite the chromite crystals have entirely disintegrated and the resulting angular chromite particles have been smeared out into streaks, often associated with sulfide particles and internal fissures. Polished. Scale bar 40 JJ.

II42 Smithland - Smithonia

Specimens in the U.S. National Museum in Washington:

12.5 g fragment (no. 1144) 120 g part slice (no. 2575,6 x 4 x 1 em)

Smithonia, Georgia, U.S.A.

34°0'N, 83°IO'W; 200m

Hexaheclrite, H. Single crystal larger than 10 em. Decorated Neumann bands. HV 165±5.

Group IIA. 5.72% Ni, 0.20% P, 65 ppm Ga, 187 ppm Ge, 34 ppm lr.

HISTORY

A mass of 70 kg (I 54 pounds) was found in I940 on what used to be the Jim Smith Plantation at a little place called Smithonia. Smithonia is in Oglethorpe County and has the coordinates given above. The mass was illustrated by Leonard ( I94 7) who also stated that it had been purchased in I942 by the Field Museum in Chicago. However, the owner, Corbett Simmons, also negotiated with other parties for the purchase. S.H. Perry acquired a 3 g fragment and labeled it Elberton in his catalog (1947); Elberton has thence passed to other catalogs as an independent meteorite, e.g., Hey (I966: I47). Henderson & Furcron (I957) had noted, however, that Elberton is nothing else than a fragment of Smithonia. Roy & Wyant (1950a) described the main mass in Chicago and gave several photomicrographs but unfortunately classified it as a nickel-poor ataxite. Henderson & Furcron (I957) rightly noted that Smithonia was a normal hexahedrite - but severely weathered - so that recognition of the stmctural elements was impaired. Berkey & Fisher (I 967) presented photomicrographs of both near-surface oxidized material and of material from the unaltered interior. They found that chlorine was strongly concentrated towards the sur­face, ranging from less than O.OI ppm at a depth of 2 em to I 0,700 ppm at a depth of 0.3 em. They gave convincing arguments for the chlorine being of terrestrial origin, rather than being derived from any cosmic lawrencite, a conclu­sion which the present author can fully support. Voshage (I967) found by the ~j41K method a cosmic ray irradiation age of 90±80 million years.

COLLECTIONS

Chicago (69.0 kg main mass, 1.2 kg slices, a~ cording to Horback & Olsen I965), Washington (589 g). Evidently the mass must have originally weighed about 7I kg rather than the reported 70 kg.

DESCRIPTION

The maximum dimensions were 49 x 27 x I7 em, and the shape was thigh- or ham-shaped (Leonard I947). The

mass is weathered and irregularly covered with I-IO mm thick crusts of terrestrial oxides. No fusion crust and no heat alteration zone are present. In several places the surface is disintegrating along parallel planes so that 0.5-2 mm thick, coin-sized fragments may be easily de­tached. It was one such fragment which Perry acquired and which gave rise to the meteorite "Elberton"; see above.

Smithonia is a normal hexahedrite with Neumann bands extending from edge to edge. They are, however, modified by some gentle, cosmic annealing. Their bound­aries are very straight, and they are often hacked up in short periods. Also, they have served as precipitation sites for numerous small rhabdites, thereby providing significant nickel and phosphorus gradients in the adjacent matrix (sensitization). The kamacite has a hardness of I65±5.

Schreibersite occurs as a few, scattered blebs, ranging from I x 0.5 to 0.4 x 0.1 mm in diameter. They have often been nucleated by a small sulfide bleb. The hardness is 900±20. Rhabdites are very common, mainly as tetragonal prisms, S-IS J1 in thickness. Some plate-shaped rhabdites, typically 2.5 x 2 x 0.02 mm in size, are also present but far from so common as in the North Chilean hexahedrites. The conspicuous arrangement of large rhabdites in parallel planes, as in Hex River, is absent.

Troilite occurs as I-I 0 mm nodules with significant amounts (10-20%) of daubreelite. The daubreelite has a hardness of 350±10. The troilite is monocrystalline, except at phase boundaries where slight recrystallization has occurred. The smaller blebs are lamellar stacks of alternat­ing troi!ite and daubreelite, and the individual lamellae are frequently only I-5 J1 thick. Slight plastic deformation has compressed and bent the lamellae. Schreibersite precipi­tates, 20-50 J1 thick and with a hardness of 820±20, occur as discontinuous rims of many sulfide inclusions.

Locally a small amount of cohenite has precipitated as I 0 J1 thick rims upon schreibersite or troilite.

In the kamacite matrix there are numerous, oriented platelets, IO x I /1, of chromium nitrides. They are often embedded in 5-IO J1 thick rhabdites and must, therefore, have precipitated before these had reached their full size or perhaps before they even started to precipitate. The chromium nitrides are very stable against corrosion, and their surroundings are not selectively corroded, indicating that only minor segregational gradients occur around them.

It has been noted that the corrosion in Smithonia follows cubic cleavage planes (Leonard I947; Henderson & Furcron 1957), but this is not quite correct. The corrosion mainly, or perhaps exclusively, follows certain sets of Neumann bands, and it penetrates to a depth of at least 2 em in places. Neumann bands are not usually attacked in

SMITHONIA - SELECTED CHEMICAL ANALYSES

percentage ppm References Ni Co p c s Cr Cu Zn Ga Ge Ir Pt

Roy & Wyant I950a 5.58 0.60 0.20 100 1000 400 300 Wasson 1969 5.86 65.3 I87 34

this way, except if microsegregation is present. This is the case with Smithonia where two or three sets of bands are decorated along both sides with 0.5-I J.1 thick precipitates of rhabdites, in much the same way as described in Scottsville. The near-surface part of the Neumann bands slowly develops into wide, funnel-shaped areas while the attack continuously proceeds deeper inwards, following in the beginning only the two sides of the Neumann bands. The attack is clearly electrochemical in nature, governed by the sensitization of the bands. Near-surface rhabdites are enveloped in 5-20 J.1 wide halos of terrestrial oxides, while the few larger schreibersite crystals have 40-80 J.1 wide halos. The near-surface troilite shows pentlandite veins. The daubreelite, the phosphides, and the chromium nitrides remain unattacked. These minerals may often be found unaltered in the heavy, oxidic crust.

Smithonia is a normal, slightly annealed hexahedrite which is related to Scottsville, Bruno and Edmonton. Chemically, it is a typical group IIA iron as shown by Wasson (I969).

Specimens:

12 g rhomboidal, plate-shaped fragments (no. 1393, from Corbett Simmons to U.S.N.M., 1941)

3 g plate-shaped fragment (no. 1648, from C. Simmons to S.H. Perry, 1941: Elberton)

574 g part slice, now subdivided (no. 1481, 9 x 9 x 0.8 em; from Chicago)

Smith's Mountain, North Carolina, U.S.A.

Approximately 36°25'N, 79°57'W; 300m

Medium octahedrite, Om. Bandwidth 0.63±0.15 mm. <-structure. HV 280±20.

Group IIIB. 9.56% Ni, about 0.7% P, 17.4 ppm Ga, 30.6 ppm Ge, 0.023 ppm lr.

HISTORY

A mass of 10 pounds IS ounces (4.97 kg) was found in I863 by Mr. Peters, at Smith's Mountain two miles north of Madison, in Rockingham County. The mass was lying on the surface of an old field which had been out of cultivation less than 20 years, and for that reason it was supposed to have fallen recently. It was acquired by the state geologist of North Carolina, W.C. Kerr, and described by him (1875) and by J.L. Smith (I877). From a crevice Smith extracted a small fragment of what he described as a solid chloride. Since the determination is insufficient and later investigators have never seen the lawrencite, I think that the chloride, as usual, is from terrestrial contamina­tion. Cohen (1905: 358) reexamined the iron, and Brezina & Cohen (1886-I906: plate 37) presented two photomacro­graphs. Mauroy (1913 : figure I7) gave another photo-

Smithonia- Smith's Mountain Il43

macrograph, while Perry (I 944) published five photomicro­graphs. Reed (I965a, b; I969) examined the various phases with the electron microprobe. The kamacite was found to have 7.3% Ni and 0.1% P in solid solution. The location, Smith's Mountain, cannot be found on present maps, but it appears to have been a few kilometers east of the town Mayodan with the coordinates given above.

COLLECTIONS

State Museum, Raleigh, North Carolina (987 g), Har­vard (809 g), New York (485 g), Chicago (458 g), Ti.ibingen (224 g), Vienna (124 g), London (70 g), Washington (57 g),

Gottingen (54 g), Amherst (54 g), Copenhagen (53 g), Buda­pest (49 g), Vatican (40 g), Tempe (35 g), Bonn (14 g), Leningrad (I 3 g), Stockholm (I I g), Berlin (8 g), Yale ( 6 g).

DESCRIPTION

The mass had the approximate dimensions of 20 x 9 x 7 em, but since it was sliced repeatedly, first by Kerr, then by Smith, and finally by Ward in I902, the largest piece extant now is the endpiece in Raleigh that measures 8 x 7 x 5 em. The mass is weathered and covered with O.I-I mm thick crusts of terrestrial oxides. However, the heat-affected a 2 zone is preserved locally as a I mm thick rim. On the average the mass appears to have lost about 2 mm by corrosion. It is, therefore, terrestrially old and can not have fallen about I850 as supposed by Smith (1877). The a 2

structure has a hardness of 200± 10; in the recovered transition zone inside a 2 the hardness rapidly increases, until it, at a depth of about 6 mm, attains a value of 280±20, corresponding to a shock-hardened, little annealed £-structure (hardness curve type I).

Etched sections display a medium Widmanstiitten

.,w·- -~ () • ' ' . _');). ,_ ... . ~: / . ' \.

.it/. .. ·.

t--- ·---~-----1.

Figure 1656. Smith's Mountain (U.S.N.M. no. 94 of 57 g) . Medium octahedrite of group Ill B with a large schreibersite crystal at center. Note the number , 110, stamped into the edge by J. Lawrence Smith in his characteristic way of identifying samples. Etched. Scale bar 20 mm.

SMITH'S MOUNTAIN -SELECTED CHEMICAL ANALYSES

percentage Reference Ni Co P c s Cr

Scott et al. 1973 9.56

Cu ppm Zn Ga Ge Ir Pt

17.4 30.6 0.023

1144 Smith's Mountain

Figure 1657. Smith's Mo untain (U .S.N.M. no. 94) . Shock-ha tched kama cite and unresolvable black plessite. Island-arcs of schreibersite crystals. Deep-e tched . Scale bar 400 ll · (i>e rry 1-950 : volume 4.)

structure of undulating, long (W ~ 25) kamacite lamellae with a width of 0 .63±0.15 mm. The kamacite is of the densely hatched, acicular E-variety which indicates a significant shock event of an intensity of above 130 k bar (HV 280±20). Both hardness and structure correspond closely to Bear Creek and Narraburra.

Taenite and plessite cover about 45% by area, mostly as comb and net plessite and as dark-etching, martensitic fields. A typical field will have a yellow or tarnished taenite rim (HV 340±10) followed by a transition zone in which

the martensitic decomposition products are developed parallel to the bulk Widmanstiitten directions (HV 390±30). Then follows an "annealed," duplex, almost unresolvable martensite (HV 340±25) and finally the fully decomposed net structures, where the kamacite cells are of the same hardness as the primary lamellae, suggesting a similar nickel and phosphorus content.

Schreibersite is common as irregular skeleton and rosette crystals , typically 10 x 3 x 1 mm in size . They are monocrystalline, but heavily brecciated, and shear displace­ments of 10-25 J.1. are quite common. The hardness is 890±25. Schreibersite is further common as 0.2 rom thick bodies centrally in the alpha lamellae, and as 2-20 J.1. blebs inside the plessite fields . The island arcs, composed of 10-20 J.1. thick blebs arranged in rows, 5-20 J.1. outside taenite and plessite are very characteristic. Rhabdites are quite common, particularly in the 1-1.5 rom wide rims of swathing kamacite ; the rhabdites range from 10-50 J.1. in cross section . Point counting of 45 cm2 resulted in an estim ate of 0 .7% Pas a bulk value .

In one place a round gray inclusion , 0.5 mm in diameter , occurred in the a-phase . It is probably a phosphate; see Figures 1659 and 1660.

Troilite is present as scattered nodules. A one milli­meter bleb in a microsection was noted to be shock melted. It consists of 1-10 J.1. troilite grains with a ragged border

Figure 1658. Smith 's Mountai n (U .S .N. M. no. 94). Characteristic plessite field with ac icular kamacite and island arcs of tin y schre ibersite crystals. Deep-etched . Scale bar 400 Jl . (Perry 1950: volume 4.)

Figure 1659, Smith's Mountain (U.S .N.M. no. 94). A large gray phosphate inclusion in the shock-hatched kamacite. Narrow schrci­bersite cry stals have precipitated upon it. Compare Figure 1660. Etched. Scale bar 200 ll·

EL 0 p

Na K Mn '------'

Figure 1660. Smith's Mountain . X-ray scanning pic tures of edge of the phosphate in F igure 1659. Besides the signal s shown , 0~, PKa , NaKa . (KKa) and MnKa , an FeKa signal was also obtained . Scale bar 100 ll·

against the surrounding metal which is partially dissolved. Chromite fragments, 5-25 iJ. in size , are scattered through the melt, while the schreibersite rim zone has survived almost intact.

The mass is somewhat corroded. The nickel-depleted kamacite is transformed to limonite near the surface , and the brecciated schreibersite has also been an easy prey for invading terrestrial ground water.

Smith's Mountain has previously been classified as a fine octahedrite, except by Tschermak (1872a) who classed it as a medium. The bandwidth is variable and erratic, as in most group IIIB irons ; but primary lamellae average about 0.63 mm, so it should be classified as a medium octahedrite. It is closely related to Bear Creek , Apoala, Knowles and Narraburra , all of which are medium octahedrites.

Smith's Mountain - Smithsonian Iron 1145

Specimen in the U.S. National Museum in Washington:

57 g slice (no. 94, 6.5 x 3.5 x 0.5 em)

Smithsonian Iron

Unknown origin

Coarsest octahedrite, Ogg. Bandwidth 10±5 mm. Neumann band s. HV 200±10.

Group liB, judging from the structure. Ab out 5.9% Ni and 0.25% P.

HISTORY

A mass of about 3.6 kg was briefly described by Shepard (1881b) and listed in his pamphlets , giving the status of his collections (1881 ; 1882; 1883). Shepard had discovered the meteorite in the old collection of the Smithsonian Institution, where it had been since about 1852 without any record of its origin. He assumed that the mass did not belong with previously known irons, and listed it as The Smithsonian Museum Iron, Locality Unknown. Clarke ( 1889: 257, 262), when editing the first catalog of meteorites in the Smithsonian Institution, listed the main mass as a nearly entire specimen of 3 ,510 g, and the Shepard material as an 88.3 g fragment broken from the main mass.

Brezina (1896 : 290) examined a 3 g sample obtained from Shepard and concluded that the Smithsonian Iron was nothing other than another Fort Duncan (i.e., Coahuila) specimen. The conclusion was accepted by Wi.ilfing (1897 : 79), Cohen (1905: 190), Farrington (1915 : 128), and Hey (1966: 109, 456), and provisionally by Tassin (1902a: 680, 697) and Merrill (1916a: 56, 180).

In 1970 during the present study it was discovered that the 3.6 kg mass was entirely different from all known Coahuila specimens and required a separate discussion. In cooperation with Mr. Roy S. Clarke jr. , small pieces were

Figure 1661. Smithsonian Iron (U.S.N.M. no. 64). A coarsest octahedrite, structurally related to Sikhote-Alin and other group liB meteorites. Deep-e tched. Scale bar 30 mm. S.l. neg. I 692C.

1146 Smithsonian Iron

cut and mounted for polished sections, and complete sections through the mass were provided in the spring of 1972.

COLLECTIONS

Washington (3.4 kg).

ANALYSES

Shepard (188Ib) reported 6.07% Ni, 0.54% Co, and 0.56% schreibersite, i.e., 0.09% P. From the examination below it appears to be a good analysis, except that the bulk phosphorus content may be estimated to be 0.25±0.05%.

DESCRIPTION

When the main mass was cut in 1972, it weighed 3,4 7 5 g and measured 10 x 9.5 x 7.5 em in three perpendic­ular directions. Only 90 g had been removed from the mass by Shepard (I 881 b) and possibly another 20-30 g on other occasions, so that the original weight of the entire angular block was 3.60 kg. The acquisition number, 46612, which had been painted on the specimen at an early date , helped to reidentify it among other masses of similar size and of partially unknown origin which are in the Smithsonian Collection.

There are a number of very conspicuous cracks in the surface; they are 1-3 mm wide, long and of extreme depth, clearly indicating an exfoliation along grain boundaries. A similar phenomenon may be seen on certain samples of coarsest octahedrites, such as Mount Joy, El Burro and Union County, but is usually absent in hexahedrites. The presence of these cracks was the first indication that the "Smithsonian Iron" could not be a Coahuila specimen.

Etched sections reveal a coarsest Widmanstiitten struc­ture of short, bulky (~ ~ 4-8) kamacite lamellae with a

Figure 1662. Smithsonian Iron (U.S.N.M. no. 64). The opposite side of the same cut shown in Figure 1661, and with the light source differently oriented. Schreibersite is mainly located at grain bound­aries. Deep-etched. Scale bar 30 mm. S.I. neg. 1692D.

width ranging from 4 to 16 mm . Late grain growth has caused many of the Widmanstiitten boundaries to disap­pear; equiaxial kamacite grains, 6-20 mm across, are com­mon. The kamacite is rich in Neumann bands, and these are slightly decorated along both sides with 0.5 p. precipitates. The microhardness is 200±10; but since it was measured on a small polished section near a portion of the meteorite showing hammer and chisel marks, it is not entirely certain that this hardness is representative of the unaffected interior.

Taenite and plessite occur but only in minute amounts. A typical field measured 400 x 50 p., and showed dense unresolvable 0' + r areas and acicular plessite. The taenite and plessite are situated in the grain boundaries or are engulfed by growing kamacite grains; and it is estimated that they constitute less than 0.1 % by area. It is, however, interesting to note that they do occur at all on this low bulk nickel level of 5 .9%. They indicate the positions of former plessite fields which are now almost entirely resorbed.

Schreibersite occurs as scattered cuneiform monocrys­talline skeleton crystals, e.g., 10 x 2 and 10 x 4 mm across. It also forms 50-500 p. wide, irregular grain boundary precipitates and a 0.5-1 mm thick rim around the only larger troilite inclusion noted. Rhabdites are prominent and ubiquitous, usually as tetragonal prisms 2-10 p. thick.

The troilite inclusion is a branching monocrystalline unit measuring 17 x 2 mm. It contains about 15% daubreelite, exsolved as 1-500 p. wide parallel lamellae. The troilite-daubreelite aggregate is slightly deformed and dis­plays narrow shear zones with fragmentation and breccia­tion. Small troilite-daubreelite blebs are also present , e.g., as a 200 J1 aggregate of alternating 1 p. wide troilite and daubreelite lamellae.

Cohenite occurs as 10-50 J1 wide - but discontinuous - rims around schreibersite and troilite-daubreelite. Decom­position to graphite plus ferrite has not occurred.

The cracks in the meteorite observed on the surface, penetrate the whole mass , mainly along grain boundaries. On the large sections the Neumann bands are seen to be slightly distorted - as they also are in the center of the mass. It appears that the cracks and the bent Neumann bands have been caused by a violent shock, either preatmos­pheric or, more plausibly, by the severe deceleration that occurred during the atmospheric flight. The fissured grain boundaries then provided easy access for terrestrial ground waters.

Although the mass may appear to be severely weath­ered to a casual observer - the cracks are misleading, - it is not actually the case. Along several parts of the surface, the heat-affected 0'2 zone is still preserved as a 1-2 mm wide zone with micromelted rhabdites in the exterior part. The hardness is 180±10 (hardness curve type II) . It is estimated that the meteorite has lost 1-2 mm on the average by exposure to the terrestrial surroundings.

Corrosion penetrates along the brecciated minerals partly recementing them in a limonitic matrix. The troilite

shows pentlandite veining, and the near-surface Neumann bands are selectively corroded due to the sensibi!ization by tiny phosphide precipitates.

The Smithsonian Iron is a relatively well-preserved monolith with a coarsest Widmanstiitten structure, related to Mount Joy, El Burro, Sikhote-Alin and, distantly , Union County. At this late date it is probably not possible to trace the origin of the mass. With the knowledge o( its structure it is, however, possible (i) to rule out any relationship with Coahuila and (ii) to indicate a relationship with Mount Joy and El Burro in particular. All of these irons became known after the Smithsonian Iron and there is no particular reason for believing that the Smithsonian Iron should be an early discovered fragment of one of these falls. The conclusion must be that the Smithsonian Iron is an independent meteorite, possibly found in the eastern United States, but otherwise of unknown origin.

[J .T. Wasson (personal communication, 1974): Group liB with 5.65% Ni, 55.3 ppm Ga, 169 ppm Ge and 0.05ppmlr].

Specimens in the U.S. National Museum in Washington:

1.9 kg half mass (no. 64) 0.9 kg half mass (no. 64)

50 g polished section (former number no. 64) 60 g piece (no . 1001) I 0 g polished section (no. 1001)

Smithville, Tennessee, U.S.A.

35°57'48"N, 85°50'18"W; 325m

Coarse octahedrite, Og. Bandwidth 2.2±0.5 mm. Neumann bands. HV 215 ±20.

Group I. 6.95% Ni, 0.56% Co, 0.19% P, 91 ppm Ga, 363 ppm Ge, 2.0 ppm Ir.

Cookeville is probably a Smithville fragment.

HISTORY

At least ten individual fragments of this meteorite have been recovered since 1840 when the first mass, of 36 pounds (16.3 kg), was found a few miles west of Caney Fork, near the road from Liberty to the ferry on that river (Troost 1840; 1845). Farrington (1915 : 420) and Read (1963a; 1965) have reviewed the history, and the latter has reproduced a detailed map showing the localities and the point where, in 1962, he discovered mass No.8, of 2.9 kg, with an electronic metal detector. The coordinates for this mass are given above. All finds are in De Kalb County, most of them only 2-3 km west of Smithville.

The 16 kg mass (No. 1) is the only one which may have been found 10 km further east-northeast, but the informa­tion is insufficient. Numbers 2 and 3, of 3,460 g and about 500 g, respectively, were found about 1863 and were described by Glenn (1904). Specimens No.4, of 29.5 kg, and No.5, of 6.8 kg, were plowed up in 1892, while No .6, of 3.2 kg, was discovered after diligent search in December of the same year. These three masses were purchased by

Smithsonian Iron- Smithville 1147

H.A. Ward and cut and distributed by him; material was available as late as 1931 from Ward's Natural History Establishment, according to various price lists. The speci­mens were described by Huntington (1894) who gave photographs of the exterior and of an etched slice. He believed that he had identified a few , tiny diamonds in the insoluble residue from the analysis but unfortunately the evidence is insufficient. He supported his belief in the presence of diamonds by the observation that the mass had been extremely difficult to cut. This, however , must have been caused by the numerous inclusions of sizable cohenite crystals.

Mass No.7, of 2.1 kg, is discussed separately below as the Cookeville fragment. Number 8, of 2.9 kg, was dis­covered by Read in 1962, using a Hedden-Stockwell metal detector (Read 1963a; 1965), while Nos. 9 and 10, of 4.4 kg and 1.3 kg, respectively, were discovered by the same technique a year later (Read 1964a). The three specimens were situated on, or 20 em below, the surface; and Read presumed that the specimens had been turned over many times and moved slightly by plowing. He provided photographs of the exterior and of etched sections through the masses and also noted that some were inclusion-poor, others inclusion-rich.

Hintenberger & Wanke (1964) examined the content of noble gases. Vilcsek & Wanke (1963) determined 3~1 and 39 Ar and estimated the cosmic ray exposure age to be 3 20± 100 million years. Chang & Wanke ( 1969) measured , in addition, 1Dse and revised the cosmic age to 4 7±15 million years. They estimated the terrestrial age to be 630,000 years.

From the above data it appears that Smithville was a small shower of irons that scattered 0.5-30 kg fragments over an area, approximately 1 km in diameter. The total recovered weight is about 70 kg; but more fell, of course, since many specimens may have disintegrated completely

~·

. '

Figure 1663. Smithville (Copenhagen no. 1876, 2244). Coarse octahedrite of group I. Numerous rounded cohenite crystals have precipitated in the Widmanstiitten kamacite lamellae. Comb and net plessite and subboundaries in the kamacite are prominent. Etched. Scale bar 3 mm.

II48 Smithville

by weathering during a long terrestrial exposure, and others may still be waiting for their discovery, as effectively proven by Read.

COLLECTIONS

New York (16.5 kg), Washington (3.74 kg), Chicago (3.70 kg), Harvard {3.30 kg), Paris (I ,755 g), London {1,713g), Tempe (l,274g), Vienna {1,252g), Hamburg (580g), Ottawa {51 I g), Berlin {408g), Delft {288g), Leningrad (286 g), Stockholm (203 g), Calcutta ( 17 8 g), Rome {165 g), Ti.ibingen (159 g), Budapest (138 g), Ann Arbor (130 g), Copenhagen (I20 g), Vatican (I09 g), Amherst {68 g), Yale {61 g), Prague {60 g), Bonn (26 g).

DESCRIPTION

The shapes of the masses have been described as roughly spherical or ovoidal by the various discoverers. The largest specimen in the U.S. National Museum is an endpiece, of 1.9 kg, - probably half of mass No. 6 - found in I892. It is badly weathered, with no fusion crust and no heat-affected rim zones preserved. It is exfoliating along the Widmanstatten planes and along other grain boundaries and is slowly disintegrating even under normal dry room conditions. Corrosion penetrates to the center of the mass which originally was about IO em in diameter. The external shape and the state of corrosion are typical for all Smithville specimens - except that some remain intact even

Figure 1664. Smithville (Copenhagen no. 1876, 2244). The top of a cohenite crystal (C) with schreibersite precipitate (S). Taenite (T). Kamacite with decorated subboundaries and numerous fine phos­phides in the interior. Etched. Scale bar 300 ll·

after 5-l 0 mm of the exterior surface has transformed to terrestrial oxides. The 2.9 kg mass (No.8), found by Read in 1962, was donated to the U.S. National Museum. In order to preserve it, the mass was cut in a number of slices, which were polished and etched and then completely embedded in two-component epoxy plastics in 1964. Even so, some of the specimens still corrode and are now ( 1969) slowly "exploding" the plastic.

Etched sections display a coarse Widmanstatten struc­ture of straight, short (W ~ 8) kamacite lamellae with a width of 2.2±0.5 mm. The narrowest lamellae are asso­ciated with the cohenite-rich sections. In cohenite-poor sections late grain growth has locally erased the Widmanstat­ten structure and created almost equiaxial kamacite grains, 5-8 mm in diameter. The kamacite has Neumann bands, but what is particularly characteristic for Smithville are the numerous, marked subboundaries which are profusely decorated with rhabdites, I-10 J1 thick. The hardness is 2I5±20.

Taenite and plessite cover 2-5% by area, mainly concentrated in the cohenite-rich parts. Comb and net plessite fields are common varieties, but an unusually large proportion of the taenite ribbons are decomposed to pearlitic structures. The pearlite has subparallel taenite lamellae, ranging from 0.3-2.5 J1 in thickness; the hardness ranges correspondingly from 250 to 220. The pearlite often has its kamacite component selectively oxidized to "limon­ite," thereby forming what has been termed oxypearlite (LOfquist and Benedicks 194I); the hardness is 375±50. The taenite rims are stained yellowish-black and have a hardness of 425±25. Locally, a few acicular-martensitic areas may be found in the taenite interior. Minute amounts of haxonite aggregages occur within some plessite fields.

Schreibersite is present as, e.g., I 0 x I mm skeleton crystals, enveloped in 24 mm wide rims of swathing kamacite. The hardness is 930±30. It further occurs as I0-100 J1 wide grain boundary veinlets and as 0.5-1 mm thick rims around the troilite nodules. Rhabdites are very common as tetrhaedral prisms, ranging from 0.4-10 J1 in cross section.

Cohenite is extremely common in some sections. Read ( I963: figures 3 and 4) gave a good photomacrograph showing the typical distribution, except that he believed the inclusions to be schreibersite. The cohenite forms elongated, rounded bodies, typically 3 x 0.6 mm in size,

SMITHVILLE - SELECTED CHEMICAL ANALYSES

Read (1963a) assumed that the chlorine present was due to lawrencite; but he did not actually see any lawrencite ; it is almost certain that the chloride was

References

Huntington 1894 Buchwald 1967,

unpubl. Wasson 1970a

percentage Ni Co P

7.02

7.06 6.78

0.62 0.18

0.51 0.20

c

introduced by ground water during a long terrestrial exposure.

s Cr Cu ppm Zn Ga

96 86.9

Ge Ir Pt

363 2.0

Figure 1665. Smithville (Copenhagen no. 1876, 2244). Cohen­ite (C). Kamacite with subboundaries that are anchored by small plate-shaped rhabdites. Etched. Scale bar 200 p.. See also Figure 96.

Figure 1666. Smithville (Copenhagen no. 1876, 2244). Typical pearlitic plessite with cloudy taenite edges. Etched. Scale bar 30 p..

which are aligned along the Widmanstiitten directions and are closely associated with (pearlitic) taenite. Cohenite has further precipitated upon preexisting schreibersite, both of the skeleton crystal type and of the troilite rim type. All cohenite contains inclusions of schreibersite, taenite and kamacite, the latter often severely corroded. The cohenite has a hardness of III0±40.

Troilite and troilite-graphite nodules are rather com­mon, ranging in size from I-5 em. In U.S.NM. No. 202 the three main varieties occur together: (i) almost pure graph­ite, (ii) almost pure troilite and (iii) a I: I mixture of troilite and graphite. The graphite (i) is an aggregate of palmate and fan-shaped crystallites in which I-10 J1 blebs of troilite (3%) and kamacite (2%) are embedded. The troilite (ii) is monocrystalline and contains about 5% daubreelite as narrow, parallel lamellae . Its hardness is 260±IO. Locally, along shear planes and phase boundaries it is micromelted

Smithville II49

n"l""l•"'l'll'l""lll••ltiiii iiiii""JIIIII""!'II'I''uln"I''''I"IIIII"J'"'I'n'l .. "lll ll l C M 1 2 . 3 4 5 6 7 8 9 10

Figure 1667. Smithville (Tempe no. lla). The coarse Widmanstii t­ten structure is penetrated by terrestrial corrosion. Above 6-7 a troilitc-graphite inclusion is seen. Deep-etched. Scale in centimeters. (Courtesy C.B. Moore.)

and fragments of the adjacent minerals have become suspended in the melt, that has solidified to an aggregate of I-IO J1 grains. The graphite-troilite mixtures (iii) occur in millimeter-sized patches, perhaps particularly along the exterior of the pure troilite nodules. The graphite forms flakes or chips, typically 3 x I 0 J1 in size, scattered through a monocrystalline troilite matrix.

Some of the graphite has crystallized as perfect cliftonite crystals, 30-I50 J1 across. Most of them are cubes, judging from the hexagonal and cuboid forms, appearing on sections; and they are uniformly oriented within the same kamacite grain. They are also found in the schreibersite and in the troilite-schreibersite interphase.

In one place a green, pyroxenoid mineral, 0.5 x 0.2 mm in size, is embedded in troilite. It is emerald-green under crossed Nicols and is perhaps ureyite (=kosmochlor) , described by Fronde} & Klein (1965) from Toluca and Coahuila.

Chromite is present as an accessory in troilite, as 0.2-0.5 mm crystals. A white, polycrystalline mineral, almost luminescent under crossed Nicols, occurs as 0 .1-I mm patches in some troilite crystals.

If it were not for the unfortunate, penetrating corro­sion Smithville would be one of our finest octahedrites. It exhibits the pearlitic plessite and the cliftonite crystals particularly well and appears to contain numerous other minerals. It is closely related to Cranbourne, Yardymly, Campo del Cielo, Canyon Diablo and Cosby's Creek. It is understandable that Huntington (I894) believed that Cosby's Creek and Smithville were a paired fall , since both are severely corroded group I irons. However, small, but significant differences in structure and chemical composi­tion prove them to be independent falls. The following entry, Cookeville, is, however, no doubt a Smithville fragment.

1150 Smithville - Smithville (Cookeville)

Specimens in the U.S. National Museum in Washington:

205 g part slice (no. 202, 5 x 4 x 1.5 em) 1,904 g endpiece (no. 1145, 10 x 10 x 6 em) About 1,200 g various slices fully embedded in plastic (no. 2252,

from No.8) 430 g various fragments and part slices (nos. 660, 661, 3072,

3073,3074,3075,3373)

Smithville (Cookeville), Tennessee, U.S.A.

Coarse octahedrite, Og. Bandwidth 2.3±0.6 mm. Neumann bands. HV 215 ±20.

Group I. About 6.9% Ni, 0.2% P, 91 ppm Ga, 384 ppm Ge, 2.1 ppm Ir.

Reasons are given below for considering Cookeville a fragment of Smithville.

HISTORY

An iron, originally of 2.1 kg weight, was briefly examined by Merrill (1916c) who produced photographs of the exterior and of an etched slice. Next to nothing was known about its origin, except that it had been found about 1913 and had apparently reached Ward's Natural History Establishment through some agent in Cookeville, Putnam County. While Merrill retained a pound for the U.S. National Museum, the remaining mass was sliced and offered for sale by Ward's. Perry (1944: plate 67) gave a micrograph, showing the extensive weathering of the mass, and Buddhue (1957: 1 09) discussed the oxidation characteristics.

COLLECTIONS

Washington (459 g), New York (294 g), Chicago (216 g), London (I 35 g), Harvard (79 g), Budapest (?).

DESCRIPTION

The iron was a roughly polygonal mass about 9 x 8 x 8 em in average dimensions. The endpiece (No. 518 in the U.S. National Museum) shows the extensive corrosion. It is heavily coated with oxides, forming a 1-6 mm thick, adhering crust. Spalling has, however, removed parts of the surface so that the original form may only be guessed. Already the polished sections display the coarse Widman­stiitten structure, since oxidation has primarily attacked the a-phase immediately around taenite and, therefore, has served as a natural etching reagent. The reason for this localized attack is the chemical potential between the

nickel-poor a-phase and the nickel-rich r-phase, plus the fact that the nearest 50-100 J.l of the a-phase is depleted in nickel, as repeatedly shown by microprobe techniques, on other meteorites. The corrosion further follows the schrei­bersite inclusions and selectively converts the a-phase around the rhabdites to oxides. The primary causes for these attacks appear to be the same as mentioned above, a chemical potential enchanced by nickel-depletion in the immediate surroundings. Minor attacks follow the a-sub­grain boundaries, the reason again being the sensitizing of the boundaries by the presence of micron-sized rhabdites and microsegregation. On a polished section it is hardly possible to select even as small an area as 1 mm 2 without some form of corrosion being present.

The bandwidth of the coarse Widmanstiitten structure is 2.3±0.6 mm. In cohenite-poor areas grain growth has often eliminated the Widmanstiitten pattern and created almost equiaxial grains, e.g., of 8 x 6 or 5 x 5 mm size. The kamacite has Neumann bands, and the subboundaries are decorated by minute phosphide particles. The hardness is 215±20. Plessite is scarce but may occasionally be observed as degenerated, open-meshed comb plessite. Taenite is rather common, particularly as 25-50 J.l wide veinlets in more or less direct contact with the cohenite crystals. Where the taenite swells to wedge-shaped bodies, the interior is decom­posed to pearlitic and spheroidized structures. Often the terrestrial weathering has transformed only the a-matrix of these duplex structures al}d thus created a stunning natural etching, where 0.3-1 J.l wide, light austenite lamellae are situated in a dark background of oxides (oxypearlite).

Schreibersite is present as 20-100 J.l grain boundary precipitates and as similar sized rounded inclusions in the cohenite crystals. Rhabdites are very common as 2-10 J.l

tetragonal prisms. Cohenite occurs in patches, as is usual in group I, of

typically 4 x 1 mm rounded crystals with small inclusions of kamacite, taenite and schreibersite. They are monocrys­talline and were apparently somewhat brecciated before the corrosion started. They are now penetrated by zigzagging veinlets of oxides. The cohenite has a hardness of 1110±40.

· Although troilite was not observed in the specimens in the U.S. National Museum, it is present in other sections. Merrill (I 916c) mentions in a footnote a 5 x I em troilite body with a rim of schreibersite on one of the specimens sold by Ward's.

SMITHVILLE (COOKEVILLE) - SELECTED CHEMICAL ANALYSES

Whitfield in Merrill (1916c) reported 6.38% Ni and analyzed, it is safe to assume that the original nickel value 0.17% P on severely corroded material. Since the structure was 6.8-7 .0%, but that oxidation changed the relative clearly indicates that Cookeville is related to such irons as proportions of the elements. Smithville, Cranbourne and Canyon Diablo, which are well

References percentage

Ni Co P c

Wasson 1970a 6.5±0.5

s Cr Cu ppm Zn Ga Ge lr Pt

91.4 384 2.1

Cookeville is a typical inclusion-rich coarse octahedrite, closely related to such group I irons as Cranbourne and Canyon Diablo. The probability that it is a transported fragment of the Smithville shower is very high, since the macro- and microstructure, the microhardness, the main and trace elements, and the state of corrosion are identical. Cookeville, the town in which the mass first appeared, is only 35 km northeast of Smithville.

Specimens in the U.S. National Museum in Washington:

371 g endpiece (no. 518, 7 x 7 x 2 em) 15 g oxidized fragment (no. 1587, detached from no. 518) 72 g oxidized fragments (no. 2731,4 x 2.5 x 0.8 em, and smaller)

Social Circle, Georgia, U.S.A.

33°39'N, 83° 42'W

Fine octahedrite, Of. Bandwidth 0.30±0.04 mm. Recrystallized. HV 166±8.

Group IVA. 7.61% Ni, 0.38% Co, about 0.05% P, 1.63 ppm Ga, 0.092 ppm Ge, 2.8 ppm Jr.

HISTORY

A mass of 219 pounds (100 kg) was found in 1926 while plowing was underway in a field on the plantation of W .B. Spearman, a few miles from Social Circle, Walton County. The actual location of find may have been just over the border line in the neighboring Newton County (E.P. Henderson, personal communication). The coordi­nates given above are for Social Circle. The mass was

Figure 1668. Social Circle (U.S.N.M. no. 2580). The meteorite has been cut parallel to its lengthwise direction. Deep-e tched. Scale bar approxima !ely 5 em. S.I. neg . 38600.

Smithville (Cookeville) - Social Circle 11 51

recognized as a meteorite by the state geologist of Georgia, S.W. McCallie, who acquired it from the owner. McCallie (1927) described it on the basis of a small wedge-shaped section that had been removed by a Colored man who used 11 hacksaw blades in the process. Merrill (1927b) suggested that Social Circle was related to Western Arkansas. This happened to be correct, although the premises were two analyses which by a curious coincidence were both much -but to about the same degree - deficient in nickel (5.02 and 5.12%Ni, respectively). Henderson & Perry (1951 b) and Henderson & Furcron (1957) reexamined the meteorite and produced photomacrographs of the exterior and of etched slices. On that occasion the meteorite was cut, essentially parallel to its long dimension; and several slices were taken from the smaller specimen at right angles to the direction of the long cut. They found the iron to be homogeneously granulated throughout the entire mass and suggested that the effect was due to a near-solar passage.

COLLECTIONS

Georgia State Museum , Atlanta (about 70 kg) , Wash­ington (22.2 kg), Chicago (341 g), Agnes Scott College, Decatur, Georgia (300 g), Albuquerque (68 g).

DESCRIPTION

The mass had, according to McCallie (1927) , the average dimensions of 37 x 32 x 22 em and had the shape

Figure 1669. Social Circle (U.S.N.M. no. 1675). A recrystallized fine octahedrite of group IV A. Comb , net and cellular plessite fields are common and some are unresolvable and black at this magnifica­tion. Etched. Scale bar 600 !J..

SOCIAL CIRCLE - SELECTED CHEMICAL ANALYSES

Henderson & Furcron (1957) believed that the cobalt analysis was too low and suggested 0.50% as a correct value. This appears, however, to be an erroneous assumption; a

percentage References Ni Co p c Henderson & Perry

195lb 7.44 0.38 Wasson & Kimberlin

1967 7.78

redetermination is considered necessary before the original value can be disqualified.

s Cr Cu ppm Zn Ga Ge Ir Pt

1.63 0.092 2.8