Table Of ContentMeteoritics & Planetary Scwme 31 87-96 (I996)
NASA-CR-2OIdI3
© Meteorilical Society, 1996. Printed in USA.
Pyroxene structures, cathodoluminescence
and the thermal history of the enstatite chondrites
YANHONG ZHANG I*, SHAOXIONG HUANG 1,DIANN SCHNEIDER I, PAUL H. BENOITt, .. )'_
JOHN M. DEHART 2'3, GARY E. LOFGREN 3AND DEREK W. G. SEARS I
I
CosmochemJstry Group, Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701. USA
2Energy Laboratories, Inc., Casper, Wyoming 82601, USA
3SN4, NASA Johnson Space Center, Houston, Texas 77058, USA
*Correspondence author's e-mail address: [email protected]
(Received 1995 May 30, accepted in revised form 1995 October 17)
Abstract-In order to explore the thermal history of enstatite chondrites, we examined the cathodo-
luminescence (CL) and thermoluminescence (TL) properties of 15 EH chondrites and 21 EL chondrites,
including all available petrographic types, both textural types 3_ and mineralogical types a_5. The CL
properties of EL3a and EH3a chondrites are similar. Enstatite grains high in Mn and other transition metals
display red CL, while enstatite with low concentrations of these elements show blue CL. A few enstatite
grains with >5 wt% FeO display no CL. In contrast, the luminescent properties of the metamorphosed EH
chondrites are very different from those of metamorphosed EL chondrites. While the enstatites in
metamorphosed EH chondrites display predominantly blue CL, the enstatites in metamorphosed EL
chondrites display a distinctive magenta CL with blue and red peaks of approximately equal intensity in their
spectra. The TL sensitivities of the enstatite chondrites correlate with the intensity of the blue CL and, unlike
other meteorite classes, are not simply related to metamorphism. The different luminescent properties of
metamorphosed EH and EL chondrites cannot readily be attributed to compositional differences. But x-ray
diffraction data suggests that the enstatite in EH5y,6 chondrites is predominantly disordered orthopyroxene,
while enstatite in EL6fl chondrites is predominantly ordered orthopyroxene. The difference in thermal
history of metamorphosed EL and EH chondrites is so marked that the use of single "petrographic" types is
misleading, and separate textural and mineralogical types are preferable. Our data confirm earlier
suggestions that metamorphosed EH chondrites underwent relatively rapid cooling, and the metamorphosed
EL chondrites cooled more slowly and experienced prolonged heating in the orthopyroxene field.
INTRODUCTION types a_5 reflect mineralogical closure temperatures and are quite
different for H and L chondrites.
Enstatite chondrites were formed in a very reducing There has always been a high degree of interest in the
environment and contain essentially FeO-free enstatite, high-Si cathodoluminescence (CL) properties of the enstatite chondrites and
metal and the cubic monosulfides, such as niningerite and ala- the related igneous class, the aubrites. This interest is partly
bandite (Keil, 1968; Mason 1966). While essentially chondritic in because the CL intensity is so high but also because variations in
bulk composition, variations in Fe/Si, Mg/Si and similar ratios trace-element compositions cause major differences in CL
enable the definition of discrete EH (high-Fe, high-siderophile) and properties (Derham and Geake, 1964; Derham et al., 1964; Geake
El. (low-Fe, low-siderophile) classes (Scars et aL, 1982). The and Walker, 1966, 1967; Reid and Cohen, 1967; GrOgler and
enstatite chondrites also have especially interesting thermal histo- Liener, 1968; Keil, 1968; Leitch and Smith, 1982; McKinley et al.,
ries. Several authors have used equilibria for enstatite, metal and 1984; Steele, 1989; Lofgren and DeHart, 1992a,b, 1993; DeHart
other phases to estimate equilibration temperatures, which seem and Lofgren, 1994, 1995; Weisberg et al., 1994). The luminescent
reasonable in terms of the observed textures and the petrographic properties of enstatite have been reviewed several times (e.g.,
types (Larimer and Buseck, 1968; Fogel et al., 1989; Zhang et al., Marshall, 1988; Steele, 1989). Enstatite grains with red CL (-700
1992). However, equilibration temperatures (more precisely, nm) have high concentrations of transition metal activators, such as
closure temperatures) based on minor mineral systems are very Cr and Mn, while enstatite with blue CL (-450 nm) is relatively
different from those based on the major phases and are higher for pure MgSiO 3. While the dependence of CL on composition is well
the EH chondrites than for the EL chondrites. For the cubic established, an important proposal by Reid and Cohen (1967) that
sulfides, equilibration temperatures are <400 °C for the EL and has not been discussed very much in the literature is that the degree
-600 °C for the EH chondrites (Skinner and Luce, 1971; Zhang et of stacking disorder of the enstatite also affects its CL.
al., 1992). The usual interpretation is that EH chondrites cooled While it is recognized that CL offers particular insights into the
more rapid[), than EL chondrites. The dichotomy between texture origin and history of the host rocks, most of the studies of enstatite
and equilibration temperatures determined from the sulfide and chondrites to date have concerned only single or small groups of
phosphide systems led Zhang et al. (1995) to argue that the Van meteorites with attention focused on individual grains and their
Schmus and Wood (1967) scheme, which works well for ordinary compositional profiles. The discovery of a great many enstatite
chondrites, is not well-suited to enstatite chondrites. They proposed chondrites among the Antarctic collection in the last decade or so
separate mineralogical and petrographic "types" for these classes. (Cassidy et al., 1992), including many in previously unoccupied
Thus, while the textural types 3_ reflect peak metamorphic positions in the chemical-petrologic grid (e.g., the EL3 chondrites
temperatures and are similar for IIand L chondrites, mineralogical described by Lin et al., 1991, and Chang et at., 1992), means that
87
88 Y. Zhang et al.
this is an opportune time to examine the luminescent properties of EXPERIMENTAL
enstatite meteorites as a class. We obtained a suite of EH and EL
Samples Studied
chondrites, covering the available spectrum of textural and minera-
logical types, and made photographic mosaics of the CL of entire The samples that were obtained are listed in Table I. Descriptions can
be tbund in Keil (1968) and Zhang et al. (1995). The samples included
thin sections. We discovered trends not previously recognized that representative EH and EL chondrites of all available textural types 3-6 and
we suggest are the result of differences in the thermal history of the mineralogical types _t-8. LEW88180 was classified as an Etf6 by Mason
(1990) while Zhang et al. (1995) described it as Ell5& Tll.91714 is the
EH and EL classes. To help interpret these trends, we performed x-
second known El,5; RKPA80259 was the first. Both meteorites have the
ray diffraction measurements. CI. properties of EI.6 chondrites, which are quite unlike those of EH5
The thermoluminescence (TL) process usually involves the chondritcs (Zhang et al., 1994a). There can no longer be any uncertainty as
same luminescent centers as CL, and the two techniques are closely to the classification of RKPA80259 (Weeks and Sears, 1985; Kallemeyn
related, although our "Fir apparatus is equipped with filters to restrict and Wasson, 1986); itisclearly an EL5 chondrite.
black-body radiation that results in a bias towards blue light. Cathodolumineseence
Because metamorphic history is a particularly important aspect of Catbodoluminescence mosaics were prepared from 10-2[) photographs
enstatite chondrite history and since induced "Ft. measurements have using polished thin sections, typically 1 cm × 1.5 cm, of 23 enstatite
proved useful in exploring the metamorphic history of ordinary and chondrites. A commercial Nuclide Instruments "Luminoscope" attached to
a low-magnification optical microscope was operated at 13 _+1kV and 8 _+
carbonaceous chondrites (Sears et al., 1980, 1991a, b; Guimon et al.,
1mA and the images photographed with Kodak "Gold" 400 film, exposed
1995), we included induced "FL measurements in the present study. for 20 sto 40 s. I!quilibrated El! chondrites typically required _10 slonger
"FABLE 1. Cathodoluminescence and related properties ofenstatite chondrites.*
Meteorite** Class Wtg :[: CL B/R§ B/R@ B/R$ Mn # XRD _:I:Pyroxene structuret References lbr Mn
(ppm) and pyroxene structure "tt
ALtt84170 ElI3ct B Red + Blue 0.20 - 0.06 - - twinned cpx I
ALt184206 12t13c_ A/B Red + Blue 0.20 0.23 1500 0.9 twinned cpx I
Qingzhen EII3_t Fall Red + Bluc 0.15 - 0.08 2000 0.47 twinned cpx 2
Abee EH4y Fall Blue 1.42 1.82 - <77 0.37 cpx and disordered opx 3
lndarch FIt4fl3' Fall Red +Bluc 0.19 - 0.04 1630 0.80 cpx and disordered opx 3
PfTA82518 EH4ct,fl B Red +Blue 0.28 - 0.38 400 0.73 twinned opx I
PCA91085 EH4cz B/C Red +Blue 0.36 0.33 0.07 390 089 twinned opx 1
St. Mark F|15fl Fall Blue +Yellow 1.9 0.94 1300 0.88 disordered opx 3
Saint-Sauveur 1';tt5), It:all Blue 5.0 3.0 - <77 0.83 cpx and disordered opx 3
I.1"W88180 I_H56 B/C Bhte 19 2.3 2.7 <77 0.74 highly cleaved opx,
unlike the F.L6s I
ALH85119 FI,3_t B Red +Blue 0.26 0.25 0.54 770 0.53 twinned cpx 1
MAC88136 I,;L3et A Red +Blt, e 0.29 - 0.78 700 0.97 twinned cpx 1
MAC88184 EL3_t C Red +Blue 0.33 0.62 - 380 0.76 twinned cpx 1
TIL91714 EI.5/3 C Magenta n.a. 0.38 - 100 3.04 euhedral opx 1
RKPA80259 FA.5), B/C Magenta n.a. - 600 cuhedral opx 4
ALIt81021 l!L6/3 A Magenta n.a. 0.10 - 70 1.17 euhedral px,
1% ol and 1% twinned cpx 1
Atlanta EL6fl Find Magenta n.a. 0.64 - <77 I.14 well-tryst, ordered opx 3
IItvittis EL6fl Fall Magenta n.a. 0.50 - <77 1.4 well-cryst, ordered opx 3
Khaipur EL6fl Fall Magenta n.a. 0.51 - <77 0.78 well-cr3,st, ordered opx 3
LEW87119 EL6_' C Red n.a. 0.31 - <77 1.36 some euhedral px 1
LI:W88135 FL6fl,), B/C Magenta n.a. 0.60 - 80 1.13 euhedral opx 1
Yilmia FL6fl Find Magenta n.a. 0.33 - 70 0.63 opx 5
llappy Canyon An EL Find Red n.a. 022 - 70 0.65 disordered opx 6
Bishopville Aub Fall Bluish red (I.23 - 150 1.15 disordered and ordered opx 7, 8
Bustee Aub Fall Bhdsh red - 0.I0 850 disordered and ordered opx 7, 8
Khor Tcmiki Aub Fall Bluish red - 0.05 - 310 disordered and ordered opx 7, 8
Norton County Aub Fall Bluish red - 0.12 - 150 - disordered and ordered opx 7, 8
Pena Blanca Spring Aub Fall Red 0.03 - 460 disordered and ordered opx 7, 8
*** Meteorites listed alphabetically within class, mineralogical (ct-8) and textural (3-6) type. See Zhang et al. (1995) for details. - = data not available.
Cathodoluminescence data for meteorites in italics were obtained by JMD at the Johnson Space Center; Aubrite data from Geake and Walker (1966).
Weathering category as defined by Score and Lindstrom (1990).
§ Ratio of the blue C1. to red CI_ as determined by point-counting the cathodoh, minescence images, n.a. indicates that this parameter isnot
applicable to these samples.
@ The intensity of the 450 nm to 700 nm CL (aubrite data from Geake and Walker, 1966).
$ Data from DeHart and Lofgren (1995).
# Mn contents of the enstatite were obtained without regard to CL and are thus weighted averages of grains with blue and red Cir. Typically in
unmetamorphosed (type 3cz) cnstatite chondrites, red grains contain ~2000 ppm Mn while in blue CL grains the Mn is below detection limits.
_ The ratio of the intensity of the (420), (221) doublet and (610) peak in the X-ray diffraction panern, see Pollack and Ruble (1964).
t "cpx," "opx" and "o1" re'let to clinopyroxene, orthopyroxene and olivine, respectively.
t_ References as follows: 1. Zhang et al. (1995); 2. Wang and Xie (1981); 3. Keil (1968): 4. Scars and Weeks (1984); 5. Buseck and Itoldsworth
(1972); 6. Olsen etal. (1977); 7. Watters and Prinz (1979); 8. Pollack (1966).
Pyroxcnsteructurceast,hodoluminesacnedtnhcetehermhailstoroyfenstatcitheondritcs 89
thanequilibratitle.cdhondritcTsh.elilmwasdevelopweitdhtheC-41
procesSsomoefthepresemnotsawicesroebtainaettdheJohnsSopnace
('enteursintghesamlecclmiqucAsu.britdcataweretakefnromthe
literature. Qingzhen : Hvittis
SpectwraereobtainefrdomtheC[.negativuessinagUV-visible
spectrom(!e1tPe8r45T2h).reaereaesacohf_0.c2m×Icmwersecanned; 12. EH3ct EL6_
backgroumnedassuuresdinbglanpkortioonfsthesamneegativweesre 4.0
subtracatenddt:hespectwracrcaverageTdh.edatawertehenconverted >ork"
fromabsorbantcoirccsllcctivililcbsrcompariwsoitnhtheCLprints.
6"
Repeamteedasuremonebnltasnpkortioonfsfilmprocesosveedartwo-
g_
yeapreriosdhowreedmarkacbolnestanincly]lmresponsTeh.espectra 30
obtainaegdreveerywelwl iththosoefindividugaral inosbtainbeyddirect
spectrombeytrGycakaendWalke(1r966)W. etriedtodetermaine
quantitarteivleativeestimaotfetheblucandredgrainbsypoint-cotmting
theCLmosaaicnsdbytakintgheratioosftheheighotfsthered(_7()n0m) O
o,I 20 144+0 44
andblue(-450nmp)eakinsthe('1_spectrOa.noccasitohneb,lucpeak i)
didnorteacahmaximubmelbrthceinstrumecnutta-loaffndthevaluaet
"6 i ,
450mnwatsaken.
exposure times because"['hetsheeytwaorcmbeotthhodsessenotfialdlyata r"eindtuecrtniaolnly arcealibirnasteends.i"tive to O 10 t • • •• 062+0 25 .... :L ,,), Lt
Optical Microscopy, Electron-Microprobe Analysis s "|
c
and X-ray Diffraction
The thin sections x_crc examined under the optical microscope and I I L_____
observations on the nature of the pyroxene arc reported in Table 1.
g
l!nstatite compositions lbr 12 samples _vcre determined using the ('ameca
electron microprobe at the Johnson Space Center, ttouston. The _;
accelerating voltage was 15 kV with a beam diameter of I Fm and beam
current of 17 nA. Data lbr five to ten grains were normally averaged for
each meteorite, kaersutitc I('a2(Na, K)(Mg,Fe)4TiSi6AI2022F2] was used as
FI(;. 1. Plots of the ratio of the 420, 221 doublet to the 610 singlet in the x-
a standard Ibr Ca, Ti, Fc, Si and Mg and garnet lbr Mn. Complete anabscs
are reported in Zhang et al (1995). ray diffraction pattern of two enstatite chondritcs. Four sections of each
The x-ray diffraction patterns over the 26° to 38° 20 range were sample were prepared, and each was scmmcd thrce times. According to
obtained for 20 mg nonmagnetic powder using a Philips diffractomcter with Pollack and Ruble (1964), the doublet is more intense than the singlet tbr
ordered enstatitcs while the reverse is true of disordcred cnstatitc. The
a scanning rate of 2°/rain and Cu K_zradiation. The relative heights of the
(420), (221) doublet and the (610) peak were used as an order/disorder internal average (average ignoring the highest and lowest points) is shown
lbr the peak height ratios of each meteorite.
parameter in the way described by Pollack and Ruble (1964). These authors
pointed out that lbr ordered orthop','roxcnc the doublet was ust,ally more
samples, A few grains displaying no CI, are rare cnstatites with >5
intense than the singlet, while the reverse was truc lor disordered
orthopyroxcnc. To test our methods and cvahmtc precision, wc prepared wt% FeO (l,usby et al., 1987: Weisbcrg el o1., 1994). Enstatite with
fimr samplcs each of Qingzhcn (l!lf3_z) and llvittis (l'lo6fl) and ran each almost pure blue CL was the maior luminescent phase in the Saint-
sample three times, making sure that diffizrent parts of the powder were Sauveur (Elt5y) chondrite, tlowever, in the lJ_W88180 (EIt56)
scanned each lime. The ratios of the peak heights of the doublet to that of
the singlet are shown in Fig. I "['hc prcciskm obtained in this experiment is and Abee (Etl4y) chondrites, _90% of the enstatite displays blue CL
comparable to the standard deviation of the mean peak height ratios lbr l!l[ and the remainder shows magenta CI_. While in St. Marks (H 15fly,
and El. chondritcs (Table I) and is-30% of the value measured _90% of the enstatite displays blue CI,, and the remainder is yellow
Induced Thermolu minescence CL, which is characteristic of chondrule mesostasis.
Wc performed "11. incasurcmcnts on 14 Ell chondritcs and 20 EI_ The CL Images of EL Chondrites-The l':IB(t chondrites
chondritcs of textural types 3 6 and mineralogical type (z ,8(Table 2), the resemble the 1"[13ct chondrites in their CI, properties, containing
sarnplcs wcrc those used in the recent study of bulk and mineral mostly enstatite with red CI, but occasionally grains with blue CI, or
compositions by Zhang et aL (1993, 1995). The apparatus and procedures
no C1, are lound (Plate la-h). There are very rare chondrules
were those of Scars el al (I991a). Dhajala was used as a normalization
standard and long-term stability check lbr the apparatus. All operations whose mesostases produce yellow CI_ (Plate le h). The enstatite
were perlbrmed in temperature- and humidity-controlled laboratories and grains in the EL5fl and EL6fl chondrites display, a magenta CI_ with
under red light; the I1_ nmasurcmcnts wcrc made with Daybreak Nuclear irregular and interdigitated regions whose CL appears to be brown.
and Medical apparatt,s lifted with thcmaal and blue filters and aheating rate
The RKPA80259 (H,5fl), TIL91714 (H,5/9), 1,1_:W87119 (EI,67)
of 7.5 °C/s. After draining the natural '1"1_by heating to 500 °C, the samples
wcrc exposed to a 250 in(i '_°Sr beta-ray source and the induced TL and anomalous llappy Canyon I';l,-relatcd chondritc also display,
measured. Three 4 mg aliquots of nonmagnetic, homogenized 100-mesh magenta CL.
powder were used [br the TL measurements, and three aliquots were
The CL Spectra-Some representative spectra are shown in Fig.
analyzed lot each sample. Some variation in 1I. sensitivity is possibly the
result of weathering, but acid-washing experiments as described by Benoit 2. The spectra for a given meteorite are highly reproducible, espe-
et aL (1091) indicate that this is of minor importance lbr the present cially for the higher petrographic types, with peaks at -_45() nm and
samples. _700 nm. Quantitative data, extracted from the spectra, arc shown
RESULTS in Fig 3. The relative abundance of red and blue grains, determined
Cathodoluminescence by' point counting the CL photographs, agrees well with peak height
ratios determined from spectra (Table 1). In general, l.:l, chondritcs
The CL Images of EH Chondrites-Photographs of representa- have stronger blue and red CL intensity than l'tl chondrites. The
tive CI, mosaics arc shm_n in Plate la-d. Enstatite grains with red melamorphosed Ell chondrites are noteworthy, lbr their high blue-
or blue CI. and rare areas of chondrule mesostasis with yellow CI,, to-red CI, ratios. For the Ell series, the intensity, of the blue Cl_
are observed in both 1';tt3(z and l'l[4(z chondrites. Very, occasion- increases with textural and mineralogical types. For the t':l, chon-
ally, we observed isohttcd grains with yellow CI_ that might be CaS, drites, the CI_ intensity and blue-to-red ratio are independent of
but weathering would have destroyed this mineral in most of our types.
90 Y. Zhang et al.
The X-ray Diffraction, Optical Microscopy planes, which is generally ordered orthopyroxene. The Mn content
and Electron Microprobe Data of the enstatite generally decreases with increasing textural type for
both EH and EL chondrites and is below the detection limit in
Observations based on optical microscopy, x-ray diffraction and
textural types 5 and 6.
electron microprobe analysis are listed in "fable 1and shown in Fig.
4. The intensity of the (420), (221) doublet in the x-ray diffraction Thermoluminescence
patterns is less than the intensity of the (610) peak for the EH The EH and EL chondrites display TL peaks at _140 °C and
chondrites and the EL3ct chondrites analyzed, which is indicative of _300 °C. ttowever, the _140 °C peak can sometimes be resolved
disordered orthopyroxene. The x-ray diffraction data for the EL6Fl into separate peaks at 125 °C and 175 °C and the _300 °C peak may
chondrites are fairly heterogeneous, but for eight of twelve samples show peaks at 280 °C and 330 °C (Fig. 5). The EH3ct chondrites are
thc doublet is stronger than the singlet, indicating the presence of missing the higher temperature peak. Figure 6 shows the TL
ordered orthopyroxene. Only for Khairpur and Yilmia is the sensitivities. The high temperature peak for the EL6Fl chondrites
doublet weaker than the singlet. Both EH3cl and EL3(t chondrites shows considerable scatter, which might be related to the scatter in
contain mainly twinned-clinopyroxene, in a variety of grain sizes, the x-ray diffraction data. The EL chondrites show a much larger
while Saint-Sauveur (EH5_), Abee (E[t47), St. Mark (EH5Fl) and range than the EH chondrites and extend to higher TL sensitivities
LEW 88180 (EH68) contain disordered orthopyroxene (Table 1). by factors of _4. Both TL peaks show a small increase in TL
The EL6Fl chondrites contain euhedral pyroxene without cleavage sensitivity with petrographic type within the Ell chondrites with
values going from <0.01 to 0.2 (Dhajala = 1).
TABLE2. Induced thermoluminescence andrelated properties ofenstatite chondrites.* However, there is no systematic trend in TL
sensitivity with petrographic type among the
Peak 1 Peak 2
EL chondrites. Instead, both TL peaks range
Meteorite (_'lass Wtgt ("_1)_ (DhaTjaLlas=ens1) (T_) (DhTaLjalsaens= 1) from <0.05 to 0.6 for the EL3(t chondrites and
from <0.01 to 0.9 for the EL6Fl chondrites
AI.1184170 EH3(t B 145± 3 0.061 ±0.004 - - (Zhang et al., 1994b).
ALtl84206 EH3ct A/B 150± 7 0,046 ±0.005 -
EET83322 EH3(t A/B 138± 15 0.042 ±0.005 - DISCUSSION
EET87746 Etl4ct.fl C 135± 7 0.071 ±0.007 - - Composition, Structure and CL of the
Qingzhen EH3u Fall 134± 8 0.183 +0.008 - - Pyroxene in Enstatite Chondrites
Kota-Kota EH3(t Find 140± 1I 0.040 ±0.005 312 ± 3 0.017 ± 0.003
Abee+ EH4y Fall 121±5 0.122+0.010 313+21 0.073 ±0.007 Compositional Factors-lron and several
Adhi-Kot EH4"y Fall 114+ 5 0.069 4-0.017 293 + 5 0.115:t:0.021 minor and trace elements in enstatite under-
Indarch EH4Fl,y Fall 142± 11 0.042 +0.004 313 + 6 0.024 ± 0.003 went reduction or diffusive equilibration with
PCA82518 EIl4_t,fl B 174±4 0.069+0.014 309± I 0.141 ±0.029 other phases during metamorphism and fell to
PCA91085 EH4(z B/C 122+ 9 0.163 +0.020 291 + 8 0.050±0.006 values near or below their detection limits
PCA91238 E[14_t B 169± 26 0.046 +0.006 315 ±1 7
0.137±0.040 (Keil, 1968;Table 1). A great many studiesof
St. Marks+ EH5Fl Fall 121±3 0.2304-0.030 311 +3
0.150±0.020 the CL and composition of enstatite and
I,EW88180 + EI158 B/C 111 !5 0.086+0.006 294±9 0.052 ± 0.008
profiles within individual grains have shown
AL1t85119 EL3_z B 126±6 02854-0.022 304±4 0A45 4-0.023 that high transition element abundances favor
EET90299 EL3cz C 119+6 0.223±0.020 311 ± 10 0.190+0.015 red CL while grains low in these elements
MAC88136 EL3cz A 146 +6 0.142 ± 0.020 300 ± 8 0.126 +0.016 have blue CL (Reid and Cohen, 1967; Leitch
MAC88180 EL3_z C 147 + 10 0.540 ± 0.046 302 +2
0.460+0.040 and Smith, 1982; McKinley et al., 1984;
MAC88184 EL3ct C 124 +7 0.087 + 0.008 288 +7 0.080 +0.010
PCA91020 + EL3Fl C 115+8 0.434±0.028 310+ 16 0.523+0.060 Steele, 1989; Lofgren and l)ellart, 1992a, b,
TIL91714 EL5Fl C 145+ 10 0.307+0.013 263+ 13 0.171 a:0.012 1993; DeHart and Lofgren, 1994; Weisberg et
ALIt81021 EL6Fl A 116+2 0.480+0.050 340+0 0.280+0.030 al., 1994). It, therefore, seems reasonable to
Atlanta EL6Fl Find 131 + 17 0.300:t:0.060 282+20 0.130±0.020 predict that the CL properties of enstatite
Daniel's Kuil EL6Fl Fall 119+3 0.899±0.178 317+ 11 0.485 ±0.100 chondrites would become progressively
Eaglc EL6Fl,y Fall 130+ 10 0.330+0.030 281 4.10 0.140+0.010 dominated by blue CL grains with increasing
EET90102 EL6 B/C 147 + 14 0.220 + 0.024
- metamorphism. While blue CI, is associated
Hvittis EL6Fl Fall 136+9 0.604 + 0.027 236 +12 0.460 +0.009 with low trace-element content, the mecha-
Khairpur EI,6fl Fall 134+ 5 0.420 ± 0.070 324 + 14 0.250 +0.040
I.EW87119 EL6)_ C 124+6 0.108+0.013 268+6 0.041 +0.007 nism producing the CL is unclear. The blue
LEW88135 EL6Fl,'/ B/C 141+6 0.069 ± 0004 347 +7 0.024 +0.004 CL of (t quartz (450 nm) is thought to be an
LEW88714 EL6y C 130±8 0.142+0.010 280± 12 0.047 4-0.004 intrinsic property of the SiO4 tetrahedron and
Pillistfer EL6Fl Fall 122+ 14 0.430 + 0.049 331 i 2 0.250+0.028 not due to activation by impurity ions
Yilmia EL6Fl Find 1344-5 0.088 i 0.009 333 +6 0.043! 0.003 (Waychunas, 1988). The ubiquitousness of
tlappy Canyon ELAn++ Find 1754-4 0.200 + 0.030 287 +3 0.140 ± 0.020 the blue CL in silicates is consistent with this
* Meteorites listed alphabetically within class, mineralogical ((z-8) and textural (3-6) type. idea although not proof (Steele, 1989). As
See Zhang etal.(1995) for details. Uncertainties are standard deviations based ontriplicate expected, the Ell chondrites show a trend of
_easurements of aliquots from asingle homogenized powder. - = peak absent. decreasing Mn with increasing blue/red ratio
Weathering category asdefined byScore andLindstrom (1990). (Fig. 7). However, the equilibrated EL
+ These meteorites have resolvable peaks atintermediate temperatures. The temperatures andTI,
sensitivities of these peaks are: Abee = 196+ 5,0.085 ± 0.010; St.Marks = 181+ 2,0.200 ± 0.030; chondrites, while containing enstatite with low
J_W88180 = 170+ 5,0.074 +0.008; PCA91020 = 176 + 14,0385 ± 0.030, respectively. Mn and other trace and minor elements that
An = anomalous; see Zhang elal. (1995) fordetails. are similar to those of the cquilibrated EH
Pyroxesnteructurceast,hodolumincsacncdtnhcetehermhailstoroyf,enstatcitheondrites 91
C,EHY'/
PLA'rt_1. Mosaics of the cathodoltmlinesesenee of Ftl chondrites (upper series) and EL chondritcs (lower series) inorder of increasing petrographic type.
(a) ALl184206, (by PCA91t)85 (paired with I'CA82518), (c) Saint Sauveur, (d) LEW88180, (e) A1,tt85119, (0 TI[,91714, (g) AIAIA81021 and (h)
LEW87119. The Etl3_z, El_3_tand F.H4_zchondrites (there are noknown EIA chondrites) display predominantly red CL with occasional grains of blue or
noCIr. For the Ell chondrites of higher petrologic type, theCL ispredominantly blue with relatively few grains exhibiting magenta CIr. The equilibrated
El. chondrites have enstatite with almost entirely magenta CL andafew obscure well-integrated regions of brownish CL The borizontal field of view in all
imagcs is-0.5 cm.
chondrites, display a large range in CI_ intensity and blue to red The co-existence of two polymorphs in individual enstatite
ratio, which is unrelated to the composition of the enstatite (Fig. 7). grains is sometimes referred to as stacking disorder and ix distinct
It should be mentioned that while we discuss Mn, which is widely from cation disordering (e.g., Skogby, 1992). Slacking disorder in
associated with the red CI, of enstatite, other transition metal orthopyroxene is readily' detected by' the single crystal Weissenberg
impurity' ions arc strongly correlated with Mn in enstatite, and the method because it produces both sharp and diffuse spots (Pollack,
same arguments apply' if they arc activators. Properties besides 1968). It has been suggested that fast cooling of protopyroxene
trace-element abundances determine the CL properties of the from 1000 °C to 700 °C produces disordered pyroxene sometimes
enstatite in these meteorites. with twinned clinopyroxene, while slow cooling (slower than a few
Structural Factors-Enstatitc exists as orthoenstatite, low- degrees per hour) of protopyroxene between 1000 °C and 800 °C
clinoenstatite, high-clinoenstatitc and protoenstatite, but the detailed produces ordered orthopyroxene (Brown and Smith, 1963; Smyth,
phase relations arc unclear (e.,_Z.,Brown and Smith, 1963; Mason, 1974).
1968: Smyth, 1974: Pacalo and Gasparik, 1990; Kanzaki, 1991; Reid et al. (1964) noted that the luminescence of disordered
Angel et a/., 1992). Clinopyroxene occurs in low textural types of orthoenstatite in enstatite achondrites was weaker and the ratio of
enstatite chondrites, while orthopyroxene occurs in the high types the intensity of blue to red luminescence was higher than for
(Mason, 1968: Keil, 1968). Isolated descriptions of ordered and ordered orthoenstatite. Reid and Cohen (1967) suggested that dis-
disordered orlhopyroxenc in enstatite chondrites and aubrites have ordered grains produced blue CL, while ordered grains from the
been reported by, many authors (Keil, 1968; Reid et al., 1964; Reid same meteorite produced red CI,. The x-ray powder diffraction
and Cohen, 1967), but no systematic variation with class or type has suggests that one EH4y (Abee), one EH5fl (St. Mark), one Elt5'/(St.
been reported. At low pressures, protoenstatite is the stable form of Sauveur) and one EH58 (IJ:,W88180) contains mixtures of
MgSiO 3between lf)(1()°C and 1300 °C (Smyth 1974). Orthoensta- disordered and ordered orlhopyroxene with disordered ortho-
tile and low-temperature clinoenstatite arc the common forms in pyroxene being dominant (Fig. 4). As noted above, these meteorites
meteorites since high-clinoenstatite is stable only at pressures >7 produce blue CL. On the other hand, the ELYfi and EI,6/_ chon-
(;Pa. The phase boundary between orthoenstatite and low-clino- drites arc composed predominantly of ordered orthocnslalite, and
enstatite is indistinct, largely because of the slow kinetics of the their enstatite produces magenta CIr. Thus, we suspect that Reid
translbrmation, so that it ixpossible for enstatite to occur as a mix- and Cohen's (1967) suggestion that ordered orthoenstatite lumi-
ture of orthoenstatite and Iow-clinoenstatite (Buseck et aL, 1982). nesces red and disordered orthoenstatite luminesces blue in aubrites
92 Y. Zhang et al.
EH chondrites EL chondntes
08
06
a
04
PCA91085 t-
(EH4,B) _d 02
v 0
_J
O
b ALH85119 "6 16
(EL3a)
12
¢D
C
08
C
04
C
TIL91714 0
(EL5_3)
5
O
2
d
tD 1
t-
KhaJpur (C1) 05
(EL6/_)
025
e
Saint Sauveur
c_
(EH5"It )
400 500 600 700 800 900
FIG. 3. Quantitative data tot the bulk CL properties of enstatite chondrites
Wavelength (nm) determined from CI, spectra similar to those of Fig. 2. (a) The intensity of
the blue peak increases with textural and mineralogical type in the Ell
chondrites, but the El, chondrites display no systematic trends. (b) The
intensity of the red peak does not appear to vary systematically with textural
FIG. 2. Representative cathdoluminescence spectra of bulk samples of and mineralogical type in either class but is a factor of _2 higher in the El,
enstatite chondrite. Baselines have been displaced vertically for clarity. chondrites. (c) The ratio or"the intensity of blue and rcd pcaks increases
IJttlc-metamorphosed chondrites of both the Ett (a) and I'l, (b) classes have
with type tbr the Eli chondrites and is independent of type in the EL
spcctra with strong red (-700 nm) and weak blue (-450 nm) CL peaks. ehondrites. It is higher lbr equilibrated Eli chondrites than the other ensta-
Metamorphosed El, chondrites (c and d) also have spectra with strong red tito meteorites. Blue/red ratios obtained by point counting the CL images
peaks, but the blue CL peaks can be of almost comparable intensity to the
are also shown as squares and connected to the spectral data by tie lines.
red CI, peak. Metamorphosed I'll chondrites have strong blue and weak Unless otherwise indicated, on all our figures h; experimental precisions
rcd ('1, peaks. These spectra are consistent with C[. images in Plate 1.
arc approximately twice the size of the symbols. Also shown isthe range of
is also true ['or the cnstatite in thc EL and HI chondrites. The blue to red spectral peak ratios measured for aubrites by Geake and Walker
(1966).
present x-ray diffraction data suggest that a major factor governing
the CI, of metamorphosed enstatite chondritcs is the structural state Induced Thermoluminescence Properties
of Enstatite Chondrites
of the pyroxenc.
Wc have bcgun a series of experiments in which we heat Since the mincral producing the TL signal is undergoing
cnstatite from meteorites in furnaccs and observe the changes to structural and compositional changes as a result of metamorphism, it
their CL and structure. To date, we have not been able to disorder is worth considering whcther the cnstatitc chondrites display major
cnstatite from AI,ItA81021 (H_6fl), nor change the color from changes in '1'1, sensitivity as a function of textural and mineralogical
magcnta to blue, but we did observe a change in the color of the CL typc like the ordinary chondrites. Variations in 'I'L sensitivity
from magenta to yellow-green, suggesting that the structure of the resemble those of blue CI, as determined from the spectra, including
enstatite can affect its CI, (Schneider et al., 1995). Thc technique an increase in the '1"I, sensitivity of the Ell chondrites with
best suited to exploring the structural dcpcndcnce of the CL of increasing petrologic type. llowcver, the increase is fairly modest
pyroxcnes is arguably transmission electron microscopy (TEM). (about a factor of four), partly because the blue TL is coming from
Unfortunately, such studics are very few. Buseck et al. (1982) sum- both the "primary" blue cnstatite of the sort observed in Elt3ct
marized the major properties of enstatitc with cmphasis on TEM, chondrites (Weisberg et al., 1994) and the "metamorphic" blue
and McCoy et al. (1995) reported data [br llafegh 009. llafegh 009 enstatite of the sort observed in EH68 chondritcs (McKinley et al..
is an unusual impact melt whose CL we have not examined. It 1984). The I';11 chondrites plot at the lower end of the 1I_ sensitiv-
contains a disordered mixturc of ortho- and clinopyroxcne. Another it>' range lor I,;1, chondritcs, which is consistent with their difference
promising method is single crystal XRD coupled with CL spectra (I. in CL intensity,. The mosaics of CI, from Ell chondrites require
Steele, pcrs. comm.). hmger photographic exposure times than those of the EI_ chondrites.
I'yroxensetrncturceast,hodoluminesacnedtnhcetehermhailstoroyf'enstatictehondrites 93
I I I I I I I I I I _ I I I I I I I I
Disordered enstatffe Ordered enstat/te
I I I I
ALH 84206 30, • IEHchondrites
EET 83322 30+ •
Kota-Kota 30, • ALH 84170 3Q: •
ALH 84206 30, •
Qingzhen 30, • EET 83322 30, •
Abee 4`/ • Kola Kota 36[ •
Adhl Kot 4`/ Qingzherq 3C[ •
Indarch 4_y AdhAi-Kbeoet 44""// •• •
PCA 82818 4(Z8 EET 87746 4(_B •
PCA 91085 4_ Indarch 4_'_'! •
PCA 82518 4c_ •
PCA 91238 40, PCA 91085 40, •
St Marks 5,_ jEH chondrite 4 PCSAI 9M1a2r3k8s 4508, • ••
St Sauveur 5_f LEW 88180 5_ • •
LEW 88180 5
ALH85119 3_ +' ELchondrites I
EET 92029 3at ELchondritesJ ALH 88119 30,
MAC 88136 30, EET 92029 30,
MAC 88136 3_
MAC 88184 30[ MAC 88180 30,
PCA91020 3_ : MAC 88184 30,
PCA 91020 31_
TIL 91714 5_ TIL 91714 5,_
ALH 81021 6_ ALH A8t1la0n2l1a 66_
Atlanta 6_ 4+ Danlels KuII 6_
Eagle 6_y Eagle 668,_
EET 90102 68 EET 9H01v0lf2lis 6_
Hviltls 6_ Khairpur 6_
Khairpur 68 LLEEWW8881731519 66_ "/
LEW 87119 6y
P_lllstfer
LEW 88135 6_ y LEW887Y1f4lmla i_
LEW 88714 6¥ • ,+ Happy Canyon An
Pillislfer 61_ ___ I I I I _ I I
Y,,_,a18 ,, 50 100 150 200 250 300 350
Happy Canyon An
TL peak temperature (°C)
I I I I I I
02 06 10 14 18 22 26 30 34 38
FIG. 5. Plot to compare the induced thermoluminescencc peak lcmperatures
Intensity ratio of (420),(221)/(610) x-ray peaks of enstatite chondrites. Except lbr l:.ll3_ chondriles, enstatilc chondrites
show peaks at _140 °C and -300 °C in the glow curve. In a few instances.
FIe;. 4. The x-ray diffraction data for enstatite chondrites. According to
the _140 °C peak can be resolved into two peaks (at _120 °(' and -170 °C).
Pollack and Ruble (1964), the relative imcnsity of the (420), (22 I) doublet
The EL6fl chondrites show considerable scatter in the position Hfthe high
and the (610) peak is a measurc of the stacking order in the enstatitc; the
temperature peak.
doublet peak is stronger than the singlct in ordered orthopyroxcne but
weaker for disordered orthopyroxcne. All of the Ell chondrites and the
(600 °C: Mason, 1968) and the ordered structure of their pyroxcnc.
tiL3_t chondrite appear to contain disordered enstatite, while eight of twelve
I{1,6 chondritcs contain ordered cnslatite. The Schneider et al. (1995) heating experiments indicate significant
changes in CL after heating >800 °C. There arc some exceptions to
The El, chondritcs span almost a factor of 10 in their TL this generalization. Eagle, for instance, which was considered a
sensitivity, and the spread in TI+ sensitivity values displayed by post-metamorphism breccia with inclusions by, Olsen el o/. (1988),
EI,6fl chondrites alone is comparable with the range shown by, the appears to contain disordered orthopyroxcne and. therefore, equili-
EI, chondritcs as a 'a'hole. Peak metamorphism alone, at least as bration temperatures >800 °C (Fig. 4). tlappy Canyon. which was
rcl]ectcd by' the textural type, is clearly not governing TI, considered an impact melt of El, composition by McCoy et al.
sensitivities, llowevcr, these EL6 meteorites are primarily, minera-
(1995), also contains disordered orthopyroxenc. I)espitc this high-
logical type fl, which suggests that their high TI, sensitivities are
temperature episode, the low equilibration temperatures of the cubic
associated with ordered enstatite.
sulfides for these two meteorites indicate that some systems closed
The Thermal History and Origin of Enstatite Chondrites at lower temperatures (Zhang el al.. 1992).
It seems that the transition between petrologic type 3 to 5.6 in
Probably the simplest explanation for the CI, trends in Plate I is El. chondritcs is not continuous, either because we have not yet
thai while the enstatite is predominantly clinopyroxene in the EtI3ct recovered an EIA chondrite or because there was some mechanical
and 1"I,3rz chondritcs, it is disordered orthopyroxenc in the process to separate these EI.5,6 chondrites from original H.3
metamorphosed EFI chondritcs and is ordered orthopyroxene in the chondrites before the},, underwent high peak temperatt,res that
metamorphosed H, chondritcs. This is the situation that is destroyed chondrules. Experience shows that it is probably due to
suggested, at least to some extent, by the optical and x-ray the lack of complete sampling of the enstatite meteorite classes•
diffraction data in Table 1 and Fig. 4. With this as a working The EHY,6 ehondrites-Thc Ell5,6 chondrites contain a
hypothesis, we will now summarize the available data. mixture of clino, ordered and disordered orthopyroxene (Table I).
The ELY,6 Chondrites-Thc lack of chondrulcs in the EI,6 In fact, based on their textures, the homogcneitv of their mineral
chondritcs, and the presence of only, a few chondrule relics. chemistry' and the presence of only,-10% cnstatite grains with
indicates that they experienced very high peak metamorphic magenta color, it is clear that these meteorites underwent prolonged
temperatures. Mason (1968) suggested a maximum equilibration metamorphism at high temperatures with the equilibration tempera-
temperature of 870 °C based on the presence of quartz. Peak meta- turcs for a number of mineral systems being _900 °C (Zhang el al..
morphic temperatures for EI,5.6fl chondrites lic bctwecn 600 ° C to 1992). We suggest that their equilibration temperatures (as opposed
800 °C. which arc bounded by' the stability' field of clinopyroxcne to their peak-metamorphic temperatures) are higher than those of
94 Y. Zhang et al.
EHchondrites ELchondrites
EH chondrites l EL chondrites a
1
1000
I140°C peak
0.8 g
O_
500
0.6
o
_D_ 200
0.4
II
tO0
_ 0.2 i i i i i I I' I' ; I I I
c- 1 2 02 04 06
a
v>, 0 0000 gO00 • IntensityratioofblueCLto redCL
_0,6 300°C )eak 1:I(;.7. Plot tocompare the Mn content in pyroxene in enstatitc chondrites
with the intensity ratio of blue CL tored CL obtained from the spectra. (a)
0"1 1"1tchondrites show a decrease in the Mn content of the enstatite with
C
0.4 increasing bluc CI, to red CI, ratio. (b) t':1,chondrites display no obvious
(/3 trend
d
0.2 pyroxene in E3 chondrites compared to 1{115,6 chondrites reflects
• eO the difference in metamorphic history.
Comparison Between EH and EL Parent Bodies-Several
authors have suggested that the El! and I':l, chondrites represent
different parent bodies with the arguments centering on differences
in breccia types and abundances, bulk compositions (Keil, 1989)
......... __o_2_ _2_:
_O0_mr • _ O_ and cosmic-exposure age distributions (Crabb and Andcrs, 1981).
JUJ << <:u _- _ m LU WW The I':L and Ell chondritcs also experienced different thermal
histories, and there is, thus, a different distribution of cnstatite
FIG. 6. Plot comparing the thermoluminesccnce sensitivities of enstatite chondrites over the textural and mineralogical types (Fig. 8). Most
chondrites. The _140 °C and the -300 °C peaks behave similarly, Eli chondrites are low textural and low mineralogical types,
suggesting that they are caused by the same luminescent minerals and
whereas most EL chondrites are high mineralogical and high
activators. Since our TL apparatus isbiased towards bluc wave lengths, the
variations inTL sensitivity shown by cnstatitc chondrites resemble those of textural types, which suggests that the El, chondritcs experienced a
the blue CL intensity. prolonged period of low-temperature metamorphism that the Ell
chondrites escaped. This different thermal history took place after
I':L5,6 chondrites (_800 °C) in order to produce the disordered
peak metamorphism and possibly involved breakup of tim 1"II and
structure. At the end of this metamorphic event or subsequent to it,
EL parent bodies. Virtually all the equilibrated Ell chondrites and a
most of these meteorites underwent rapid cooling (faster than a few
few t':1_chondritcs were invoh, ed in an event that resulted in rapid
degrees per hour) to produce the observed mixture of enstatite
cooling. The fragmental breccias among the Ell chondritcs (e.g.,
structural states and metal structures observed in Abce (tlerndon
Abee and St. Sauveur) may be evidence of this event. The
and Rudee, 1978). This scenario is consistent with the proposed
environment is a matter of speculation. Zhang et al. 11995) sug-
higher shock levels of I"tl5,6 chondrites compared with EI,5,6
gested that differences in thermal blanketing in a rcgolith might
chondrites (Rubin and Scott, 1995) because rapid cooling following
have been responsible.
shock would favor disordered structures. ]fowever, interpretation of
Aubrites-Thc aubrites have thermal histories that are more like
shock effects must be considered tentative until the effects of shock
I.:II chondrites than the f'I, chondrites. The aubritcs experienced a
on the various pyroxenc structures are considered.
violent breakup, which resulted in heating and rapid cooling.
The E3 Chondrites-ln type 3chondrites, the clinopyroxene has
Aubrites contain large enstatite crystals and a mixture of ordered
either blue or red CL depending on transition metal contents:
and disordered orthoenstatite. It is possible that the impact and
apparently, CL color is not uniquely identified with this structure.
rapid cooling event was associated with the event(s) that produced
Weisberg et aL (1994) proposed that chemical processes in the
the numerous breccias observed in this meteorite group (Keil.
nebula rcsuttcd in the compositional diversity (and thereby CL
1989).
colors) of the enstatitc in type 3ct chondrites and that red CL
cnstatitc grains are solid-state reduction products while blue CL CONCLUSIONS
enstatitc grains are condensates from the nebula, tlsu and Crozaz '['he C[, properties of enstatite in un- or little-metamorphosed
(I 994) argued that both types of cnstatite were produced from melts, l':ll and H, chondrites arc similar: cnstatites with high transition
based on their similar REE patterns,. Previous studies suggested metal contents have red CI,, while relatively pure cnstatites have
that clinopyroxene produced by the fast cooling of protopyroxenc is hluc CI,. Rare enstatites with >5 wt% I:eO have no CL.
usually twinned while ctinopyroxcnc produced by shearing stress Metamorphosed Ell chondritcs display' predominantly bluc CL,
from orthopyroxcnc is not usually txvinncd (Brown and Smith, while metamorphosed El, chondrites display, magenta CI, whose
1963; Smyth. 1974). The higher abundances of twinned clino- spectra contain blue and red peaks of approximately equal intensity.
l'yroxene structures, cathodoluminescence and the thermal history of enstatite chondrites 95
EH chondrites phosed enstatite chondrites. The x-ray diffraction measure-
ments and optical microscopy suggest that unmetamorphosed
3_i,lLlqHz8h4e1ll10Y1t)4a3_1s(a enstatite chondrites of both classes contain clinopyroxene (with
<600 M_,HLtSI1/178195_i Y691 [||8/146 small amounts of disordered orthopyroxene), but meta-
MH84206 morphosed Ell chondrites contain predominantly disordered
(ela Kota
orthopyroxene and metamorphosed El, chondrites contain
predominantly ordered orthopyroxene. These conclusions arc
600-800 PCA82518 51.Mafk
hldatch summarized in 'Fable 3.
The differences in the structural form of the cnstatitc in
metamorphosed Ell and EL chondrites indicate maior differ-
800-1000 Aboo
Adhi Kot 51 Sauveut ences in the thermal history of the two classes folh)wing peak
Q.
_3 >, metamorphism. The Ell chondrites cooled relatively quickly'.
t-- but the EL chondrites were heated for a prolonged period in the
8 °
stability field oforlhopyroxene (600 °C 800 °C).
[EW88180
E >1000
O Acknowledgments We are grateful to the Meteorite Working Group of
I-- NASA/NSF, I. Casanova (Field Museum of Natural Ilistory, Chicago,
EL chondrites Illinois), R. Itutchison (The Natural Ilistory Museum, lxmdon), G.
E Kurat (Naturhistorisches Museum Wicn, Vienna), M Prinz (American
0 E
Museum of Natural llistory, Ncw York), L. S. Schramm (National
3;
ALlt85119 Museum of Natural IlistoD,, Smithsonian Institution, Washington.
_. <600 EEf90_9 D.C.), L. Schultz (Max-Planck Institut ftir Chemic, Mainz) and B.
r'_ MAC881
Zanda (Museum d'llistoirc Naturelle, Paris) for supplying samples;
°_ especially to Walter Manger and Ronald Konig (University of
-I ALH81021 PilliMfe
ET Atlanta Hvittis Arkansas, Geology Department) for their continued generous loan of
PCA91020 11L91714 Blilhfield Ufa{la the cathodoluminescence and x-ray diffraction equipment; and to
III 600-70O Yilmia Khairpui
Daniel's Kuil Gordon McKay and Vincent Yang lbr access to and help with the JSC
Jaih dell KotLalu electron microprobe analysis. Wc also especially appreciate useful
Eagle reviews by Katherina Lodders and lan Steele. This work was
LEW88135 supported by a grant from NASA (NAGW 3519) while I)S was
RKPAS0259 LEWB7119
700-800 LEW88714 supported by an NSF REU award.
Editorial handling: K. Keil
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