Orientation of Pigeonite Exsolution Lamellae in Metamorphic Augite: Correlation with Composition and Calculated Optimal Phase Bo

American Mineralogist, Volume 60, pages 9-28, 1975

Orientationof PigeoniteExsolution Lamellae in Metamorphic Augite:Correlation with Compositionand Calculated OptimalPhase Boundaries

Howano W. Jerre. Perrn RosrNsoN.Ronrnr J. Tnacv, Departmentol Geology,Uniaersity of Massachusetts, Amherst, Massachusetts0 1002 eNn Mnrcor,u Ross U.S.Geological Suraey, Reston, Virginia 22092

Abstract

Optical examination of metamorphic augites coexisting with orthopyroxenes ranging in composition from Fs,u to Fs"s from the Adirondacks, the Hudson Highlands, and the Cortlandt Complex, New York, and the Belchertown Complex, Massachusetts, shows three sets of exsolution lamellae. X-ray single crystal photographs show these are orthopyroxene lamellae on (1@), and pigeonite lamellae, termed "0Ot" and "l00", oriented on irrational planes near (001) and (100). Optically observed angles of the phase boundaries of "001" and "100" lamellae with respect to the c axis of host augite vary with iron-magnesium ratio determined from electron probe analyses of 29 pyroxenes, supplemented by 76 measurements of gamma index of refraction. The angles are largest in magnesian specimensand are in near agreement with angles of optimal phase boundaries calculated from measured lattice parameters of host and one or both sets of pigeonite lamellae. Compositional control of lamellae orientation is related to compositional control of lattice parameters, which appear not to have changed significantly since exsolution in the range 800-500'C.

Introduction and (100) directionsof the host, as had beengen- (Poldervaart and fless, 1951). Many igneous and metamorphic rocks contain erally accepted establishedsolely by pyroxeneswhich have unmixed during cooling. The Thesesignificant relations were petrographic of grain mounts by exsolved pyroxenes form as lamellae which have microscopicstudy betweenthe two promi- been generally described in the past :rs being noting that the obtuseangle was 122" rather than the orientedparallel to the (100) or (001) lattice nent sets of lamellae requiredby two setsof lamellaeparallel planesof the host (Poldervaartand Hess, 1951). 105-106" (001) (100) Metamorphic augites containing sets of exsolution to and of augite. a study of these exsolution phe- lamellae in three different orientations were first Subsequently, nomena in more detail by Robinson, Jaffe, Ross, describedby Jaffe and Jaffe (1973)1 from their Klein (197l)L establishedby singlecrystal X-ray occurrence in augite-orthopyroxenegranulites and and photographsthat the lamellae parallel to (100) of amphibolitesassociated with Precambriancrystalline augite host were orthopyroxene. They found gneissesof the Monroe quadranglein the Hudson the that the two more prominent setsof lamellae,desig- Highlandsof New York (Fig. 1). These authors nated as "001" and "100" lamellae,were indeed noted that only one of the three sets of lamellae pigeonite' with phaseboundaries, separating lamel- was parallel to a crystallographicaxis, the c axis, of the host augite,whereas the two more prominent 'The definitions of pigeonite and augite used here are sets of pigeonitewere not parallel to the (0Ol) based partly on the work of Ross, Huebner, and Dowty (1973). Pigeonite is defined as a monoclinic pyroxene con- 'The paper by Jaffe and Jaffe (1973) was submitted for taining less than 20 mole percent CaSiOs component (Wo ( publication in 1969, but not published until 1973 because 20). The FeSiO" component can vary from zero to 100 of a freeze on funds at the N.Y. State Museum and Science mole percent, although for the end-membersonly, we would Service. It thus preceded the study and publication of the use the names clinoenstatite (En'.) and clinoferrosilite paper by Robinson,Jafte, Ross,and Klein (1971). (Fs"-). Pigeonite can be unambiguously identified by single 10 TAFFE, ROBINSON, TRACY,,{ND ROSS

of Bollmann, as applied to exsolution in feldspars by Bollmannand Nissen(1968). Soon thereafter, a study of glain immersion nrountsrevealed that the exsolgtion phenomenadis- covered in the two-pyroxenegranulites of the Hud- son Highlands, described above, are even more widespread in two-pyroxene-bearinganorthosites, granulites, and charnockitic gneissesin the Mount Marcy quadrangle of the Adirondacks. Detailed areasurementsof the gamma indices of refraction of orthopyroxeRescoexisting with these clinopy- roxenes indicated a wide range of iron-magnesium 1sfi6s-n36ely, 100 (Fe2** Fe3. * Mn)/(Mg + Fe2* * Fe8* * Mn)-fronr 40 to 95 (Tabtre1, Fig. 2). Pyroxene assemblagesricher in magnesium were obtainedfrom the BElchertownfnfiusive Com- plex of central Massachusetts(Fig. 1; Emerson, 1898, 1917; Guthrie and Robinson,1967; Guthrie, 1,972; Hall, L973) and the Cortlandt ultramafic complex of southeasternNew York (Fig. 1; Shand, 1942;Tracy, l97O). Mea$ure(nentsof the gamma EXPLANATION index of retraction of orthopyroxenesfrom these rocks indicated iron-magnesiumratios of 15 to 35 lF.n Mesozoicond CenozoicCpver (Table 1). Thus pyroxene pairs were available for study in which the orthopyroxenesrange in com- T;-l Belchertown,Corllondl Complexes position from Fsrs to Fse5(Fig. 2). In all cases these orthopyroxenescoexisting with augite con- tain augite exsolutionlamellae parallel to (100). D'Kl Anorthosite Although of different primary origins, all of the pyroxene pairs under discussion are believed to ffi.,:iln Precombrion have equilibrated under generally similar meta- Ftc. l. Generalized geologic map slrowing locations from morphic conditions of high tqmperature, inter- which coexisting pyroxene$ \ilere obtained: (1) Monroe mediate pressure, and low humidity equivalent to quadrangle, Hudson (2) quad- Highlands; Mpunt Marcy the granulite facies.Hudson Highlands and Adiron- rangle, Adirondacks; (3) Belphertown Compkx; (4) East- ern part of the Cortlandt Complex. dack specimensappear to have been involved in Precambriangranulite facies metamorphismof re- gional extent, although the Adirondack specimens lae host, parallel from that are not oriented to a also contain some relict featuresof earlier plutonic specific rational plane lattice of the host. These events.The specimensfrom the BelchertownCom- authors also demonstrated that the orientation of plex came from a Devonian batholith. Subsequent exsolution lamellae pigeonite of in augitenand also to syntectonic emplacernent,the batholith under- that of monoclinic amphibolesin each other, was in went hydration during continued regional metamor- accordancewith the optimal phaseboundary theory of composition Wog (En, Fs)* clinohypersthene. Yet crystal X-ray diffraction by ( I ) its space group symmetry pigeonito on cooling may unmix enough augite to give it Hh/c at room temperature, and (2) by having a B angle this low calcium content. of greater than 107.5' (usually 108-109") at room tem- Augite is defined as a elinopyroxene having greater than perature. The names clinoenstatite, glinobrEnzite, clino- 20 mole percent CaSiO' (Wo ) 2O) aad Fs content from hypersthene, and clinoferrosilite are not used for naturally zero to E0 mole perc€nt. Augite is unambiguously identified occurring pyroxenes because there is no consensuson the by having space group symmetry C2/c ar:.d a p angle of range of Fs or Wo content to be attached to those names. less than 107.5' (usually 105.5-106.5"). The names diop Some would prefer to call a clinopyroxene of cornposition side and hedenbergite are reserved for the augite end- 'bigeonite" Wo$ (En, Fs)* but would call a clinopyroxene members CaMgSLOo and CaFeSLOorespectively. PIGEONITE EXSOLUTION LAMELLAE IN METAMORPHIC AUGITE 11

HE

EN t5 Frc. 2. Compositions of co-existing pyroxenes from the Hudson Highlands (open circles), Adirondacks (closed circles), Belchertown Complex (open squares), and Cortlandt Complex (closed squares). All compositions were determined by electron probe, except for thosemarked by the smaller closed circles, which were determined optically. Solid tie lines'indicate specimens for which X-ray single crystal data have been obtained; long-dashed tie lines, only optical data on exsolution lamellae. Short-dashed lines indicate trend of zoning in angites 447,T65, and A21. Stippled pattern indicates limits of mutual solid solution of synthetic Ca-Mg-Fe augite and orthopyroxene at 810"C as determined by Lindsley, King, and Turnock (1974). phism that left the central core with the relict stageof the microscope.According to Deer, Howie, granulite facies mineralogy of an orthopyroxene- and Zussman (1963), each atomic percent of augiterronzodiorite. The interior of the easternend (Fe'z.* Fe3** Mn) leadsto an increaseof O.@125 of the Ordovician Cortlandt Intrusive Complex in the gamma index of refraction of orthopyroxenes seemsalso to have preserved essentiallygranulite from both igneous and metamorphic parageneses. facies assemblagesalthough undergoing recrystalliza- This correlation incorporates data for orthopy- tion during later regional metamorphism (Tracy, roxenes from plutonic rocks (Hess, 1952, 1960), 1970). Further detail concerningthe specimensand from volcanic rocks (Kuno, 1954), and from high- their geologic setting is given in the Appendix. grade metamorphic rocks (Muir and Tilley, 1958; Howie, 1955), all of which are plottedon Figure 3. Optical Properties and Composition of Pyroxenes Electron probe and optical data obtained during The compositionsof coexistingpyroxenes in 38 this study show lessscatter and a new curve relating specimenswere estimatedby measurementof indices iron-mapesium ratios with gamma indices of re- of refraction in immersion oils (Table 1) and fraction was constructed (Fig. 3). This was used verified by thirteen sets of electron probe analyses to obtain the iron-magnesiumratios listed in pa- (Table 2). Optical curves based on the new probe renthesesin Table 1. analyses (Figs. 3, 4) proved more reliable than curve.sbased on literature data and were used for Augite all optically determined compositions reported in Ordinarily, one measuresthe beta index of re- this paper. fraction of augite in order to obtain an estimateof the iron-magnesium ratio as suggestedby Hess Orthopyroxene (1949). However, an evaluationof the optical data Measurementof the gamma index of refraction for analyzedaugites listed by Hess (1960) andDeer of orthopyroxenes in fragment mounts is simple et aI (1963) shows that the gamma index of re- becauseboth the excellent {210} cleavageand the fraction yields estimates of the iron-magnesium god {100} parting provide numerousplates ori- ratios comparableto those obtained from the beta ented with the Z-vibration direction parallel to the index. It is much more informative and convenient t2 ]AFFE, ROBINSON, TRACY, AND ROSS f'esrE 1. Optical Measurements of Angles of Exsolution n"l Lamellae of Pigeonite in Host Augite, .y Indices of Re- fraction, and Compositions of Coexisting Pyroxenes from the Adirondacks, Hudson Highlands, BelchertownComplex, and Cortlandt Complex'r' r78

t7 Sanp]e ]-Aug fe -Aue y-opx fe -Opx erttoloecrs, n. t.'' Co-17 i{k 7r5 (26) N.D. It6.5' l0' 126 5" Pir- 3 1rt (29) L.714 (40) N.D. IZ' 1r1 (29) 1,7r8 (43) r19" 11 5' 130.5' ;{a-5 JLIS (3C) L 120 (44t 117' 7.s' r24.5" }ID-I 7195 (32) I. 7I8 (43) 117' 11" 128"

Jo-8 1.720 33.0 (33) 1.720 !L9 (44) 11i" I2' t29' Pr-l r,720 (33) r.72O (44' 117" 1,2' 129. co-4 1.120 lq.: (33) r.727 49.2(49\ I15' 123' sb-2 r.124 (39) I.721 (49) 116' Po-r 1.126 !l{ Cr,zl r.731 53.9(s4) 117' 7" r24' t720 sl-5 1,726 (42\ \.7295 (51) 1r5. 5o 8.5' 12l' :{a-3 r,127 (44, r.729 (5r) II5 " L22" vr{-2 I.127 (44' 1.730 (s1) 116.5 " 8,5' 125. tTto ca-6 r.729 46.9 (47\ 1.738 :Zjl (57) rl5' 5' 120' T65 cD 1 ,733 (53) r.738 (s7) 115' I2t ' 'o | 700 ca-17 r.736 ll:7 (57> ,r.752 99:L (68',, 112.5. 3' rr5. 5' T526 ciantii 1.735 (s7) L759 (74') 113" 2' tt5' JB-2 L.14I (65) r.757 (72',) II2. 1" 113' I OYU Go-2 I.146 (72) I.773 (85) 111.50 0,5" tr2' sb-r r.7485 (75) r.77r (83) 111.s' 0.5"

Po-13 1.7s05 gLf Qe' 1.776088.9 (88) 1u,5"i/d 11t. 5. | 680 sc-6 L759 87,5 (92) )-,785 923 (96\ 11r.## 0" 1I I' Po-17 r.760 91.4 (93) 1,786 95.s (97) rrr" I I | 670 llUDSoNl{IGHltr\NDS, N, Y.

J51s 1. 7r3 (23) r.7r7 (12) 115' 124. J241S L7r6 (27) 1.71s (40) 115' l0' 125' J223 r,720 35.4 (33) r.729 501 (sr) 116' r22' J431P I 722 (36) r.731 (52') 114' 6' 120' lo 20 30 40 50 60 70 80 90 loo BELCHERTOWN,MASS. (Fezt+ (Mg+ 1ps3++ 1.104 12.9 (9) !.682 14{ (16) 722" 22" 74t' IOO Fe3*+Mn) / Fe2t Mn) lr5 1.714 (24\ 1.703 (32) 1r9' 13. 132' Il0 r ,7r4 (24\ r.7o4 (32) rr9,5o 12,s' 732" .y tl3 r.116 (27' r.7o25 (3r) r20" 13" Frc. 3. Variation of the index of refraction and the A21 I.716 26J \27, r,705 llf (33) r20" 12,5" ratio 100 (Fe* + Fe''. + Mn)/(Mg { Fe',*+ Fe'. + Mn) C0RTLANDtCoyPLDX, N. Y. for orthopyroxenes. Data from Deer, Howie, and Zussman

162 r tot5 (r4) r.69t (21\ 120' 16" 136' (1963) and present study (labelled points). Indices of rs8 I 708 (15) r ,6935 (25t r2z' r7' r39' (1.658) (1.789) T25 I1r4 (24) 1.701 (32' 119' 16' 135' pure syntheticenstatite and orthoferrosilite \52 1.718 23.1 (30) r.696 22 I (26\ 122.5" 19.5' IL2" from Stephenson,Sc.lar, and Smith (1966) and Lindsley, 165 t.124 2L.O (39) r 1o2 25,7 (3r) r23" 17" 140" MacGregor, and Davis (1964), respectively.Solid curve

* All nicroscopic measuremenEs by I{. il. Jaffe. fitted visually to data points of present study (except T65) n* - least re 100(r.2++Fs3+1y6;71yg+pg2++pe3++un; derermt,ed by erectron probe and synthetic end members. Dashed line is linear analysis (underlined) or oprically using Figure 3 or Ftgure 4, rhis pape! squares best fit of same data points. The equation for this ri Adirondack specimens excepc Ciant are fron )lount;4arcy 15r quadrangle. Cidnt is from adjoining ElizaberhroDn quadrangt€. line is:

+r; llccenE optical re-exrmination of Po-13, SC-6 and Po-]7 has revealed a very very frne second ser of "00I'r pigeoniEe Ianellae orlenred ar It2ol n7 - Fee Mn)/ lL2 5', and lI2 5'respectively These appear to represenr a second 1.6626+ 0.1297[(Fe* + + episode ot exsolution at loaer renpera!ure, slmilar Eo second and third sets of "0CI" ramellre observed 1n sone igncous au8ires (Ross, Robinson, (Mg*FeP*fFe"**Mn)1. and l.ffe, r972). to measure the gamma index because grains that lie parallel to the optic plane (010) yield, in ad- the extensive substitution of Alrv, {lvr, pss*vr, Tivr, dition to the gamma index of refraction: (1) maxi- and NaYrrr in augites, and a plot of published data mum relief between exsolution lamellae and host. (Fig. a) shows a considerable scatter of points. (2) maximum exsolution angles, "001" n c and Electron probe and optical data obtained during "100" n c, and (3) the relative orientation of the this study show much less scatter and a new curve Z-vibration direction with respect to exsolution relating iron-magnesium ratios with gamma indices lamellae and the c-crystallographic axis (see Jaffe, of refraction was constructed (Fig. 4). This was Robinson, and Klein, 1968). Neither B or y, how- used to obtain the iron-magnesium ratios listed in ever, yields accurate iron-magnesium ratios due to parenthesesin Table 1 PIGEONITE EXSOLUTION LAMELLAE IN METAMORPHIC AUGITE l3

pigeonite lamellae parallel to the surface. In all three examples (Fig. 2), the chemical zoning trend of augitelies on the Fe-rich side of the augite-ortho- pyroxene tie line. This suggeststhat pigeonite lamellaeare more Fe-rich than the coexistingortho- pyroxene,but does not rule out the possibility of primary zoning, which is supported in the case of 447 by some variation in the index of refraction (1.700 to L.704). In this casethe highestindex i'r??3, Go-4 correspondingto the highest Ca content was em- ployed for optical determinativework. Keeping in mind the uncertainties discussed above, lhe cas values from probe analysis of co- '""-o ro 20 30 40 50 60 70 80 90 roo existing augite and orthopyroxenecan be used to estimatetemperature conditions of exsolutionon the loo (Fe2*+Fe3* + Mn)/(Mg+ Fez*+Fe3t + Mn) basis of experimentalwork in the pyroxene quad- Ftc. 4. Variation of the 7 index of refraction with the rilateral, particularly the 810'C isotherm (Fig. 2) ratio 100 (Fe* Fe"* Mn)/(Mg Fe2* + + f + Fes* + Mn) of Lindsley, King, and Turnock (1974). Un- in augites coexisting with orthopyroxene. Data from Hess (1960); Deer, Howie, and Zussman (1963); and present fortunately,unlike the lunar specimensevaluated by study (labelled points). Line is both visual and least squares these authors,only four of the specimensreported best fit to data points of present study, excluding T65. here contain lessthan 3 wt percentof "nonquadri- Equation for line is: lateral" components.Nevertheless, the plottedtrends - n7 1.6977+ 0.0669[(Fe* + Feil + Mn)/ of the Adirondack specimensin Figure 2 arc gen- (Mg+Fe',*1Fs"**Mn)]. erally parallelto the experimentalsolvi and suggest temperaturesof equilibrationlower than 810"C. Composition ol Coexisting Pyroxenes As is to be expected,the orthopyroxenegenerally Electron probe analyses and structural formulae has a higherFe/Mg ratio than the coexistingaugite. of coexisting pyroxenes are listed in Table 2 and Distribution of Fe and Mg between the coexisting presented graphically in Figure 2. In doing the pyroxenesmay be describedin terms of a distribu- analyses an effort was made to avoid putting the tion coefficientKD (Kretz, 1963) wherc electron beam on large concentrations of exsolution -A Opx ug (Mg/Fe + Mg)oo*.(Fe/Fe + Ms)o'" lamellae. When the beam was deliberately aimed at KD _ concentrations of lamellae, the analysis usually gave Mg- Fe (Fe/Fe * Mg)o"- 1Mg7Fe + Ms)""* some intermediate composition along the tie line formulation was employed in calculating connecting augite and orthopyroxene compositions. This simple values of Ko Table 3. More elaborate formula- Thus, in general, analyseswith maximum and mini- the in of full site occupancy mum cd8 values were considered to represent the tion involving consideration (Saxena, 1971) was rejected at this time because compositions of the augite and orthopyroxene, re- of uncertainties in site assignmentrelated to analyti- spectively, on which the optical measurements were knowledge of the made. Exceptions to this occurred in augites 447, cal uncertainties and inadequate oxidation state of Fe. T65, and A21 (Fig. 2) in which analyses on aI> by specimen parently clear grains yielded a range of ca\ values The effect of Fe3* on Kn is illustrated oxidized rock in between distinct upper and lower limits. The A21, which is probably the most titano- analyses,with low ca3 values of 38.7, 39.1, and the suite, in that the pyroxenes coexist with purified augite 37.0 respectively, can be interpreted in two ways: hematite. Wet chemical analyses of (Ashwal, yield (a) they represent distinct primary chemical zoning and orthopyroxene separates 1974) 0.085 re- in these metamorphosed igneous augites, or (b) ratios of Fe1+fFe2+* Fes* of 0.333 and (Table they represent analyses of places on the polished spectively, and a much lower Kn value 3). made from electron surface of the augite crystal occupied by thin Estimates of Fes* content can be probe analyses, by summing the formula to four 'ca = lMa/(Ca * Mg f Fe * Mn) cations, but this method is heavily dependent on t4 IAFFE, ROBINSON, TRACY, AND ROSS

Taers 2a. Electron Probe Analyses of Coexisting Pyroxenes

447-1 .r52 441-2 T65-r a65-2 A2l-I A2L-2 Jo-8 Go-4 J223 Po-l Ca-6 Ca-I7 Po-I3 AUGITE sio^ 51 18lt 52.40il 47.6b* 47.17 * 48.6I* 51.41il sZ.rTtl 49.88* 49.66+ 50.29t| 48. 95* 49.04t 48.72* 47,17* 48.18* 47,53* Tio^ .31 .J6 .36 1.61 r.66 .31 .10 .18 . t8 .02 .2t .r7 .16 .14 .08 ,21 [-o" 3.61 4.]I 6.97 6.28 5.\2 2.27 1 99 2.77 2.44 1.39 2.77 2 50 1.E6 r.24 .89 1.08 cr^o" .98 .63 ,29 ,09 .I5 .o7 .o2 .46 ,62 02 .44 33 ,18 .07 .00 , tl Yso 15.27 17.51 I 4 98 13.r3 14.81 11.63 15.05 12.76 12.63 12 96 11.12 I0-42 8.31 3, 11 2.2r r,23 Nio .23 , t4 .08 ,18 .I8 .I8 zno ,24 .29 ,46 Feo 1.81 5.b8 7,70 1.2L 10.12 8.35 rt.47 to.92 12,52 72,49 14.01 16.L2 19.51 26.04 27.47 30.39 Ilno .18 .09 .32 .19 ,25 -23 .24 ,o .58 .31 .35 .44 Cao 22,52 1a.22 2L 10 22.45 18.4 3 2t.29 11.72 22,L5 2r.56 21,1l 2r,62 20.24 r9,91 19.68 19.24 18.29 BaO .01 ,03 .03 . 02 .04 .03 Na"0 .91 .90 .67 .68 .65 .89 . 85 55 . 68 .14 .77 r.05 .85 .78 .11 .57 l("0 .01 .00 .00 .o2 .03 ,02 .02 .04 .00 . 00 .00 Total 99.t L 99 90 IOO.l1 98,80 99.79 98 42 99.5r 100.15 r00.82 99.r7 100.36 r00.40 r00.33 98.77 99.44 100.3r

ORTHOPYROXENE

s 102 53,72* 5r.37* 50.98* 52.5511 49.92* 48.97* 50,731j 48.17* 50.27* 48. 05# 44,92* 45. 05* 45.r1* Tl.o2 .08 .I4 36 .03 . t0 lt .0r .t2 .09 ,06 .10 Izo: 4.25 1.20 1. 59 r. 05 1.06 r. 60 1.18 ,69 .38 ,26 .47 ct2o3 .10 05 .07 .03 . 00 .16 ,00 .15 .O4 . 00 .05 .07 .09 Mco lr. 28 28.09 26 45 24.00 19.83 18.06 r1,20 15.97 14. 50 10.43 3.44 2.34 l. 35 Nio .04 -28 .05 zno .45 .56 .62 FeO 9.16 14.40 16 19 20.63 28.08 30.36 28 81 32,69 33.96 38.65 44.32 49.40 50.14 MnO .32 .35 2L .74 .58 .87 1 88 50 ,41 r,03 .a2 .83 I 30 CaO 1.06 r 21 1.11 -53 .60 61 74 ,66 .68 .68 .12 .15 Ba0 .00 . 00 .02 .00 '04 Na .r0 00 2O I2 00 .04 .06 0L .06 .00 .01 ,00 .00 08 K2o 03 00 .00 .o2 .02 . 04 .00 .00 ,00 0{ Total 98 21 100.48 99,75 99.64 100.75 r00.54 100.37 100,13 r0r.1r 99,60 99.16 100 35 roo.14

'R J. Tracq, analgst; on MC uodel 4OA ekctron probe, using sxandard Bence-Albee correctjon pracedurcs, at lrstituae af Materiafs Science, unjversixg of Connecticut, Starrs' except T65, Opx Ca-6, Po-13, {-5, Po-17 at Deqrtrent of Earth and Planetarg Scjerces. dassac}usetCs Insxitute af rechnoloqu, #Nelson Eickling, analgst; on ARL-EMX electron probe, osing standard correction pracedures, at U,S. Gealogical Surveg, Washington, D C.

T,rst-p 3. Distribution Coefficientsof Fe and Mg in the accuracy of the SiO2 analysis and was not used Coexisting Augite and Orthopyroxene for this paper. It thus appears likely that much of the scatter of Sanple Fe / F e+MB KD Sanple Fe/ Fe+Mg KD Kn values in the range 0.577 to 0.671, (specimens

Aug ,123 Aug .324 Po-17) is a function of analytical diffi- 447-1 'd)4 Jo-8 Jo-8 through .t4l oPx oPx .441 culties. They are, however, all lower than the gen- Aug Aug 447-2 1.roe Go-4 qon eralized value of Kr : 0.690 suggested for the opx :i;i oPx 810'C isotherm by Lindsley et al (1974), and 147-l Aug , 123 Aug .351 J223 ')/l tt47-2 Au8 opx .1484 hence consistent with equilibration at a lower tem-

't52 Aug .224 Aug .414 perature. On the other hand, the extremely high 17L Po-1 . olq opx opx , )J) values of KD exhibited by specimens 447-1, T52,

Aug Au8 .465 T65-I .as'/ Ca-6 . oor and T65-1 appear to be related to very high Al2O3 oPx ?19 opx andfor TiO2 content in augite, possibly combined '165-2 Aug .277 Aug .568 Ca-I7 oPx oPx with higher temperature of formation of these ultra- mafic rocks. The still higher values for 447-2 and T65-l Aug Aug ' 806 ' oul T65-2 Aug .'rli oPX .aai T65-2 are related to the low ca' values of these

Aug .256 Aug .874 augite analyses.If the interpretation is correct that A2l-1 ,/r) sc-6 ' )60 oPx oPx ,g22 these low Ca analyses are due to pigeonite exsolu- Aug Aug A2l-2 .886 Po-17 tion lamellae, then they may be combined with the opx .\;; oPX .954 high Ca analyses to yield a Kn which might be A21-1 A"g .807 A2I-2 Aug .'r33 interpreted as that between augite and pigeonite during the exsolution (Table 3). A2L Aug ,L94 q

Tanr-r 2b. Formulas Per Six Oxygensfor Coexisting Pyroxenes

't52 ca-Ll Po-I3 441-2 T65-1 T65-2 Jo -8

AUGITE 1. 931 I 880 t. 895 r.gr4 L.946 r.978 1.962 si 1, 909 1 909 r,116 r.784 7.822 1. 948 1. 955 1,891 r . 884 . 120 . r05 .086 , 054 ,O22 . 018 l.r .091 .091 .224 216 . r78 .O52 .045 . 109 .109 . 063 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2,000 2,000 2.000 2.000 1.993 1.994 2 . 000 .005 .009 . 006 . 021 .014 Ar .066 .086 .082 . 064 .048 .049 . 043 .015 . 006 .005 . oo5 . oo4 .002 . 007 ri- 009 .0I0 . 010 .045 .046 .009 .003 . 005 005 .001 .013 .010 .006 , 002 .004 cr" .O29 .018 .009 . 002 .004 . 002 . 000 ,014 .019 .001 ,637 .600 ,487 ,191 .136 '075 Mg 839 .951 .832 .740 .827 .769 .841 .72r .7r4 .142 . 006 .006 Zn.OO7 '009 .014 Ni .007 .004 ,o02 .006 .52r ,64r .899 .944 1.049 Fe- .1I8 ,I73 .240 .228 .3r7 .264 .359 .346 ,397 .401 ,450 .009 ,019 .01r ,012 '015 Mn- .006 .O03 .010 .005 ,007 . 007 .008 .009 .013 ,O24 .008 .890 .838 .838 .870 .847 . 809 ca .890 .7II .842 . 909 .739 .859 .711 .900 .a77 .869 .000 . 00r .001 Ba .001 ' 00r . 001 .057 .079 .065 .062 .058 046 Na .066 .064 ,048 .O49 .046 .065 .062 ,040 .050 ,010 .000 ,001 K .00r .001 ,00r 2,072 2,O19 # zn52 2.us zn, 2.023 2.016 2.082 2.042 2.O34 2.024 2,027 2.056 2379 2n48 41.8 46.9 57.s 82.7 87 5 93'4 fe** I2.9 15.6 23.1 24.O 28.1 26.1 30.4 33.0 Jb. ) 44.8 42.6 42.2 44.L 43.7 4L 5 call{l 48.0 38.1 43.8 48.3 39,1 45.3 37 0 45.5 43.8 42,1

ORTITOPYROXINE r.959 r.962 I .954 r. 982 r,961 si t.924 1.846 r 953 1. 908 1. 9C6 1. 959 1.901 .041 ,033 .019 .013 -O24 AI ,076 .r54 .143 .o47 ,o72 .048 . 041 -074 2,000 1.995 r 973 1 . 995 1. 991 2-000 2.000 2.000 2.000 r.980 1.954 2,000 1.975 .0rl AI ,018 .046 .039 ,006 .007 ,002 .003 .004 .004 .002 .004 ,009 ,001 .003 . 003 .000 .004 .002 ,000 .O02 .003 .003 Cr- 003 . 001 .001 . 00r .005 .005 ,635 .223 ,150 .088 r,670 1.504 1.436 1.329 r,r29 1, 048 .990 .939 .a42 Zn 'OI4 .0r8 . 020 Ni . 004 .004 . 001 .009 .002 1,107 L.320 1,758 r.718 L.826 274 .433 .493 .641 ,898 . 988 .930 r.079 .036 . 030 .030 . 048 Mn- :010 ,011 .005 .023 .019 .029 ,062 017 .0r2 ,o21 .010 0t2 .033 .035 Ca 04r .o47 ,042 .018 ,o22 .O25 .028 .031 Ba .000 .001 . 005 .001 .006 Na .007 . 008 .003 ,005 .001 .002 .001 .001 . 001 .o02 2.003 2,024 2.062 2.016 2.O32 2.030 2,050 2.033 2.Or9 2,076 2.113 2.018 2.085 58.1 88,9 92.3 95 5 22,8 25.1 3l 3 44.a L9,2 50.1 53.9 57.r r 1.5 r.6 L.7 1,8 2.O 2,3 2.L r.0 1.4 1,5 3

** fe = I00(Fe+Mn)/(Fe+MnfMg) //// ca = 100 cal(Ca+re+Mn+Ms) @ Last three values in lhis rou are for Zn

concerning the behavior of several other elements. axes are parallel or nearly parallel to b and c, Mn is concentrated in orthopyroxene over augite, respectively,of the host augite. These two pyroxenes as is Zn in the three specimens analyzed. Cr, Al, thus are orientedwith their (010) and (100) planes and Ti, with rare exceptions, are strongly preferred parallel to those of augite. The "001" pigeonite q in augite over orthopyroxene. Na, without exception, (Fig. 5C) is oriented with its and b axes parallel follows Ca in augite. or nearly parallel to a and b axes respectively, of the augite; the (001) and (010) planes of both Exsolution Lamellae in Augite phases being parallel. A slight divergence of the a or c axes of host augite and exsolved pigeonite (0.25 C ry st aIIograp hic O rientatio n to 0.45o), due to lattice rotation about the common The relative lattice orientations of host augite and b direction, was observed in four specimens (Table the three types of unmixed pyroxenes, (100) ortho- 4), and is discussedbelow. pyroxene, "100" pigeonite, and "001" pigeonite, X-ray single crystal precession photographs were were determined by single crystal precession pho- taken of eleven representativespecimens from a wide tography to an accuracy of i0. 1 to 0.2 degrees. range of compositions from which lattice parameters, Orthopyroxene and "100" pigeonite (Fig. 5A and relative orientations, and relative amounts of lamel- 58, respectively) are oriented so that their b and c lar phaseswere determined (Table 4)' The low con- l6 IAFFE, ROBINSON, TRACY, AND ROSS

Dauo gAUG tent of pyroxene lamellae in most of the sDecimens Section bopx Seclron gPIG necessitatedlong exposuretimes, in some casesup Itoc -Ltoc to 2OO hours before reflections of lamellar phases could be satisfactorily observed. In spite of this, in the more magnesian specimens (447, T52, AZl, ',001" Jo-S, Go-4, Po-l), in all of which both and "100" pigeonite lamellae were detected optically, ,ooPx oiue,oirc only the "100" pigeonite lamellae were sufficiently -r^ uoAnG abundant to yield a measurable diffraction pattern. "AUG Similarly, three of the specimens (A21, J223, Ca-6) lroce (lOO) showed no trace of orthopyroxene (100) lamellae F4g6;toe"phose although such lamellae could be seen optically. In nearly all casesthe lamellar pyroxenes appear to have an identical b dimension to the host augite, or at least so close to the host that it cannot be separately resolved. The single exception is speci- men Po-13 in which the b dimension of ortho- pyroxene lamellae is slightly smaller than b of the host. In those single crystals in which lattice pa- rameters for both "001" and "10O" pigeonite lamellae were obtained (J223, Ca-6, Ca-17, Po-l3, X Po-17), the two sets of lamellae may have slightly +trsq difrerent a, c, and F values. It is not known whether these differences are due to differences in chemical composition, or represent physical con- tot' ot'n l*rn J"orn straint on the parameters of the lamellae by the A. B. host. In pyroxenes from high temperature lunar Seclion ll lo (OtO) Seclion ll to (OtO) basalts,Ross, Huebner, and Dowty (1973) noted that the "001" lamellae within host augites and host pigeonites have their a dimensions constrained to a of the host, whereas the "100" lamellae have their c dimensions constrained to c of the host.

M or pholo gical O rientation All of the unmixed pyroxenes in augite grew as lamellae 0.2 to 3.0 pm thick. the contacts of

Frc. 5. Crystallographic, morphological, and optical orientations of exsolution lamellae (stippled) in augite. A. Augite with (100) orthopyroxene lamella. dtre A aopx - 16', Doro A Dopx = 0", ceuc I coex - 0". Phase boundary of orthopyroxene lamella is parallel to (10O) of augite. B. Augite with "100" pigeonite lamella. aeuo A duc I3', b-oon 71 berc:0o, cero /\ cerc t 0'. Phase boundary of pigeonite lamella is parallel to 6, but at an 8" angle to c of augite ("100" I ceuc= 8.). C. Augite with "001" pigeonite lamella. aeuc A apls - 0", Deuc cauG A c. Drro = 0', c.rucA cpro - 3o. Phaseboundary of pigeonite lamella is parallel to D, but at a 115. Sectionll to (OtO) Section lt to (lOO) angle to c of augite ("001"ncerc-115'). PIGEONITE EXSOLUTION LAMELLAE IN METAMORPHIC AUGITE t7 which are referred lo as phase boundaries. The lamellae on (010). In examples considered in this morphological orientations of the phase boundaries paper, the angle between the trace of the "100" of the three types of exsolved lamellae were de- lamellae and the a axis of the host, and the angle termined by optical examination of single augite between the trace of the "001" lamellae and the c crystal fragments in immersion oils. Orientations axis of the host is greater than the B angle of are most successfully measured in grains oriented augite. Both sets of lamellae thus lie "in the acute In with (010) parallel to the stage and immersed in angle B" (Robinson et al, 1971, Fig. 5-1, 5-4). an oil of refractive index close to that of the host. some amphiboles and pyroxenes these angles are In this position the lamellae are viewed edge on, less than the B angle of the host so that the lamellae indicating, within the accuracy of optical measure- lie "in the obtuse angle B" (Robinson et al, 1971, ments, that they are parallel to the D axis of the Fig. 5-3, 5-6; Ross, Robinson, and Jaffe, 1972).ln augite host. Because the phase boundaries are grains lacking both "100" orthopyroxene lamellae oriented parallel or nearly parallel to b of the and {110} cleavagetraces, the angle between the augite, their absolute orientation may be defined "100" and "001" lamellae ("1.00" A "001") may by the angle between the trace of the lamellae as be measured as a separate but related angle. seen in the (010) plane and the c direction of the to ComPosition augite. These angles are designated "100" A c for Relation "100" pigeonite,and "001" A c for "001" pigeonite. The exsolution features under consideration were Orthopyroxenes (Fig. 5A) are oriented so that first studied in augites of intermediate Fe/Mg ratio the phase boundary is always p,arallelto the (100) from the Hudson Highlands (Table 1) with "001" plane of the augite. The "100" pigeonite lamellae angles of 114-116" and "100" angles of 6-10' possessphase boundaries that deviate from being (Fig. 68). Pyroxenes of similar composition from parallel to (100) of the host by 0 to 22" (Table 4). the Adirondacks showed similar or slightly larger An example of "100" pigeonite with "100" A c = angles (Table 1). With increasing iron content in 8o is shown in Figure 58. The "001" pigeonite the Adirondack specimensthe angles of both "001" lamellae have phase boundaries which deviate from and "100" lamellae decrease (Fig. 6C, 6D) until the (001) plane of augiteby 5 to 17o ("001" A c = for the most iron-rich specimen obtained, an augite 111o to 123", Table 4). An example of "001" for which le = 93.5, the "001" angles are l11o pigeonitewith "001" A c = 115'is shown in Fig- and the "100" pigeonite lamellae are parallel to the ure 5C. Where "001" pigeonite is in an orientation c axis. Since the most magnesianAdirondack pyrox- so that the phase boundary is parallel to (001) of ene pair has augite le : 33, more magnesian under augite, "001" A c : B augite. pyroxene pairs, believed to have formed Measurement of the angles of exsolution lamellae ti-ilat conditions, were sought and found in the was accomplished to a precision of f 1o on crystal Belchertown and Cortlandt Complexes (Table 1). fragments oriented with (010) perpendicular to the The most magnesian and most dramatic of these is optical axis of the microscope. Such grains are also specimen 447 (Fig. 6,4,) with angles of 122" and most suitable for measuring the gamma index of 22o and a total angle between the two sets of refraction and the angle between the Z vibration pigeonite lamellae of 144". direction and the c axis. The c direction of augite The relations between the composition of augite could usually be found by observing either the and angles of pigeonite exsolution lamellae in all summarized in trace of the { 110} cleavageor the trace of the ( 100) augite specimens from Table 1 are orthopyroxene exsolution lamellae. Figure 7. The two insets in each part illustrate The orientation of the phase boundaries of the schematically typical patterns involving the two iron- "001" pigeonite exsolution lamellae is defined by pertinent sets of lamellae for magnesian and the obtuse angle ("001" A c) measured between rich compositions. Figure 7 shows moderate scatter the c axis of the host and the trace of the lamellae and some interesting difterencesbetween the Hudson on (010). The orientation of the phase boundaries Highlands and Adirondack specimens, and among of the "100" pigeonite exsolution lamellae is de- the Cortlandt specimens;these may represent either fined by the acute angle ("100" A c) measured differences in composition or differences in condi- between the c axis of the host and the trace of the tions of formation. Overall the correlation between 18 TAFFE, ROBINSON, TRACY, AND ROSS

--+O r' I I I *I

a

FE I3 50A^ t22-22 FE 36 50Atn

FE 57.5 l125-3 FE72 50 Lrm 50an

Flc. 6. Tracings from photomicrographsof augiteswith different iron-magnesiumratios showing patterns and anglesof exsolution lamellae. Scale and directions of augite a and c crystallographic axes indicated. FE indicates value of the ratio 100 (Fe'* * Fes+* Mn)/(Mg * F"t* * Fet* * Mn) from Table 1. A' Specimen447 from Belchertown Complex. Crystal twinned on (100) with single orthopyroxene lamella on twin plane. Thin and thick stubby lamellae are pigeonite "001" lamellae at 122" to the c axis. Thin abundant lamellae forming herringbone pattern acrosstwin plane are pigeonite "t00" lamellaeat22o to the c axis. (D axis is nearly normal to plane of paper, but slightly misoriented,so that anglesshown are slightly lessthan maximum observedangles of 122" and 22o.) B. SpecimenJ223ftomHudsonHighlands(originalphotomicrographisFig.lofRobinsonetal, lgTl).Coarsevertical Iamellae are (100) orthopyroxene. Thick stubby lamellae are "001" pigeonite at 7160to the c axis. Very thin lamellae are "100" pigeoniteat 6o to the c axis. C ' SpecimenCa-17 from Adirondacks. Vertical lamellaeare (100) orthopyroxene.Thick stubby lamellaeare "001" pigeonite at ll2.5o to the c axis. Thin lamellae are "100" pigeonite at 30 to the c axis. D. SpecimenGo-2 from Adirondacks. Thick vertical lamellae are (100) orthopyroxene. Thick tapered lamellae pigeoniteat 111.5oto the c axis. Very thin lamellaeare "l00" pigeoniteat 0.50to the c axis. composition and angle of exsolution lamellae is plotted against the composition of the coexisting excellent. Indeed, in routine thin section petrography orthopyroxene. This has some practical value be- reasonably careful measurements of exsolution cause the composition of orthopyroxene can be lamellae in metamorphic augites can be applied to more accuratelyestimated from optical data. It can Figure 7 to yield rough estimatesof Fe/Mg ratio. be justified only to the extent that the composition Figure 8 is similar to Figure 7 but shows the of the coexistingorthopyroxene resembles the com- angles of pigeonite exsolution lamellae in augite position of the pigeonite lamellae that participated "oor"Ac

oJ O.' o a,. ,:, ,:, a t.a o o a.ooooo

I "loo"nc

T I

o"tni i ao o a

T I

I

.q BU a

a o' o

loO (Fez'*Fe3' * Mn)/(Fez'+Fe3'*Mn * Mg )

Frc. 7. Angles of pigeonite exsolution lamellae in metamorphic augitesplotted against 100 (Fe* + Fe"* + Mn)/(Fe" * Fe"* * Mn * Mg) of augite determined by electron probe analyses(large symbols) or by measurementof the gamma index of refraction (small symbols). Insets to left and right show some exsolution patterns of Mg-rich and Mg-poor augites re- spectively.Closed circles: Mt. Marcy area, Adirondacks. Open circles: Monroe quadrangle, Hudson Highlands. Closed squares: Cortlandt Complex, New York. Open squares: Belcher- town Intrusive Complex, Central Massachusetts. IAFFE, ROBINSON, TRACY, AND ROSS

there will be planes of dimensionalbest fit between the lattices that in nature tend to form the boundary between them. In the case of simple monoclinic latticeswith identical b dimensionsand fairly similar a and.c dimensions,and B angles,very slight rela- tive rotations about the D axis will produce optimal phase boundaries that are perfect with respect to "roo"n c the dimensionalposition on the boundary surface of equivalent points in the two lattices (Robinson *:|. et al, L971,).The orientationof theseoptimal phase ".. boundaries can be calculated with reasonableac- [.r curacy from any pair of lattice parameters using simple trigonometric equations programmed for computer (check Robinson,Jaffe, Ross, and Klein, Erratum, 1,971). "roo"n"oor" The qualitative effects of relative variations in the c and c dimensionson the orientationof "001" and "100" lamellaehave been coveredpreviously (Robinsonet aI, l97l). Basedon computercalcu- lations, the quantitative effects of variation in a and c dimensionsand of the ,B angle are shown in Figure 9. It should be emphasizedthat the absolute values of the parametersare much less important than their differencesor misfit (symbolizeda). If c and F are held constantat 5.27 A and 1O9ofor augite and 5.22 A and 105" for pigeonite, the angle "001" A c for "001" lamellae increases markedly with aa (Fig. 9, left) whereasthe angle '(100" loo(Fe2** Fe3*+ Mn)/( Fez*+Fe3' + Mn + Mg ) "L00" A c for lamellaehardly changesat all. If a andB are held constantat9.76 A and 109ofor Ftc. 8. Angles of pigeonite exsolution lamellae in meta- augiteand at 9.68 A and 105' for pigeonite,how- morphic augitesplotted against (Fe* 100 * Feq + Mn)/ ever, "100" A c changesmarkedly with ac but (Fe'* + Fe* + Mn + Mg) of coexisting orthopyroxene. Plot is justified to the extent that orthopyroxene mimics "001" A c doesnot (Fig. 9, center). For a and c tho composition of pigeonite that was involved in the held constant,the anglesof both setsof exsolution exsolution process. Symbols same as in Figure 7. lamellae-that is. "100" A c and "001" A c- increasemarkedly as AnBdecreases (Fig. 9, right). This last effect was not emphasizedin our previous equally with augite in the exsolutionprocess. The work, but its geometricsense may be deduciblefrom pigeoniteand orthopyroxenelamellae observed in studyof Figure4 of Robinsonet aI (1971). the present study are much too thin to analyze Figure 10 shows,for analyzedaugites, the Aa's, quantitatively with the electron probe. In the one Ac's, and ,ArB'sfrom the lattice parameters (Table case from the Bushveld Complex where probe 4) plotted againstthe iron-magnesiumratios. De- analysescould be made of both hosts and lamellae spite some scatter, the generalpicture is clear. The (Boyd and Brown, 1969), host orthopyroxeneand magnesianpyroxenes have larger angles of exsolu- pigeonitelamellae in augitehave fairly similarcom- tion lamellae because they have larger a and c positions. misfits and also because they have smaller AB's (see Smith, 1969, p. 23). Even without the lattice Calculated Optimal PhaseBoundaries parameterspresented in Table 4, this was deduced According to the principle of "optimal phase by calculating "best fit" exsolution angles for the boundaries"(Bollmann, 1970; Bollmann and Nissen, synthetically matched pure end member pairs di- 1968), when two latticesare superimposedin space opside-clinoenstatite(CaM$SirO6-MgzSizOo) (127o, PIGEONITE EXSOLUTION LAMELLAE IN METAMORPHIC AUGITE 2l

5 278 - ^^9 -<-

...O'

o o2.o4 06 08 lo o 02 04 06 08 lo o | 2 3 4 5 6 A or8' A c,A, APet

Frc. 9. Quantitative effect of variation of differencesrn a, c, and B (La, Ac, AB) on the anglesof "001" and "100" pigeoniteexsolution lamellae.Pairs of values at double circle denote representative absolute values of the indicated parameter (a at left, c at center, B at right) for augite (upper value) and pigeonite (lower value). These values also were the ones held constant when varying a parameter in each of the other columns. Left column: efiect of varia- tion of "a misfit" (Aa), with c and B of augite and pigeonite held constant at 5.27 and 5.22, and at 105" and 109', respectively.Center column: effect of variation of "c misfit" (Ac), with a and P of augite and pigeonite held constant at 9.76 and 9.68, and at 1O5" and 109'. Right column: effect of variation of AB, with a and c of augite and pigeonite held constant at 9.76, 9.68, and 5.27, 5.22, respectively.

22') and hedenbergite-clinoferrosilite (CaFeSirOu- are calculated: from pigeonite "100" parameters, Fe2Si2O6)(1I7", 4" ) (Robinson et al, l9Tl, Table "100" actual and "001" hypothetical; from pigeonite 3, Nos. 1 and 3). It is dramatically shown in plots "001" parameters,"001" actual and "100" hypo- of a and c dimensions for synthetic pyroxenes in thetical. It will be noted that for the many specimens the Di-Hed-Clinoen-Clinofs quadrilateral (Turnock, in which only pigeonite "100" lamellae were sum- Lindsley, and Grover, I973). ciently abundant to obtain lattice parameters, the The results of calculations of optimal phase calculated hypothetical "001" orientations are close boundaries are given in Table 4. For each pair of to the optically measured orientations of "0O1" lattice parameters, for example, augite host and lamellae. pigeonite "100" lamellae, orientations are calculated Figure 11 gives a comparison of calculated and both for "100" lamellae and also for hypothetical observed angles of exsolution lamellae in the pyrox- "001" lamellae (in parentheses) with identical lat- enes in Table 4. Despite some scatter the correlation tice parameters. For augites for which lattice pa- is excellent and gives very strong confirmation that rameters have been measured for both pigeonite the lamellae did indeed f;orm on optimal phase "001" and "100" lamellae, two pairs of orientations boundaries. Slieht differencesbetween observed and 22 JAFFE, ROBIN.'ON, TRACY, /ND ROSS

TasLe 4. MetamorphicAugites and Included ExsolutionLamellae: Lattice Parametersfrom X-Ray SingleCrystal Photographs, Calculated Optimal PhaseBoundaries, and Calculated and ObservedLattice Rotations

i'IOO" "001" LAMELLAE HELLAE Dlff. ,. Rotatlon** .- Roration#// Aug fron Sanple fe* ,1"B Angle Angle Calc. obs. Angle Angle CaIc. Obs. 12 9 Auglte host (90%) 9.726 8.874 5.251 106.1ro Pigeonite "100rr (I0Z) 9,62I 5,193 r08.6r' (1.60)#(165'' (r22,6"') r22" (,42') orthopyroxene(100) (tr)

23 1 Ausire hosr (802) 9,138 8.882 5.275 106,030 Plseontce "100" (202) 9,644 5.205 108.62' (r.86) (14.s') (\2a.6"' r22.5o (,39') 4.65 19.0' 19.5' .45' .45+.L' orthopyroxene(100) (r!)

26.1 Augite host (902) 9.748 5,262 tO5 95' Pi.geonire "100" (t0Z) 9.659 5 2I4 108,73" (2.30) (12.0') (r17.9") 120' (.29") 7.91 12.0' 12.5' .29' .28+.05"

Jo-8 33.0 Ausite host (802) 9.j43 LgI4 5,255 105.950 Piseonlte "I00" (52) 9,649 5.2r7 IO8.62. (2.12) (r2.9"') (118.8') 117' (.28') 9,61 9.9' 12' .24' .0+ r' 0! chopyroxene(100) (I5Z) 18. 28 5.222

36.5 Ausire host (902) 9.j55 5.9L4 5.249 r05,91" Piseonlte "100" (42) 9.666 5. 227 108.58" (2.34) (r1.7') (117.6') 115' (.22'' 17.50 5,7' 8' ,13" .0+.1' orthopyroxene(10C)(62) 18. 30 5.245

36'4 Augire host (902) 9,i76 B,Bgo 5.252 r05.910 Piseonlre "I00" (52) 9,694 5. 248 108.83. (2 e4) (9.s') (115.4") (.16') 111.34 .9" 5" .03' .0+.2" Pigeonite '1001" (22) 9 695 5.236 108.55" 2.60 10.7" 116.6' 116' .19' .25+.1' (24.36' G,r') (.10")

41.8 Augire hosc (8OZ) 9.759 8.936 5.266 r05.920 Piseonite "100t' (r0Z) 9.682 5,239 I08.63" (2.76) (r0.1') (1r6.0') tr7" (.20') rA 54 6.8' 7" .16' ,0+,I' orEhopyroxene(100) (102)18. 33 5.256 Ca-6 46.9 Augite hosr (9OZ) g.i49 8,gI4 5.264 106.02" Piseonite "I00" (32) 9.6jI 5.236 roa.73. (2.71) (10.3') (116.3') (.21") 13,94 7.0" 5' .16' .Gr.2" Piseonire "OOL" (1i() 9 699 5.232 r08.73. 4.32 6,6" 112.7" rr5" .14' .0+.2" (12.s0) (7.8') (.15')

57.5 Austre hosc (772) 9.770 8.943 5.256 105.48" Pigeonire "100,, (52) 9.693 5.236 r08.80" (3.s0) (8 r") (1r3.6') (.18') 24,9a 4.0' 3' .I0' .0+.2' Pigeonite "00I" (82) 9.j16 5,227 IOA.18" 4 97 5.8' 111.3" 1r2.5' ,14' .0+,2' (17.2r) (s.8') (.14') 0rthopyroxene(100)(102) 1S. 4O 5.249 Po-13 82.7 Ausite host (84%) 9.782 Lgj6 5.252 rO5.40' Plgeonire "r00" (42) 9,706 5. 256 108,50" (3.45) (8.2') (1r3.6") (.13") 100 -0.9" 0' -.020 ,0+,2' PlseoniEe "001" (82) 9,j24 5.236 108.48' 4.38 6.5' 11r.9' 1rr.5"+.r20 .0+.20 (29-s2) (3.4'I (.07" orthopyroxeoe(100)(42) I8,42 8.972 5,246

Po-I7 93.4 Auglce host (852) 9.781 9.OO2 5 245 rO5,42' (52) Piseonite "100" 9,742 5.245 108.53' (s.eo) (4.9") (110.3') (.08") - 0. 0" 0' ,00' .0+. 20 PlgeonlEe (52) "001" 9,725 5,233 108.40" 3,97 7.2" 1r2.6' I11'+ .13' .0+.2" <38.2) (2.7") (.05.) orthopyroxene(100)(52) 18,43 5.242

* comPoslEion of = auglte hosc; fe lOO(Fe+Mn)/(Fe+Mn+Mg) derernined by elecrron probe analysls (Tabre 2). // Parentheses ,'hyporhetlcal indlcare values calculated fron Iamellae,, (see text). ** Relatlve counter-clockuise !otatlon of latttc€ of p1geontre "00t" tamellae wirh reepecc ro (OOt) of augire hosr. //# Relarlve clockwise rotatlon of lattice of pigeonire "100" tanertae wirh respecr to (100) of auglte host. 1 Recent oPttcal re-examlnation of Po-I3 and Po-17 has revealed a very very fine second set of "001" pigeonire lanellae orienred ar 112" and 112.5" resPectlvely' These aPPear Eo rePresent a second eptsode of exsolurion ar lower temperatuie, slnltal ro second and ihlrd sets of "O0l', Ianellae observed In sone lgneous pvroxenes (Ross, Robinson and Jafie, 1972). The observed angles of rhese lare flne teellae of II2" and 112.5" agree nosE closely wlrh angres of 1r1.9'and rr2.6'carcurared fron lattice Daraner€rs.

calculated angles of "100" lamellae cauld be due evaluated earlier, this rotation as calculated from to changes in parameters since exsolution, particu- lattice parameters was so small, generally 0.15' or larly reduction of Ac; but at the moment this inter- less, that it could not be accurately measured in pretation is highly speculative. From the evidence precession photographs.a It was natural, then, to at hand the relative lattice parameters appear to seek evidence for such lattice rotation in the more have changed only very slightly since exsolution took magnesian specimens with large exsolution angles, place. and in this we were successful. Detection and measurement of lattice rotation in Lattice Rotation X-ray single crystal photographs is dependent on In our earlier paper (Robinson e/ al, 1971) and size of crystal used and abundance of lamellae. above, we pointed out that slight relative rotation Small angular separations of spots only show up in of pyroxene lattices allowed improvement of the photographs of small undistorted crystals. In many fit between lattices. We also presented in detail the qualitative eftect of relative parameters on the direc- 'Precision in measuring angular relationshipsfrom the tion of relative lattice rotation. In the examples we X-ray precessionphotographs is 0.05' to 0.2'. PIGEONITE EXSOLUTION LAMELLAE IN METAMORPHIC AUGITE 23

5l I "ool"n c o\ -IT tDt "'1 I 1 t- ll -l .J jo t "loo"n c '1

ol o "roo"n"oor"

lo 20 30 40 50 60 loo (Fez'+Fe3'+ Mn)/(Fe2'+ Fe3'* Mn * Mg)

Ftc. 10. Aa's, trc's, and AB's from X-ray single crystal data in Table 4 plotted against 100 (Fe% * Fe"' * Mn)/ (Fe'* + Fe"* + Mn f Mg) of augite host. Vertical lines connect data points derived from single crystals in which lattice parameters for two sets of pigeonite lamellae were loo (Fez*+Fe3'+ Mn)/(Fe2'+Fe3'+Mn + Mg) obtained. In Aa row large symbols indicate ..actual,'values derived from parameters of "001" lamellae, small symbols Ftc. 11. Comparison of optically observed angles of indicate "hypothetical" values derived from parameters of "001" and "100" pigeonite exsolution lamellae in augites "lfr)" lamellae. In Ac row large symbols indicate .,actual" with angles calculated from measured lattice parameters values derived from parameters of "100" lamellae, small for several different iron-magnesium ratios. Vertical lines symbols indicate "hypothetical" values derived from param- connect observed and calculated values for a single speci- eters of "001" lamellae. Aa and Ac are larger, and Ap is men. Solid circle: Optically observed angle of exsolution smaller with lower iron content, resulting in larger angles Iamellae. Thick cross bar: Calculated angle based on lattice of exsolution lamellae. parameters of "actual" lamellae. Thin cross bar: Calculated angle based on lattice parameters of "hypothetical" lamel- *1t" lae. Double crossbars: Calculatedangle for "001" n such crystals exsolution lamellae are not sufficiently based on combination of "actual" and "hypothetical" re- sults. abundant to give measurable X-ray reflections, hence larger crystals must necessarily be used with resultant loss of definition. For these reasons, in the majority counter-clockwise rotation of the "001" pigeonite of cases, small lattice rotations could be neither lattice of 0.25 :L 0.1o (case 1-1 ) in close agreement detected nor ruled out, but the evidence for lattice with the calculated value of 0.19'. For "100" lamel- rotation is decisive in four cases (Table 4). lae calculated rotations all fit into case 4-l of Fig- It will be seen that for "001" lamellae, calculated ure 6 (earlier paper) with relative clockwise rotation rotations all fit into case 1-1 of Figure 6 (earlier of pigeonite lattices, except for Po-13 which, by paper) with relative counter-clockwise rotation of 0.02', is in caseGl with counter-clockwiserotation. pigeonite lattices. Of the four augites in which X-ray The augites from the three most magnesianspeci- reflectionsof "001" pigeonite lamellae were ob- mens (447, T52, andA21) show relativeclockwise tained, the most magnesian(J223) showsrelative rotations(case 4-1) of 0.42 -f 0.05o,0.45 :t O.l' )1 IAFFE, ROBINSON, TRACY, AND ROSS and 0.28 t 0.05' in close agreementwith calcu- Tenre 5. Relation between Relative Lattice Parameters, lated rotations of 0.43o, 0.45o, and 0.29" respec- Irrational Intercepts,and Irrational Miller Indices of Phase tively. Thus in the four caseswhere lattice rotations Boundaries in Monoclinic Pvroxenes are large enough to be detected,they are in re- .AUG t tPrc = tPrG ' markably good agreementwith rotations calculated "-A.uc "AUG "Prc independentlyfrom lattice parameters.This agree- Int ercepts +wol l-w@I ment is a further confirmationof the optimal phase Miller Indices 1ori 00llOu boundarytheory. t = tPrc teuc ' "AUG "PrG "AUG "Prc Notation of PhaseBoundaries Int ercept s f't+"1o-1@-w In this paper we have used the notations"001" i,liI1er Indices w01 100w01 and "100" to designatethe relative orientationof the crystal latticesof the host and unmixedphase. As we have previouslystated, the phaseboundaries is the idea that pigeoniteexsolves from augiteonly of these"001" and "1.00" lamellaeare not usually under temperatureconditions where pigeonite is parallelto the (001) or (100) host latticeplane, the stable Ca-poor pyroxene above the pigeonite- 3'001" Thus,in the generalcase, the or "100" phase orthopyroxeneinversion loop. In the case of the boundaryplane must be describedas being parallel specimensconsidered here, the metamorphic Ca- to an irrationallattice plane. poor pyroxene coexistingwith augite is orthopyrox- In Table 4, in additionto the calculatedangles of ene.The paucityand fine scaleof exsolutionlamellae "001" and "100" exsolutionlamellae and the differ- in thesespecimens show that the initial solid solu- encesfrom ,B for "001" lamellae,a number w is tion was very limited as compared to igneous listed for each calculatedorientation. The value ur pyroxene pairs of similar iron-magnesiumratio, in- is the non-integralnumber that representsthe posi- dicatingthat the metamorphicrecrystallization took' tion in the first row of lattice points abovethe origin place at much lower temp€ratureswith subsequent that is common to both lattices (Robinson er a/, slight amountsof exsolutionduring further cooling. I97I, p.923). Anotherway to look at this is that There seemsno question that the pigeonite and w is the irrational intercept of an "001" phase orthopyroxenelamellae exsolvedat a temperature boundaryon the c axiswhen the interceptis 1 on the well below that of the pigeonite-orthopyroxenein- c axis or vice versafor "100" boundaries."Miller" version curve. Two hypothesesare suggestedas indicesfor the phaseboundaries may be derivedfrom possibleexplanations: (1) Nucleation and growth theseirrational intercepts. Because "001" boundaries within the monoclinic structureof the augite host may have either positive or negativeintercepts on favors metastableformation of pigeonite under a, and,"100" boundaries,positive or negativein- conditionswhere orthopyroxene is the stablephase. tercepts on c depending on the relative values (2) Pigeonite (or clinohypersthene)is again the of aao* and op1q, and clus and cplq respectively stable phase at lower temperatures of exsolution (Robinsonet ql, 1971,Fig. 5), the orientationof (Kuno, 1966; Boyd and England, 1965; Sclar, the lamellae with respectto both lattices may be Carrison,and Schwartz,1964; Smith, 1969; Brown, accuratelydescribed in termsof the irrational Miller 1968,1972; Grover, 1972). indicesas shown in Table 5. The interestedreader The hypothesisof metastableformation in the may wish to ink in the appropriateMiller indicesin augite host is very attractive.The nucleationand each of the six sectionsof Fizure 5 in the earlier growth of pigeonite lamellae, particularly "001" paper. lamellae, requires only migration of the 6- and 8-coordinatedcations without major disruption of Comment on Equilibrium Relations During the monoclinic tetrahedralnetwork that would be Crystallizationand Exsolution of required for formation of orthopyroxene. The hy- Metamorphic Augite pothesisof metastabilityallows simultaneouspre- Implicit in the work on igneousaugites of Polder- cipitation, for which there is some textural evidence, vaartand Hess (1951, p. 483), Hess(1960, p. 40), of pigeonitelamellae on "001" and orthopyroxene Preston(1966, p. I23O), Boyd and Brown (1968, on (100) where the monoclinic and orthorhombic p. 358; 1969,p. 212), and Smith (1969, p. 24) lattices have their best mutual fits respectively. PIGEONITE EXSOLUTION LAMELLAE IN METAMORPHIC AUGITE 25

Simultaneousprecipitation of pigeoniteon "100" is of optimal phase boundaries calculated from also a possibility. This explanation would also be lattice parametersof augite host and pigeonite applicableto metamorphicrocks containing coexist- lamellae. In the most magnesianspecimens rela- ing calcic clinoamphibole and orthoamphibole, in tive lattice rotations measuredfrom precession which the exsolutionlamellae in the calcic amphibole photographsare in nearly exact agreementwith are alwaysmonoclinic cummingtonite (Ross, Papike, relative lattice rotations calculated from lattice and Shaw,1969). parameters.These two facts give strong evidence A further argumentfavoring metastableformation that the pigeonitelamellae nucleated on "optimal has to do with the optimal phaseboundary theory phase boundaries" and that the relative lattice itself. Orthopyroxenecan have a good fit on augite parametersof host and lamellaehave not changed only on (100), and this only providedthe c and b a gteat deal since nucleation of lamellae under dimensionsof the two phasesare nearly identical. metamorphicconditions. This is in stark con- Pigeonite, because of its monoclinic lattice, can trast to relations observedin some igteous py- achieve nearly perfect dimensional fit with both roxenes(Ross el al,1972). "001" and "100" lamellae provided it has a b 4. A simplenotation has been devisedmaking use dimension in common with the host. Indeed, the of irrational Miller indices that succinctly de- potential for better fit of pigeonite on irrational scribes the orientation of the phase boundaries. "100" planes as comparedto poor fit of ortho- 5. Pigeonitelamellae in augitehosts coexistingwith pyroxeneon (100) could be used to explain the orthopyroxene clearly exsolved below the pi- relative abundanceof pigeonite'(100" lamellaein geonite-orthopyroxeneinversion temperature.It mostspecimens. is not clear whether pigeonite nucleated meta- In the case of the most iron-rich specimens stably in augite hosts becauseof best fit consid- (Po-13, Sc-6, Po-17), the argumentgiven in the erations or whether pigeoniteis again the stable preceding paragraph can be reversed. In these phase under conditions of low temperature ex- specimensthe fine "100" pigeonite lamellae are solution. essentiallyparallel to (100) and to coarser (100) orthopyroxenelamellae (Table 1, Fig. 7), so that Appendix: Description of Specimensand Their there is little or no misfit of the c dimensionsof . Geologic Setting pigeoniteand augite host. In this case one might Hudson Highlands ask what advantagemonoclinic pigeonite essentially The Hudson Highlands of New York consist of a belt of on (100) would haveover orthopyroxeneon (100) high-grade, regionally metamorphosed,essentially concordant, and consideragain the possibilitythat the pigeonite Precambrian gneissesand granulites that represent a north- is indeed the stable phaseunder the lowest exsolu- eastern extension of the Reading Prong and the Blue Ridge tion temperatures. geomorphic and petrographic provinces. According to Jaffe and Jaffe (1973), the gneissesand granulites of the Hudson Summaryand Conclusions Highlands represent a eugeosynclinalsequence of sedimentary and volcanic rocks that attained a largely isochemical meta- 1. In augitesof augite-orthopyroxenepairs believed morphic equilibrium under conditions of the hornblende to have exsolvedin a metamorphicenvironment granulite facies. According to these authors metamorphism pressures there is a strong correlation between the angles took place at temperaturesof 700-800oCand in the relatively conditions with PH,6 pigeonite range of 2-4 kbar under dry of "001" and "100" exsolutionlamel- considerablylower than P.o.,1. Similar estimateswere obtained lae and the iron-magnesiumratio, with the largest by Dodd (1965)and Dallmeyerand Dodd (1971)from a study anglesin the most magnesianaugites. of mineral assemblagesin adjoining terrane. Exsolution in 2. The angles of exsolution lamellae with respect pyroxenes, as well as in feldspars, of necessity commenced to the a and c crystallographicaxes are related below these maxima, although not necessarily much below these values. A further episode of unmixing in the solid state to the misfits of the a and c lattice parameters at still lower T and P conditions could have occurred in and differencesbetween the p anglesof host and Paleozoic and/or Triassic time associatedwith orogenic events lamellae. In magnesianspecimens misfits of a that mildly affectedthis region (Hall, 1968).Variation in exsolu- c p tion textures, such as irregularities in size, distribution, and and arc larger, differencesbetween anglesare *100" smaller. percentagesof both "001", pigeonite, and (100) hyper- sthene lamellae in grains of host augite, from the same small 3. The optically measuredangles of the exsolution specimen suggest that solid state exsolution may have taken lamellae are in close agreementwith the ansles place in more than one stage. 26 IAFFE, ROBINSON, TRACY, IND ROSS

J-241S. Pyroxene plagioclasegranulite, similar to I-437P, Ma-3. Similar to Ca-6. below. Ca-17. Microperthite-pyroxene granulite; microperthite J-515. Orthopyroxene-quartz-andesinegneiss, quartz 33Vo, 5OVo, orthopyroxene 75Vo, zugite 35Vo. K-feldspar 17o, andesine(An,') 55Vo,orthopyroxene 8%, lb-z. Microperthite granulite; blue megacrysts of micro- arsgite7Vo, biotite l%, magnetite 7Vo. perthite in a matrix of oligoclase (An*), orthopyroxene, J-223. Equigranular pyroxene granulite, orthopyroxene augite, garnet, magnetite, ilmenite, apatite. 30Vo, augite 507o, hornblende 5%, quartz 75%o, Po-13. Ferromangerite gneiss; microperthite 37Vo, oligo- l-437P. Pyroxene plagioclase granulite, andesine (An",) clase (An.e) 33Vo, qtartz 47o, alu9ite6Vo, orthopyroxene 65Vo, orthopyroxene l6Va, augite ll7o, hornblende 17o, 4Vo, hornblende 4Vo, garnet 77o, magnetite { ilmenite magnetite4Vo, qtartz 7Vo,K-feldspar ZVo. 3%, apatita 2%. Go-2, SC-6, Po-17, Giant, Sb-I. Similar to Po-13. A tlitotttlucl;s Belcltertown Complex The Adirondack province of northern New York con- sists of a circular area of Precambrian crystalline rocks The pyroxene-bearingspecimens from the Belchertown that occupy approximately 30,000 square kilometers (Fig. Intrusive Complex are from the extreme inner portion of 1). The northeasternquarter of this circular region contains the batholith that has largely escaped major metamorphic a heart-shapedmassif of anorthosite, intimately associated hydration during the Acadian kyanite-grade regional meta- with charnockitic gneissand minor amounts of gneissand morphism that affected the country rocks. The pyroxene- granulite of sedimentary and volcanic ancestry (Kemp, bearing rocks thus represent an igneous mineralogy that 1921; Buddington, 1939; Crosby, 1969; de Waard, 1970, has undergone a slight but as yet uncertain degree of Davis, 1971). All of the exsolved pyroxenes from the metamorphic recrystallization. The fact that pelitic schist Adirondack region described in this report are from this inclusions in the hydrated part of the batholith up to anorthosite-charnockitecomplex (Table 1, Fig. 2). hundreds of feet thick and several miles long contain Metamorphic relations in the anorthosite-charnockite sillimanite (and sillimanite pseudomorphs of andalusite) complex of the northeast Adirondacks are complicated shows that the batholith strongly disturbed the local by the coexistence of apparent pyroxene-hornfels facies thermal structure and that metamorphic hydration and contact metamorphic assemblages(Turner, 1968, p. 224, recrystallization took place at metamorphic temperatures 235, 242) such as calcite-monticellite-forsterite-augite- considerably above those of the kyanite zone. The speci- garnet-spineland wollastonite-diopside-grossularwith gran- mens are from two localities collected by David J. Hall ulite facies regional metamorphic assemblagessuch as (1973) in a regional study of geophysics and petrogaphy, orthopyroxene-augite-garnet-plagioclaseand quartz-calcite- and the mineralogy and petrology of both are being studied diopside-grossular.It is not yet known whether the former in greater detail by Lewis D. Ashwal (1974). The ex- represent contact metamorphic assemblagesformed by tremely magnesian compositions of the pyroxenes from anorthosite magma in contact with argillaceous dolomite the Belchertown monzodiorites as compared to rocks of that has survived the later regional granulite facies meta- comparable feldspar composition in the Adirondacks ap morphism or whether these assemblagesresulted from pear to be due to the extremely high activity of oxygen special localized conditions of low activity of COz that under which they crystallized, as indicated by the pres- may have existed during regional metamorphism (Walter, ence of primary high temperature titano-hematite. 1963). The extremely iron-rich orthopyroxenes reported 447. ultramafic inclusion 700 feet across de- here suggest,according to the experimental data of Smith Core of Emerson ( 1898, l9l7) as "cortlandtite;" (1971), that pressuresmay have been in excessof 7 kbar. scribed by very coarse a[gfte 5A-75Vo, pargasite lO-4AVo, and Jo-8. Anorthositic gabbro, (Annu) orange-brown biotil.e 3-l4Eo enclosing grains of augite, sub-ophitic; andesine - 57Vo, andesine antiperthite I9Vo, artgrte l7Vo, ortho- orthopyroxene l-77o, olivine (Fa', ? 1.694) SVo and rutile. pyroxene 5Vo, ilmenite f biotite I apatite 2Vo. Ph-3, IV-AI, MD-1, VI-Nk. Similar to Io-8. 113, 115, 1lO, A2l. Pyroxene monzodiorites typical of pink (An+a) Pr-l and CD. Gabbroic anorthosite; andesine (Anc), core of batholith; clouded oligoclase quartz garnet, orthopyroxene,augite, hornblende,ilmenite, mag- 4O-47Vo, orthoclase microperthite ll-l8Vo, 4- zl--8%, netite in a matrix of andesineand minor K-feldspar. l3Vo, arugite7-18%, orthopyroxene hornblende Go-4. Pyroxenite; augite 65%, orthopyroxene 25%, 0.5-2%, biotite 6-147o, titanohematite and magnetite andesine 3Va, microperthite 'Vo, quartz 3Vo, magnetite O.5-1.07o. l/2%, biorite 1/2%. Ma-5. Similar to Go-4. Cortlandt Complex Po-1. Gabbroic anorthosite gneiss;andesine (Ana) mega- The Cortlandt Complex near Peekskill, New York, is crysts l3Vo in a matrix of andesine(An*) 58%, ortho- a mafic to ultramafic layered intrusion of probable Ordo- clase 6Vo, augite 1Vo, orthopyroxene 3%, hornblende vician age, according to Long and Kulp (1962). It is 4Vo, garnel 3Vo, llmenite 4Vo, magnetite lVo, apalite 77o. intruded into sillimanite-grado schists and gneisses of the Sb-2. Sl-5. VH-l. VH-2. Similar to Po-l. northern Manhattan Prong and displays a thermal meta- Ca-6. Melagabbro granulite; andesine28Vo, microperthite morphic aureole. Ratcliffe (1968) suggestedthat the Com- 5Vo, orthopyroxene 23Vo, augite llVo, garnet 7OVo, plex was intruded after most Taconic age deformation and ilmenite l3Vo, magnetite 4Vo, apatite 6Vo. metamorphism had occurred. Tracy (1970) found features PIGEONITE EXSOLUTION LAMELLAE IN METAMORPHIC AUGITE 27 in the eastern, ultramafic end of the complex indicative of BuoorNcton, A. F. (1939) Adirondack igneous rocks and later metamorphic recrystallization such as alteration of their metamorphism.GeoI. Soc. Am. Mem.7r 354 p. pyroxenes to hornblende and biotite, and exsolution of Cnossv, Pnncv (1969) Petrogenetic and statistical implica- pyroxenes and oxide phases. This was ascribed to an tions of modal studies in Adirondack anorthosite. N.Y. Acadian reheating event, proposed by l-ong and Kulp State Mu:s. Sci. Seru. Mem. lEr 289-344. (1962) to account for hybrid K-Ar ages in the Man- DeLrrrevrn, R. D., lNu R. T. Dooo (1971) Distribution hattan Prong. and significanc€ of cordierite in paragneisses of the Hudson Highlands, southeastern New York. Contrib. T62, T58, T52, T65. Olivine pyroxenites; orthopyroxene Mineral. Petrol. 33, 289-308. 2O4OVo, augite 30-50%, olivine (Fa^) 5-35Vo. Devrs, B. T. C. (1971) Bedrock geology of the St. Regis T25. Hornblende-biotite pyroxenite; orthopyroxene 40Vo, quadrangle, New York. N. Y. State'Mus. Sci. Sero., Map angita 30Vo, hornblende 76Vo, biotite 87o, minor ser- and Chsrt Ser. No. 16, 34 p. pentine probably after olivine. DeBn, W. 4., R. A. HowrE, eNo Jlcr ZussueN (1963) Rock-Forming Minerals, Yol. 2, Chain Silicates. lohn Acknowledgments Wiley and Sons,New York, 379 p. DE WAARD,DIm (1970) The anorthosite-charnockite suite Optical work, computations, and manuscript preparation of rocks of Roaring Brook Valley in the eastern Adiron- at the University of Massachusetts and electron probe dacks (Marcy Massif). Am. Mineral. 55,2A63-2075. analyses at the Institute of Materials Science, University of Doro, R. T., Jn. (1965) Precambrian geology of the Connecticut, and at the Department of Earth and Planetary Popolopen Lake Quadrangle, southeastern New York. Science, Massachusetts Institute of Technology, were sup- New York State Mus. and Sci. Seru., Map and Chart ported by National Science Foundation Grant GA-31989 Ser.No.6,39 p. (to Jaffe and Robinson). X-ray studies were done at the Elraensow, B. K. (1898) Geology of Old Hampshire U. S. Geological Survey. Electron probe analyses at the County, Massachusetts comprising of Franklin, Hamp- U. S. Geological Survey were done by J. Stephen Huebner shire, and Hampden Counties. U.S. Geol. Suro. Mon. and Nelson Hickling, and data reduction was carried out 29, 790 p. by Mary Woodruff. Field work in the Hudson Highlands (1917) Geology of Massachusettsand Rhode (Jaffe and Jaffe) was supported by the New York State Island. U.^9.GeoI. Suru. Bull. 597,289 p. Museum and Science Service. Geological information and Gnovrn, JouN (1972) The stability of low-clinoenstatite specimens from the Belchertown Complex were obtained in the system Mei,SLOe{aMgSLOu. Trars. Am. Geophys. under the guidanceof David J. Hall and Lewis D. Ashwal. Union, 53, 539. David B. Stewart and Karen Wier Shaw provided percep- Gutunte, J. O. (1972) Geology of the northern portion tivo reviews of the manuscript. To each of the above of the Belchertown Intrusive Complex, west-central persons and institutions we express our grateful acknowl- Massachusetts.Geol. Dept., Unia. Mass., Contib. No. t, edgment. 110p. AND PETER RonrNsoN (1967) Geology of the References northern portion of the Belchertown Intrusive Complex. Guidebook for Field Trips in the Connecticut Valley ol AsHwAL, L. D. (1974) Metamorphic Hydration ol Augite- Massachusetts, 59th Annu. Meet., New England Inter- O rthopy roxene M onzodiorite to Hornblende G ranodior ite collegiate Geol. Conl., Amherst, Mass., p. 143-153. Gneiss, Belchertown Batholith, Massachusetts. M.S. Hrrr, D. I. (1973) Geology and Geophysics of the Belcher- Thesis, University of Massachusetts,116 p. town Batholith, llest-Central Massachusetts. Ph.D. Bot-rueNN, W. (1970) Crystal Defects and Crystalline In- Thesis, University of Massachusetts,110 p. te rf ace s. Springer, Berlin. Heu, L. M. (1968) Times of origin and deformation of eun H. -U. 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