An introduction to pallasites, their mineralogy, … introduction to pallasites, their mineralogy, texture and origin Lena Boeck Institut für Mineralogie, TU Bergakademie Freiberg,

An introduction to pallasites, their mineralogy,

texture and origin

Lena Boeck

Institut für Mineralogie, TU Bergakademie Freiberg, Germany

supervised by Professor Gerhard Heide

Piece of the Springwater pallasite (photo by Lena Boeck)

2 Boeck, Lena

Table of Contents

1. Introduction ......................................................................................3

2. The sources of meteorites..................................................................4

3. The classification of meteorites.........................................................4

4. Pallasites............................................................................................7 4.1. Definition ...........................................................................................7 4.2. The Mineralogy of Pallasites...............................................................7

4.2.1. Olivine ...............................................................................................7 4.2.2. Kamacite and Taenite..........................................................................8 4.2.3. Minor phases ......................................................................................8

4.3. Texture ...............................................................................................9 4.4. The classification of pallasites........................................................... 10

4.4.1. Main group pallasites ........................................................................ 10 4.4.2. Eagle station trio (Eagle station pallasites)......................................... 11 4.4.3. Pyroxene-pallasite grouplet ............................................................... 11

4.5. Formation of pallasites...................................................................... 12

5. The pallasite problem ..................................................................... 12

6. Summary......................................................................................... 14

7. References ....................................................................................... 15

8. Appendix ......................................................................................... 17

A Glossary........................................................................................... 17

B List of meteorite groups and their main properties........................ 18

C Petrologic type................................................................................. 21

D Shock stage...................................................................................... 22

E Grade of weathering ....................................................................... 23

F Figures............................................................................................. 24

G References ....................................................................................... 27

Pallasites – Their origin and composition 3

Abstract. Some of the most beautiful meteorites found on Earth are pallasites.

These meteorites contain only two major phases, olivine and FeNi-metal. The ei-

ther rounded or angular, pure, yellow to yellow-green olivine crystals are swim-

ming in a matrix of silver FeNi-metal like raisins in a cake. Based on differences

in the chemistry, three pallasite groups were distinguished by meteoritists: the

main group pallasites, the Eagle Station pallasites, and the pyroxene pallasites. It

is suggested that the unlike association of olivine and metal was generated by

metal melt intruding into fragmented olivines.

1. Introduction

When scientists want to obtain information about the very beginning of our solar

system including Earth, they do not study Earth’s interior but look into the sky. In

other words, they do research on extraterrestrial material – meteorites. In one year

nearly 40,000 tons1 of cosmic material enter Earth’s atmosphere and fall as dust

particles, which usually can not be collected, or meteorites. Unfortunately, a lot of

meteorites are not discovered because they fall in oceans or uninhabited areas. But

the theoretical number of samples is very huge.

The formation of chondrules (all words written in italics are explained in the glossary), which are found in meteorites, can be dated to a time ~4,564.7 ± 0.6 Ma

ago.2 Contrary to meteorites, the oldest terrestrial material is only 4.2 billion years

old, measured by dating the ages of grains of the mineral zircon3. So meteorites

are the oldest witnesses of the origin of our solar system. After Wood and Chang

(1985), the materials which formed meteorites are residues of the planetesimals.

Most of the planetesimals incorporated afterwards into the planets4. The most

primitive meteorites made of this primordial material are the carbonaceous chon-

drites. Especially CV3-, CO3-meteorites and UOCs represent the earliest solar

nebular material.

Some constituents of chondritic meteorites such as chondrules, CAIs and dark

inclusions can be dated to measure the time of the accretion. After that a period of mild to moderate heating followed. Holdovers from this time are undifferentiated

meteorites (= chondrites).

Some asteroids were partly or totally melted (e.g. Vesta) and differentiated into

an inner and an outer part which show similarities to the core and the mantle of the

Earth. Pieces of this core- and mantle-like material fall as meteorites to Earth.

1 Web 1 2 Davis, 2004, p. 437 3 Kleinschrot, 2003, p. 30 4 Sears, 2004, p. 13

4 Boeck, Lena

2. The sources of meteorites

Meteoriticists distinguish between different sources of meteorites. There are

Martian meteorites, meteorites from the Earth’s moon and asteroidal meteorites.

Four kinds of Martian meteorites are known: shergottites, nakhlites, chassigny,

and ALH 84001. From the compositions and properties of these meteorites, it is possible to develop and understand the geological and planetary evolution of

Mars.5

The majority of the lunar meteorites are pieces of the lunar highlands. The lu-

nar meteorites are divided into the types LUN A (lunar anorthositic (feldspathic)

breccia), LUN B (lunar mare basalt or gabbro, basaltic or gabbroic breccia), LUN

M (mingled breccia (basaltic, and anorthositic clasts)), and LUN K (lunar KREEP

basalt, or KREEP-rich mafic breccia)6.

Asteroidal meteorites have their source in the asteroid belt which is located

mostly between the orbits of Mars and Jupiter (2 - 4 AU from the Sun). By reflec-

tance measurements, 26 different asteroid groups were defined. Through spectral

reflectance most meteorites can be linked to different, although not necessarily

specific, asteroids. Also it is possible to track incoming meteors and meteoroides and calculate the orbits of these objects. These orbites lead to the asteroid belt.7

Sometimes a fourth source of meteorites is assumed, namely leftovers from com-

ets, but this is unproven.8

3. The classification of meteorites

Scientists are used to classify all objects to make it possible to compare them with

other objects. Classifications system of meteorites were created very early in the

history of science. Besides various classification schemes there are two long used

systems based on different properties. One is the Rose-Tschermak-Brezina classi-

fication. It divides the meteorites by their composition and texture.9 The second

system is the Prior classification. It is based on variations of the iron-nickel metal

and the iron incorporated in olivines and pyroxenes. These early systems were re-

placed after the first microanalysis of meteorites was preformed by Klaus Keil and Kurt Fredriksson. This new tool of analysis enabled a classification unattached to

the structure and nickel content alone.10

Today’s classification includes different properties which are both primary and

secondary. The first step is to characterize the chemical type of the meteorite. The

chemical classification divides meteorites into undifferentiated and differentiated

5 Lauretta and McSween, 2006, p. 6 6 Web 2 7 Lauretta and McSween, 2006, p. 5 8 Bevan and de Laeter, 2002, p. 48 9 Norton, 2002, p. 73f. 10 Norton, 2002, p. 75f.


meteorites. The undifferentiated meteorites are also called chondrites and include

different classes, groups and clans. The classification is given in Figure 1.

Undifferentiated Meteorites

(chondrites)

Ordinary Chondrites (O) Carbonaceous Chondrites (C) Enstatite Chondrites (E)

H-Chondrites(high iron)

L-Chondrites(low iron)

LL-Chondrites

(low metal,low total iron)

CI-Chondrites CM-CO-Chondrites CV-CK-Chondrites CR-Chondrites

CM-Chondrites

CO-Chondrites

CI-Chondrites

CV-Chondrites

CK-Chondritess

CR-Chondrites

EH-EL-Chondrites

EH-Chondrites

EL-Chondrites

class

clan

group


(chondrites)




LL-Chondrites



CM-Chondrites

CO-Chondrites

CI-Chondrites

CV-Chondrites

CK-Chondritess

CR-Chondrites

EH-EL-Chondrites

EH-Chondrites

EL-Chondrites


(chondrites)




LL-Chondrites



CM-Chondrites

CO-Chondrites

CI-Chondrites

CV-Chondrites

CK-Chondritess

CR-Chondrites

EH-EL-Chondrites

EH-Chondrites

EL-Chondrites

class

clan

group

Figure 1: Classification of undifferentiated chondrites11

To the given groups 2 further classes must be added: the R-chondrites and the

K-chondrites whereas the latter group is only a grouplet, because it only consists

of 2 members. Chondrites are the most primitive and most common meteorites. Of

all known meteorites ca. 85 % are chondrites.12 The primary classification parame-

ters mentioned above are the bulk chemical composition, the oxidation state, the

bulk oxygen isotopic composition, the bulk nitrogen and carbon abundance, as

well as their isotopic composition. The secondary classification parameters are the petrologic type, the shock stage and the grade of weathering. The petrologic type

was developed by Van Schmus and Wood 1967 and describes the degree of the

aqueous and thermal alteration the meteorite experienced.13 The shock stage, clas-

sified by Stöffler, Keil and Scott, 1991, results from hypervelocity collisions on

their parent body.14 Finally the affect of terrestrial weathering can be character-

ized with the grade of weathering after Wlozka. This grade correlates with the ter-

restrial age of the meteorite.15 For further informations see appendix C to E or va-

rious literature (e.g. Norton; 2002; p. 73ff., Davis; 2004; 89ff.).

The remaining 15 % of all meteorites are differentiated meteorites and belong

to achondrites, irons or stony-irons. The parent bodies of differentiated meteorites

did undergo total or partial melting and started to differentiate as described in the

introduction. In Figure 2 the structure of a basaltic parent body and Earth are compared. The classification of differentiated meteorites is given in Figure 3.

11 Norton and Chitwood, 2008, p. 80; Lauretta and McSween, 2006, p. 21 12 Norton and Chitwood, 2008, p. 113 13 Davis, 2004, p. 87ff. 14 Stöffler and Keil, 1991, p. 3845 15 Wlotzka, 1993, p. 460

6 Boeck, Lena

Figure 2: Structure of the terrestrial planet compared to a basaltic parent body16

A list of all groups of meteorites can be found in the appendix B.

Differentiated Meteorites

Irons Achondrites Stony-irons

Martian Primitive achondrites Asteroidal achondrites Lunar

Shergottites Nakhiltes Chassigny ALH 84001

Acapulcoites Lodranites Winonaites

Impact

breccias

Mare

basalts

Basaltic Angrites Aubrites Ureilites Brachinites

Eucrites Diognites Howardites

Differentiated Meteorites

Irons Achondrites Stony-irons

Martian Primitive achondrites Asteroidal achondrites Lunar

Shergottites Nakhiltes Chassigny ALH 84001

Acapulcoites Lodranites Winonaites

Impact

breccias

Mare

basalts

Basaltic Angrites Aubrites Ureilites Brachinites

Eucrites Diognites Howardites

Figure 3: Classification of differentiated meteorites17

The meteorites, this paper is dealing with, belong to the stony-irons. The stony-

iron class includes mesosiderites and pallasites. Mesosiderites are breccias which

have approximately equal proportions of silicates and FeNi-metal. The silicate

component is made of minerals and lithic clasts of fine-grained fragmental or ig-

neous matrix.18 The pallasites will be described in more detail below.

16 Norton and Chitwood, 2008, p. 114 17 Norton and Chitwood, 2008, p. 115ff. 18 Davis, 2004, p. 112


4. Pallasites

4.1. Definition

Pallasites are stony-iron meteorites composed of 2 major phases: Olivine and

FeNi-metal. The 2 phases are physically and geochemically dissimilar and for this

reason unlikely to be related to each other.19 The olivine to metal ratio of an ‘aver-

age’ pallasite is 2.40 by volume or 1.02 by weight. Pallasites also contain some

minor phases, but their abundance is limited. Minor phases are troilite, schreiber-

site, chromite, phosphoran olivine, and pyroxenes. The composition of an ‘aver-

age’ pallasite would be 64.9 vol. % olivine, 31.0 vol. % metal, 2.3 vol. % troilite,

1.2 vol. % schreibersite, 0.4 vol. % chromite, and 0.2 vol. % phosphate.20 Palla-

sites were the first material, which was recognized and accepted as extraterrestrial

material.19 A picture of a pallasite can be found in Figure 8 (appendix F).

4.2. The Mineralogy of Pallasites

4.2.1. Olivine

One of the main phases of pallasites is olivine. The composition of the olivine var-

ies from Fa11 – Fa20 21, but in a single pallasite the olivine composition does not

vary22. The crystals are very homogenous and show no zoning.21 Besides in di-

mensions of some 100 µm23 around the edges they show a significant chemical gradient in minor elements.24 The shape of the olivine can either be rounded or

angular. The olivine appears as transparent yellow to yellow-green crystals. The

green olivine known from terrestrial rocks is surprisingly rare in pallasites.25

Until today no correlation between the shape and chemical features could be

found. Only a correlation to some evidence of deformation is suggested, because

the non-rounded olivine is thought to be a product of an event or a series of events

of deformation.21

19 Buseck, 1977, p. 711 20 Buseck, 1977, p. 738 21 Buseck, 1977, p. 712 22 Scott, 1977b, p. 693 23 Huss and Tomiyama, 2005, Introduction 24 Huss and Tomiyama, 2006, Introduction 25 Norton, 2002, p. 203

8 Boeck, Lena

4.2.2. Kamacite and Taenite

The second main phase of pallasites is the iron-nickel metal. The FeNi-metal can

appear as 3 different minerals. If the nickel content is low (between 4 and 7.5

wt.%) the mineral is called kamacite. Kamacite is mostly found in irons and stony-

irons and only as a minor mineral in some achondrites.26 The Ni-content of the

second mineral varies from 27 to 65 %. It is known as tenite and occurs usually as

thin lamellae around kamacite in iron meteorites. If the kamacite and the taenite

grow into each other they are forming plessite, the third FeNi-mineral. Plessite can

be found in octahedrite and some chondrites.27

In pallasite all three minerals can be found. The composition of the FeNi-metal

is similar to that of iron meteorites. Etching of pallasitic metal shows that it has a Widmannstätten structures .28

Lovering et al. (1957) suggested a genetical linkage between pallasites and

“group III” irons. This was confirmed by different studies of e.g. the cooling rate

which is similar to the IIIB irons29 or the content of Ga, Ge, Au, As, Ir, Ni and W

which overlaps the IIIAB irons field in plots.30

4.2.3. Minor phases

Pyroxene is a widespread mineral in meteorites. In pallasites they were unknown

for a long time. However, modern analytical techniques lead to their discovery as

a minor phase with grains only a few µm in diameter. Pyroxenes commonly occur

in 2 kinds of symplectic intergrowth. The first one is as peripheries along olivine

crystals. In this case they are in contact with kamacite or schreibersite. Also it can be found in sharply defined contact areas between two olivine grains. The pyrox-

enes in pallasites have a low Ca-content.

Also a common mineral found in meteorites is troilite. It can be found in pall-

sites next to olivine crystals or as polycrystalline grains in eutectic (or eutectic-

like) intergrowth with kamacite.

Another mineral found in pallasites is schreibersite. Schreibersite is an iron-

nickel phosphide which is common in iron and stony-iron meteorites. 27 In palla-

sites it is always surrounded by kamacite, because of that there are suggestions

that the schreibersite incorporated the nickel of the metal. In the metal the size of

the grains is around 1 mm and smaller. But schreibersite can also occur as mm- or

cm-sized grains adjacent to olivine.31 One of the most interesting minerals in pallasites is phosphoran olivine. Be-

cause phosphor is generally not a constituent of olivine it is unusual that olivines

in pallasites can contain sharply defined areas, up to 20 µm wide and several mm

26 Norton, 2002, p. 312 27 Norton, 2002, p. 313 28 Buseck, 1977, p. 726 29 Scott, 1977a, p. 354 30 Scott, 1977a, p. 349 31 Buseck, 1977, p. 727


long, with P2O5 contents between 3.8 and 4.9 wt.%. Based on studies of the

charge, size and stoichiometry of the phosphorus it can be concluded that it substi-

tutes for silicon. The question of the compensation of the different charge is still

open. Known is that the source of the phosphor is external to the olivine as sug-

gested by the appearance of the P-rich areas near the edges.32

Besides these already described minor phases pallasites can contain accessory

phases such as chromite, rutile, magnetite, pentlandite, native copper and copper

sulfides, sphalerite, graphite and many more. Usually these phases are known in a limited number of pallasites and not common in all pallasites.33

4.3. Texture

Pallasites have a very unique texture that is dominated by olivine and FeNi-metal.

The mixture of olivine and metal can vary from nearly pure olivine regions to

nearly pure metal, in numbers the FeNi-content differ from 28 to 88 wt.%.25 With

this the density of pallasites has also a wide range. E.g. the pallasite Mount

Vernon has variations from 4.0 to 6.0 g/cm3 even in samples of only 1 kg.34 The olivine crystals in pallasites are either

rounded or angular. Angular means in this

case fragmental, not euhedral.22 It must be

added that the angular olivines appear also

rounded in microscopic scales (see Figure 9,

appendix F). It is thought that the rounded

shape was produced by an event called grain

boundary migration which reduces the

boundary energy of the olivine. The grain

boundary migration is caused by the unusual

high surface energy of olivines in metal that is much higher than the surface energy of

olivine in silicate melts. It is expected that

this also produces the crystal faces found in

pallasitic olivines.36 Terrestrial olivines have

only three dominating crystal faces (faces af-

ter {010}, {110}, and {021}). After Kolomensky et al. olivines in pallasites can

have up to 21 different faces.37

The FeNi-metal of the pallasites is a complete connected network. Primary

metal is kamacite showing also thin lamellae of taenite and plessite. The Widman-

32 Buseck, 1977, p. 724f. 33 Buseck, 1977, p.727f. 34 Scott, 1977b, p. 705 35 Kleinschrot, 2003, p. 28 36 Scott, 1977b, p. 708 37 Scott, 1977b, p. 703

Figure 4: Widmannstätten

structures in different cutting posi-

tions (a - octahedral face, b - cube

face, c - rhombbododecahedron

face, d - optional35

10 Boeck, Lena

stätten structure of the metal appears as a medium octahedrite structure (see Figure

4).25

Pallasites are strongly influenced by weathering which oxidize the metal and

olivine. The largest intact pallasite is the Huckitta pallasite. To his main mass of

1411 kg a 900 kg iron shale was discovered nearly completely converted to hema-

tite and a opaque black mass of former olivine.25

4.4. The classification of pallasites

Meteoritists defined three separate pallasite types based on different silicate min-

eralogy and composition as well as the metal composition and the oxygen isotopic

composition. By these scientists concluded that pallasites formed at least on three

different parent bodies.38 Further the three types will be presented.

4.4.1. Main group pallasites

The main group pallasites contain besides FeNi-metal and olivine minor amounts of

low-Ca pyroxenes, chromite, phosphates (e.g. farringtonite, whitlockite), troilite, and schreibersite.38 The composition of the olivine varies from Fa10.5 to Fa13 and the Ni

content of the metal is between 8 and 12 wt.%.22

No. Ni (wt.%) Ga

(µg/g)

Ge

(µg/g)

Au

(µg/g)

Fa

(mole%)

Main group 19 7.8 – 11.7 16 – 26 29 – 65 1.7 – 3.0 11 – 13

Eagle station

trio 3 14 – 16 4.5 – 6 75 – 120 0.8 – 1.0 19 – 20

Table 1: main properties of the main group and Eagle Station pallasites30

In Table 1 the concentration of different elements in the metal are given based

on studies of 34 pallasites. In a Ga-Ge plot, the IIIAB iron field overlaps with the

field where the main group pallasites are plotted30 (plots are given in Figure 10,

appendix F). Another sharply defined field can be found in the bulk oxygen iso-

topic composition. In Figure 5 the plots of the main group pallasites are high-

lighted in blue. Even if the oxygen isotopic composition of the main group palla-

sites is alike to this of HEDs it is unlikely that both come from the same parent

body.

38 Davis, 2004, p. 113


Figure 5: Bulk oxygen isotopic composition of primitive achondrites and differenti-

ated meteorites39

4.4.2. Eagle station trio (Eagle station pallasites)

The red highlighted plot in Figure 5 characterizes the Eagle Station pallasites

which have similar mineralogy as the main group pallasites but different chemical

compositions. For example, the metal is richer in Ni, Ir, and Ge22 (see Table 1) and

therefore close to the composition of IIF iron meteorites. Meteoriticists suggest

that they formed in the same region of the solar nebular. 40 The fayalite content of

the Eagle Station pallasites is higher than that of pallasites belonging to the main

group pallasites, and the Au, As, and Ga contents are lower.22 Also with trace

element plots (like Ga-Ge) the Eagle Station pallasites can be easily distinguished from the main group pallasites.41

4.4.3. Pyroxene-pallasite grouplet

This group was defined after sufficient volumes of pyroxene were discovered in

two pallasite. These are referred to as members of a grouplet because at this time,

only two members, Y-8451 and Vermillion, are known. These meteorites have be-

tween 0.7 to 3 vol.% mm-sized pyroxene grains. The contents of olivine in this

grouplet is between 14 to 63 vol.% and the metal content is reduced to 30 to 43

vol.%. The classification can also be made with both the oxygen isotopic composi-

tion (highlighted in orange in Figure 5 and Figure 6) and the metal composition

which indicates a formation on a third pallasite parent body.40

39 Davis, 2004, p. 106 40 Davis, 2004, p. 114 41 Scott, 1977a, p. 352ff.

12 Boeck, Lena

Figure 6: Bulk oxygen composition (enlarged from Figure 4)

37

4.5. Formation of pallasites

In pallasites meteoriticists found evidences for three different events which influ-

ence their formation. The first event was the fracturing of an earlier formed olivine

mass with crystals that were at least 30 cm in size. It is thought that this was

closely related to the second event where the olivine fragments were mixed with

the FeNi-metal. In this step the olivines were separated and reoriented. Finally the metal started to solidify while the olivine-metal boundaries migrated and formed

macroscopically or microscopically rounded shapes.42

5. The pallasite problem

Many scientists suggested for a long time that pallasites represent the core-mantle-

boundary of asteroids. But in today’s literature there is extensive discussion about

the formation of the pallasites including the mechanism as to how olivine can oc-

cur next to metal. So actually four different regions inside a parent body are de-

scribed where pallasites formed. First, near the surface, second close to the center

of the asteroid, third inside of isolated metal pools which had a contact zone to an

olivine layer and finally at the core-mantle boundary.43

Buseck (1977) suggests that at least one or more melts formed the pallasites.

First a silicate melt which formed the olivines and second a metal melt forming

the surrounding network of FeNi-metal. But isolated metal grains proof that the metal melt already existed when the olivine crystallized. The higher melting point

of pallasitic olivine speaks for a coexistence of solid olivine and a FeNi-melt. Also

a residual melt is proposed because of the presence of troilite and phosphate.44

42 Scott, 1977b, p. 705ff. 43 Web 4 44 Buseck, 1977, p. 733


The pallasite problem itself in-

cludes not only the formation of the

pallasite minerals but also the

mechanism of mixture. Because of

the immiscibility of silicate and

metal melts olivine and FeNi-metal

have a strong tendency to separate

quickly from each other. The Ency-clopedia of meteorites describes two

possible mixture processes. Either a

shock wave induced driving of oli-

vine into the metal core or a convec-

tive instability which forced the

metal into olivine cumulates above

(see also Figure 7). Both are only

short-lived and temporary processes after which the separation would start again.

So they conclude that the pallasites “[…] formed after differentiation but before

complete solidification of the core. …”.45

Scott 1977b gives 3 explanations for the pallasite problem. The first is after

Wood 1963 who suggested that the separation of floating olivine crystals and a metal melt was prohibited by an overlying olivine layer or a solid crust of metal.

The second explanation suggests that the metal was already solid and acted plastic

while it intruded into the fractured olivine as developed by Merrill 1928. The third

and by Scott most favored idea is that the metal was close to the freezing point or

maybe even supercooled while the olivine was marginally cooler, so that differen-

tiation and segregation was prevented.46

Today’s research is focusing on the minor and trace element zoning of palla-

sitic olivine as well as different isotopic analyses. The Widmannstätten structures

found in pallasitic FeNi-metal are known to develop between 700 – 800 °C and

400 – 500 °C with a cooling rate of a few degrees per million years47 (0.5 – 2

°C/106 a48). Cooling rates based on the olivine zoning were measured as 20 – 200 °C/a from 1100 to 600 °C. The wide range of these cooling rates are caused by

different best fitting calculated cooling rates for different analyzed elements.49

From these cooling rates a thermal history of a two step cooling was suggested.

Miyamoto (1997) discusses two different interpretations. First a high-temperature

formation of chemical zoning, where the first step is characterized by fast cooling

at high temperatures and the formation of the chemical zoning. And the second

step is a slow cooling at low temperatures event formed the Widmannstätten struc-

ture. The other interpretation is a low-temperature formation process to form the

chemical zoning. This theory explains the chemical zoning by a migration of ele-

45 Norton, 2002, p. 206 46 Scott, 1977b, p. 706 47 Huss and Tomiyama, 2005, Discussion 48 Miyamoto, 1997, p. 21,613 49 Miyamoto, 1997, p. 21,615

Figure 7: Formation of Pallasites45

14 Boeck, Lena

ments into the olivine during grain boundary diffusion.50 A large number of papers

(e.g. Huss and Tomiyama, 2005 and 2006 or Chen and Papanastassious, 2009, all

Lunar and Planetary Science Conference) are dealing with different analyses of

trace elements and isotopic compositions to clarify the thermal history of palla-

sites and with this the formation. But all authors are pointing to continue their

work to clear up the issue.

After all the discussion is still open and offers some future work to do.

6. Summary

Pallasites are very unique stony-iron meteorites with a relatively simple mineral-ogy. They can be divided into main group pallasites, Eagle Station pallasites and

pyroxene pallasites, all more or less similar in mineralogy but different in the

composition of the metal and of the oxygen isotopes. So that the three groups ap-

pear as well defined clusters in plots of this properties. Most scientists believe that

pallasites were formed at the mantle-core boundary of differentiated asteroidal

parent bodies. Nowadays the formation of pallasites is a subject of discussion. Dif-

ferent ideas can be found in papers by Scott, Buseck, Norton, Miyamoto and much

more. Because of much work currently is in progress, no firm conclusions could

be drawn on the origin of these enigmatic meteorites.

50 Miyamoto, 1997, p.21,617


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ton: 2006.

Miyamoto, M.: Chemical zoning of olivine in several pallasites. Journal of geo-

physical research. Vol. 102, No. E9: 1997

Norton, O. Richard: The Cambridge Encyclopedia of Meteorites. Cambridge Uni-

versity Press, Cambridge: 2002.

Norton, O. Richard; Chitwood, Lawrence A.: Field Guide to Meteors and Meteor-

ites. Patrick Moore’s Practical Astronomy Series. Springer-Verlag, Lodon: 2008.

Sears, Derek: The Origin of Chondrules and Chondrites. Cambridge University Press, Cambridge: 2004.

16 Boeck, Lena

Stöffler, Dieter; Keil, Klaus; Scott, Edward R. D.: Shock metamorphism of ordi-

nary chondrites. Geochimica et Cosmochimica Acta, Volume 55. Pergamon Press:

1991.

Scott, Edward R. D.: Pallasites – metal composition, classification and relation-

ship with iron meteorites. Geochimica et Cosmochimica Acta. Volume 41. Per-

gamon Press: 1977a.

Scott, Edward R. D.: Formation of olivine-metal textures in pallasite meteorites.

Geochimica et Cosmochimica Acta. Volume 41. Pergamon Press: 1977b.

Wlotzka, F.: A weathering scale for the ordinary chondrites. Meteoritics 28: 1993.

(abstract)

8. Appendix

A Glossary

CAIs calcium-aluminium-rich inclusions1 containing also

titanium, which are highly refractory and suggested

to be the first mineral formed in the solar nebular2

chondrules approximately spherical elements of meteorites, which show evidence of partial or complete melting1

dark inclusions also dark clasts

are rock fragments of a size of 100 to 1000 µm,

which contain aqueously altered material3

HEDs howardite-eucrite-diogenite clan of meteorites,

which belong to the asteroidal achondrites4 and

trought to have formed on the asteroid 4 Vesta5

planetesimals bodies made of rocks or ice which formed in the primordial solar nebular6

UOCs unequilibrate ordinary chondrites7

Widmannstätten structure structure found on etched regions of iron meteorites

showing large bars of kamacite surrounded by small

fields of taenite8

1 Lauretta and McSween, 2006, p. 908 2 Norton and Chitwood, 2008, p. 269 3 Krot and Scott, 2005, p. 31 4 Lauretta and McSween, 2006, p. 910 5 Norton and Chitwood, 2008, p. 273 6 Lauretta and McSween, 2006, p. 913 7 Web 8 Lauretta and McSween, 2006, p. 917

18 Boeck, Lena

B List of meteorite groups and their main properties

Undifferentiated meteorites Enstatite chondrites EH high total iron, highly reduced,

minichondrule-bearing

EL lower total iron, highly reduced, mod-

erately size chondrules

Ungrouped e.g. LEW 87223

Ordinary chondrites O

Olivine-Bronzite H high total iron

Olivine-

Hypersthene

L low total iron

Amphoterite LL low total iron, low metallic iron

Rumuruti chondrites R high total iron, highly oxidized, δ17O-

rich

Carbonaceous chon-

drites

C

CI primitive chondrites, contains phyl-

losilicates, chondrules-free, volatile-

rich, aqueously altered

CM contains little chondrules (< 0.5 mm)

and phyllosilicates, aqueously altered

CR contains metal, mm-sized chondrules and phyllosilicates, aqueously altered

CH similar CR, contains smallest chon-

drules, volatile-poor

CO contains metal and chondrules (0.1 –

0.4 mm)

CV contains bigger chondrules (0.5 – 2

mm) and CAIs, partially aqueously al-

tered

CK highly oxidized, large chondrule-

bearing, darkened silicates

Ungrouped e.g. Coolidge, LEW 85332

IAB/IIICD silicates Subchondritic composition, chon-

drule-free, planetary-gas-bearing

Ungrouped chondrites e.g. Deakin 001

K-chondrites


Differentiated meteorites Primitive Achondrites

Acapulcoites ACAP chondritic abundance of plagioclase and troilite, medium-grained

Lodranites LOD subchondritic abundance of plagio-

clase and troilite, coarse-grained,

higher olivine content than ACAP

Winonaites WIN IAB-silicate-related, highly reduced

recrystallized silicates

Ungrouped e.g. Divnoe

Asteroidal achondrites

Eucrites EU basalts

Diogenites DIOG magnesium-pyroxenites

Howardites HOW brecciated mixture of basalts and or-

thopyroxenites

Angrites ANGR fassaitic-pyroxene-bearing basalt

Aubrites AUB enstatie achondrites

Ureilites UR olivine-pyroxene-carbonaceous ma-

trix-bearing

Brachinites BRACH equigranualar matrix of olivine, py-

roxenes and plagioclase

Martian meteorites SNC

Shergottites SHE basalts and lherzolites

Nakhlites NAK cumulus-augite-bearing pyroxenites

Chassigny CHS dunite

ALH 85001 ALH orthopyroxenites

Lunar meteorites

Mare basalts basalts, gabbros, anorthosites

Impact breccias anorthositic- and mare-dominated re-

golith and fragmented breccias

Stony irons

Pallasites PAL metal plus olivine

main-group pallasites (MG)

Eagle-station pallasites (ES)

Pyroxene pallasites

Mesosiderites MES metal plus basaltic, gabbroic and or-thopyroxenitic silicates

Ungrouped e.g. Enon, Mt. Egerton

Irons

Chemical classification

Magmatic groups IC, IIAB, IIC, IID, IIF, IIIAB, IIIF,

IVA, IVB

Nonmagmatic IAB/IIICD, IIE

20 Boeck, Lena

groups

Ungrouped IRUNGR e.g. Britstown, Denver City, Guin,

Sombrerete

Structural classification

Hexahedrites HEX Neumann lines (IIAB, IIG)

Octahedrites O Widmanstätten lines

Coarsest octa-

hedrites

Ogg 3.3 – 50 mm Kamacite bandwidth

(IIAB, IIG)

Coarsest octa-

hedrites

Ogg 1.3 – 3.3 mm Kamacite bandwidth

(IAB, IC, IIE, IIIAB, IIIE)

Medium octa-

hedrites

Om 0.5 – 1.3 mm Kamacite bandwidth

(IAB, IID, IIE, IIIAB, IIIF)

Fine octahedrites Of 0.2 – 0.5 mm Kamacite bandwidth

(IID, IIICD, IIIF, IVA)

Finest octahedrites Off < 0.2 mm Kamacite bandwidth (IIC,

IIICD)

Plessitic Opl < 0.2 mm Kamacite bandwidth, spin-

dles (IIC, IIF)

Ataxite D > 16.0 % Ni (IIF, IVB)

Table II: List of classes, groupes and subgroups of meteorites9

9 Kleinschrot; 2003; p. 19 and Norton; 2002; p. 307f.


C Petrologic type

Petrologic type Criterion

1 2 3 4 5 6

Homogenity of oli-

vine and pyroxene

compositions

- > 5 % mean devia-

tion < 5 % homogeneous

Structural state of

low-Ca pyroxene -

Predominantly

monoclinic

> 20 %

monoclinic

< 20 %

monoclinic Orthorhombic

Feldspar - Minor primary grains

only

Secondary,

< 2 µm

grains

Secondary,

2 – 50 µm

grains

Secondary,

> 50 µm

grain

Chondrule glass -

Altered, mostly

absent a

Clear, iso-tropic,

variable

abundance

Devitrified, absent

Metal: max. bulk

Ni, wt% -

< 20 %;

taenite

minor

or ab-

sent

Kamacite and taenite in exsolution relationship

> 20 %

Sulfides: mean Ni

content -

> 0.5

wt% < 0.5 wt%

Matrix

All

fine-

grained,

opaque

Mostly

fine,

opaque

Clastic

and minor

opaque

Transparent, recrystallized,

coarsing from 4 to 6

Chondrule-matrix integration

No chon-

drules

Chondrules very sharply defined

Chondrules well de-

fined

Chonrules readily de-

lineated

Chondrules poorly de-

fined

Carbon, wt% 3 – 5 0.8 –

2.6 < 1.5

Water, wt% 18 – 22 2 - 16 0.3 - 3 < 1.5

Table III: Petrologic type after Van Schmus and Wood (1967), with modifications

from Sears and Dodd (1988) and Brearley and Jones (1998)

a Chondrule glass is rare in CM2 chondrites, but is preserved in many CR2 chondrites.

22 Boeck, Lena

D Shock stage

Effects from equilibration peak shock pressure Shock stage

Olivine Plagioclase Orthopyroxene

Shock pres-

sure GPa

b

unshocked

S1

Sharp optical extinction, irregular fractures

very weakly

shocked S2

Undulatory extinction, irregular

fractures

Undulatory extinc-

tion, irregular and some planar fractures

weakly

shocked

S3

Planar fractures,

undulatory extinction,

irregular fractures

Undulatory

extinction

Clinoenstatite lamel-

lae on (100), undula-

tory extinction, planar

and irregular fractures

moderately

shocked

S4

Mosaicism (weak),

planar fractures

Undulatory

extinction,

partically iso-

tropic, planar

deformation

features

strongly

shocked

S5

Mosaicism

(strong), planar frac-

tures + planar defor-mation features

Maske-

lynite

Restricted to local regions in or near

melt zones

very strongly

shocked

S6

Solid state

recrystallization

and staining,

ringwoodite,

melting

shock melted

(normal glass)

Majorite, melting

Shock melting Whole-rock melting (impact melt rocks and melt breccias)

< 4 -5

5 – 10

15 – 20

30 – 35

45 - 55

75 - 90

“Shock effects in ordinary chondrites are characterized by effects in olivine and

plagioclase; shock level in carbonaceous chondrites are characterized by effects

mostly in olivine (Scott et al.,1992); shock levels in enstatie chondrites are charac-

terized by effects mostly in orthopyroxene.” The prime shock criteria for each

shock stage are underscored.10

b Shock pressure can be used for ordinary chondrites only.

10 Davis, 2004, p. 93


E Grade of weathering

WO • no visible oxidation of metal or sulphide • usually fresh falls

W1 • minor oxidation rims aroud metal and troilite

• minor oxidation veins

• same fresh falls are al-

ready in this stage

W2 • moderate oxidation of metal

• 20 – 60 % of metal grains are affected 5,000 – 15,000 a

W3 • heavy oxidation of metal and troilite

• 60 – 95 % replacement 15,000 – 30,000 a

W4

• complete (>95 %) oxidation of metal and

troilite

• no silicate alteration

20,000 – 35,000 a

W5 • beginning alteration of mafic silicates

(mainly along cracks) 30,000 - >45,000 a

W6 • massive replacement of silicates by clay

minerals and oxides

• terrestrial ages correlated with meteorites found in Roosevelt county (New

Mexico) and caused by that only useable for meteorites found in regions with

similar climate conditions11

11 Wlotzka, 1993, p. 460

24 Boeck, Lena

F Figures

Figure 8: A piece of the Springwater Pallasite from the collection of the UH, Uni-

versity of Hawaii in Manoa, Honolulu (picture by Lena Boeck)


Figure 9: Angular olivine crystals in the Eagle station pallasite. (a) Scale bar: 5 mm

(b) and (c) enlarged areas marked in (a), reflected light microscope picture, Scale bar:

200 µm. Olivine (O) – dark grey, plessite (P) – light grey, Kamacite - white.12

12 Scott, 1977b, p. 698

26 Boeck, Lena

a) b)

c) Figure 10: Logarithmic plots of different pallasites. Pallasitic fields are compared

to iron meteorite fields. a) Au-Ni-plot, b) Ga-Ni-plot, c) Ge-Ni-plot13

13 Scott, 1977a, p. 352f.


G References

Web: http://www4.nau.edu/meteorite/Meteorite/Book-Chondrules.html (February

2009)

Kleinschrot, Dorothée: Meteorite – Steine, die vom Himmel fallen. Beringis Son-

derheft 4, Würzburg: 2003.

Krot, Alexander N.; Scott, Edward R. D.: Chondrites and the Protoplanetary Disk.

Vol. 341. ASP Conference Series: 2005.

Lauretta, Dante S.; McSween, Harry Y.: Meteorites and the early solar system II.

The University of Arizona Press, Tucson and Lunar and planetary institute, Hous-

ton: 2006.

Norton, O. Richard; Chitwood, Lawrence A.: Field Guide to Meteors and Meteor-

ites. Patrick Moore’s Practical Astronomy Series. Springer-Verlag, Lodon: 2008.

Scott, Edward R. D.: Pallasites – metal composition, classification and relation-

ship with iron meteorites. Geochimica et Cosmochimica Acta. Volume 41. Per-

gamon Press: 1977a.

Scott, Edward R. D.: Formation of olivine-metal textures in pallasite meteorites.

Geochimica et Cosmochimica Acta. Volume 41. Pergamon Press: 1977b.

Wlotzka, F.: A weathering scale for the ordinary chondrites. Meteoritics 28: 1993.

(abstract)

Documents

An introduction to pallasites, their mineralogy, … introduction to pallasites, their mineralogy, texture and origin Lena Boeck Institut für Mineralogie, TU Bergakademie Freiberg,