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American Mineralogist, Volume 66, pages 127-141, 1981
Exsolutionrelationshipsin a clinopyroxeneof averagecomposition
Cao.orMno.uoMgo.rrSirOu:
X-ray diffraction and analytical electron microscopyr
WBrqnv A. GoRDoN,' DoNALD R. PEAcoR,Pnrup E. BRowN, Enrc J. EsseNe
Departmentof GeologicalSciences
Universityof Michigan,Ann Arbor, Michigan 48109
AND LAWRENCE F. ALLARD
Department of Materials and Metallurgical Engineering
University of Michigan, Ann Arbor, Michigan 48109
Abstract
A pyroxeneof averagecompositionCaoa.Mno
from Balmat, New York is exs2Si2Ou
"rMg6refinementswere performed in part to
solvedto coexistingP2r/c and,A]c phases.Structure
limit the compositionsof the individual phases,as were plots of cell parametersand average
Ml-O and M2-O bond distancesrs. composition.Site occupanciesshowedthat the phases
are manganoandiopside and kanoite. Transmissionelectron micrographsrevealedthat the
phasesare lamellar, share(001) as the interphaseboundary, and are 2000A wide. X-ray energy dispersiveanalysisof individual lamellae yielded CaaurMno*MgorrSirOufor the A/c
phaseand Cas,2Mn,orMgo,.SirOufor the F2r/c phase.Dark-field imaging revealedthe presenceof antiphasedomain boundariestn the F2r/c phase,indicating the existenceof a A/cP2t/c ttansition for the Ca-poor phasein the Ca-Mn-Mg pyroxene system.In situ heatttg
experimentsyield a temperatureof the A/c-F2,/c inversion of 330+20oC.Observationof
antiphaseboundary (APB) positionsbefore and after heating through the inversion temperature showedthat the APB positionswere essentiallyunchanged,indicating stabilization of
APB's by the concentrationof Ca or somedefect at the APB's. Lattice fringe imagesacross
the lamellar boundary (001) indicate that the interfaceis semi- or completelycoherent.The
featuresof this exsolutiontexture are consistentwith initial exsolutionby spinodal decomposition or nucleation and subsequentslow cooling in a regionally metamorphosedphase.
Introduction
X-ray diffraction photographsconfirmed that rl did
In the investigationof a magnesianrhodonite from indeed consist of two pyroxenes.We report results
Balmat Mine No. 4, New York (Peacoret al., 1978), obtained primarily through single-crystalX-ray difa pyroxenecoexistingwith the rhodonite was qualita- fraction and further characterization of the two
tively analyzed and determined to have major Mn phasesby scanningtransmissionelectronmicroscopy
analysis.
and Mg with minor Ca. This is near the ideal compo- in conjunctionwith X-ray energy-dispersive
provided
generosity
two
samples,
by
the
Our
of
sition of kanoite, MnMgSirOu (Kobayashi, 1977).
primarily
David
Dill
the
mine
staff,
Dr.
of
consist
of
Quantitative wavelength dispersiveanalyseson the
pink
rhodonite,
buff-colalternating
bands
of
bright
electronmicroprobewere obtainedfor this pyroxene
(Table l). A pale blue schiller observedin the Mn- ored pyroxene,and qufitz. They were found by minpyroxeneindicatedthe presenceof a periodic micro- ers at BaLnat and were savedonly becauseof their
structure,and preliminary single-crystaland powder unique and colorful appearance,and are from a complexly interlayeredsequenceof metamorphosed,impure, siliceouscarbonatesand evaporitestypical of
rContribution No. 361 from the Mineralogical Laboratory, Dethe southwestpart of the Grenville Province (Engel,
partmentof GeologicalSciences,The University of Michigan.
2Presentaddress:ExxoN Co., U.S.A., P,O. Box 1600,Midland,
1956).The najority of the units consist of calcitic
Texas79702.
and dolomitic marbles with variable amounts of
0003-004x/8 1/0 102-0r27$02.00
127
128
GORDON ET AL.: CLINOPYROXENE EXSOLUTION RELATIONSII/PS
Table l. Electron microprobeanalysisof Balmat clinopyroxene
oxidewt.%
si02 52.09
l,,toiar atio/2Si
2.00
AI 03 0.25
2
0.01
l 4 n 0 2 1. 0 6
0.69
l'490 | 4 .21
o.a?
Feo
0.28
0 .0 t
Cao
10 . 4 1
0.43
sum
98.30
l' S t a n d a r d s
usedwere synthetic tephroite for lln, Irving clinop y r o x e n ef o r C a a n d S i , I n g a m e l l ' sa l B a n d i n ef o r A l a n d F e , a n d
l ' l a r j a l a h t i o l i v i n e f o r M g . D r i f t , a t o m i c n u m b e r ,f l u o r e s c e n c e ,
a b s o r p t i o n , a n d b a c k g r o u n dc o r r e c t i o n s w e r e a p p li e d t o t h e
d a t a , u s i n g t h e p r o g r a nE M P A DVR
I I ( R u c k l r d g ea n d G a s p a r r i n j ,1 9 6 9 )
diopside, tremolite, phlogopite, calcite, anhydrite,
and primary talc. Scattered Mn-rich pods have
yieldedthe amphiboletirodite (Brown et al.,1979)as
well as the assemblagesMn-pyroxene-Mg-rhodonite-talc-quartz and Mn-pyroxene-Mg-rhodonitemanganoan calcite-quartz-anhydrite. The Balmat
ore deposithas been regionally metamorphosed,the
peak metamorphicevent there being estimatedat P
: 6.5+.5 kbar and T : 625+25oC,as calculated
from sphaleritebarometry (Brown et al., 1978),twofeldspar, iron-titanium oxide, and calcite-dolomite
thermometry(Bohlen and Essene,1977;Brownet al.,
1978),and from the presenceof sillimanite in the adjacent metapelites.
Determinationof lattice parameters
Cleavagefragments of the pyroxene were examined by Weissenbergand precessiontechniques.Two
reciprocal lattices were observed in the diffraction
photographs;characteraationof the extinction rules
combinedwith identification of the phasesas pyroxenesled to the assignmentof spacegroupsF2,/ c and
A/c. The class(b) (h+k:2n+l) reflectionsof the
primitive phase were sharp, with no diffusenessor
streaking.
A powder diffractometer pattern obtained with
quartz as an internal standard, CuKa radiation, and
graphite monochromator,showed only single unresolvedpeaks,and a combinationof single-crystaland
powder-diffractiontechniqueswas required to determine accuratelattice parameters.Least-squaresrefinementof the averagelattice parametersfor the two
phaseswascarriedout with the program Lclse, written by CharlesW. Burnham. Estimatesof the lattice
parametersfor the individual phaseswere then ob-
precession,and Weissenbergtained by Weissenberg,
diffractometertechniques.Thus, preciseabsolutevaluescould not be obtained,but differencesin the valuesof correspondingparameterswere appted to the
averagevalues as determined with the powder diffractometer,resulting in the final valuesa : 9.78(2),
b : 8.93(9),c : 5.32Q)4, F : 108.60(7)'for the
primitive phase, all:da : 9.76(2), b : 8.93(9),c :
5.27Q)4, B : 106.44(4)ofor the C-centeredphase.
The magnitudesof the b axesof the two phasesare
virtually identical, their a-axis magnitudesare only
slightly different,but a significantdifferenceexistsin
their valuesof c. The c axesdiverge by an angle of
2.16(2)",and a and b of the two lattices are parallel.
We therefore predicted an intergrowth of these
phaseswith (001) as the common plane of least
strain, a relation verified by direct indexing of the lamellar interface.
Crystal structure refinements
Refinementsofihe crystalstructuresof the coexisting pyroxeneswere carried out in part to determine
the nature of the individual site occupancies,which
in turn restricts the compositionsof the individual
phases.Theserefinementsalso provide data on the
generalnature of pyroxenestructuresas a function of
composition,and specifically for two compositions
which appearto have formed under equilibrium conditions.
The measurementof intensity data was difficult
becausethe class (a) diffraction maxima from the
separatelattices are poorly resolved.Intensities for
thesediffraction maxima could not be measuredutilizing an automatedsystem,and were determinedby
graphically recording intensity profiles and integrating the areas using a planimeter. Even this
method failed to provide resolution for diffraction
maxima with a relatively large value of the index h,
and the data sets were significantly smaller than is
usually the casefor pyroxenes.Class(b) diffractions
for the primitive phasewere measuredwith the Supper-Pace automated system, which employs background counts on each side of a continuous peak
scan.The class(b) and (a) diffraction maxima were
placedon the samescaleby comparing class(b) diffraction intensitiesasdeterminedwith both methods.
A crystal approximately 0.16 x 0.08 x 0.39 mm
was selectedand mounted for rotation about the b
axis of the Weissenberg-geometrydiffractometer.
MoKa radiation monochromatedwith a flat graphite
crystal was used. For the C-centeredphase,374 ntensities were measured,while for the primitive
r29
GORDON ET AL.: CLINOPYROXENE EXSOLUTION RELATIONSHIPS
phase we determined the intensities of 982 diffractions, of which 386 were class(a). Theseinclude 139
unobservedintensitiesfor the class(a) diffractions of
the primitive phase,27 for class(b) of the primitive
phase, and 2l for the C-centeredphase. Intensities
were determinedfor reflectionshaving sind lessthan
0.5.All data were correctedfor Lorentz, polarization,
and absorptioneffects(p : 38 .--'), by a modified
versionof the programABsRpwritten by C. W. Burnham.
The structure was refined using the program
RFINE2(Finger and Prince, 1975).Form factorswere
those of Doyle and Turner (1968). The weighting
schemeof Cruickshank(1965,p. lla) was used,and
all reflections with individual discrepancy factors
greater than 0.5 were rejected. For the primitive
phase,separatescalefactors were used for the class
(a) and (b) diffractions.
Form factorswere initially chosenfor Ml and M2
of both phases,basedonly on generalpyroxenecrystal-chemical principles in relation to the average
composition as determined by electron microprobe
analysis.For the C-centeredphaseMl was assumed
to be occupiedby Mg and Mn, and M2 by Ca and
Mn. For the primitive phaseMn and Mg were both
assignedto Ml and M2. Theseare only approximations to true site occupancieswhere three different
cations may occupy a given site. This is especially
significantfor M2 of the primitive phase,which must
contain someCa in addition to Mn and Mg.
Refinementproceededwithout complicationthrough
the refinement of anisotropic temperature factors.
The final R values as computed including all data,
both observedand unobserved,are 5.7Vofor the
primitive phaseand 5.4Vofor the C-centeredphase.
Structurefactorsare listed in Table 2.'Atom coordinatesand temperaturefactors are listed in Table 3,
thermal ellipsoid data in Table 4, selected interatomic distancesin Table 5, and site occupanciesin
Table 6. The distanceswere calculatedwith the program oRFFE(Busing et al., 1964} utilizing the full
variance-covariancematrix of atomic parameters
and standarderrors of lattice parameters.
Discussionof refinementresults
The site occupancies(Table 6) are consistentwith
phasesnear diopside and kanoite in composition.
The generalnature of pyroxene structure variations
as a function of compositionis well known; Clark et
al. (1969) determined the details of the diopside
structure, and Ghose (written communication) has
3To receivea copy of Table 2, order Document AM-81-147
from the BusinessOffice,Mineralogical Societyof America, 2000
Florida Avenue,NW, Washington,DC 20009.Pleaseremit $1.00
in advancefor the microfiche.
Table il. Atom coordinatesand anisotropictemperaturefactors for A/c m:rnganoandiopside (above) and F2r/c kanoite (below)
Atomxyz
B lI
M1
0
M2
0
Si
0l
0 . 2 1 r 4 ()r
0 . 4 0 7 5 )( t
3/4
0.7635(3)
o . 4 r 2( 3
r)
0.8ss7(8)
02
03
0.382e(3)
0.r358(2)
0.r489(2)
MI
M2
0 . 2 5 0 5 )( l
0.2534(o)
SiA
S'iB
0tA
0 . 0 4 r 3 ()1
0 . 5 4 6 6I ()
0.8674(3)
0 . 6 5 4 42()
0 . 0 2 3 5 )( i
0 . 3 4 i l( 2 )
0lB
024
028
0.3726(3)
0 . r 1 8 (r 3 )
03A
038
0.623e(3)
0.r034(2)
0.6038(2)
0.0e2e(2)
0. t 0 7 2 ( 1 )
0.2510(3)
0.482
r (3)
0 . 8 3 8 82()
( 4)
0 .3 3 8 3
0.8382(4)
0.500e(4)
0.eer6(4)
0 . 2 5 e 03()
0 . 7 r 6( 3
r)
3/4
r3(2)
28(r)
r 8 ()r
822
rr ( 3 )
5 r( 2 )
24(2)
r8(2)
o.67rs(8) 3r(3)
38(4)
2 8 ( 4)
0 . e 9 e 4 ( 7 ) 2 1( 2 )
46(4)
0 . 2 3 s(72 )
0 . 2 3 3 2) ( 1
0 .2 7 1(03)
0.2408(3)
0 .r 6 5 2 ( 9 )
0.r367(e)
0.3255(9)
0.358e(e)
0.s707(6)
0 . 4 e 0( 5r )
1 7( 2 )
1 3 ( )r
2 3 ( )l
r 7 ()r
r 6 ()r
1e(2)
2s(3)
3 r( 3 )
1e(2)
2 1( 2 )
23(2)
3 r( r)
re(2)
21(2)
23(4)
22(4)
26(4)
38(4)
4 r( 3 )
44(3)
62(12)
8 6( 8 )
812
B l3
823
0
6(3)
0
0
-4(2)
-4(5)
- 4 ( 6)
r64(r7)
0(2)
-t (2)
3(2)
12(2)
il(4)
2 7( 4 )
80(r6)
-4(2)
7(4)
-r6(6)
2(2)
6 ( r)
4 ( r)
4(1)
e6(s)
I (0)
3(0)
- 2 ( t)
rr ( 2 )
-6(2)
e6(5)
-t (t)
il (2)
16(5)
-6(2)
-4(2)
- 7( 2 )
re(5)
r6(4)
e(4)
r2 ( 3 )
ro(5)
-r2(s)
-r7(5)
r7(4)
4(2)
5(3)
34(4)
e5(8)
112(20)
72(7)
ro5(4)
r38(r3)
r6(r3)
124(12)
r4s(14)
r30(ro)
i l 2 ( 10 )
0
- 2 ( t)
2(2)
I (2)
-8(2)
e(5)
130
GORDON ET AL.: CLINOPYROXENE EXSOLUTION RELATIONSHIPS
Mgor.SirOufor the C2/c phase, and MnoerMg,o,
SirO.for the P2,/c phase,ascalculatedbasedonly on
occupancyfactors,which were artifically constrained
as describedabove.The relative volumes of the two
A n g l ew i t h
phaseswere determinedfrom preliminary transmis90
sion electronmicrographs,which confirmed that the
r r( r 5 )
2 r ( r 5 ) phaseswere indeed lamellar in nature. The computed averageweightedcomposition,consideringrel30(5)
6 0 (5 )
ative volumes,did not, however,agreesatisfactorily
90
with the observedaveragemicroprobe composition.
80(6)
r05(e)
The compositionsare plotted in Figure 1. Part of this
Ie(8)
discrepancyis related to the limiting assumptions
7 7( B )
made about cation distributions for the two phases.
r 4 0 ( 3)7
l27(38)
We thereforeconcludedthat although the site occu85(e)
panciescorrectlyindicated that the phaseswere near
86(7)
diopsideand kanoite in composition,details regard7(7)
ing site occupanciesremainedto be determined.
38(r8)
60(20)
The average Ml-O and M2-O bond distances
112(7)
were also usedto determine site occupancies.Interatomic distancesas functions of composition were
57(5)
examinedfor compositionalrangesof Ml and M2 of
e 6 (r 0 )
Table 4. Thermal ellipsoid data for manganoan diopside and
kanoite
l v l n - D i o pi d
se
Atom
Axi s
t4l
1
2
3
0.066(7)
0 076(4)
0 . o e (' t7 )
r' r5' r82( (rr55))
0
90
90
I
2
3
0 . 0 e 8 ()
0 . 12 5 ( 2 )
0 .r 4 4 ( 2 )
60(s)
r 5 o ( 5)
90
90
90
0
1
2
3
0 .0 8 8( 2)
0.098(3)
0.il2(4)
1e(e)
7 r( e )
e5(5)
74 ( a )
r56(9)
r08(9)
0.090(5)
0.il8(8)
0.'r28(7)
r3(B)
81(il)
8 1( 7 )
e r( 5 )
r28(40)
38(40)
0.r07(7)
o.r3(5)
0.r46(7)
.92\32)
1 75 ( 2 4 )
86(6)
6(r7)
93(33)
9s(8)
0.089(B)
0.r05(5)
0.r42(6)
58(22)
147(22)
e 0 ( 4)
72(6)
i8(r0)
22(7
)
l'1'l
0 . 0 72 1 4 )
o .o B 4( 4)
0'r05(3)
43(r0)
il5(il )
58(6)
r4(r4)
154(r3)
100(s)
M2
0.094\2)
0.il4(2)
0.125(2)
42(3)
il0(4)
s5(3)
r12(4)
r58(3)
e 4( 4 )
56(3)
ee(4)
14 5 ( 3 )
0 . 0 8 (r 3 )
0.08e(2)
0.il4(3)
50(r8)
r 3 6 (B
r )
r04(4)
46(rB)
46(r8)
r 0 3 ( 4)
7 r( 4 )
e t( 3 )
re(3)
( 3)
0.083
0.092(3)
0.il4(3)
28(12)
65(13)
r03(4)
7 0 (r 3 )
r53(10)
r07(5)
7 t( 4 )
r00(7)
2 1\ 4 )
014
0.089(6)
0 . 0 9 6 () 7
0 .r 3 6 ( 6 )
36(46)
124(48)
r 0 r( 6 )
(48)
'rr24r6
( 4 5)
7 8 ( 7)
7 4 1)7
86(16)
r6(6)
02A
0 089(8)
0.r2s(5)
0 . 13 r( 6 )
6 4( 8 )
r53(r5)
99(35)
35(B)
' r6
' 13o(( r26r )
)
6e(7)
8 9 ( 3 )7
2 r( B )
03A
0 . 0 8 (75)
0 .r ' r 4 ( 5 )
0 . r5 0 ( 4 )
2 6 ( 5)
7B(8)
i r2(3)
65(6)
1 2 71 7)
4 7( 6 )
3e(6)
5t(6)
0lB
0 089(9)
0 .r 0 2 ( 6 )
o . r2 5 ( 7 )
' r76e9( (2r08))
028
0 .0 8 8 6( )
0 . r1 8 ( 7 )
0.148(6)
30(9)
65(e)
r05(5)
Si
03
Rmsampli tude
A n g l ew ' i t h
abc*
90
Anglewith
Kanoite
siA
5iB
038
0.084(5)
0 . r ' 1( 4 )
e 4 ( r) r
62(8)
1 4 s ( )7
r03(4)
2 11 1z )
81(19
7 r( e )
7 4( 8 )
r 4 7( B )
r8(8)
lre(5)
rrB(6)
42(5)
r46(6)
r08(9)
97(il)
le(e)
66(6)
il 0(9)
32(7\
42(4)
7717\
5 t( 4 )
refined a pyroxene structure near ideal kanoite in
composition.We thereforeshall not describethe details of the structuresbeyondthe parametersgiven in
the tables,exceptto point out the well-defined7-fold
coordinationof M2 in kanoite.
The structurerefinementswere performed in part
to limit the compositionsof the individual phases.
The refined compositionscorrespondto CaorrMn",n
Table 5. Selected cation-oxygen interatomic distances for kanoite
and manganoan diopside
MnD
- io p si d e
Kanoi te
M r- 0 2 A
028
0rA
0 rB
2.045(4)
( 3)
2.066
2 . 0 7(75 )
2.087
\4)
t4r-02
0t
0t
ave.
2x2.056(3)
2x2.068(4)
2x2.14013)
2 .O B B
0rA 2.166(4)
018 2.reo(4)
ave.
2..l05
M2-028 2.1??(4)
0 2 A 2 . r 6 1( 4 )
0 rB 2 . 1 7(74 )
0 r A 2 . 1 e (73 )
0 3 A 2 . 4 ? (13 )
o3B 2.7rs(3)
uJb
^
2.495
^ . F t ^ \
2.380
)14-UZA
t.f,vJ(+J
o'rA 1.612(4)
03A r.648(3)
o3A r.682(3)
.634
S iB - O 2 B l . 5 B B ( 4 )
0tB r.613(4)
038 r . 6 7 3 ( 3 )
038 L-62!13)
ave.
ave.
4)
2 x 2 . 2 7( 1
? x 2 . 3 0(73 )
2x2.613(3)
2x2.7BB(3)
4.do3tJ,,
ave.
ave.
M?-02
01
03
03
1.637
si-02
0r
03
03
r.se0(3)
r.605(3)
r.666(3)
1. 6 7 (73 )
ave.
I .635
13l
GORDON ET AL.: CLINOPYROXENE EXSOLUTION RELATIONSHIPS
Table 6. RefinedMl and M2 occupanciesfor maganoandiopside
and kanoite
Mn-Diopsi de
Mz
K a n oi t e
M n0 . 0 6 ( 1)
l 4 g0 . 9 4
MI
C a0 . 8 7 ( 2 )
M n0 . 1 3
142
l 4 n0 . I 0 ( l )
Ms 0.90
Mn0.86(l)
M g0 . 1 4
each phase.Linear variation of bond distanceswith
compositionwas assumed.
For the P2,/c phase,the range clinoenstatite-synthetic kanoitewas usedfor both Ml-O and M2-O.
Data for bond distanceswere obtained from Morimoto et al. (1960)for clinoenstatiteand from Ghose
(written communication)for synthetickanoite; these
are the only pyroxenesfor which data exist in the Capoor edgeof the Ca-Mn-Mg quadrilateral.For MlO the inferred occupancy of the Ml site correspondedto (MgornMn ,,), which is in excellentagreement with the refined site occupancy.For M2-O the
refined bond distance value fell just beyond the
range representedby the clinoenstatite-kanoitevalues. However, as some Ca must be present in the
P2,/c phase(as it coexistswith a manganoandiopside),the slightly larger value of the M2-O distance
(2.30vs.2.2564 for synthetickanoite) is entirely reasonablewhen the presenceof Ca in M2 is considered.
For the A/c phase,the compositionalrange diopside-johannsenitewas chosenfor Ml, and diopsideCZ/ckanoite was chosenfor M2. Bond distancedata
from Clark et al. (1969)for diopside and Freed and
Peacor(1967)for johannsenitewere used.An M2-O
interatomic distancefor a theoretical A/c kanoite
wasgeneratedfrom the linear regressionequationsof
Ribbe and Prunier (1977)for A/c pyroxenesand effective ionic radii of Shannonand Prewitt (1969).
For Ml-O the inferred site occupancycorresponded
to (MgornMno,,),
which agreeswell with the refined
value for that site. This method was not entirely applicable to M2 of the C2/c phase,as Ribbe and Prunier (1977) have shown that the M2-O distance in
C2/c pyroxenesis not a function of the ionic radius
of M2 alone.Rather,it is a function of the ionic radii
of both the Ml and M2 sites.Thus, as the composition of the Ml site does not change appreciably
in the range diopside-kanoite, the value obtained
for the M2 site occupancy of the C2/c phase
(CaorrMnoor)does not reflect any appreciableionic
substitutionbetweenCa and Mn in M2. The overall
results of this technique were in reasonableagreement with thoseof the structurerefinements.
For cell parametersthis same basic method was
applied, with the compositional ranges clinoenstatite-MnrSirOufor P2,/c and diopside-kanoitefor
A/c.This methodhad the samegenerallimitations
as the previous procedure. First, for the Ca-poor
edgeof the Ca-Mn-Mg quadrilateral,cell parameter
data exist only for ctnoenstatite (Stephensonet al.,
1966),syntheticMnrSirO. (Syono et al., l97l), znd
synthetic kanoite. Second,the compositional range
clinoenstatite-MnrSirO.does not account for small
amountsof Ca, which are likely to be presentin the
P2,/c phase,and which will have a significant effect
on the cell parameters.Thus, the value shown in Figure I for the composition of the P2,/c phase from
this method is only a rough estimate.For the A/c
phasethe estimateof compositionwas more tightly
constrained;average Ml-O and M2-O bond distanceswere generatedfrom the observedcell parameters and Shannon and Prewitt's (1969) effective
ionic radii in the linear regressionequationsof Ribbe
and Prunier (1977) for A/c pyroxenes.From these
equations,Ml and M2 ionic radii were computedfor
the A/c phase,which in turn allowed a more accurate estimateof composition to be made fot A/c
than for Y2r/c.The resultsof this methodare shown
in Figure l. The compositionsdetermined for each
phasecorresponded
to Ca"rrMno,Mg,oSirOufor the
/
En
l\l9SiO3
\
Rh
lvlnSiO!
Fig. 1. Plot of Balmat C2/c and P21lc clinopyroxene
compositions in the Ca-Mn-Mg quadrilateral. The points labeled
A and B are average compositions for the Balmat clinopyroxene as
determined from electron microprobe analyses. A corresponds to
the average composition of the sample analyzed using srru; B is
included to show the range of average pyroxene compositions of
the Balmat samples. I and A represent compositions determined
by structure refinements and plots of cell parameter v.t
composition, respectively. The points labeled X, Y, and Z
represent average compositions determined from the results of
each method.
r32
GORDON ET AL.: CLINOPYROXENE EXSOLUTION RELATIONSHIPS
C2/c phase, and MnosrMg,,rSirOufot the P2,/c
phase. The averagepyroxene composition determined by this method, consideringrelative volumes,
corresponded
to Cao3oMfurrMg,orsirou,
in only fair
agreementwith the observed average microprobe
composition,although in reasonableagreementwith
the averagerefinement composition of Ca"rrMno.,
Mg, ooSirOu.
Nonetheless,the resultsof theseattempts
clearly underscoredthe necessityfor somemeans of
determining the lamellar compositionsdirectly,
which led to analysisof this material by techniques
of analyticalelectronmicroscopy.Fortunately, at this
stage of the study appropriate instrumentation becameavailableto us.
Analytical electronmicroscopy
The electron microscopewas a JEoL JEM l00cxASID outfitted with a Princeton Gamma-Tech energy-dispersivesolid-statedetectorwith an analytical
resolution of 146 eV at 5.9 keV. This instrument
combinesthe imaging and diffraction capabilitiesof
the conventional(fixed beam) transmissionelectron
microscope(crnu) with the analytical capabilities
offered by a scanning electron beam instrument
(sreu). It has the capability of 2.0A line resolution
in the transmissionmode, using a side-entry goniometer and the tilted beam technique for lattice
fringe imagery.Incident beam diametersof 50A and
lesscan routinely be obtained.The actual spatial resolution for analysis,however, may be considerably
increasedin relatively thick samples.A thick nonbeam-defining Mo aperture positioned above the
specimenwas used to eliminate most of the hard Xrays generatedby the beam-definingaperture in the
condensersystem.A graphite specimenholder was
usedto further minimize X-rays generatedin the vicinity of the sample.NonethelessCuKa was detected
in all spectra,but was subsequentlydetermined to
arise from the specimenrod beneath the graphite
holder. A liquid-nitrogen-cooledanti-contamination
devicewasemployedat all timesto minimize organic
contaminantsformed at the beam-specimeninterface. All electronmicroscopywas performed at 100
rectly observableon the optical level, but could easily be detectedwith a reflected light source by the
schiller. To ensurethat the lamellar boundary [predicted to be (001)l would be as nearly parallel to the
incident electron beam as possible,only those crystals showingcenteredor near-centeredoptic normal
figures were selected.3-mm-diameterbrass washers
were then attachedto the crystals with a non-acetone-solubleepoxy. The crystals were subsequently
removedfrom the slide by dissolving the glasswax
with acetone,and thinned by ion bombardment using standard conditions (7 kV, l00pa Ar ion beam
current, and inclination of the sample to the beams
of -15").
From conventional transmission electron micrographs,the lamellae were seento be approximately
20004 wide. The interphaseboundary was directly
of loindexedas (001)within error of measurement
2". This was accomplishedthrough determination of
the direction, luvwl, of the lamellae boundary for
each of severaldifferent orientations. Further confirmation of this index was obtained by measuring
the anglebetweenthe lamellar boundary and (ll0)
cleavagesteps seen in transmissionimages, which
was76o45'.Robinsonet al. (1971)studiedthe orientations of exsolutionlamellae in clinopyroxenesand
showedthat the interphaseboundary of lamellae exsolvedon a plane that is apparently(001)may not be
parallel to (001);that is, the interfaceand (001) may
in fact differ by an angle of as much as l0o. Therefore, the interphaseboundary is actually someplane
of irrational index that representsthe plane of dimensionalbestfit betweenthe two latticesat the time
of exsolution. Our index determination, therefore,
while consisientwith the interphaseboundary being
(001),may by analogybe up to lo-2o from the (001)
plane.
Quantitativeanalysis
The Cliff-Lorimer (1972)simple ratio method for
thin foils wasusedto determinethe individual pyroxene compositions.This method is extremely convenient if the sampleis suffi.cientlythin that atomic
kv.
number, absorption,and fluorescence(ZAF) effects
Specialthin sectionsof the Balmat sampleswere are minimal and may effectively be neglected.The
made, using a glass wax that was acetone-soluble, ratio of characteristicX-ray intensities of two elerather than a conventionalthin-sectionepoxy. Single ments is thereforerelated to the concentrationratio
crystalsof pyroxeneexhibiting schiller were then se- of those two elementsby the expression(C"/C) :
lectedoptically.The optical orientation of this pyrox- k"b(I,/Ib). We have taken the ratio of each element
ene appearsin generalto be the sameas that of the presentin the unknown (Mn, Mg, and Ca) to Si, ascommonclinopyroxene,i.e. the optic normal is paral- sumingthat only Si occupiesthe tetrahedralsite (this
lel to the D axis. Generally the lamellaewere not di- was also assumedfor all of the standardsused).The
133
GORDON ET AL: CLINOPYROXENE EXSOLUTION REI'.ATIONSHIPS
Nuclear Data 6600 computer systeminterfaced with
the detectorincluded a program which calculatedthe
area under the curve for a chosenpeak, so that the
final intensity value corresponded to peak-background.Upper and lower energylevelsfor eachpeak
were set manually. These computed areaswere the
actual valuesusedin the intensity ratio calculations.
The simple ratio method has been used successfully
by other researchers
on geologicalmaterials(Lorimer
and Champness,1973;Copley et al., 1974;Nobugai
et al.,1978).
Somepotentialproblemsof chemicalanalysiswere
contamination buildup and mass loss due to volatilization, both associatedwith the stationary spot
mode used for analysis.To check for errors from
theseeffects,ratios of characteristicX-ray intensities
were taken from the same spot every 200 seconds
over a l5-minute period. This was done on the unknown and on eachof nine standards.No detectable
errors were found from either effect, as intensity ratios remainedconstant.We point out, however,that
this was determined only for the materials used in
this study. Care should be taken with other minerals
(most notably feldsparsand carbonates)to check for
this effect,as masslossmay lead to seriouserrors in
compositiondeterminationsfor some phases.As an
additional checkfor beam damage,micro-micro diffraction was performed on the individual pyroxene
lamellae(1.e.diffraction from areasof approximately
1000x 1000A;Mardinly, personalcommunication).
If specimendamage was occurring, the diffraction
pattem should have gradually deteriorateddue to alteration of the specimenin the beam. The resulting
diffraction patternswere observedin the microscope
over a period of severalminutes,and were found not
to change.Therefore, we concluded that sample
damagefrom the beam was negligible.
Analysisdependson the measurementof k, a constant for each element ratio which includes corrections for the efficiencyof X-ray detectionby the detector (Cliff and Lorimer, 1972). To determine k
values,X-ray intensity pairs for CalSi, Mn/Si, and
Mg/Si were measuredfrom nine homogeneouswellanalyzed standards routinely used in microprobe
analysesat the University of Michigan. The standards used and k values are listed in Table 7. This
procedureyielded three intensity ratios for Mn/Si,
four for Mg/Si, and six for CalSi. The k valueswere
determinedin two ways: (l) by simple calculation
from the above expression,using known values of
element/Si concentrationin element weight percent
and observed X-ray intensities; and (2) by least-
squaresrefinementof the straight lines resulting from
plots of concentration ratio vs. intensity ratio; the
slopesof these lines are k values. There was very
good agreementbetweenk values determined from
both methods.23 analysesper standardwere made,
counting for 200 secondseach.The specimenholder
was tilted at an angle of approximately 30o during
analysis.The standardswere analyzed as crushed
crystalssuspendedon holey carbon films, becauseinsufficientamountsof eachwere availablewith which
to prepareion-thinned standards.Care was taken to
anzlyzeonly thin edgesof crystal fragmentsin order
to eliminateZAF corrections.The plots of concentration vs.intensityratios alsoservedascheckson specimen thickness;beyond a critical thickness,ZAF effects must again be considered.As long as the
relationship between concentrationand intensity is
linear, the simple ratio method is applicable. Note
that k values must be determined for every instrument, as efficiencyof X-ray detection may vary
between detectors,with operating conditions, and
with data reduction methods.
The lamellar compositions were determined by
summing the concentrationsof Ca, Mn, and Mg to
one for each phase, such that the following expressionscould be used:
C.":
[k.".'(Ic"/Ist)]Csr
(l)
C'":
[kM"st(IM"/Isi)]Csi
Q)
C.r:
[k',",(I'rlI.')]C",
(3)
Each equation is the original expressionrewritten to
solvefor the concentrationof the octahedral-siteelements. Thus written, the observed intensity ratios
and k valuescan be usedto determinethe Si content
of eachphase:
[kc"s,Gc",/I",) + kM"si(IM"/Isi)
+ kMesi(I.s/I.')lC"' :
(4)
I
This value of C", was then used in expressionsl, 2,
and 3 to solvefor the contentsof Ca, Mn, and Mg in
eachphase.Approximately 50 analysesof eachof the
Balmat pyroxeneswere made to determineelement,/
Si intensity ratios. Standard statistical analyses
yieldedthe final valuesof k to be k."., : 1.27+ 0.09,
k'.r' : 1.399i 0.002,and k'r", : 0.47 + 0.04.The
resulting errors in the STEMcompositionsare representedby the circles shown in Figure l. The results
of this procedure yielded compositions of
for the C2/c phase, and
Cao.rMnoooMgorrSirOu
for the F2'/c phase.The averCao,rMn,orMgoruSirO.
age composition,considering relative lamellar vol-
t34
GORDON ET AL.: CLINOPYROXENE EXSOLUTION RELATIONSHIPS
Table 7. Standardconcentrationand intensity ratios used in
determinationof k values
CalSi
Conc.
I
lvln/Si
Conc.
I
Mg/si
Conc.
I
0.445 0.46
0 . 2 4 40 . 3 4
0.2540.36
2 . 0 0 02 . 8 0
0 . 4 6 50 . 1 9
0 . 4 6 50 . 2 3
0 . 4 7 00 . 2 3
0 . 5 ' l 70 . 2 5
0.490 0.62
0.495 0.51
0.495 0.66
0.675 0.77
0.995 1.27
'Standards
u s e dw e r e s y n t h e t i c t e p h r o i t e , B r o k e n
H iI I b u s t a mtie , a n d W a b u s h
c u m mn ig t o n it e f o r M n / S ;i
B r o k e nH i l 1 b u s t a m i t e ,D u c k t o wdni o p s i d e , G o l d i c h
d i o p s i d e , A N Uh e d e n b e r g i t eA, N Uw o l l a s t o n i t e , a n d
I r v i n g c i i n o p y r o x e n feo r C a l S i ; W a b u schu m m i n g t o n i t e , D u c k t o wdni o p s i d e , G o l d i c hd i o p s i d e , a n d I r v i n g
c l i n o p y r o x e n ef o r M g / S i . C o n c e n t r a t i o nr a t i o s
f o r t h e s e p l o t s w e r e c a l c u l a t e d i n n r o l ep e r c e n t
f r o m s t a n d a r da n a i y s e s .
umes,correspondsto CaoruMnorrMgourSirOu,
which
agreesreasonablywell with the observedaveragemicroprobecomposition.As predictedearlier in considering the refined compositions,the P2,/c phasedoes
indeed contain some Ca (approximately six mole
percent). The two compositions are analogous to
those found in coexistingaugites and pigeonites of
the Ca-Mg-Fe pyroxenequadrilateral.The most obvious differencein the Ca-Mg-Mn systemis the relative enrichment of the P2,/c phase in Mn. This is
probably due to the marked preferenceof Mn for the
larger M2 site,as noted by Ghoseet al. (1975)in a
study of site preferencesof transition metal ions in
syntheticpyroxenes.They found that, based on site
occupancyrefinements,Mn showed a much greater
affinity for M2 than did Fe. Another major differencebetweenthe two quadrilateralsis the apparent
smaller width of the solvus in the Ca-Mn-Mg system. A probable explanation for this lies with the
similarity in ionic sizebetweenCa and Mn; thus the
impetus for unmixing would not be as great as that
expectedfor Ca vs.Mg and Fe, where cation sizesare
more disparate.The results of all methods used to
determinelamellar compositionsare shownin Figure
1. The compositionsdetermined with srnu clearly
give the best agreementwith the observedaverage
microprobecompositions.
Antiphasedomains
The scaleof antiphasedomains(APD's) in pigeonites has been used by severalresearchersto indicate
relative cooling rates (Bailey et al., 1970;Christie el
al., l97l; Brown et al., 1972;Ghoseet al., 1972;LalTy
et al., 1972,1975;
Brown and Wechsler,1973;
Nord et
al., 1973).At high temperatures,pigeonitespossess
C2/c symmetry,in which two chains of linked SiOo
tetrahedra are identical. With decreasing temperature, the coordinationof the cationsin the M2 sites
changesin concert with shifts in the chains. The
chains are no longer related by symmetry, and the
spacegroup is P2,/c. In pigeonitesthe temperature
of this transformation is controlled by composition
(Prewittet al.,l97l). The resultingmicrostructureis
composedof regions of the P2t/c structure (APD's)
related to each other across antiphase boundaries
(APB's) by a translationof (a + b)/2 (Morirnoto and
Tokonami, 1969).The structureis thus C-centeredat
the boundaries.
APD size should be a function mainly of cooling
rate through the inversion temperature,as the
C2/c-P2,/c inversion in pigeoniteseemsto occur by
random nucleationand growth of lhe P2,/c phasein
the C2/c phase.However, heterogeneousnucleation
may occur and theseAPD's may be larger than if
formed by random nucleationalone, and thus would
give an erroneousindication of cooling rate (Lally et
al., 1975).With a relatively slow cooling rate, large
APD's with fairly straight boundaries may be expected, becausethe few random Y2,/c nuclei that
form in Ihe A/c phasehave more time to grow, and
straightening of boundaries is one way in which
APD's can becomeenlarged(Cupschalkand Brown,
1968).Few instancesof APD's with straight boundaries have been reported in pigeonites (Champness
and Lorimer, l97l; Lally et al., 1975). Carpenter
(1978) describesslowly-cooledterrestrial pigeonites,
which exhibit not only large APD's but also relatively straight boundaries.He cites evidencefor the
enrichment of some APB's in Ca, as predicted by
Morimotoand Tokonami(1969).He alsopresentsdirect evidencefor theseAPB's being good nucleation
sitesfor augite,as noted by Lally et al. (1975).
As mentionedearlier, APD's on the order of 200A
or less(Morimoto and Tokonami, 1969)are revealed
in single-crystalX-ray diffraction photographsby the
presenceof diffusenessin the class(b) reflectionsof
the P2,/c phase.However,the class(b) reflectionsof
our F2,/c phasewere quite sharp.This was explained
by bright- and dark-field transmissionelectron micrographswhich revealedthat not only were APD's
present in the primitive phase, but that they were
large,rangingfrom -400 to -3500A in wdith. As no
nucleation of other phaseswas seen in either the
P2,/c or A/c phases,this coarseAPD sizealone in-
GORDON ET AL.: CLINOPYROXENE EXSOLUTION RELATIONSHIPS
135
dicatesa slow cooling rate. In any case,the large domain sizeallows the APD's to difract independently,
consistentwith the sharp quality of the class (b)
Y2r/c reflections.Figure 2 showsdark-field transmission electronmicrographsof the BaLnat pyroxenelamellae,showingonly diffraction of tll:eP2,/c phase.
The APD texture is readily apparent.The reflections
used for imaging were sharp, single class(b) reflections, representedby the circled spot in the accompanying diffraction pattern (inset),which is the a*c*
plane. Clearly, from this microtextural evidence,the
Ca-poor pyroxene in the Ca-Mn-Mg quadrilateral
undergoesa C2/c-Y2,/c inversion analogousto that
of pigeonitesin the Ca-Mg-Fe system.
The temperatureof APD inversion was qualitatively determined with a special sample heating
holder in the srBu. An ion-thinned fragment of pyroxenewas enclosedin a folding Cu grid, which was
then placedin the heating holder. Immediately prior
to heating,the APD texture was imagedin dark-field
mode.Both the APD texture and the sharp class(b)
imaging reflection were monitored during heating.
The rate of heating and cooling was rapid (approximately 50oC per minute). As the inversion temperature was approached,the APD texture was seento
disappear quickly and continuously; concurrently,
the imaging reflection was seen to decreasein intensityuntil it disappearedcompletely,as seenin the
sequenceshown in Figure 3. The samplewas maintained above the inversion temperature(at temperaturesas high as -360"C) for no more than one to
two minutes before cooling. This procedure was
performedseveraltimes within a period of about 2030 minutes. Care was taken to determine whether
changesin sampleinclination during thermal cycling
might be causing the disappearanceof both APB
contrastand diffraction intensity. Bright-field observation of extinction contours, which are extremely
sensitiveto changesin sampletilt, indicated that the
specimen remained stable during temperature
changes.The results of these experiments yield a
temperature of inversion corresponding to 330 t
20"c.
Ca enrichmentof APB's
In somecases,enrichmentin Ca of APB's can be
detectedby the broadeningof APB's or by the nucleation of a Ca-rich phase at the APB's (Lally et al.,
1978).We observedneitherof these
1975;Carpenter,
effects. but instead noted the distribution of the
APB's which relatesto the distribution of Ca. before
and after heatingthrough the inversion temperature.
In Figure 4, which representsthese conditions, the
Fig. 2. Dark-field transmission electron micrographs of Balmat
clinopyroxene taken at 100 kV using h+k odd reflections: (A)
shows diffraction pattern (inset) of a*c* plane, with imaging
reflection circled; note the relatively large APD's and fairly
straight boundaries. (B) and (C) also show "pinch-out" lamellae
(arrowed) in both the P2,/c (light) and A/c (dark) lamellae.
GORDON ET AL.: CLINOPYROXENE EXSOLUTION RELATIONSHIPS
APB's are generallysimilar, even identical in several
areas.This indicatesthat somethingis controlling the
distribution of the APB's, giving rise to a "memory"
effectafter rapid heating and cooling through the inversiontemperatur€.We concludedthat Ca rrust be
concentratedat the APB's to some extent, causing
this phenomenon,although it is possible that it is
causedby the presenceof defects.As heating and
cootng were rapid, the likelihood of appreciablediffusion of Ca away from the APB's while the sample
was at a temperatureabove that of the inversion is
small.
In an attempt to directly confirm the presenceof
Ca at the APB's, Ca X-ray imagesof lamellae showing the APD texture were recorded.A window was
placedon the emissionspectrumaround the Ca peak
in order to permit the detection of CaKa photons
only. Theseimages(Fig. 5) qualitatively illustrate the
enrichmentof the A/c phasein Ca with respectto
n,/c.Withlnthe Y2r/c phase,however,it is not possible to determineif the few Ca X-ray photons detectedcorrelatedirectly to the APB's. The amount of
Ca presentat the APB's is simply too small to be detected relative to the inefficiency of the method.
However,in light of the "memory" effectseenin the
APB orientationsfrom the heating experiments,we
concludethat some Ca is probably concentratedat
the APB's, and this is very likely the factor controlling the distribution of the APB's.
Coherencyof the interphaseboundary
Fig. 3. Sequence showing the disappearance of p2r/c h+k odd
imaging reflection during heat of clinopyroxene: (A) 135"C; (B)
260"C; (C) 350"C. Diffraction patterns were taken in dark-field
mode, with the electron beam tilted so that the diffracted beam is
parallel to the optic axis of the microscope. All micrographs taken
at 100 kv.
From examinationof the unit-cell parameters,we
predictedthat the index of the interphaseboundary
would be (001); this was subsequentlyverified by
electrondiflraction and transmissionelectron micrographs.This prediction was basedon the fact that rhe
greatestdifferencein magnitudesof parameterswas
betweenthe c axesof the two pyroxenes.Both phases
have the b axis in common, and a and D of the two
phasesare parallel. tf both the a and D parameters
were identical, the interphaseboundary would be a
coherentunstrainedboundary. As the b parameters
are equal within the error of observationand the a
parametersare nearly equal, we thought that the lamellar interface in the sample might be such a
boundary.To investigatethe coherencyof the interface, high-resolution electron microscopy (lattice
fringe imaging)was performedusing the (200) reflection.
Figure 6 showsa tilted beam lattice fringe image
of (200) planescrossingthe lamellar boundary. The
accompanying difraction pattern shows the a*c*
plane, with the characteristicextinctions apparent.
GORDON ET AL.: CLINOPYROXENE EXSOLUTION RELATIONSHIPS
Fig. 4. Dark-field micrographs of APD's of P21/c phase: (A) before heating; (B) after rapid heating (to 350'C) and cooling. Note the
extremely similar orientations of APB's in both micrographs (100 kV).
GORDON ET AL.: CLINOPYROXENE EXSOLIITION RELATIOI{S.rIIPS
closest-packedoxygen anions. From these observations, it appearsthat this interfacemust be a largely
coherentboundary.The lattice parametersparallel to
the interfaceare so similar, however,that a minimum
strain condition would exist if the interface was
largely coherentwith dislocationsseparatedby approximately 10004, a situation which would require
that the interfacebe defined as semicoherent.
Discussionof the exsolutiontexture
The Balmat clinopyroxenelamellae are homogeneous phases,share (001), and are approximately
20004 wide. The interphaseboundary, as described
above,is largely coherentwith little strain, and the
presenceof APD's indicatesthe existenceof a C2/cF),/c inversion for the Ca-poor phase.The Balmat
samplesexhibit an exsolution phenomenonformed
completelyas a product of regional metamorphism,
with a well-definedand simple thermal history. The
parent material for this rock was shallow marine
sediment.The probable geologic sequenceinvolved
heating and crystallizationof sedimentsby regional
metamorphism to the peak temperature of 625"C
and pressureof 6.5 kbar (Brown et al.,1978);formation of the pyroxene,which waspresumablyhomogeneousat the peak of metamorphism;and subsequent
slow cooling from peak metamorphic conditions
which permittedthe pyroxeneto unmix continuously
to two A/c phases,one Ca-rich and one Ca-poor.
Further slow coolingthen allowedthe Ca-poor phase
to invert from C2/c to P2,/c symmetry,with the associated formation of relatively large APD's. Another feature of the exsolutiontexture is the "pinching-out" of lamellae (Fig. 7), which occurs in both
Fig. 5. (A) Scanning transmission image of clinopyroxene
phases.A possiblemechanismfor formation of this
lamellae (lighter lamellae arc A/e;
(B) scaming transmission Ca
feature is describedby Copley et al. (1974); it inX-ray image of same area as in (A), showing A/c
lamellae
cludes phase nucleation and subsequent lamellar
enriched in Ca; 100 kV.
growth by the propagationof ledgesalong the interphasesurface.In our sample,however,these ledges
may simply be a strain-releasemechanismafter the
The centralbeamand the (200)reflectionusedto im- formation of a dislocation,and may not necessarily
age the 4.6A periodic fringes are circled. In this im- be the result of heterogeneous
nucleation.
age,it is possibleto follow thesefringes without inWe cannot assign a specific mechanism of exterruption acrossthe interface, at which point they solution using only the observedmicrostructure,albend slightly. The angle between the fringes from though spinodal decompositionis certainly possible.
one phaseto the other is 2.5o,which is equal to the Featuresof the microstructureare consistentwith the
angle betweenthe c axes within error of measure- known geologichistory of the sample,especiallythe
ment. Viewing the structure in this orientation @* mostly coherentlamellar boundary, the large size of
parallel to the incident electron beam) is essentially the APD's, and the apparent Ca-enrichment of
equivalent to observing the tetrahedral-octahedral APB's. Of special interest is the small amount of
layers of the pyroxene structure, i.e. the planes of strain associatedwith the lamellar boundary, which
GORDON ET AL: CLINOPYROXENE EXSOLUTION REI-/ITIONSHIPS
Fig. 6. Lattice fringe image ofinterphase boundary (001). Ditrraction pattern (inset) shows the atc* plane; circled are the central beam
and the (200) reflection used to image the lattice fringes. Note the slight bending of the (200) planes across the interface' Dark-field
micrograph, 100 kV.
indicatesthat the a and b parametersof both phases
must havebeenas similar at the time of exsolutionas
they are now. In general,the exsolutionrelationships
exhibited by the Balmat clinopyroxene provide
markersin the cooling history of the rock.
In sum, techniquesof analytical electron microscopy have provided information complementaryto
that determinedby the more conventionaltechniques
of electronmicroprobeanalysisand singlecrystal Xray diffraction. The exsolution relationships for
clinopyroxenesin the system CaSiOr-MnSiOrMgSiO. are strikingly similar to thoseof the common
rock-formingsystemCaSiO.-FeSiOr-MgSiO..
GORDON ET AL: CLINOPYROXENE EXSOLUTION RELATIONSHIPS
References
Fig. 7. Ledges (arrowed) along interphase boundary. ,.pinchout" lamella is arrowed in (A). Bright-field
rransmission
micrographs, 100 kV.
Acknowledgments
We thank Professor W. C. Bigelow, John Mardinly, and Steve
Krause of the Department of Materials and Metallurgical Engineering of The University of Michigan which houses the sTEM
and the electron microprobe for continuing operational assistance
with the instruments and for many valuable discussions. We also
thank Craig Johnson for the final electron microprobe analysis of
the pyroxene. We are grateful to St. Joe Minerals Corporation
who provided summer support for one of us (pEB), and particularly to Dr. David Dill who saw that the rhodonite- and pyroxenebearing samples were saved. We are also indebted to Dr. Gordon
L. Nord, Jr., of the U. S. Geological Survey for his careful review
of the manuscript. This research was supported in part by a Sigma
Xi Grant-in-Aid of Research and a Geological Society of America
Research Grant to Wendy A. Gordon and NSF grant EAR 7g22758 to Donald R. Peacor.
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Manuscriptreceived,February1, 1980;
accepted
for publication,Seplember15, 1980.