American Mineralogist, Volume 7 1, pages 1 3 2 2-l 3 36, I 986

Complex exsolution in inverted pigeonite: Exsolution mechanisms andtemperatures of crystallization and exsolution

Wrr-r-rlvr A. RlNsoNDepartment of Geology, Furman University, Greenville, South Carolina 29613, U.S.A.

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The results of electron-microprobe and optical examination of Ca-poor pyroxenes fromanorthositic and intermediate rocks of the Nain complex, Labrador, show a range in hostcomposition from lO0Fe/(Fe + Mg) : 34 to 77 and three distinct exsolution types. TypeI pyroxenes have Bushveld-type exsolution with fine (100) augite lamelle and were primaryorthopyroxenes. Type 2 and type 3 pyroxenes are inverted pigeonite with Stillwater-typeexsolution. Type 2 varieties display irregular and unoriented lamellae and blebs of augite.Type 3 pyroxenes show complex exsolution features including broad "00l" and "l00"augite lamellae, multiple sets of pigeonite lamellae exsolving from "00l" augite, fine (100)augite lamellae, and patches of uninverted pigeonite. Stacking faults, which are present inorthopyroxene and pigeonite host and "00l" augite lamellae, bend 2-4 it a counterclock-wise direction as they pass from host into "001" augite.

Exsolution occurred by the mechanism of nucleation and gFowth for all three pyroxenetypes. Observations suggest that homogeneous nucleation produced exsolution in type Iand type 3 pyroxenes but that heterogeneous nucleation is responsible for the irregularexsolution in type 2 pyroxenes. Conventional estimates of crystallization temperatures areunrealistically low for type I pyroxenes, probably because ofgranule exsolution that pro-duced discrete pyroxenes that were not included in microprobe analyses. Type 2 and type3 pyroxenes crystallized at temperatures between I 100 and I 000.C at approximately 3-kbarpressure and began to exsolve augite. During initial cooling, strain developed along augite-pigeonite interphase boundaries, and stacking faults formed to relieve the strain. Finestacking faults that propagated into the host from augite lamellae provided the mechanismfor the pigeonite to orthopyroxene transformation. Some constrained patches of host pi-geonite failed to invert. Coarse augite exsolution continued down to temperatures perhapsas low as 600"C. Pigeonite exsolution in "001" augite likely began before inversion andcontinued metastably to low but uncertain temperatures.

rnrnooucrro* i#,i:1H:11';?:?Ji :l',:?li:T;trii'";11'"'l'lli;Pyroxenes that have exsolved during cooling have been of these studies confirmed that (100) lamellae in augite

reported in both igneous and metamorphic rocks from a hosts were orthopyroxene and that the other two sets werewidevarietyofenvironmentsofformation.Recently,con- in fact pigeonite, which were designated as "00l" andsiderable attention has been given to the orientation of "100" lamellae, after the usage of Robinson et al. (1971),exsolution lamellae with respect to the host crystal. Jaffe which refers to orientations not quite parallel to (100) andand Jaffe (1973) studied pyroxene granulites ofthe Mon- (001), respectively. Jaffe et al. (1975) described exsolutionroe quadrangle in the Hudson Highlands of New York lamellae in various metamorphic augites and related theand reported three sets of lamellae in augites. Of these orientation of the lamellae to composition. Robinson etlamellae, only one set had a crystallographic axis that al. (1977) turned their attention from metamorphic tocoincided with a crystallographic axis of the host, in this igneous augites, which experienced slow cooling from highcase the c axis. Optical observations of grain mounts con- temperature. They showed that the orientation of exso-firmed that the remaining two sets of lamellae were not lution lamellae in igneous augites is dependent on high-parallel to the (100) and (001) directions as proposed by temperature lattice parameters and suggested that lamel-Poldervaart and Hess (1951). Since this study, more de- lae orientation can be an effective geothermometer.tailed investigations of exsolution features have been made The present study describes a wide variety of exsolutionby Robinson et al. (1971), Jaffe et al. (1975), and by Rob- types in Ca-rich and Ca-poor pyroxenes, with emphasisinson et al. (1977), who made use of single-crystal X-ray on augite exsolution in inverted pigeonite, and presentsphotographs to confirm the structures of lamellae and to optical and microprobe data on pyroxenes as well. All ofdevelop the principle ofthe optimal phase boundary (Boll- the pyroxenes discussed in this report are from the Nain

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RANSON: COMPLEX EXSOLUTION IN INVERTED PIGEONITE r323

Fig. l. Geologic map of Nain complex showing boundaries of Puttuaaluk Lake area and locations of pyroxene specimens. SeeApp. I for specimen descriptions. Map compiled by D. de Waard based primarily on field work of E. P. Wheeler II.

r324

complex> Labrador, specifically from the Puttuaaluk Lakearea (Ranson, L978,1979, l98l). This area transects theHelikian Nain complex, which consists of greater thanl0 000 km' of anorthositic rocks and a nearly equal areaofintermediate to granitic rocks (Fig. l). The anorthositic,intermediate, and granitic rocks of the Nain complex areunmetamorphosed, and all of the investigated pyroxenesare the products of slow cooling from their temperaturesof crystallization at pressures of 3 to 4 kbar (Bery, 1977 ,and pers. comm., 1983).

The petrography of the pgoxene-bearing rock typesfrom the Nain complex is briefly discussed in the nextsection. Detailed descriptions of the nine specimens con-sidered in this report are found in Appendix l.

PBrnocru,prrv

Seven ofthe nine pyroxene specimens investigated arefrom anorthositic rocks; the remaining two samples arefrom granitic and intermediate rock types. Anorthositicrocks have been divided by Ranson (1979, l98l) intothree mappable units: anorthosite, leuconorite, and oxide-rich anorthosite. Pyroxenes occurring in these three an-orthositic units are different in composition, grain size,habit, and in textural relationship to plagioclase. The an-orthosite unit has a color index less than 10 and containspredominantly Ca-rich pyroxenes, which occur as fine-grained, anhedral, interstitial phases. The leuconorite hasa color index averaging between l0 and 30 and carriesCa-poor pyroxenes as the dominant mafic mineral. Ca-poor pyroxenes are primary orthopyroxene in leucono-rites of magnesian composition and inverted pigeonite inmore Fe-rich leuconorites. Most pyroxenes in leuconoritesoccur as medium-grained, subhedral to euhedral phases,suggesting simultaneous crystallization of plagioclase andpyroxene (Ranson, 1981). The oxide-rich anorthosite, withcolor index between 5 and 30, is distinguished by 5 to150/o Fe-Ti oxide minerals in addition to chiefly Ca-poorpyroxenes. For all of the oxide-rich anorthosite specimensstudied, the Ca-poor pyroxene is inverted pigeonite ratherthan primary orthopyroxene.

Granitic and intermediate rocks in the Puttuaaluk Lakearea include granite, quartz monzonite, monzonite, quartzmonzodiorite and diorite, all of which show strong Feenrichment. Ca-rich p)'roxenes may be present in all ofthe above rocks, whereas Ca-poor pyroxenes (invertedpigeonites) are restricted to quartz monzodiorite, mon-zodiorite, and diorite. In the more Si-rich rocks approach-ing granite in composition, Ca-rich pyroxenes are reducedin abundance, and Fe-rich amphibole is the major maficmineral. Fayalitic olivine is commonly associated vrithCa-rich pyroxenes but rarely coexists with inverted pi-geomtes.

Or"rrc,l.r, pRopERTTES AND coMposrrroN oFPYROXENES

Determinative methods

Measurement of n, of Ca-rich and Ca-poor pyroxeneswas made on crushed grains in oil-immersion mounts.

RANSON: COMPLEX EXSOLUTION IN INVERTED PIGEONITE

Some specimens contained only minor or interstitial Ca-rich pyroxene that could be analyzed by electron micro-probe in thin section but did not appear in grain mounts,and hence r?1 was not obtained for these pyroxenes. Forreasons cited in Jaffe et al. (1975), n, rather than nu wasmeasured for Ca-rich Pyroxenes.

Chemical analyses of pyroxenes were obtained by elec-tron microprobe. Microprobe analyses were performed atl5-kV accelerating potential and an aperture current of0.03 pA on an automated electron microprobe at eitherthe University of Massachusetts (Erec autoprobe) or theMassachusetts Institute of Technology (uec 500 probe).Standards used to report element weight percent valueswere as follows: synthetic pyroxene (Di65Jd3s) for Na andSi (source: F. R. Boyd, Geophysical Laboratory); syntheticCr-rich diopside for Mg, Si, and Ca (source: J. F. Schairer'slab, Geophysical Laboratory); chromite for Al and Cr(source: Stillwater complex, 52-NL-11); sanidine for Siand K (source: R. V. Dietrich, Leilen Kopf, Germany);Mn-bearing ilmenite for Ti, Mn, Fe (source: USNM sam-ple 96189). Matrix corrections were made following theprocedure of Bence and Albee (1968), using the alphafactors of Albee and Ray (1970). An electron-beam di-ameter of 0.5-1.0 pm was used for analyzingbroad exso-lution lamellae and for the host pyroxene in between la-mellae. Where exsolution lamellae were too fine to beresolved with the electron beam, a broader beam with adiameter of 10-12 pm was used. Bulk compositions forexsolved pyroxenes were calculated by first determiningthe proportion of host to lamellae from enlarged photo-micrographs of pyroxene crystals. Paper tracings of hostand lamellar portions of single grains were cut apart andweighed to obtain the host: lamellae volumetric ratio.Probe analyses of host and lamellae were then used in theindicated proportions to determine the bulk composi-tions. The compositions taken for host and lamellar phas-es are averages of 10 to 30 spot analyses.

Results

Exsolution lype, n, and composition of Ca-poor andCa-rich pyroxenes are posted in Table l. The n, for augiteis listed for only two samples in Table I for the reasoncited above (see App. I for complete sample descriptions).Complete chemical analyses of hosts, lamellae, and bulkgrains with formulae for both Ca-poor and Ca-rich py-roxenes are tabulated in Table 2. Figure 2 shows the vari-ation of n, and the atomic ratio 100(Fe, + Mn)/(Mg +Fq + Mn) for orthopyroxenes and inverted pigeonites.Scatter about the visually fit curve and the least-squareslinear best fit probably reflects the difrculty of obtainingaccurate analyses ofthese highly exsolved host pyroxenes.Despite the scatter, the curves do not difler greatly fromthose obtained by Jaffe et al. (1975) for metamorphicorthopyroxenes.

Even with the difficulties involved in obtaining mean-ingful analyses offinely exsolved pyroxenes, most oftheanalyses posted in Table 2 have cation totals of 4.00 +0.02 based on six oxygens. Nonquadrilateral components,

Table 1. Exsolution type, n , and composition of pyroxenesfrom the Puttuaaluk Lake area

opx Aug

Comp.-' Comp..'

1 . 7411 7085

1.73731.74261.722't.748

1 .7651.763

. Refers to the three types of exsolution observed in Ca-poor pyrox-enes: see text for descriptions.

** Composition: 100(Fq + Mn)/(Mg + Fe, + Mn) for host.

including Al2O3, TiOr, KrO, NarO, MnO, and CrrOr, areslight in amount, usually summing to less than l.50wt0/0.For all microprobe analyses, total Fe was determined andreported as FeO. Wo, En, and Fs contents, as indicatedin Table 2, were calculated according to the method ofLindsley and Andersen (1983) for application oftheir py-roxene geothermometer.

Exsor,urrox rN Ca-poon PyRoxENES

Morphology

Inasmuch as exsolution features in Nain augites arenearly identical to those described elsewhere (Robinsonet al., 1977; Rietmeijer, 1979; Huntington, 1980), thesepyroxenes will receive only brief mention. The major de-scription and discussion will be devoted to the exsolutionin Ca-poor pyroxenes. Ca-poor plroxenes from the Put-tuaaluk Lake area have been grouped into three types onthe basis of optically observed exsolution features (Ran-son, 1978). Type I pyroxenes are primary magnesian [Fe, *Mn)/(Mg * Fe, * Mn) < 601 orthopyroxenes and lackexsolution or more typically contain only fine (100) augitelamellae (Fig. 3). This type of exsolution, described byHess and Phillips (1940) as the Bushveld type, is believedto have formed upon cooling of orthopyroxene precipi-tated directly from the liquid. The magnesian composi-tions are in accord with the findings of Poldervaart andHess (1951) and Wager and Brown (1967), who showedthat for magnesium compositions, crystallization takesplace within the orthopyroxene stability field, whereas forFe-rich compositions, crystallization takes place withinthe stability field of monoclinic pyroxene. Many factors,in addition to Fe enrichment, play a role in determiningwhether orthopyroxene or pigeonite is the pyroxene form-ing on the liquidus. Some ofthese factors will be addressedin the discussion section.

The second category ofCa-poor pyroxenes, type 2, con-sists of pigeonites that have exsolved augite and under-gone transformation from the C2/c or P2,/c structure ofpigeonite to the Pbca structure of orthopyroxene. Exsolvedpyroxene in type 2 pyroxenes takes the form ofirregularlamellae, granules, and blebs of augite 40-100 pm in width.Blebs of augite are variable in size and shape and show

1325

no preferred crystallographic orientation relative to thehost. Irregular lamellae are generally aligned subparalleland usually go to extinction simultaneously in cross-po-laized light, indicating like crystallographic orientationwith respect to each other. Exsolved augite may itselfcontain finely exsolved pigeonite lamellae of "100" and"001" orientation. Some type 2 pyroxenes contain fine(100) augite lamellae more characteristic of type I pyrox-ene. Granules of augite at the margins oftype 2 pyroxenesprobably have resulted from granule exsolution. Photo-micrographs of pyroxene crystals illustrating type 2 exso-lution are shown in Figure 4.

Type 3 pyroxenes contain the most interesting and com-plicated exsolution features> some of which have not beenpreviously described to the writer's knowledge. Like thetype 2 plroxenes, type 3 pyroxenes are inverted pigeon-ites, and both types belong to the category referred to asStillwater-type pyroxenes by Hess (1941).

The most distinctive features of the type 3 pyroxenesare the broad (40-50 pm in width), regularly spaced augitelamellae oriented parallel to an irrational plane near (001)of the host inverted pigeonite. Because these lamellae areanalogous to the broad "001" (Pig I) lamellae in hostaugite described by Robinson et al. (197l, 1977), they areclassed here as Aug I. These broad "001" augite lamellae(Aug I) are typically continuous across entire crystals andslightly curved at the ends (Fig. 5a). Robinson et al. (1977)have pointed out that curved lamellae record changinglattice parameters associated with changing environmen-tal conditions of pressure, temperature, and composition.Up to two sets of "001" pigeonite lamellae have in turnbeen observed enclosed in the coarse Aug I lamelle, abroad set (Pig II),0.5-1.0 pm in width, and a finer set(Pig III) 0.05-0.1 pm in width, at a slight angle to thebroader set (Fig. 5b). These are the Pig II and Pig IIIlamellae of Robinson et al. (1977). Coarse Pig I lamellae(thickness 5-12 pm) of Robinson et al. (1977) were notobserved. Both Pig II and Pig III lamellae are short anddiscontinuous across enclosing augite lamellae. Pig III la-mellae are more numerous than Pig II lamellae and areconcentrated away from them (Fig. 5b). In addition to thebroad set of augite lamellae oriented on "00l" (Aug I), asecond set of augite lamellae occurs with a "100" orien-tation. Aug "100" lamellae are 5-10 pm in width andcontact broader Aug I lamellae but do not penetrate them.Aug "l00" lamellae are straight and typically continuous,although they may be confined to certain portions of anindividual grain, as seen in Figure 6. Also, Aug "100"lamellae are most abundant in portions of crystals whereAug I lamellae are absent (upper half of Fig. 6). Fine (< Ipm) (100) augite lamellae of the Bushveld-type may bepresent in the orthopyroxene host and represent low-tem-perature exsolution after the original pigeonite had in-verted to orthopyroxene.

Some type 3 pyroxenes, notably those found in PL- l9 I(see App. l), conlain a second, much narrower set of "001"augite lamellae (Aug II). The Aug II lamellae are ratheruncommon, are discontinuous across the host crystal, and

RANSON: COMPLEX EXSOLUTION IN INVERTED PIGEONITE

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o ro 20 30 40 50 60 70 80 90 tooI o o (Fe?Fe3i Mn) /{u9* re2i reri N n )

Fig. 2. The ratio 100 (Fd* + Fe3* + Mn/Mg + Fe2+ + Fe3+ +Mn) versus n, for Ca-poor pyroxenes from Puttuaaluk Lakearea. Dashed line is result oflinear regression analysis with 12 :0.969. Theequationforthislineis n,: 0.1235(Fe, + Mn/(Mg +Fe, + Mn)l + 1.6689. Solid curve is visually fit to the data. Opencircles are indices ofsynthetic enstatite (1.658) and orthoferro-silite (1.789) from Stephenson et al. (1966) and Lindsley et al.(1964), respectively.

occur midway between broad Aug I lamellae at a slightlysteeper orientation (Fig. 6).

Another interesting characteristic of type 3 pyroxene isthe presence ofpatches ofpigeonite in the host regions ofthe crystal, which suggest that transformation of the hostto orthopyroxene was not complete. Patches of pigeoniteare referred to as Pig O since these remnants of originalhost pigeonite preceded pigeonite exsolved from broadAug I lamellae. Pig O patches, as seen in Figure 6, appearbordered on all sides by Aug I, Aug II, and Aug "100"lamellae and are thus isolated, at least in the plane of thethin section, from the rest of the host. Aug II lamellaehave not been observed penetrating pigeonite patches.

Other features characteristic of type 3 pyroxenes arenumerous, fine stacking faults similar to those describedby Robinson etal. (1977) in Bushveld augites containingpigeonite exsolution. These stacking faults, discernible onlyat high magnification, are oriented on ( I 00) and cut throughorthopyroxene host, patches of uninverted pigeonite host,and broad "00l" augite lamellae (Fig. 5b). Stacking faultsrotate 2-4 in a counterclockwise sense as they pass fromhost and pigeonite patches into broad "001" augite la-mellae but have the same orientation in both the ortho-pyroxene and pigeonite host regions. In pyroxenes fromPL- 19 l, stacking faults are particularly numerous and ap-

RANSON: COMPLEX EXSOLUTION IN IIWERTED PIGEONITE

Fig. 3. Photomicrographoftype I (Bushveld-type)exsolutionin leuconorite sample PL-150. Exsolution consists offine (100)augite and parallel Fe-Ti oxide minerals. Scale bar is 0.10 mm.

pear to be accompanied by fine, parallel exsolution la-mellae, probably Aug (100).

In some anorthositic specimens, notably PL-187 (seeApp. l), Ca-poor and Ca-rich pyroxenes appear to becompletely intergrown. In such cases, sizable patches oforthopyroxene are penetrated and bordered by coarse"001" augite lamellae. The bordering augite lamellae arenot actually lamellae but extend outward to form the hostportion of the augite crystal.

Discussion

Type I pyroxenes crystallized as orthopyroxenes belowthe pigeonite - orthopyroxene + augite transition tem-perature. Ross and Huebner (l 979), through their heatingexperiments, have shown the dependency of the temper-ature of this reaction upon Fe/Mg ratio. At I bar theydetermined that the upper temperature limit for the min-imum stability temperature of pigeonite varied from1284'C at Mg/(Mg + ZFe) :0.92 (atomic) to 901"C at0.04. In the same study Ross and Huebner also comparedtwo orthopyroxenes in order to show the relationship be-tween temperature and the Ca content of orthopyroxeneinvolved in this reaction. A second purpose was to illus-trate the interface reaction mechanism. Their orthopy-roxene sample 3 (-Caoo,rMg, rorFeorrrSirOu), which con-tains lamellae of augite (orientation not given) 7 to 20 pmthick, had pigeonite first appearing in the run productsheated to 1259C. A second sample, sample 29, consistsof chemically homogeneous orthopyroxene (Caoo,r-Mg, uruFeo rrrSirOu), which contains no lamellae of augite.

PL-2 | |

RANSON: COMPLEX EXSOLUTION IN INVERTED PIGEONITE 1329

Fig. 4. Photomicrographs illustratlng type 2 exsolution in inverted pigeonites. (a, left) Exsolved augite in the form ofblebs andgranules in sample IR-31. (b, right) Inverted pigeonite sample PL-I17 with irregular exsolved augite. Scale bar is 0.10 mm.

Being less magnesian than sample 3, it was expected toform pigeonite at a lower temperature. In fact, pigeonitedid not appear in the run products until 1375t. Ross andHuebner concluded from these experiments that the firstappearance ofpigeonite is dependent on notjust the Fe/Mg ratio but also on the local Ca content and the presenceof augite lamellae, which serve as favorable nucleationsites. Results of experiments on sample 3 clearly showthat orthopyroxene reacted with adjacent augite lamellaeto form pigeonite + melt.

Other factors may play a role in determining whetherorthopyroxene or pigeonite is the pyroxene forming onthe liquidus. Huebner and Turnock (1980) have shownin a series of melting experiments on synthetic and naturalpyroxenes that minor elements, especially Al and Ti, areimportant in determining the position of the solidus.Using synthetic compositions in the system CaSiOr-MgSiOr-FeSiOr, their experiments show that the solidusat constant Wo : l5olo lies at temperatures above the pi-geonite - orthopyroxene + augite transition, thus allow-ing Fe-free pigeonite to exist. In contrast, their results inmelting experiments on natural pyroxenes reveal that thesolidus occurs at lower temperatures than for the puresystem and below the pigeonite - orthopyroxene + au-gite transition for magnesian compositions [Fe/(Fe +Mg) < 0.201 (see their Fig. l2).

Pressure also affects the pigeonite - orthopyroxene +augite relationship, producing an increase in reaction tem-perature of about 7 deg/kbar (Brown, 1968). However,pressure differences among the Nain pyroxenes are prob-ably slight.

Thus it is to be expected that differences in mrnor-element concentrations and whole-rock chemistry will re-sult in shifts in the stability frelds of pigeonite and ortho-plroxene. In general, the more magnesian pyroxenes wouldbe expected to crystallize as orthopyroxene, but a reviewofTables I and2 reveals that this is not always the case.It should be noted that of the type I pyroxenes, onlysample PL-l I was saturated with augite as orthopyroxenecrystallized. All type 2 pyroxenes contain minor augite,but not one ofthem appears to have been saturated withaugite. Finally, only sample PL-187 of the type 3 pyrox-enes was saturated with augite.

As expected in primary orthopyroxene hosts, exsolutionof augite as lamellae is restricted to the (100) plane. Forthese two structurally distinct pyroxenes, (100) is not aplane of exact fit, but it does offer the best dimensionalmatch (Robinson, 1980). The two possible processes re-sponsible for the initiation ofexsolution in pyroxenes arespinodal decomposition and nucleation and growth. Inthis case the spinodal mechanism can be ruled out becauseof the diflerence in structure of the two pyroxenes(Champness and Lorimer, 197 6).

Type 2 and type 3 pyroxenes are similar to one anotherin that both crystallized as pigeonite, exsolved augite, andthen the host portion, or most of it, underwent transitionto orthopyroxene. The major differences between the twopyroxene types are the form and orientation ofthe exsolvedaugite, which most likely reflect the cooling history andcomposition of the original pigeonite. Further explanationofthese differences will be considered later in the reportwhen mechanisms of exsolution are discussed. Compar-

l 330 RANSON: COMPLEX EXSOLUTION IN INVERTED PIGEONITE

gItr

u g I

r l

Fig. 5. Photomicrographs and tracings ofexsolved pyroxenes in type 3 Nain pyroxenes. (a, upper) Broad, regularly spaced "00l"

augite lamellae (Aug I) in PL-210. Scale bar is 0.10 mm. (b, lower) Detail of broad "001" augite lamellae (Aug I) in PL-l9l showingfine pigeonite lamellae, Pig II and Pig III, and stacking faults that rotate counter-clockwise as they pass through Aug 1 lamellae (see

text for details). Scale bar is 10 pm.

ison of original pigeonite compositions (Table 2) showsthat type 3 pyroxenes are enriched both in Fs and Wocontent relative to type 2 pyroxenes. Type 3 pyroxenesoccur in rocks that appear to be the final or extreme dif-

ferentiate of the anorthosite-leuconorite suite, whereas type2 pyroxenes occur in less differentiated, more typical rocksof the suite (see App. l). The regular lamellae, both "00l"(Aug I and Aug II) and "100" (Aug "100"), represent the

RANSON: COMPLEX EXSOLUTION IN INVERTED PIGEONITE 133 I

Fig. 6. Photomicrograph and tracing of type 3 pyroxene PL- l9l showing four generations of augite exsolution: broad "001" AugI, fine "001" Aug II, Aug "100", and Aug (100). Note the light patches of pigeonite host (Pig O) constrained by augite lamellae,which failed to invert. Aug I lamellae have exsolved two sets of pigeonite lamellae, Pig II and the finer Pig III. The host orthopyroxeneis at extinction. Fine straight lines parallel to (100) are stacking faults. Scale bar is 50 pm.

two planes of exact dimensional fit, one near (001) andthe other near (100), in accordance with the exact phaseboundary theory (Bollman and Nissen, 1968; Jaffe et al.,1975). The single most important factor determining theorientation of "001" lamellae is the misfit of the a di-mension of pigeonite and augite. Similarly the orientationof "100" lamellae is a function of the misfit of the cdimension of pigeonite and augite. In the same mannerthe "001" and "100" orientations of pigeonite lamellaein exsolved augite of type 2 or type 3 pyroxenes may beexplained. Although augite exsolution from pigeonite intype 2 pyroxenes was irregular, pigeonite exsolutionachieved the "exact dimensional fit" described above,presumably because the pigeonite exsolved from the au-gite lamellae at alater time and at a lower temperature.

A further difference between type 2 and type 3 pyrox-enes is that in most type 2 pyroxenes stacking faults arenot present or are at least not visible with the opticalmicroscope. The reason for this is not clear, especiallysince both pyroxene types are inverted pigeonites andstacking faults appear to figure prominently in the pi-geonite to orthopyroxene transition, as discussed below.Without the benefit of electron microscopy, the absenceof stacking faults in most tlpe 2 pyroxenes is open tospeculation. The following discussion assumes their pres-ence in all inverted pigeonites.

Robinson et al. ( I 977) explained stacking faults in Bush-

veld augites as minute normal faults developed on (100)that relieve the strain produced by the unequal change ofunit-cell parameters by host augite and pigeonite. Theunequal change in the unit cell is brought on by the C2/ c +P2r/c transition and means that the unit cell of pigeonitechanges more rapidly than that of augite during cooling,and as a result the pigeonite lamellae are severely strained.The faulting allows the crystallographic angle 6 of a pi-geonite lamella to equilibrate to lower temperatures foreach small, faulted lamella segment, thereby reducing thestrain for the entire lamella. Robinson and coworkers alsopointed out that faults are most abundant in the thickestpigeonite lamellae, which follows since fault density shouldcorrelate with the amount of lattice change a pigeonitelamella has undergone. Hence high-temperature, early-formed, thick lamellae with a long cooling history werefound to contain a greater density of stacking faults bythese authors than thinner lamellae formed at a lowertemperature.

An analogous situation seems to be present in Naintype 3 pyroxenes, but with pigeonite being the host andaugite the exsolved guest. Like the Bushveld augites, theNain pigeonites and their exsolved augite lamellae musthave experienced strain as cooling occurred and the rateof change of the pigeonite and augite unit cells differed.Strain was again relieved by small-scale faulting on (100).Unlike the Bushveld augites, stacking faults are found not

1332

only in the lamellae, but also in the host regions. For thehost, faults have the same orientation regardless of wheth-er the host is pigeonite or orthopyroxene.

It seems likely that the faults are the mechanism bywhich the P2,/c - Pbca transilion occws for the host.Robinson et al. (197 7) attributed the production of a stripof orthopyroxene in Bushveld augite to stacking faults,and Coe and Kirby (1975) reported that shear on (100)produces the transformation of orthoenstatite to clinoen-statite. However, their experiments show that the trans-formation is mechanically reversible. Champness andCopley (1976) stated that the (100) stacking faults in pi-geonite lamellae exsolved from augite may initiate theinversion reaction. Faults preferentially nucleate at growthledges at interphase boundaries, according to Champnessand Copley, because these are the regions ofhighest strainenergy and some ofthis strain energy can be used to coun-teract the activation barrier opposing fault formation.These same authors cited evidence by Morimoto and Koto(1969) that the orthoenstatite structure could be producedfrom that of clinoenstatite by twinning of every unit cellon (100), and they stated further that although clinoen-statite and pigeonite are slightly different structurally, a(100), l/6 [001] fault is a possible means of generatingorthopyroxene from pigeonite.

In Nain pyroxenes, patches of uninverted pigeonite maystill contain stacking faults, but these patches are alwayssurrounded by "001" and "l00" augite lamellae (Fig. 6)and are thereby isolated from the rest ofthe host. Theywere apparently constrained so that the change frommonoclinic to orthohombic lattice could not occur com-pletely in these areas, despite the presence of stackingfaults. Smith (1974) also observed uninverted pigeonitebounded by augite lamellae in orthopyroxene from ada-mellite of the Nain complex (Smith's Fig. 3D). He citedthe persistence of pigeonite as evidence that it was difficultfor new phases to nucleate within these Nain pyroxenes.

The counterclockwise rotation or bending of (100)stacking faults in Nain pyroxenes as they pass from or-thopyroxene host to "001" augite lamellae is analogousto the bending of orthopyroxene lamellae described byRobinson et al. (1977) for Bushveld augites. In that case(100) orthopyroxene lamellae bend as they pass through"001" pigeonite lamellae in order to become parallel to(100) of the lamellae. In the present example, stackingfaults bend as they enter augite lamellae in order to be-come parallel to (100) of the augite.

Champness and Lorimer (1976) and Busecket al. (1980)have very thoroughly reviewed the rnechanisms for exso-lution in silicates in general and in pyroxenes in particular,and the reader is referred to these discussions. Basicallythese authors have distinguished between two processeswhereby exsolution may occur: (l) nucleation and growthand (2) spinodal decomposition. Buseck and co-workerspointed out that the later stages ofspinodal decompositioneventually lead to exsolution features similar to those pro-duced by nucleation and growth. Verification of spinodaldecomposition involves the observation of the change of

RANSON: COMPLEX EXSOLUTION IN INVERTED PIGEONITE

amplitude and wavelength of the compositional modu-lations over an interval of time and requires sophisticatedsmall-angle X-ray scattering experiments. The observa-tion of periodic compositional modulation of a coherentlattice by electron observation may also serye as evidencefor spinodal decomposition. In its initial stages, spinodaldecomposition is characterized by coherent modulationsas shown in Figures 9 and l5b of Champness and Lorimer(1976). Textures ofthis nature have not been observed inNain pyroxenes, although it is likely that exsolution hasproceeded beyond the initial stages where such textureswould be visible. In addition, judgrng from time-temper-ature-transformation (TTT) diagrams in Champness andLorimer (1976,Fi9.7) and Buseck et al. (1980, Fig. 20),spinodal decomposition requires rapid cooling rates moreappropriate to a volcanic or hypabyssal environment rath-er than the plutonic environment of the Nain complexwhere cooling was demonstrably slow. The requirementof rapid cooling is especially true for pigeonites, which areusually far from the crest ofthe solvus and hence are wellbelow the temperatures for both pigeonite "inversion"and binodal exsolution before they enter the spinodal re-gion (Lindsley, pers. comm., 1985). For these reasons,exsolution in Nain pyroxenes most likely proceeded by amechanism of nucleation and growth.

Nucleation can be either a heterogeneous or a homo-geneous process, and the textures and microstructures ob-served for Nain pyroxenes suggest that both processesoperated. The specimens categorized as type 2 pyroxeneshave irregular exsolution, and it is suggested that heter-ogeneous nucleation occurred at a pre-existing defect suchas a dislocation in the structure, a grain or subgrain bound-ary where strain or surface energy is reduced. In contrast,type 3 pyroxenes display regularly spaced, quite uniformexsolution features suggestive of homogeneous nucle-ation. This process implies that the original pigeonite hostreached a point in its cooling history where it was super-saturated with augite components, that compositionalfluctuations grew to critical size (see Buseck et al., 1980,Fig. l5), and that nucleation occurred uniformly through-out the crystal. Another possibility for type 3 pyroxenes,discussed by Yund and McCallister (1970), is that of dis-continuous or cellular precipitation. In discontinuous pre-

cipitation, a duplex cell consisting ofthe equilibrium com-position ofthe host and a new phase, nucleates and growsfrom the supersaturated matrix with the incoherent in-terface advancing into fresh matrix. This type of grainboundary diffirsion is described as being many orders ofmagnitude faster than volume diffusion, and so this mech-anism can produce a rapid exsolution rate as comparedto gowth of nuclei of a new phase-for example, augite-by long-range volume diftrsion (Yund and McCallister,1970). Operation of this mechanism has never been prov-en for pyroxenes (Yund, pers. comm., 1986), but it is animportant exsolution mechanism in alloy systems in whichtextural relations are very similar to those in pyroxenes.

Again with reference to the TTT diagrams of Champ-ness and Lorimer (1976) and Buseck et al. (1980), het-

RANSON: COMPLEX EXSOLUTION IN INVERTED PIGEONITE r 333

Table 3. Temperatures of magmatic crystallization andcessation of maior exsolution for pyroxenes from the

Puttuaaluk Lake area

Temperature ofcessation of exsolu-

tion based onEx-solu-tion

Sample type

Temperature of pyroxenecrystallization

Ca-poor'- Ca-rich--lamel-lae** host.'

Fig. 7. Pyroxene compositions from the Puttuaaluk lake area,Labrador, taken from Table 2. Temperature contours and plot-ting procedure are from Lindsley and Andersen (1983). Solidsymbols represent calculated bulk compositions connected bysolid tie lines. Open symbols are compositions of exsolved py-roxenes and host connected to bulk compositions by dashed tielines. Heavy dashed curve on Fe-rich side of quadrilateral ispolythermal boundary of"forbidden zone" at 5 kbar after Linds-ley (1983).

erogeneous nucleation is favored at high temperatures andslow cooling, whereas homogeneous nucleation is favoredby lower temperatures and intermediate cooling rates ofa fairly narrow temperature range. All of the samples con-taining type 3 pyroxenes (PL-187, PL-191, PL-210) werecollected near the margin of the anorthositic massif (Fig.l) and are fine to medium grained, suggesting a moderatecooling rate. By comparison, samples with type 2 pyrox-enes (IR-31, PL-117, PL-153, PL-2ll) come from theinterior of the massif and are coarse to medium grained,indicating slower cooling. Results of pyroxene thermom-etry presented in the following section suggest that thetype 3 pyroxenes crystallized at somewhat lower temper-atures than the type 2 pyroxenes. These observations sup-port the suggestion that the type 2 pyroxenes are productsof heterogeneous nucleation and the type 3 of homoge-neous nucleation, and that the observed differences inexsolution can be explained in this manner. Furthermore,regular pigeonite lamellae exsolved from irregular augitein type 2 pyroxenes were initiated at a lower temperatureand probably under conditions of more rapidly fallingtemperatwe than that experienced by the host originally.Hence, the pigeonite lamellae underwent homogeneousnucleation, in contrast to the host augite formed throughheterogeneous nucleation.

PvnoxnNn THERMoMETRy AND ExsoLUTIoNCHRONOLOGY

Temperatures of magmatic crystallization and temper-atures marking the cessation of coarse exsolution havebeen estimated using the pyroxene thermometer of Linds-ley and Andersen (1983). Using this thermometer an un-derstanding can be gained ofthe cooling history ofthesepyroxenes and the rocks that include them. The nine sam-ples investigated are plotted in Figure 7, which shows thepyroxene quadrilateral and isotherms of Lindsley and An-dersen (1983). Reconstructed bulk compositions, com-positions of coarse "001" and "l00" augite lamellae, andhost compositions are plotted in Figure 7. Estimates of

Note.'Temperatures in degrees celsius at 3-kbar pressure.. Samples saturated with augite.

'- [ ] : Fe/(Fe + Mg) for bulk composition, lamellae, or host as indi-

cated.

temperatures of crystallization and of cessation of exso-lution are given in Table 3. Only two of the nine samplescontain primary augite, so all but two of the crystallizationtemperatures recorded in Table 3 are minimum temper-atures of crystallization. Temperature estimates for ces-sation of exsolution of coarse "001" and "100" augitegiven in Table 3 are based on both lamellar and associatedhost compositions. Close agreement between host andlamellar temperatures suggests that the estimates are re-liable. Disaggeement, as in the case of PL-191 and espe-cially PL-210, is probably the result of finely exsolvedaugite in the host which is not resolvable with the electron-microprobe beam. These fine lamellae raise the Wo con-tent and hence the temperature estimate for these hostregions. Discounting the temperatures associated with hostregions ofPL-l9l and PL-210, cessation ofaugite exso-lution took place in the temperature range from 773 to557'C for type 2 and type 3 pyroxenes. Fine pigeoniteexsolution found within "001" augite and usually not re-solvable with the electron beam, probably took place downto lower temperatures.

Temperature of crystallization plotted versus l00Fe/(Fe + Mg), for Ca-poor pyroxenes in Figure 8, shows asharp distinction between inverted pigeonites and primaryorthopyroxenes. Inverted pigeonites form a high-temper-ature-of-crystallization trend with temperature falling asFe/(Fe * Mg) increases, whereas primary orthopyroxenesplot at lower temperature. The surprisingly low temper-atures of crystallization for PL- I 50 and especially PL- I Imay reflect granule exsolution of augite from orthopyrox-ene that was not accounted for, even though these ortho-pyroxenes appeared to be primary and contain only (100)augite exsolution. Alternatively, postcrystallization re-equilibration at submagmatic temperatures may have oc-curred in the form of (Mg,Fe) = Ca exchange. The ex-periments of Lindsley and Andersen (1983) suggest suchan explanation for submagmatic temperatures for someSkaergaard augites. Sample IR-31, which has exsolved

PL-1 | .PL-1 50tR-31PL-117PL-211PL-1 53PL-21 0PL-1 87'PL-1 91

1 686 [0.59] 765 [0.4s]1 821 [0.34] 710 [0.23]2 1OO7 IO.s2l2 101 1 [0.49] 906 t0.3912 997 [0.5512 1117 [0.4413 1009 [0.61]3 956 [0.77] 9s0 [0.72]3 976 [0.76] 1021 [0.73]

760 [0.37] 692 [0.s3]704 [0.3s] 773 [0.50]73410.431 72310.571649 [0.32] 711 [0.45]557 [0.s01 866 [0.63]688[0.69] 710[0.78]658 [0.69] 817 [0.771

r334

| 2 0 0

I t oo

I ooo

T o c( 3 k b o r ) r O O

8 0 0

7 0 0

P L-t5 3

" r , , ,

- - \ P L 2 t o

o o o - o - - P L r g l

r R 3 r - p L 2 r - _ _ o

e r - r a z !

aP L - r 5 0

PL- I I

aa T Y P E IO T Y P E 2t r T Y P E 3

F e / ( F o + M q ) r o oFig. 8. Temperature of crystallization at 3-kbar pressure plot-

ted against l00Fe/(Fe + Mg) for Puttuaaluk Lake pyroxenes. Py-roxene bulk compositions were reconstructed from host and la-mellar compositions as described in the text, with the exceptionof PL- 1 I and PL- I 50, which contain only fine (100) augite exso-lution. Temperatures are minimum temperatures except for PL-11 and PL-187, which are saturated with augite. High-temper-ature crystallization trend is that ofType 2 and Type 3 pyroxenes.Dashed line is visually fit to the data. Pyroxenes from PL- I I andPL-150 plotting at low temperature are Ty'pe I pyroxenes. Seetext for discussion.

pyroxene characteristic ofboth type I and type 2 pyrox-enes, may possibly have crystallized both primary ortho-pyroxene and primary pigeonite since the inferred tem-perature is very close to the minimum temperature forpigeonite of this Fe/(Fe + Mg) ratio (Lindsley, pers.comm., 1985).

Jaffe (pers. comm., 1984) has suggested that by usingthe relationships obtained for Bushveld specimens as il-lustrated in Figure 9 and Figure 10 of Robinson et al.(1977) and the angular relationships for host and lamellaereported in Appendix l, it can be shown that coarse exso-lution began at a temperature above the C2/c = Pzt/cinversion point. According to the data of Robinson et al.and assuming a reasonable B angle for the pigeonite hostof from 109 to I I 0', Aug I n cei, : 108" and Aug " 100" nc",r: -6 indicate ap;e) erue and ceis ) ceue, which ispossible only above a temperature of 800'C, the approx-imate C2/c = P2t/c inversion temperature (see Figs. 9and 10, Robinson etal., 1977).

The experiments of Lindsley and Andersen (1983) mayalso be used as an independent check on the pressures ofcrystallization for Nain anorthositic rocks. The two mostFe-rich inverted pigeonites, having the lowest tempera-tures of crystallization, are PL-l9l and PL-187. PL-l9lcontains the assemblage inverted pigeonite + fayalite +minor augite and PL-187 contains the same assemblagewith the addition of quartz (see App. l). It is apparentfrom these assemblages that Fe-rich orthopyroxene (in-

RANSON: COMPLEX EXSOLUTION IN INVERTED PIGEONITE

6 0 0 -

20 706 05 0

verted pigeonite) is reaching its limit of stability and isgiving way to augite + olivine * quartz. These pyroxenesplot just to the left of Lindsley's polythermal boundaryfor the "forbidden zone" al 5 kbar (Fig. 7) and henceindicate maximum pressures of 3 to 4kbar. Hence, thesepressures based on pyroxene composition confirm thepressure estimates of Berg (1977 and pers. comm., 1983)based on metamorphic mineral assemblages in the contactaureole of the anorthositic Nain complex.

In summary, most Ca-poor pyroxenes crystallized aspigeonite at temperatures around I100 to 1000'C (min-imum temperatures for most samples) at 3-4-kbar pres-sure and underwent exsolution of augite by a mechanismofnucleation and growth. Coarse exsolution began aboveabout 800"C, the approximate C2/c = P2r/c inversiontemperature, and continued down to temperatures per-haps below 600"C. The formation of stacking faults, whichinitiated the transformation from pigeonite to orthopy-roxene, occurred as the temperature fell and the resultantstrain on the pigeonite-augite phase boundary increased.The pigeonite to orthopyroxene transition was not alwayscomplete as witnessed by small, metastable patches ofpigeonite sandwiched between augite lamellae in type 3pyroxenes. Pigeonite probably began exsolving from "001"augite lamellae before inversion and continued to formas fine metastable exsolution even after the host becameorthopyroxene. The lower temperature limit of pigeoniteexsolution is not known.

AcrNowr-BocMENTS

The author wishes to thank S. A. Morse for providing boththe opportunity and financial support for field work in Labrador.H. W. Jaffe and P. W. Ollila reviewed an early version of themanuscript and contributed to its improvement. J. S. Huebnerand D. H. Lindsley also provided perceptive reviews. M. D.Leonard assisted with the electron-microprobe facility at the Uni-versity of Massachusetts. To each of these persons the authorgratefully extends his thanks. Support was provided by the Geo-logical Society ofAmericagrant-in-aid2245-77 to W. A. Ransonand by Earth Science Division, National Science FoundationGrants EAR 73-00667 and EAR 74-22509. both to S. A. Morse.

RnrnnnNcns

Albee, A.L., and Ray, Lily. (1970) Correction factors for electronprobe microanalysis of silicates, oxides, carbonates, phos-phates, and sulfates. Analytical Chemistry, 42, 1408-1414.

Bence, A.E., and Albee, A.L. (1968) Empirical conection factorsfor the electron microprobe analysis of silicates and oxides.Journal of Geology, 7 6, 382-403.

Berg, J.H. (1977) Regional geobarometry in the contact aureolesof the anorthositic Nain complex, Labrador. Journal of Pe-trology, 18,399-430.

Bollman, W, and Nissen, H.-U. (1968) A study of optimal phaseboundaries: The case of exsolved alkali feldspar. Acta Crys-tallographica, 424, 546-557 .

Brown, G.M. (1968) Experimental studies of inversion relationsin natural pigeonitic pyroxenes. Carnegie Institution of Wash-ington Year Book 66, 347-353.

Buseck, P.R., Nord, G.L., Jr., and Veblen, D.R. (1980) Subsol-idus phenomena in pyroxenes. Mineralogical Society of Amer-ica Reviews in Mineralogy, 7 , ll7-211.

Champness, P.E., and Copley, P.A. (1976) The transformationof pigeonite to orthoplroxene. In H.-R. Wenk, Ed. Electron

Microscopy in Mineralogy, 228-233. Springer-Verlag, NewYork.

Champness, P.E., and Lorimer, G.W. (1976) Exsolution in sili-cates. In H.-R. Wenk, Ed. Electron microscopy in mineralogy,17 4-204. Springer-Verlag, New York.

Coe, R.S., and Kirby, S.H. (1975) The orthoenstatite to clinoen-statite transformation by shearing and reversion by annealing:Mechanism and potential applications. Contributions to Min-eralogy and Petrology, 52,29-55.

Hess, H.H. (1941) Pyroxenes of common mafic magmas. Amer-ican Mineralogist, 26, 5 I 5-535, 57 3-594.

Hess, H.H., and Phillips, A.H. (1940) Optical properties andchemical composition of magnesian orthopyroxenes. Ameri-can Mineralogist, 25, 27 l-285.

Huebner, J.S., and Turnock, A.C. (1980) The melting relationsat I bar of pyroxenes composed largely of Ca-, Mg-, and Fe-bearing components. American Mineralogist, 6 5, 225-27 l.

Huntington, H.D. (1980) Anorthositic and related rocks fromNukasorsuktokh Island, Labrador. Ph.D. thesis, University ofMasachusetts, Amherst.

Jaffe, H.W., and Jaffe, E.B. (1973) Bedrock geology of the Monroequadrangle, New York. New York State Museum and ScienceService, Map and Chart Series No. 20, Albany, New York.

Jaffe, H.W., Robinson, Peter, Tracy, R.J., and Ross, Malcolm.(1975) Orientation of pigeonite lamellae in metamorphic au-gite: Correlation with composition and calculated optimal phaseboundaries. American Mineralogist, 60, 9-28.

Lindsley, D.H. (1983) Pyroxene thermometry. American Min-eralogist, 68, 47 7 -493.

Lindsley, D.H., and Andersen, D.J. (1983) A two-pyroxene ther-mometer. Proceedings of the Thirteenth Lunar and PlanetaryScience Conference, Part 2. Journal of Geophysical Research,88, Supplement, 4887-4906.

Lindsley, D.H., Mac Gregor, I.D., and Davis, B.T.C. (1964) Syn-thesis and stability of fenosilite. Carnegie Institution of Wash-ington Year Book 63, 174-l'16.

Morimoto, N., and Koto, K. (1969) The crystal structure of or-thoenstatite. Zeitschrift fiir Kristallographie, 1 29, 6 5-83.

Poldervaart, Arie, and Hess, H.H. (1951) Pyroxene in the crys-tallization of basaltic magma. Journal of Geology, 59, 472-489.

Ranson, W.A. (1978) Exsolution in Ca-poor pyroxenes from an-orthositic rocks of the Nain complex, Labrador. GeologicalSociety of America Abstracts with Programs, 10, 476.

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-(1981) Anorthosites of diverse magma types in the Put-tuaaluk Lake area, Nain complex, Labrador. Canadian Journalof Earth Sciences, 18, 26-41.

Rietmeijer, F.J.M. (1979) Pyroxenes from iron-rich igneousrocksin Rogaland, S.W. Norway. Ph.D. thesis, University of Utrecht,Netherlands.

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Robinson, Peter, Ross, Malcolm, Nord, G.L., Jr., Smyth, J.R.,and Jaffe, H.W. (1977) Exsolution lamellae in augite and pi-geonite: Fossil indicators of lattice parameters at high tem-perature and pressure. American Mineralogist, 62, 857 -87 3.

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Stephenson, D.A., Sclar, C.B., and Smith, J.V. (1966) Unit cellvolumes of synthetic orthoenstatite and low clinoenstatite.Mineralogical Magazine, 3 5, 838-846.

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MlNuscnrpr REcETvED Snprnrrasen 11, 1985MnNuscnrpr AccEPTED Jwv 8, 1986

ApprNorx 1. Sorpln DEScRTPTToNPL-l1

A medium- to fine-grained gabbroic anorthosite with 600/o an-desine (Anoo), 260/o a,tgite, 100/o type I orthopyroxene, 2o/o Fe-Tioxides, 2olo hornblende and trace biotite. The two pyroxenes arevery closely associated and are commonly intergrown. Ortho-pyroxene [0.59]' forms fine-grained crystals interstitial to coarserplagioclase. Exsolved pyroxene consists offine (<0.1 pm thick)( I 00) augite lamellae characteristic of type 1 orthopyroxene. Au-gite [0.46] also forms fine-grained crystals interstitial to coarserplagioclase and contains exsolved orthopyroxene and pigeonite.Exsolved pigeonite has orientation "00 1 " with respect to the hostaugite and consists of l-2-pm-thick lamellae generally continu-ous across host crystals but tapered at the ends ("001" n c:I 13"). Orthopyroxene lamellae are about 0.2 pm in width, havea (100) orientation, and are discontinuous across the host augitecrystals.

PLl50

Coarse- to medium-grained leuconorite with 600lo labradorite(An,r) (including antiperthite), 380/o type I orthopyroxene, 10/oaugite, 10/o Fe-Ti oxides. Coarse-grained, subhedral orthopyrox-enes in this rock contain fine (<l pm) augite lamellae orientedparallel to (100) ofthe host. Fine oxide exsolution lamellae arealso present with an orientation of(100). Fine-grained, anhedralaugite crystals occur at the maryins of large orthopyroxenes ormore rarely are included by orthopyroxene, suggesting an originfor the augite by granule exsolution. Augite contains fine (<1pm) (100) Iamellae of orthopyroxene.

IR-31Medium-grained leuconorite with 850/0 andesine (Anrr), 100/o

orthopyroxene, 40/o augite, l0lo magrretite. The orthopyroxenesbear similarity to both type I and type 2 proxenes. Many appearto be primary and contain only fine (100) augite lamellae. Otherexamples have augite granules within and adjacent to the ortho-pyroxene crystals, making accurate determination of compositionand classification difrcult. Augite also occtus as an interstitialphase apart from orthopyroxene and has two sets of pigeonitelamellae with orientations of "001 1' and " 100" ('00 t " A " 100" :

1 19"). Lamellae oriented at "001" arc 2 to 3 pm thick and havean indistict, granular appearance. " 100" lamellae are fine (l pm)and discontinuous along the length of the host.

Pl-ll7Medium- to coarse-grained, oxide-rich leuconorite with 77o/o

labradorite (Anru), 100/o type 2 inverted pigeonite, 5o/oaugste,5o/oilmenite, 3oh qwrtz-plagioclase intergtowths, 10/o biotite. Invert-ed pigeonite [0.50] forms coarse-grained (5-7 mm) and fine-grained(l-3 mm) crystals containing coarse (up to 0.1 mm in width)irregular lamellae and blebs ofaugite ofnonuniform orientation.

' Values in brackets are the atomic ratio (Fe, + Mn)/(Mg +Fe, * Mn).

RANSON: COMPLEX EXSOLUTION IN INVERTED PIGEONITE

l 336

Some inverted pigeonites also contain fine (100) augite lamellaemore characteristic of type I low-Ca pyroxenes. Augite crystals[0.38] form at the margins of, or within, large inverted pigeonitecrystals and are anhedral. Three sets of "001" pigeonite lamellaewere observed in host augite: a coarse set (10 pm thick) contin-uous across the grains ('001" A c : 102'), a fine set (0.05 to Ipmthick; "00l" A c : I08"), and avery fine set(0.0I pm thick).In addition, fine (1-2 pm) discontinuous (100) lamellae of or-thopyroxene are present. Yet another set ofexsolution lamellaewith a " 100" orientation is barely resolvable with the microscopeand probably constitutes a fourth set of pigeonite lamellae.

Pr-2ll

Coarse-grained leuconorite with 800/o andesine (Anou), 140lo type2 inverted pigeonite, 30/o Fe-Ti oxides, 30/o K-feldspar. Invertedpigeonite [0.57] forms coarse (5-8 mm), anhedral crystals con-taining abundant exsolved augite in the form ofirregularlamellae,blebs, and granules adjoining the host margins. In thin section,irregular lamellae have a variable orientation with respect to thec axis depending on the orientation ofthe host crystal in relationto the plane of the thin section. However, in immersion mounts,cleavage plates have augite lamellae perpendicular to the c axis.Broad (40-50 pm) augite lamellae with irregular boundaries con-tain fine (1-3 pm) pigeonite lamellae with "00l" orientations andequally fine (100) orthopyroxene lamellae. Granules of augiteexhibit the same patterns of pigeonite exsolution.

PLI53

Medium-grained, oxide-rich anorthosite with 850/o labradorite(Anr.), 80/o type 2 inverted pigeonite, 60/o ilmenite, l0lo augite.Inverted pigeonite forms crystals between 0.5 and 1.0 mm inlength and contains irregular lamellae and blebs of exsolved au-gite at nonuniform orientations typical of type 2 low-Ca pyrox-enes. Exsolved augite blebs and lamellae range in width from 40to 60 pm. Equant augite crystals averaging about 0.1 mm acrossoccur at the margins of large inverted pigeonites and may rep-resent the result ofgranule exsolution. Augite contains both " 100"and "001" pigeonite lamellae. "001" lamellae are continuousand l-2 pm in width, whereas " 100" lamellae are discontinuousand <l rrm in width.

PL2IO

Medium- to fine-grained leuconorite with 640lo labradorite(An,,), 300/o type 3 inverted pigeonite, 20lo augite, 4olo Fe-Ti oxides,trace apatite. Inverted pigeonite forms crystals up to 2 mm inlength containing broad (30 pm) "001 " augite lamellae (Aug I ncng : l07o). Augite lamellae contain fine pigeonite lamellae par-allel to "00l" and "100" ofthe augite, although the "l00" setis rare. Stacking faults or unresolvable lamellae of uncertain com-position cut the host inverted pigeonite but do not penetrate the

RANSON: COMPLEX EXSOLUTION IN INVERTED PIGEONITE

augite lamellae. Rectangular patches ofuninverted pigeonite areconstrained between adjacent broad augite lamellae and are acommon characteristic of these pyroxenes.

PLT87

Medium-grained monzodiorite with 50o/o andesine (An.r) (in-cluding antiperthite), 25o/o perthite, 70lo hornblende, 4o/o atgste,3o/o type 3 inverted pigeonite, 5o/o fayalite (ForrFarorTe, u), 50/oqtarlz, lo/o apatite. Inverted pigeonite crystals (0.5-1.0 mm inlength) have exsolved "00l" augite lamellae (Aug I) 30 to 40 pmin thickness (Aug I n c",, : 108"). Most augite lamellae zue con-tinuous and have sharp straight boundaries, although irregularlamellae and blebs of augite do occur in some inverted pigeonite.Vertical patches ofaugite connect adjacent "00 I " augite lamellaein some inverted pigeonite crystals. Discrete augite crystals, whichare closely associated with inverted pigeonite, have two sets of"001" pigeonite lamellae, a set of coarser (-0.8 pm), continuouslamellae ("001" A c : 108") and a set of fine (<0.1 pm) discon-tinuous lamellae across the host crystal ("001" A c: 112"). Athird set consists of pigeonite lamellae that have a "100" ori-entation and are thin (< I pm) and less common than the "001"lamellae. Composite grains of augite and inverted pigeonite oc-cur, usually consisting oflarge patches ofinverted pigeonite con-tained in augite.

PL191

Fine-grained ferrodiorite with 52o/o andesine (An.r) (includingminor antiperthite), 39o/o type 3 inverted pigeonite, 60lo Fe-richolivine, 4o/o Fe-Ti oxides; traces of clinopyroxene, hornblende,K-feldspar. Inverted pigeonite crystals are irregularly shaped, av-erage about I mm in length, and contain broad (4G-50 pm),evenly spaced and sharply bounded "001" augite lamellae (AugI). These lamellae are usually continuous across the host and aretapered (Aug I 1 crie : I 08'). Exsolving from broad "00 l " augitelamellae are two sets of fine (<l pm) pigeonite lamellae withslightly different "001" orientations. A second 6-10-pm set ofaugite lamellae (Aug "100") have a "100" orientation and arerestricted to portions of crystals where there are no Aug I lamellae(Aug "100" I cr,e: -6'). Aug "100" lamellae stop abruptly atthe phase boundaries of Aug I lamellae. Finally, a third, narrower(3-5 pm) set of augite lamellae (Aue U) has orientation of "001"but occurs at a slightly steeper angle than Aug I lamellae (AugII n cpis : 109). Fine stacking faults and accompanying fine la-mellae ofaugite (?) oriented parallel to (100) ofthe host are bentas they pass from host into broad augite lamellae.

Minor interstitial clinopyroxene has two sets of exsolved pi-geonite lamellae with a "001" orientation: a coarser set with2-prn-thick lamellae ("001" n c : I10.) and a finer set with 0.5-pm-thick lamellae C'001" A c : 114'). Orthopyroxene lamellaeparallel to (100) are also present and are l-2 p.m in width.