Exsolution lamellae of kirschsteinite in magnesium-iron olivine from an angrite meteorite

7/27/2019 Exsolution lamellae of kirschsteinite in magnesium-iron olivine from an angrite meteorite

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A m e ri ca n M i ne ra lo g is t, V o lu m e 8 0 , p a ge s 585 -592 , 1 995

Exsolution lam ellae of kirschsteinite in m agnesium -iron olivine from an angrite m eteoriteTAKASHI MIKOUCHI, HIROSHI TAKEDA, MASAMICHI MIYAMOTO

M ineralogical Institute, G raduate School of Science, U niversity of Tokyo, H ongo, Tokyo 113, JapanKAzUMASA OHSUMI

Ph oton Factory, N ational L abo ratory for H igh Energy Physics (K EK ), Tsukub a, Ibaraki 30 5, JapanGoRDON A. McKAy

SN 4 N ASA /Johnson S pace C enter, H ouston, T exas 77058, U .S.A .ABSTRACT

Exsolution phenomena of kirschsteinite (CaFeSi04) in olivine have been studied bysingle-crystal X -ray diffraction techniques and scanning electron m icroscopy (SEM ) oforiented polished thin sections (PTS ) of three single crystals separated from the A ntarcticangrite LE W86010, supplem ented by m icro-area X -ray diffraction w ith the Laue m ethod(MXL) by synchrotron radiation (SR) for PTS of a rock chip of LEW8601O. The celldim ensions of the host olivine and exsolved kirschsteinite are a = 4.79(3), b = 1 0.3 9(5 ),and c = 6.06(3) A , and a = 4.87(5), b = 11.14(10), and c = 6.36(5) A , respectively, fromthe precession photos. The PTS of olivine single crystals oriented parallel to (100) showexsolution lamellae of kirschsteinite up to I 0 ~m in width. The two sets of lamellae aresym metrically related and parallel to (031) and (031). E lectron m icroprobe analysis gaveSi02 33.1, Ti02 0.07, Al203 0.03, FeO 49.4, M nO 0.61, M gO 13.4, CaO 2.2, Cr203 0.02,V203 0.01, NiO 0.05 (sum 99.4 wt%) for the host olivine and Si02 33.3, Ti02 0.03, FeO31.5, MnO 0.39, MgO 5.4, CaO 28.5, Cr203 0.02, NiO 0.05 (sum 99.2 wt%) for theexsolved kirschsteinite. The results from M XL for the olivine crystals on the rock PTSare com patible w ith the observation on the single crystals that the lam ellae are parallel to(031) and (031). The (031) and (031) planes have been know n to be tw in planes for olivine,and the tw inning is by reticular pseudom erohedry based on a quadruple lattice. A lthougho th er re po rte d e xso lv ed p re cip ita te s in m ete oritic o liv in es e xist a s in clu sio ns, k irs ch ste in itein LEW 86010 olivine takes the form of lamellae. Our explanation is that LEW86010olivine is Fe-rich and that lam ellar precipitates are m ore easily form ed than inclusionsb ecau se ex so lu tio n lam ellae alo ng {0 31 } in F e-rich oliv in e m ain tain lattice co heren cy.

INTRO U TION

E xso lu tion textures are frequ ently ob served in m ineralsof igneous and metamorphic rocks and often take theform oflam ellar precipitates. E xsolution occurs w hen anoriginally hom ogeneous phase enters a two-phase field,u su ally a s a result o f redu ctio n in tem peratu re. T he g row thof an ex s olved phase causes a redistribution of elem ents,w hich can give som e inform ation on the cooling history.T hese exsolution textures are com monly observed in pla-g io cla se s, p yro xe ne s, amp hib ole s, a nd o th er ro ck -fo rm in gm inerals (Putnis, 1992). The exsolution of N i-Fe m etalin Fe m eteorites is w ell know n (W idm ansH itten pattern),and Goldstein and Short (1967) calculated the coolingrates of these m eteorites assum ing the N i-rich lam ellaew ere form ed by diffusion. M iyam oto and Takeda (1994)calculated the thickness of the crust of R ED (horw ardite,eucrite, diogenite) parent body from augite lam ellae inpigeonite host crystals in a cumulate eucrite (MooreCounty). A lthough olivine is one of the m ost im portantrock-form ing m inerals and is com mon in both terrestrial0003-004 X/9 5/0506-0585$02.00

and extraterrestrial m aterials, exsolution phenom ena ino liv in e h av e n ot been w ell characterized . A shw orth (19 79 )found two kinds of exsolution of chrom ite in chondriticolivine. Petaev and Brearley (1994) and Petaev et aI.(1994) reported a lamella-like texture produced in theolivine in the D ivnoe achondrite. D odd (1971) identifiedfe rroan m onticellite (kirsch steinite) in sets in c ho nd riticolivine, but they are not lam ellae. Prinz et aI. (1977) re-ported kirschsteinite inclusions in the Angra dos Reisachondrite. Prior to the recovery ofL EW 860 I 0 A ntarctican grite, the presence o f forsterite lam ellae in a m onticel-lite host (C aM gSi04) w as the only instance of exsolutionlam ellae in olivine ever reported (W illem se and B ensch,1964; T racy et aI., 1978).A ngrite are unusual m eteorites, of w hich the first sam -ple discovered w as A ngra dos R eis. A ngrites are basalticachondrites with fassaite, olivine, and anorthite. In thelast eight years three additional angrites w ere recoveredfrom A ntarctica (M ason, 1987, 1989; Yanai, 1991). An-grites are no te d fo r their v ery o ld cry stallization ages (4.5 5b.y.) and are said to be one of the oldest known basalts

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M IKOUCHI ET AL.: KIRSCHSTEINITE LAM ELLAE IN OLIVINE

Fig. 1. O ptical photom icrograph of LEW 860 10 thin section(L EW 86 01 0,6 ) (p ara lle l p ola riz ed lig ht). Z on ed fassa ite (F s), o l-ivine w ith kirschsteinite lam ellae (01), and anorthite (A n) arem ajo r c on stitu en t m in era ls. K irsc hste in ite la me lla e o ccu r in tw oorientations. The scale bar is 1 m m.

in the solar system (Lugmair and Galer, 1992). One ofthe m ost rem arkab le characteristics of ang rites is th at th eyare conspicuously rich in refractory elem ents and poor inv olatile elem ents. T he o rig in s of the ang rite are still un derd ispu te (M ittlefeh ld t et aI., 19 90 ).LEW 8601O, discovered in Antarctica in 1986, is thesecond angrite to be identified (M ason, 1987). O livinewith a high bulk Ca content is one of the major constit-u ent m in erals ofL EW 86 01 O, and it exsolved kirsch stein -ite (ideally C aF eS iO .) d uring coo ling (M cK ay et aI., 1 98 8;G oodrich, 1988; Prinz et aI., 1988; Delaney and Sutton,1988).A lth ou gh M ason (1 98 7) first d escribed k irschsteinitelam ellae in the LE W860 10 olivine, and Prinz et ai. (1988)and M cK ay et ai. (1989) described the lam ella directions

on non oriented PTS of a rock chip, this report and ourp relim in ary o ne (M iko uch i et aI., 19 93 ) represent the firstcry stallog raph ic stud y of the relatio nship betw een th e h ostand the lam ellae. This exsolution texture is the only caseknown in which kirschsteinite lamellae formed in a Ca-poor olivine (fayalite) host, as is com monly observed inp yro xen es. T he coex isten ce of oliv in e an d ex so lv ed p hasesprovides us with information on the phase relationsbetween Ca-poor and Ca-rich olivine (Davidson andM ukhopadhyay, 1984), and its cooling history can be de-duced by the method applied to other minerals (Gold-stein and Short, 1967; M iyamoto and Takeda, 1994).E mploying C a diffusion profiles of kirschsteinite in thisangrite olivine, M cKay et ai. (1989, 1993) estimated aco olin g rate for th is m eteo rite.Although kirschsteinite was recently found in a car-

PF R ing~ 8 0j 80 1'l ll 8 0x l8 01 'l ll2350 380BlAB j___n n j n_n_o 166f nu n 18 ra b

Vac uum Cha mb er

c dFig. 2. A schematic illustration of the optical system and

diffraction equipment of M XL. PF ring is a storage ring for SRexperiments. Letters a, b, and c indicate the first and second slitand third pinhold, respectively. Sam ple (thin section) and im-aging plate are indicated by d and IP, respectively. The num berssituated below the horizontal dotted line are the distances fromthe source point (0).

bonaceous chondrite (K im ura and Ikeda, 1994), it is ex-tremely rare on the Earth and is known to be producedin only a few places in the world (Sahama and Hyt6nen,1 95 7, 1 95 8; K onev et aI., 1 97 0). M agn esian k irschstein-ite, reported by Sahama and Hyt6nen (1957), occurs in amelilite nephelinite, North Kivu, Zaire, in which it isa sso cia te d w ith k als ilite , g 6tz en ite , so da lite , c ombe ite , a ndperovskite. M agnesium kirschsteinite has also been de-scribed from calcareous skarn xenoliths in the alkali sy-enite, T azheran, R ussia (K onev et aI., 1970).W e exam ined the ex so lu tion lam ellae o f kirsch stein itein oriented olivine single crystals from the LEW 860 10an grite u sing X -ray d iffractio n, op tical m icroscop y, S EM ,an d electron m icro pro be an aly sis. B ecause the cry stallo-grap hic o rien tatio n o fkirsch stein ite lam ellae o btain ed b yo rien ted sin gle crystals is different fro m th at determ in edby optical microscope observation of the PTS (Prinz etaI., 1988; M cK ay et aI., 1989), olivines in the LEW 860 10PT S w ere analyzed by m icro-area X -ray diffraction w iththe Laue method (MXL) developed by Ohsumi et ai.(1991, 1993) using synchrotron radiation (SR) at beamline 4B (BL-4B) of the Photon Factory (PF), NationalLaboratory for H igh Energy Physics (KEK) in Tsukuba,Japan. The lam ella directions determ ined by M XL wereco mpared w ith the results of o rien ted sin gle cry stals. T hisn ew te ch niq ue a llows in -s itu c ry sta llo gra ph ic m ic ro an al-ysis of m inerals in thin section at spatial resolutions com -p arable w ith th e electro n m icrop rob e.

SAMPLES AND EXPER IMENTAL METHODSSample desc ri pt ion

A ntarctic an grite L EW 86 01 0 w eig hs o nly 6 .9 g (M aso n,1987) and has approxim ate dim ensions of 1.5 cm on eachsid e. F iv e o liv in e g ra in s s ep ara te d from LEW860 10 ,1 0,1 4by L. E. Nyquist and coworkers (NASA Johnson SpaceCenter), were used for this study as a part of the consor-tium study led by G. M cKay. Each grain is about 0.5 mmin diam eter and, when viewed through an optical m icro-scope, appears to be a yellow ish single crystal. O nly three


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M IKOUCHI ET AL.: KIRSCHSTEINITE LAM ELLAE IN OLIVINE 58 7ofthe crystals w ere used for this study, and the rest werekept for a future study.

In addition to three single crystals, a polished thin sec-tion of a rock chip (PTS LEW 8601O,6) (Fig. 1) was ex-amined by MXL at the PF, at KEK in Tsukuba, Japan.The m odal abundances of m inerals in this PTS are 43.3%py ox ene, 31 .8% plagioclase, 20 .3% o livine, 3 .2 % kirsch -steinite, 0.7% troilite, 0.3% w hitlockite, and 0.1 % her-cynite (M cK ay et aI., 1988). O livine occurs as subhedralto anhedral crystals. A ll olivines show exsolution lam el-lae of kirschsteinite, which are mostly parallel to twoc ryst al log raphi c d ir ec ti ons.S in gle -c ry st al X -r ay d if fr ac ti on

F or d eterm inatio n o f th e cry stallog rap hic o rien tationof the olivine grains, a single-crystal X -ray diffractiontechnique (precession cam era) w as em ployed, using N i-filtered CuKa (X = 1.5418 A ) and Z r-filtered M oKa (X =0.7107 A) radiation. At first, each grain was carefullymounted on a glass fiber using oxybenzon. The hkO)*and hOt)* reciprocal nets w ere photographed by a pre-cession cam era with a O.I-m m slit. Exposure tim es w ere50-120 h at 40 kV, 15 mA. The cell dimensions of thehost olivine phases were calculated from the distancesbetween the pair of reciprocal lattice rows measured onthe photographs by a com parator. After the orientationof the host olivine w as defined, other reflections that donot belong to the host were searched for, and a matchw as s ou gh t w ith re fle ctio ns from e xso lv ed k irs ch ste in ite .Then, the grain was m ounted in resin with its crystallo-graphic a axis perp end icu lar to th e plane o f th e g lass slide,since the reported lam ella directions were all parallel tothe a axis (Prinz et aI., 1988; M cKay et aI., 1989). Thecrystal w as polished parallel to the (100) plane, w hich isperpendicular to the lam ella planes. T he depth of polish-ing was controlled to get the w idest cross section. Crys-ta llo gra ph ic o rie nta tio ns o f th e lame lla e w ere d ete rm in edwith respect to the crystallographic band c axes of thehost, as determ ined by the precession m ethod. The widthof the exsolution lam ellae and the spacings betw een themwere measured through an optical microscope. After Ccoating, the grain was observed by a JEO L JSM -840 SEMequ ip ped w ith an energ y-d isp ersive X -ray sp ectrom eter(ED S). The chem ical com position of kirschsteinite exso-lution lamellae and host olivine were obtained with theJEOL Super Probe 733 at the Ocean Research Instituteof the University of Tokyo with a 12-nA beam currentan d 15 -kV accelerating v oltage.M icrodiffraction on a thin section bys yn ch ro tr on r ad ia ti on

The olivine crystals in the PTS of LEW86010 wereanalyzed by SR at BL-4B of the PE The Laue method(M XL) was used to record the diffraction patterns withan imaging plate (Ohsumi et aI., 1991). Although thismeasurement yielded a m inimum beam diameter of 8p.m, a beam diameter of about 50 p.m was used for theMXL measurement to cover both the lamella and the

F ig . 3. X -ray diffraction ph otos of hkO)*an d hot)* planesby p re ce ss io n c ame ra . F ain t r ef le ct io ns f rom k ir sc hs te in ite a reo bserved ju st insid e th ose fro m the ho st o livine. T he intenseki rschs tein ite reflect ions correspond to the reflect ive powder di f-f rac tion intensi ties for ki rschs te ini te .

host. The Laue patterns of the LEW 860 I 0 olivines weretaken w ith lO-m in exposures, with the storage ring cur-rent at around 250 mA and a ring energy at 2.5 GeV. Acom puter softw are system developed for analysis of m i-crom eter-sized single crystals (Hagiya et aI., 1993) hasbeen revised and used for data reduction. A schematicillustration of this entire system is show n in Figure 2, anda detailed description of the m ethod is given by Ohsum ie t a 1. ( 1994 ).The orientation of the sample with respect to the co-ordinate system of the equipm ent is calculated based onthe positions of the Laue spots on the im aging plate usingindices and cell dim ensions of olivine obtained from theprecession m ethod. W e calculated the angles betw een thetwo sets of lamellae that would be observed in the planeof the PTS, assum ing that the lamellae are parallel to(031) and (O JI), and com pared the calculated angles w iththose actually observed on the PTS.

REsULTSC ry sta llo gra ph ic d ata

H ost o livin e. T he oliv in e in th e fo rsterite-fay alite so lid -solution series is orthorhom bic w ith space group Pbnm.Kirschsteinite belongs to the same crystal system andspace group as olivine. The cell dimensions were mea-sured on the precession photographs taken with CuKaradiation. These sets of a, b, and c axis precession pho-tographs give the cell dim ensions of the host olivine as a= 4.79(3), b = 10.39(5), and c = 6 .0 6(3 ) A b y re fin em en tof about 50 pairs ofreciprocallattice row s (Fig. 3). U singa diagram of cell dimensions vs. Mg/(Mg + Fe) atomicratio s for th e fo rsterite-fay alite so lid-so lu tion series (A k-imoto, 1976; Hazen, 1977), the Mg/(Mg + Fe) atomicratio o f a pp rox im ately 0 .3 (F 03 o) i s ob tained for th e ab ov ecell dimensions without taking the Ca content into ac-

-.-


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KirschsteiniteHost olivine lamellaSiO, 33.1 33.3TiO, 0.07 0.03AI,03 0.03FeO 49.4 31.5MnO 0.61 0.39MgO 13.4 5.4CaO 2.2 28.5Na,O 0.01K,OCr,03 0.02 0.02V,03 0.01N iO 0.05 0.05P,O,Total 99.4 99.2

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M IKOUCHI ET AL.: KIRSCHSTEINITE LAM ELLAE IN OLIVINE

CaFe(Kst)

Mg(Fo) Fe(Fa)Fig. 4. Chem ical composition (in m ole percent) of exsolved

k irsc hstein ite a nd h ost o liv in e p lo tte d in th e c aIc iu m-m ag ne si-u rn -i ro n o li vi ne quadr il at er al .

Fig . 5. B ack-scattered electron (B SE ) im ages oftw o orientedoliv ine crystals (O livine I and 0Iivin e2). B oth oliv ine grains in-clude exso lution lam ellae o f kirschsteinite parallel to (031) and(031). The thickness of the lamellae is at most 10 ,..m and thespacing is generally several tens of m icrometers. The blacksp he ric al m in era l in O liv in e I is a no rth ite . E xso lv ed p re cip ita te sa re o bse rv ed a t th e c ry sta l b ou nd ary . S ev era l b la ck lin es c ro ssin gth e lam ella e are th e tra ce s o f m ic ro pro be a na ly sis. A n: a no rth ite ,0 1: h ost o liv in e, a nd K st: k irsc hste in ite la me lla .

TABLE1 . E le ct ro n m ic ro pro be a na ly sis ( in weig ht p er ce nt ) o f t heh ost olivine and exsolv ed kirschstein ite in ang riteLEW86010

count. This r atio is in re la tiv ely good a gr eement w ith th evalues (F032) later obtained by microprobe analysis asw ell as p revio usly rep orted value s (F 03 1_ 33 )M cK ay et aI.,1988; Delaney and Sutton, 1988; Prinz et aI., 1988;G oodrich, 1988). Chem ical com positions of the host ol-ivine and the exsolved kirschsteinite are listed in Table1, and Ca-Fe-M g atom ic ratios are plotted on the olivineq ua drila te ra l (F ig . 4 ).Exsolved phase. The faint reflections just inside theh ost o livine reflectio ns w ere reco gnized on the hkO)* and hOl)* photographs (Fig. 3). G enerally, the higher the Fecontent of olivine, the greater the absorption of X-rays.These olivines have high Fe contents (Fo32)' A s a result,the exsolved phase was easier to detect on the photostaken w ith M oKa radiation than on those with CuKabecause of the weaker absorption of M oKa X -rays. B e-cause the exsolved and the host phases have the samecrystallographic orientations, w e can obtain the cell di-mensions of the exsolved phase as a = 4.87(5), b =11.14(10), and c = 6.36(5) A , which are in close agree-m ent w ith th e rep orted cell d im en sion s for k irschstein ite(Saham a and Hytonen, 1958). Furtherm ore, the indicesof intense reflections found am ong the list of the intensereflections in the powder diffraction file reported forkirschsteinite are also intense on the precession photos(Saham a and Hytonen, 1958) (Table 2).O rien tatio n of th e ex so lu tio n lam ellae

Examination by an optical microscope and SEM re-vealed that all olivine crystals polished parallel to (100)contain two sets of distinct exsolution lamellae up to 10p,m in w idth and perpendicular to (100) (Fig. 5). Spacingsbetw een the lam ellae are typically a few tens of m icrom -eters. T he lam ellae looked perpendicular to the observedplane (100). The observed angles betw een the tw o lam ellad ire ctio ns w ere a ll a bo ut 58-65. The tw o s ets o flame lla eare sym metrically related w ith respect to the crystallo-graphic band c axes. The lam ellae position did not m ovewhile the sections were polished. All those results indi-cate that the lam ellae are parallel to the a axis.There is good agreement between the observed angle


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T LE 2 . X -ra y crys ta llo gra ph ic d ata fo r kirsc hste in iteKirschsteiniteCaFeSiO.

d A 1/1, hkl5.58 50 02 04.22 35 0213.895 5 10 13.672 60 11 12.957 10 0 13 02.788 30 0402.687 90 13 12.612 95 1122.559 20 04 12.421 35 14 012 22.176 10 13 21.835 60 24 022 21.610 35 06200 4

Lamellae Angle (0 ) Lamellae Angle (0 )1 / (011)and (011) 119.5 11(001)and ( 011 ) 2811(021)and (021) 81.2 1/ (001)and (031) 5511(031)a nd ( 03 1) 59.5 1/ (011)and (011) 881/ (041)and (041) 46.4 1/(031)and (031) 61

M IKO UCHI ET AL.: KIRSCHSTEINITE LAM ELLAE IN OLIVINE

Note: system = o rt ho r homb ic ; s pace group = Pbnm; a = 4.875, b ~1 1 .1 55 , C = 6 .438 A .. F ro m S ah am a a nd H yto ne n (1 95 8).

betw een the lam ellae (5 8-65) and the ang le b etw een (03 1)and (O J I) (59 .6), calcu lated u sing the o bserved ho st-ol-ivine cell dimensions. The agreem ent indicates that thelamellae are indeed parallel to (031) and (031). The dif-ference between the observed and calculated angles arewithin error because the plane of observation m ay not beexactly parallel to (100). T able 3 show s that other poten-tial o rientations d o not sho w sim ilar agreem ent.A lthoug h w e cannot detect th e textural relationsh ip b e-tw een the lam ellae and the host from only the diffractionimages taken with the Laue method (M XL) by SR, thoseim ages provide inform ation on the crystallographic di-rections of the crystals in the X-ray beam . Using the ori-entation matrix obtained by M XL for the olivine in thePTS, and assuming the lam ella directions, we can calcu-late the lamella angle on the plane of the PTS and com-pare it with the observed angle.

One olivine grain on the LEW86010 PTS shows thatthe angle between the two lam ellae is about 60 (Fig. 6).The orientation matrix from the MXL is rx : -171.3, ry :-65.26, and rz : 128.65. If the lamellae are assum ed tobe parallel to (031) and (031), the lamellae angle is cal-culated to be 61 on the PTS. This angle is in good agree-ment with the observed values for the crystal (Table 4).

DISCUSSIONOur results indicating lamellae parallel to (031) and(031) do not agree w ith the previously reported orienta-

T LE 3. Calculated ang le s b etween two lamella e u sin g c elld im ens io ns o f th e obs er ved o liv in e

58 9

O.5mmF ig . 6 . Op tic al p ho tomi cro gr ap h o f a n o liv in e c ry sta l in PTSLEW8601O ,6 .T h e a ng le b etwe en t he two s et s o flam ella e is 6 0 .An : ano rt hi te ; 01 : o li vi ne .

tion parallel to (OIl) and (all) (M cKay et aI., 1989), orto (001) and (OIl), or to (001) and (031) (Prinz et aI.,1988). Each angle between a set of two directions isroughly 60, except for (001) and (all) pairs (Fig. 7). Thew idth of the lam ellae previously reported is about 15-20~m , which is wider than the values of this study (up toI 0 ~m ), indicating that the kirschsteinite lam ellae of theprevious study are not exactly perpendicular to the planeof the PT S. O ne po ssible ex planation of th ese d ifferencesis that the previous thicknesses were m easured on non-oriented thin sections, for w hich the lam ellae tend to givelarger values, and the researchers might have m istakentheir crystallographic ax es. T here is also a possib ility th attheir crystallographic axes m ight have been rotated 60under m icro scopic observation. Fo r exam ple, ifthe b axisis exchanged with the c axis, (OIl) and (OIl) nearly agreew ith (031) and (031). Furtherm ore, if the Okl) p lan e (in -cluding band c axes) is rotated by 60, the (00l) and (031)pair are also in good agreem ent with (031) and (031).

MXL also indicated that the result from the orientedsection is consistent with the lam ellae being parallel to(031) and (031). W e cannot exclude the possibility thatlam ellae of several directions m ay truly exist. H ow ever,a consideratio n of the atom ic structu re o f olivine su ggeststhat the planes parallel to (031) and (031) are stable di-rection s for lam ellae, as is discussed later.T LE 4 . Ca lc ula te d ang le s b etween two lamella e obs erv ed on

the PTS using the data from SR analysis

T h e o bs er ve d a n gle s b et ween tw o l am e lla e a re 5 8- 65. . T h e o bs er ve d a ng le betw een tw o lam ellae o n the section is 60* .

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59 0 M IKOUCH I ET AL.: K lRSCHSTEINITE LAM ELLAE IN OLIVINE

a

b

c c

b

b

b

Fig. 7. Schem atic illustrations of the lam ellae directions re-ported by previous w orkers and com parison w ith this w ork. (a)This w ork [parallel to (031) and (031)], (b) M cK ay et aI., 1989[parallel to (011) and (O il)], and (c) Prinz et aI., 1988 [parallelto (001) and (031)]. A ll reports indicate that the angles betw eenthe tw o sets of lam ellae are nearly 60 .

o o~

t i 0.~o

o

o o \~-,_..@f)'

o

1/ 031

.-I ) 0\'dJnp b

// 031

(031)Fig. 9. Schematic illustration of olivine cells with the twin

plane of (031). The (031) plane defines a quadruple lattice fortw inning by reticular pseudom erohedry because the ratio of thecell dim ension of the b axis and c axis is almost the square rootof 3: 1 .

o

Figure 8 depicts the crystal structure within a unit cello ffo rsterite p erp en dicu la r to th e a a xis (B ra gg a nd B ro wn ,1926). The plane of dense 0 distribution is parallel to the(031) and (011) planes. Twinning in m onticellite is notcommon, but {03l} are known to be twin planes (Larsenet aI., 1941) and olivine also has twinning on (031). Fig-ure 9 show s a schem atic illustration of a larger scale w ithtw in plane of (031). The quadruple lattice can be takenfor a twinning by pseudom erohedry, because the ratio ofthe cell dimension of the b axis and e axis is almost thesquare root of 3:1.Exsolved precipitates in Angra dos Reis and Sharpschondrite do not take the form of lamellae, but they existas inclusions (Prinz et aI., 1977; D odd, 1971). Taking thecell dimensions of the band e axes of olivine at variousFo contents (Hazen, 1977), we calculated the bl e r atio sand the angles (8) (tan 8 = ble). The cell dimensions at1000 C w ere em ployed because the exsolution w as esti-mated to occur at this temperature (Davidson and Mu-khopadhyay, 1984). As the Fa content increases, the 11value also in creases an d app ro aches 60 (F ig. 10), at w hichpoint the ideal quadruple cell can be taken. This resultshow s that m ore Fe-rich olivine w ill give a quadruple cellw ith sm aller tw in obliquity (K asper an d L onsdale, 197 2),which also indicates the possibility that exsolution la-

o


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59.78

Inclusion Lamellae59.76 11 0/0) II {OJl}59.74

59.72 LEW86010

.-., 59.7~C D59.68

59.66 Sharps~chondrite59.64

59.62

59.6 0 10 20 30 40 50 60 70 80 90 100F a C on te nt

M IKOUCHI ET AL.: KIRSCHSTEINITE LAM ELLAE IN OLIVINE 59 1

Fig. 10. The relation between Fa content and () correspond-ing to the bl e ratios. T his figure indicates that the m ore Fe-richolivine can take the quadruple cell w ith sm aller tw in obliquity.m ellae along {031} in Fe-rich olivine m aintain lattice co-herency. This is in line with the natural occurrence ofex so lved precip itates. T he m ost F e-rich olivine (F 03 ,) h asexsolution lam ellae and the less Fe-rich olivines, A ngrados Reis (Fes3) and Sharps chondrite (F07s), do not havelam ellae, bu t instead h ave ro und ed in clu sions of ex so lv edphases (Prinz et al., 1977; Dodd, 1971). The texturaltransition boundary between lam ella and inclusion m aye xis t a ro un d Fe.D. F orsterite la me lla e (F 09 0-9 .) p ara lle l to(0 I 0) have been reported in a monticellite host (Traceyet aI., 1978). In this case we have to consider anotherm echanism to explain the form ation of (0 10) lam ellae inm onticellite. W e also calculated tw in obliquity at roomtemperature and obtained a sim ilar result. This showsthat com po sitio nal d ifferen ce plays a m ore im portant ro lethan therm al expansion at high tem perature. H ow ever, itis difficult to reach a definite conclusion because w e havefew ex am ples of th e ex solutio n p hen om eno n of oliv ine. A ne xp erim en ta l stu dy th at p ro du ce s e xs olv ed p ha se s o f o liv in eat vario us F o con ten ts is required for furth er d iscu ssion .F uku da (1 99 2) calculated the o rientation o f th e invari-an t p lane in b elite (C a2S iO .) assum in g un disto rted p lanesw hen an initial unit sph ere of the origin ally hom ogeneo usphase is distorted into an ellipsoid of the exsolved phase.The calculated planes are in good accordance with theobserved ones. In the case of kirschsteinite lam ellae andhost olivine in the LEW 86010 olivine, the cell dimen-sions ofkirschsteinite are alw ays larger than those of ol-ivine. Thus, we cannot obtain any unique solution byt hi s m e th od .

ACKNOWLEDGMENTST he M eteo rite W ork in g G ro up g enerou sly supp lied us w ith the sam pleof LEW86010 as part of the consortium study. We thank H. Mori at

E him e U niv ersity fo r his technical sup port fo r m aking oriented th in sec-tio ns. T he discu ssio ns w ith the staff o f th e M ineralo gical Institute o f th eU niversity of Tokyo and K Fukuda of the N agoya Institute of Technol-

ogy were very helpful. We are also indebted to O. Tachikawa and T.T ak as e o f th e M in era lo gic al I ns titu te fo r th eir te ch nica l a ss is tan ce , an d J .W ag staff an d L. Le o f L ockh eedlE SC O fo r th eir helpfu l sug gestio ns. W ethank the O cean R esearch Institute of the U niversity of Tokyo for m i-c ro pro be a na ly sis . T he m an us crip t b en ef itte d f ro m co ns tru ctiv e re vie wsb y J.H . B erg, A . B rearley , and an an on ym ous rev iew er.

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M IKOUCHI ET AL.: KIRSCHSTEINITE LAM ELLAE IN OLIVINEte rn to a na ly ze th e s tru ctu re o f a s ub micr om ete r- siz ed s in gle cr ys tal b ys yn ch ro tro n X .ra y d if lfa ctio n. J ou rn al o f A pp lie d C ry sta llo gra ph y, 2 4,340-348.

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M AN uS CR IP T R EC ET VE D AU GU ST 9 , 1 99 4M AN uS CR IP T A CC EP TE D JA NU AR V 1 1, 1 99 5

Documents

Exsolution lamellae of kirschsteinite in magnesium-iron olivine from an angrite meteorite