Crystal-Chemical Characterization of Omphacites1

THE AMERICAN MINERALOGIST, VOL. 53, MAY JTINE, 1968

CRYSTAL-CHEMICAL CHARACTERIZATION OF OMPHACITES1

JoaN R. Cr-enr euo J. J. Parrre, U. S. Geologi,calSurvey, Wosh'ington,D. C. 20242.

ABSTRACT

The clinopyroxene, omphacite, can be defined compositionally within the system Jd- Ac-Di-He-Tsch, the chemical formula being expressed as (M2)(M1)(Si,AI)zOo; M2 represents Ca and Na, the ratio of Na/(Na*Ca) being between 0.2 and 0.8, and Jl1 l represents the octahedrally coordinated cations, Mg, Fe2+, Al, Fe3+, the ratio of AI/(A1*Fe3+) being greater than 0.5 Omphacites are found to have space-group symmetries C2/c or P2, although this distinction cannot be made by examination of X-ray diffraction powder data. The P2 omphacites have a restricted compositional range. The results of a crystal-structure refinement of California P2 omphacite show that the cation distribution is largely ordered, (Mg,Fe2+)-octahedra alternating with (Al,Fss+;- octahedra in the octahedral chains, and the larger polyhedra alternating in contents between major Na and major Ca. The presence of sufficient octahedrally coordinated AI to initiate the ordering process is considered necessary to obtain P2 omphacite. Appearance of P2 omphacites in blueschist facies at widely separated localities along the Circumpacific mountain belt suggests that these omphacites form at the low temperature-high pressure conditions considered characteristic of this facies of metamorphism.

INrnopucrroN Modern X-ray diffraction techniques can be used to obtain crystal- structure refinements of numerous rock-forming silicates, each sample being selectedfor its chemical and petrological significance.Such studies taken singly provide data on the chemicalbonding and cation distribution in individual crystals, and, when consideredjointly, give valuable information on phase characterization,which is useful in the studies of mech- anisms, energetics and kinetics of mineral reactions. A systematic re- searchprogram of this kind is in progressat this laboratory, and the subject of the present paper, one of a series,is the eclogitic clinopyroxene, omphacite. A definition of this mineral is proposed that is based on its chemical composition and removes the requirement of association with eclogite; a brief review of the occurrenceand composition of omphacites and aegirine-augitesis included. The results of single-crystal, X-ray diffraction studies of various omphacites and aegirine-augitesare presented, and a crystal-structure refinement of an omphacite from eclogite associ- ated with glaucophaneschists in California is reported. Finally, the im- plicationsof this researchfor pyroxenecrystal chemistry and for eclogite formation are considered.

1 Studies of silicate minerals (7). Publication authorized by the Director, U. S. Geologi- cal Survey. 840 OMPHACITES 841

Hrsronrcar,q.un GBorocrcer BecrcRouND Omphacitein mineralogy.The name omphar in connection with a mineral first appears in the treatise De Lapidibu.s by Theophrastus (circa 315 B.C.). In their commentary on the text, Caley and Richards (1956)sug- gest that omphar may be prehnite, or possibly chrysoprase,since the name indicatesthat the stoneresembled an unripe grape,i.e. that it was greenin color and botryoidal in its natural state; Theophrastusrefers to it as a stone from which sealsare cut. Unfortunately, there are apparently no omphar sealssurviving, and so identification of the classicalomphar must remain conjectural. Probably Werner (1812,as cited by Dana, 1892)was familiar with the classicalnames and thought omphazitappropriate to the greenpyroxene he found near Beireuth. We have been unable to ascertainwhether he believed omphazitto be the same as the omphar mentioned by Theophras- tus. At any rate, the name omphazit or omphacitehas been used by mineralogistsfor nearly 150 years to designate"a green clinopyroxene occurring in eclogitesand related rocks" (Deer, Howie and Zussman, 1963).In 1865,Breithaupt remarked that omphacite is a mineral that not enoughis known about as yet. Nearly one hundred years later, this remark was still valid, despite a large number of publications on the mineral. Other mineral names have been used for clinopyroxenes of compositions similar to those assignedto omphacites (Deer et al., 1963), such as diopside-jadeite,and chloromelanite. This complication is discussedby Hashimoto (1964), and we agree with his conclusion that retention of the name omphacite appearsdesirable (especially in view of its classical significance),provided the restriction of eclogitic associationis removed.

Omphaciteanil eclogites.Eclogite was defined by Haiiy in 1822 as a rock with the essentialcomponents "la d,iallage,le grenat, et le d.isthine" (pyroxene,garnet, and kyanite, respectively).Sometime in the next half- century, omphacite became accepted as the characteristic eclogitic pyroxene even while remaining itself ill-defined. In fact, a circular definition evolved, so that eclogite is described as the rock containing omphacite and garnet, and omphacite is describedas the pyroxenic constituent of the garnet-rockeclogite (Rice, 1954).Confusion is further compounded becausethe assemblagegarnet-clinopyroxene occurs in rocks other than eclogites,a point discussedby White (1964). Some of the problems associatedwith formation of eclogiteshave beendescribed by Yoder (1950), Yoder and Tilley (1962), Coleman et al. (1965), Forbes (1965) and Green and Ringwood (1966).We prefer not to enter controversiesconcerning 842 TOANR, CLARK AND T. T. PAPIKE eclogites; for the present paper we shall adopt the defi.nition given by Green and Ringwood (1966), which is in harmony with much modern usage of the rock name, and use the classification of eclogitesinto three groupsaccording to Colemanel al. (1965). Green and Ringwood (1966) define eclogite as " . . . a rock of basaltic chemistry consisting mineralogically of garnet (almandine-pyrope solid solution) plus clinopyroxenewith or without qtartz, kyanite, hypersthene or olivine as minor minerals. Plagioclase is absent as a primary phase from rocks strictly classifiableas eclogites and in addition the clinopyroxene of eclogites contains jadeite solid solution and a high jadeite/ Tschermak's silicate ratio (White , 1964)." As we shall show, the chemical composition of omphacite can be defined so that it is essentially com- patible with this definition of the clinopyroxene associatedwith eclogite. Colemanet al. (1965)refer to eclogitesas Group A, B or C types. The Group A eclogites are defined as "inclusions in kimberlites, basalts, or layers in ultramafic rocks...... [having] a very deep-seatedigneous or metamorphic origin." Omphacites associatedwith such eclogites are not consideredin the present paper, but will be the subject of a later study. The Group B eclogites are "bands or Ienses within migmatic gneissicterrains", and Group C eclogitesare "bands or lenseswithin the metamorphic rocks of the alpine-type orogeniczones . . . . locally forming isolatedblocks when associatedwith glaucophaneschists." Most of the omphacites considered in the present paper are from Group C eclogites but some data are also given for omphacites from Group B eclogites.

Clinopyroxeneuystal strwcture.The original determination of this structure was reportednearly forty yearsago by Warren and Bragg (1928)for diopside, CaMgSizOo,which has the space group C2f c. Other clinopyrox- eneswere examined by Warren and Biscoe(1931), who concludedthat hedenbergite,augite, clinoenstatite,acmite, jadeite, and spodumeneare all similar to diopside in structure. This conclusion remains valid insofar as gross structural features are concerned,but clinoenstatite was shown by Morimoto (1956) to have the space grotp P21fc, and spodumenehas been found to have spacegroup C2 (Appleman and Stewart, 1966).Re- cent refinements of the crystal structures of jadeite, NaAISi2O6,by Prewitt and Burnham (1966), and of johannsenite, CaMnSi2O6,by Freed andPeacor ( 1967),c onfirmC2 f c symmetry for theseclinopyroxenes. An omphacite from Group B eclogite (Warner, 1964)has C2f c symmetry, but the Group C omphacites are found to have space group P2, and a preliminary note on the P2 structure has been published (Clark and Papike,1966). OMPHACITES 843

DorrNrrroN on Orupnacrre The compositions of most omphacites, as the term is now used, can be represented within the system NaAISLOo-NaFe3+SizOe-CaMgSi206- CaFe2+SizOo.However, some omphacites have been reported to contain significant amounts of Al in tetrahedral coordination (White, 1964; Velde, 1966). Although such omphacites do not fit the definition quoted from Green and Ringwood (1966) for the eclogitic clinopyroxene, never- thelessthis feature must be consideredin arriving at a satisfactory definition of omphacite. In order to represent such pyroxenescompositionally, a fifth component, CaAI(SiAl)06, must be added and we must therefore consider the system Jd-Ac-Di-He-Tsch. We suggestthat within this system the omphacite compositional field be defined as lying between the boundary planes0.2 < Na/(Na*Ca) (0.8 and limited to the regionwhere the ratio, Al/(Alf Fe3+)>0.5 (Fig. 1; octahedrallycoordinated Al only). The portion of this volume with AI/(Al*Fer+;(0.5 is occupiedby the aegirine-augites.Considering tetrahedrally coordinated Al, the volume boundedby the planeshaving a ratio of Na/(Na* Ca) between0 and 0.2 can be divided into two regions. One region locates the aluminous diopside-hedenbergiteswith Al/(Al+Si) (0.2, and the second locates fas-

J D NoAlSi2O6 Ac NoFe!*siro"

1No No+ Co

I GoAl(Si Al)O3 FAS SAI TE

UIII{OU8 DIOPSIDE- HEDEI{BERCITE

Dt+HE Co(M9.Ff)St205

Frc. 1. Schematic view of the omphacite compositional field (shaded region) within the system Jd-Ac-DifHe-Tsch, showing its relationship to the fields for aegirine-augite, fas- saite, and aluminous diopside-hedenbergite. 844 JOAN R. CLARK AND T. J. PAPIKE

DI+ HE+ TSCH

xP 9x x a

AEGIRII{E-AUGITE

At JAOEITE AEOIRINE

A rl JO

Frc. 2. Compositions of omphacites, jadeites, aegirine-augites and some diopside- hedenbergitesplotted on the plane Jd-Ac-DitHe*Tsch. P2 omphacites are indicated by dots, others, by circles; diopsidic pyroxenes, by crosses;jadeitic pyroxenes, by triangles; aegirines, by solid rectangles; and aegirine-augites, by open rectangles. The field of P2 omphacites is indicated by dashed lines Numbers correspond to analyses in Deer et al. (1963) for jadeites, omphacitesand aegirines(aegirine-augites), respectively; other symbols as follows: C and NC, Coleman et al (1965); Co, D. H. Green and J. P. Lockwood, written communication (see Table 1); F-NG, Nicholas and Gagny (1964) F-V, Velde (1966); G. Foshag (1955);J, Hashimoto (196D;L, Klein (1966);N, Warner (1964);P, W. R. Church, written communication (seeTable 1); SA, Williams (1932). Total amounts of components other than those shown are minor. See Table 1 for analvtical data on some of these samples.

saites with Al/(Al+Si))0.2. These divisions are in accordancewith modern usage. Somenatural compositionsof clinopyroxenesfalling within this system and between the boundary planes0.2

AI fffirr rcffar.,roo 80 60 40 20 r.'.jr.ll I olrrPHACtrEI eeetRlt{E- 20 I lue tre

I 40 I I 60 I

I le 80 cil | L-lt- ! !-lllE

tg.oAl., lT,'.i]

F'rc. 3. Compositions of omphacites (.dots,P2 omphacites; circles, CZ/c omphacites) and aegirine-augites (rectangles) plotted on the plane Di50Jdb0-Hdb0Jdro-HdsoAcso-DisoAcro. Only analyses with tetrahedral Al<0 05 atoms per formula unit are shown. Numbers are keyed to analyses in Deer et ol. (1963) for omphacites and aegirine-augites, respectively; others, as follows: C and NC, Coleman et al. (1965); Co, D. l[. Green and J. P. Lockwood, written communication (see Table 1); F-NG, Nicholas and Gagny (1964); G, Foshag (1955); J, Hashimoto (19&);L, Klein (1966);N, Warner (1964); SA, Williams (1932). Analytical data for some are given in Table 1.

The P2 omphacites to be described in the following sections of this paper are expectedto be restricted to a fairly narrow compositional range with the ratio Na/(Na*Ca) =0.5, and the ratio for the octahedrally coordinated cations, Al/(Alf Fe3+)>0.6 (Figs. 2, 3). Substitutions among octahedrally coordinated cations may occur in natural systemsby elementssuch as Mr'*, Cr3+,and V3+,among others. Adjectival qualifica- tions, such as chromian omphacite, are preferable to assignment of new mineral names, and Ca- or Na-rich omphacites may conveniently be designatedcalcic or sodic omphacites,respectively.

SruorBs or Vanrous Olrpuecrms ANDAEGTRTNB-Aucrrns E*per'i,mentaltechniques. Analyzed sampleswere available f or most of the minerals studied, and the analytical results are given in other publications. Therefore, only the formula contents, in terms of six oxygen atoms 846 JOAN R. CLARK AND J. T. PAPIKE per formula unit, are given in Table 1. Electron-probe examinations of a few selectedcrystals were carried out during the present study and veri- fied the homogeneity and the bulk analysis figures. Single crystals were examined with Zr-filtered Mo radiation on the Buerger precessioncam- era, and powder samples were studied both by diffractometer and film methods using Ni-filtered Cu radiation. Preliminary unit-cell parameters wereobtained from measurementsof the precessionpatterns. The powder data, obtained from measurementsof diffractometer patterns prepared with CaFzor Si as an internal standard,were used in a computer least- squaresrefinement program (Evans, Appleman and Handwerker, 1963), starting with the preliminary unit-cell parameters, to arrive at the final valuesgiven in Table 2.

Omphacites.The single-crystal patterns of some omphacites reveal the presenceof weak reflections violating the C2/c space-groupcriteria that require hhl to be present only if h+h:2m, and hJl, only it l:2n (and h:2n). The hol photographof such an omphaciteis illustrated in Figure 4; the weak reflections are sharp and precisely aligned, so the diffraction pattern is representative of a single, well crystallized phase. Assignment of P2/m or P2 symmetry is required to fit the observations,but atomic coordinatesfor a structure closely analogousto the known C2fc strtcture (Warren and Bragg, 1928), as indicated by the intensity distribution, cannot be assignedin P2/m. The true symmetry of these omphacites must therefore be P2. Although there are few such reflections on the precessionphotographs, an appreciable number can be measured for a single crystal by the greater sensitivity of scintillation counting, and these data were available for the structure refinement. However, reflections with hlh odd have not been observedon X-ray diffraction powder patterns of these omphacites (seeTable 3, especiallyfootnote 1) and the powder data for all omphacites are satisfactorily indexed nsing the C2/ c symmetry. Therefore, X-ray diffraction powder data cannot be used to distinguish between C2/ c and P2 omphacites. Crystals from six different samplesdescribed by Coleman et aI. (1965) from Group C eclogitesassociated with glaucophaneschists in California, U.S.A., and in New Caledoniaall proved to be P2 omphacites.Because the chemical analysesare similar for the six samples,data are given in Tables I and 2 only for sample 100-RGC-58, from which the crystal selectedfor structural analysis was taken. X-ray diffraction powder data for this samesample are listed in a paper by Colemanand Clark (1968), in which the relationships of jadeite, jadeitic pyroxenes, and omphacites in the blueschistfacies of California are considered.Geological features of the Californialocalities were discussedby Colemanand Lee (1963). OMPHACITES 847

Somecrystals of a diopside-jadeitefrom a boulder tomb, Kaminaljuyri, Guatemala, Central America, which were describedby Foshag (1955) and were obtained from his samples at the U.S. National Museum, arc very nearly pure Nas.5Cas.5Mgo.5AIo.5Si2O6,and are found to be P2 omphacite. The results of electron-probeanalysis of two crystals of this diopside-jadeite agree well with the values obtained from the bulk analysis (Table 1). Because the sample contains so little iron, it is virtually a corner member on the plane Na6 5Ca6.5in the Jd-Ac-Di-He system (Fig' 3), and therefore the indexed X-ray diffraction powder data for this sample are given in Table 3. A crystal-structure refinement of this mineral is in progress. Other crystals that proved to be P2 omphacites came from Group C eclogitecobbles, Guajira Peninsula,Colombia, South America, (D' H' from Green and J. P. Lockwood, written communication, 1966) and Group B eclogite (Duen type) describedby Eskola (1921) from Vanelvs- dalen, Norway. An omphacite with C2f c symmetry was studied by Warner (1964)' who gave single-crystal and X-ray diffraction powder data for the mineral. This omphacite came from Group B eclogite in the Eiksundsdal Com- plex, Hareidland, Sunmfre, Norway (Schmitt, 1963), and, unlike the other omphacitesmentioned above, is associatedwith orthopyroxene. we examined crystals from this sample and confirm Warner's results. Elec- tron-probe analysis of a crystal of this omphacite also checks the values obtainedfrom the bulk analysis(Table 1). A possibleigneous"omphacite," from garnet-bearingari6gites of the French Pyrenees(church, 1966), also has c2f c symmetry; however, it is not an omphacite according to the present definition and should.preferably be called a sodium-rich alumi- nousdiopside (Table 1; Fig. 2).

Aegirine-augites.An aegirine-augitesample from the Wabush Iron For- mation, Labrador, Newfoundland, Canada (Klein, 1966) has a composition very nearly Nao.tCao.s,Mgo.rFe3+o.rSizOsand it is therefore approximately a corner member on the plane Nae.5Ca65 in the Jd-Ac-Di-He system (Fig. 3). The crystals were examined to determine whether they would exhibit the P2 symmetry, but no weak reflections violating C2/c symmetry were found. A similar result was obtained for another aegirine- augite sample of different composition from the sameformation' 848 JO.4N R. CLARK AND J. J. PAPIKE

NN + <+1 NOn ONN NON N\O€ L NN q 7:i ! 'tr d Di\O NDT u F.1 o v st3 *io4 1nr NN

l- 440 \o6<1 -v A..

r4 o:.- =.= i;q Dno @rD no4 44O ar. 60o 4+O € E.!od OIEN ;: : o ()op UE ioN () P 2

:ih @No @\oH vtvR,e *ro4 .+'O$ <1 oO o5 NN z E v b0 9- !n O N i;4

F'l d o 7.=143 O ts Q FA-O rl O E E F-x Odi

(-) a-

F oE :r il z ia 4 dtN\O 6Ni ONr d !x 404 6 vl 3 U NN ":-a ;-i-; F B3

s, \o @NO DO€ Oco@ no4 co:o ^^^6O@ @Ho\ o rl v

+ + z ue. I .+ =,Y _:- ,5

+o \\ I

lol

to t. -c-.o *t' ToT.

Frc. 4. (a) Precession photograph taken with Zr-filtered Mo radiation for the h0l net ot a P2 omphacite from Guajira Peninsula, Colombia (see Table 1 for analytical data). (b) Schematic diagram showing indexing of the precession photograph.

' Analytical data available in given references, converted on the basis of six oxygen atoms per formula unit; tetrahedral contents made up to 2.00 by addition of AI. b A possible "igneous omphacite"; analysis figures from W. R. Church (written communication, 1967). " Analysis for a "chloromelanite" (Eskola, t921,p.32); conversion as giveninDeer et al'. (1963, p. 156, no. 1). Sample examined during present study is from the same locality, was obtained from the University of Cambridge, and electron-probe scans of single crystals show that it is comparable in chemistry to the Eskola sample. d Average of electron-probe analyses for two crystals. Total Fe divided between Fe2+and Fe3+ as found in chemical analysis. " Conversion of analytical data as given in Deer et al'. (1963, p. 103, no. 9). r Written communication, 1966. e Sample contains 0.09 atoms Mn2+. h Samnle contains 0.19 atoms Mn2+. 850 JOAN R. CLARK AND I. T, PAPIKE

ON€ -- aa\o 9?qNcj ? oo fl +l \-e d +l+l+l;-N+l Or.!€: ! \o @$€o N i €@Nro 4 U o € 4 o€43 O

.t d o rl -a-i-1i il 1 - o fl v\-v +l +l +l: fl o *l +DoXoQ O\4€o O ts €@N€N z .OO O\0Oni<1 cO o

cv z 5 o r F z su) ! O H

o od N ! ?9qb9 c.t) --N:; da O Y,----Tl O z D\oOi.i+r o O€€o ts R 4TN€$ FA< z B o.i@nH$ R oo 4*r\O Fr -a: -i '. i E () + il Y6 (, o +t +l +t - +t x-.o irn (, rr Fn- "c:;3o\'*j6r€o O nrN€)o z o..€Di<'

F -a

.o v b/i -^^ P p "

vvJ o A.= B€ saeadd f, /-\a Te.sn 3. X-nev DrllnecuoN Powoon Dlra lon P2 Oulrr.lcrrr, Nao oCao5A16 5Mgo ssizOorrnou KlurNer,luvri, Guernuer'n' Monoclinic, true space-group P2, pseudo space;group for-indexing o^fpolvCer patterns, a2 / c : a: 9.559, b': 8.762, c : 5.145 A, B : 106o55', v : 420.3 A3

Calculated" Observedb

Peak 20, CuKot dr,xl!l (degrees) d,,*r(A) heig\t

110 13.98 6 .330 0.7 200 19.37 4 577 0.0 T11 20.23 4.385 o.7 020 20.25 4.381 8.0 4.37| 10 111 1A t7A 3.596 0.2 02l 26.99 3 .300 to.7 3.299 7 220 28.17 .t. loJ 19.7 3.162 20 22r 30.18 2.959 100.0 2.956 100 310 31.00 2.882 33.8 2.877 45 311 31.23 2.862 33.4 2.86r 30 130 32.L4 2.782 0.1 202 35.41 ?

have calculated I(2.0. b Measu_rementsfrom difiractometer patterns with NaF as internal standard; CuKa1, I:1.5405 A. Broad peaks are marked B. 852 JOAN R. CLARKAND J. J. PAPIKE

petrologic significanceof these results is discussedin another section of this paper.

Cnysrar Srnucrunp ANarysrs Erperimental d'eta'ils.The omphacite crystal selectedfor structural study was from Group c eclogite associatedwith glaucophane schist on the Tiburon Peninsula, Marin County, California (Sample 100-RGC-58, Coleman et al., 1965;see present Table 1 for chemical data). The pale greenprismatic crystal was about 0.2X0.07X0.04mm with {010} the dominant form. For collection of the diffraction data, the crystal was mounted with b parallel to the @ axis of the single-crystal, manually operated goniostat. Three-dimensional intensities for reflections with 20<60" were measured with a scintillation counter using the 2l-scan technique;the scan range was calculatedaccording to the equation for MoKa radiation suggestedby Alexanderand Smith (1964).Background counts of 50 secondsduration were made for each reflectionat the be- ginning and end of the scan range. unfiltered Mo radiation was used for most of the measurements,but approximately 60 reflections were also measuredwith Nb-filtered Mo radiation (0.002, thick Nb foil used). A total of 1297independent reflections were recorded; 830 of thesehad scan counts greater than twice the background standard deviation, and only this group was used in the refinement. The measurementswere corrected for Lorentz and polarization effects, and for absorption (tt:22 cm-1), using computer programs listed in an appendix. None of the reflections appears to be affected by extinction.

Ref.nementof the structwre.The refinement calculations were handled by computer methods which are describedin an appendix. In the C2/c structure of jadeite, the unit cell contains four formula units, NaAlSizOe. The asymmetricunit consistsof one si and three oxygenatoms, all in the generalpositions 8f plus two cations, Al(MI) and Na(M2), eachlocated on the twofold axesin specialpositions 4e. The unit cell of omphacite arso contains four units of similar formula, although with varying kinds of cations, but the generalpositions in p2 areof twofold multiplicity (2e), so that four si and twelve oxygenatoms must be locatedin thesepositions. The specialpositions on the twof old axesof P2 are single ( 1o, lb, lc) , ld,, so that oneMt and one M2 cation must be assigned'toeach of thesefour special positions. The initial atomic coordinates ior p2 omphacite were derived from those for correspondingequivalent positions in jadeite, using the asymmetric set given by Prewitt and Burnham (1966). These derived coordinates were then modified in two ways, first by adding )MPHACITES ess

+0.250 to allz coordinatesin order to place the P2 origin on a twofold axis, and second, by arbitrarily altering the r, 1, a values slightly (+0.010) in order to avoid the exact centrosymmetricmodel. The coordinates oI oneM2 position were fixed to Iocate the P2 origin. As a first approximation, all Ml and,M2 cations were assignedthe Nao scattering factor. The initial residual R was 0.43, but the value dropped rapidly after a few cyclesof least-squaresrefinement, becoming 0.21 with fixed' ind.ividual,isotropic temperature factors, and 0.16 after refinement of individual, isotropic temperature factors. The silicon and oxygen atoms were then assumedto be reasonablywell located, and.attention was directed to the problem of cation distribution among the eight available sites. Some trial structure-factor and bond- Iength calculations showedthat division of the eight sitesinto lour ol M2 type, suitablefor the Iargecations Na and Ca, and four of M1 type' suit- able for the smallercations Mg,Fe2+,Al,Fe3+,was reasonable'Because a site refinement program was not immediately available, two other ap- proacheswere tried. First, someindividual structure factor contributions were examined to determine the effect of various cation distributions. A number of modelswere tested,and during this study, the calculationsfor Z0l structure factors with h and I odd revealed that Na and Ca must be

TAer-r4. INuvrouar- Arolrrc CoNtrunutloNsTo rHE 101 SrnucrunB F.tcrbn or P2 Onpn,tcrrr

cos2r(- xilzi) F' (calc.t^ Atoms e "r'(J;+Ni t -5.71 Oxygens(24) 7.24 -0 788 -t Silicons(8) t2 29 -0.152 87 M Cations(4) +0.68 Ml 0.8Mg*0 2Fe 13.26 +1.000 Mt(1) 0.95A1*0.05re 11.96 -1.000 M1H 0.8Alf 0.21'e t3.87 -1 000 M1(1)H0.8Ms*0 2F-e 13 25 +1 000

a 11 M2 Cations (4) M2 0.64Nat0 36Ca t2.50 +1.000 Mz(r) 0 36Na*0 64Ca 14.67 - 1.000 - M2H 1.00Ca II.4J 1.000 M2\1)H0.64Na*0.36Ca 12.50 +1.000 -14.0 CalculatedF ObservedF 12.9

'F'(calc.):L"-ar'(f1lAf') cos 4o(-rilz); atomic parametersas in Table 6, s(T01):0.0097A-r. 854 JOAN R. CLARK AND I. T. PAPIKE

Telr,n 5. Vlnrous N.t-Ca.Onoemnc Moonr,s Tosrnn

Final site Cation Site Model refinement

123456Na Ca M2 Na Ca Ca Ca Na Na 0.64 0.36 M2(1) Na Ca Na Na Ca Ca 0.36 0.64 M2H Ca Na Ca Na Na Ca 100 M2(r)H Ca Na Na Ca Ca Na 0.64 0.36

Residual Factorsu Initial 0.34 0.38 0.35 0 50 0 35 0.34 Final 0.25 0.28 0.22 0 29 0 24 016 0.11

"For 299 hhl,having hlh*2n; initial coordinates after 12 cycles of least-squaresrefinement using all hhl,. Final residual factors after six cycles of refinement varying x, y, z for all atoms but using fixed isotropic temperature factors.

largely ordered if thesereflections are to have observableintensities. The individual atomic contributionsto the 101reflection, calculated using the final atomic parameters and site occupancies, illustrate this feature (Table 4). Variousmodels having orderedNa and Ca werethen subjected to least-squaresrefinement using only the 299 data having Z*ft odd. The results (Table 5) unambiguously favored one model, which was then adopted for further refi.nementusing all data. However, the temperature factors of the cations and of some oxygen atoms persistently became negative when all parameters were refined together. This effect was at- tributed to the large correlations (0.85 and up) that were found among various pairs of atoms. Accordingly, when the site refinement program became available, the atoms were divided into two groups, and refinement of parameters was carried out within one group at a time; parameters of the other group were held fixed. High correlations between the site occupancy and temperature factor for the same cation were also found, so the final site refinement, assumingfull occupancyof eachsite by two atomic species,was carried out with fixed isotropic temperature factors, using only the data having h*k odd. The full data set was then used with the site occupanciesfixed during successiverefinement of the other atomic parameters. By these gradual methods a successfulrefinement was achieved, and the final atomic parameters and site-occupancyfigures are given in Table 6, where they are compared with the corresponding parameters in the jadeite structure.The atomic designationsused in Table 6 and in the description of the structure follow the schemeoutlined by Burnham et al. OMPHACITES 855

'fesln 6. Arourc Peneurrnns eNl CltroN Srrt-OccupeNcv F'ccrots ron P2 Oulrecrrn, Coupatm wrrn Srurr-aeDerl ron Jeorrm

Omphaciteu Jadeiteb Prewitt and . Site present study Burnham (1966) Atomc occupanq/' o1(1)A 1.0 0.rr2 0.088 0.864 0.1090 0.0763 0.8775 o1(2)A .110 .922 .405 .1090 .9237 .3775 o1(1)C .386 .567 .103 .3910 .5763 .1225 o1(2)C .385 .4rr .620 .3910 .4237 .6225

02(1)A .364 .263 .066 .3608 .2630 .0429 o2(2)L - J+t .747 .551 .3608 .7370 .5429 02(1)C .133 .749 .939 .1392 .7630 .9571 o2(2)C .135 .2M .M6 .t392 .2370 .4571

03(1)A .360 .o22 .757 .3533 .0070 .7558 03(2)A .350 .994 .252 .3533 .9930 .2558 03(1)C 1

SilA 1.0 .2890 .W72 .9774 .2906 .0934 .9777 Si2A .2881 .9135 .4820 .2906 .9066 .4777 SilC .2137 .5880 .0196 .2W4 .5934 .0223 Si2C .2t03 .4027 .5232 .2094 .4066 .5223

0.81 MI i**'. .9122 0 0 .9060 0 IFe'+ 0.19

AI3+ 0.95 Mr(r) .1002 .500 .0940 .500 Fe3+ 0.05

iep* 0.82 M1H 1 .4045 0 .500 .4060 0 lFea+ 0. 18 r' " Coordinatesfor spacegroup P2. Standarderrors in coordinates:oxygen 1+0.001, z+0.002; siliconr, y, z and Ml, MZ 1,+0.0005.Individual isotropictemperature factors asfollows: oxygens 0.4*0.1 .AP;silicon 0.17t0.05 A'?; Ml andMl(l)H 0'2+0'1 A'; Ml(l) ard.M1H0.3t0.1 A'; a2610.7+0.1 A2;other M20.8+0.1 f*. b coordinates convertedby present authors from those of basic atorn set; M1 is Al' M2 is Na. Individual isotropictemperature factors as follows:0l,0'36 l'2; 02, O'aSf*; 03, O.4gA'; Si, 0.41 ,S; Al, 0.a0 A'; Nu, 0.95 fu. " Nomenclatureaccording to Burnham el'al,. (1967)- d Total occupancyof 1.00 assumed.The method does not distinguish between Fez+ and Fe3+;assignment of Fe3+with Al and Fez+with Mg is arbitrary. 856 JOAN R. CLARK AND J. J. PAPIKE

Taer,n 6 (continued)

Omphacite' Jadeiteb Prewitt and Site present study Burnham (1966) occupancyd

.5957 .500 .500 .5940 500

o.6+ {** .3036 0 .3009 0 lcu'* 0.36 irvu* 0.36 I .7017 .500 .699r .500 Icat+ o64

0.03 I,I2H ')I*u* .8009 0 .500 .8009 Lcu'*097 f*u* 0.64 M2(r)H 1 1996 .500 .500 .1991 .500 Icu'* 0.36

(1967).The final R is 0.08 for the 830 observedstructure factors,and the observedand calculatedstructure factors are comparedin Table 7.1

Description of structure. There are two characteristic features of the omphacite structure that distinguishit from other pyroxenestructures. First, the octahedral chains along c, formed by Ml octahedra sharing edges,alternate (AI,Fe'+)-octahedraand (Mg,Fe2+)-octahedra.Similar octahedral chains occur in the structure of the amphibole, glaucophane (Papike and Clark, 1966). Second,the large M2 polyhed.rathat link the octahedral chains into layers are alternately occupied dominantly by either Na or ca (Fig.5). Layers adjacent along the o direction stack so that the (Mg,Fe2+)-octahedraof one layer have (Al,FeB+)-octahedra above in the next layer, and a similar staggeredarrangement prevails for I Table 7 has been deposited as Document No. 9896 with the ADr Auxiliary publications Project, Photoduplication Service,Library of congress, washington, D.c. 20540. A copy may be secured by citing the Document number and by remitting in advance $2.50 for photoprints or $1.75 for 35 mm microfilm. Make checks or money orders payable to: Chief, Photoduplication Service, Library of Congress, 1MPIIACITES 8sz

TeelB 8. BoNn Drsr.tlcos rN Tr{E Srr,rcn'tn Trrrlrmou or ?2 Oupnecrrr

Distancesu(41, tetrahedron Atoms SilA Si2A SiiC

si-o1 .63 164 1.59 1.60 si-o2 .63 t.5/ 1.60 157 si-o3(1) .65 1.70 1.64 r.67 si-o3(2) 66 t.66 t.oJ 1.68 Average .04 1.64 1.62 1.63

o1-o2 2.80 266 2.82 2.73 o1-o3(1) 2.66 2.72 2.61 2.66 o1-o3(2) J 71 2.73 2.58 2.6+ o2-o3(1) 2.66 2.63 2.61 2.66 o2-o3(2) 2.57 2.69 2.51 2 6l o3(1)-O3(2) 265 2.64 2.65 2.64 Average 2.67 2.68 2.63 2.66

si1-si2 3.06,3.11 3.07,3.12

, Si-Odistances:L0.02A, O-o distances*0.mA, Si-Sidistances*0.01 A' the M2 cations. The four crystallographically distinct silicate tetrahedra are joined, in pairs to form the two difierent tetrahedral chains. Within the restrictions of the structure, this relative freedom permits the best possible oxygen coordination for the variable cation content. Adherence to c2/c symmetry would preclude such accommodation, and the lowering of the s1'mmetry restrictions to P2 is thus reasonable.

Sili.con-orygenbond.ing. The sodium metasilicate structure (Grund and Pizy,1952), which contains single silicate chains comparable to those in the pyroxenes, was recently refined by McDonald and Cruickshank (1967) with particular attention given to the Si-O bonding. In a discus- sion of zr-bondingin second,-rowelements, Cruickshank (1961) suggested that there should be observabledifferences between the lengths of bridging and non-bridging atoms in chain silicates.NolI (1961)discussed the same feature from the viewpoint of electronic theory. The Si-O distances in omphacite (Table 8) are thus most meaningful when considered according to whether the oxygens are bridging (designated03rr according to the nomenclature scheme of Burnham et at,., 1967) or non-bridging (}lxx, 02xx), and comparison with the corresponding values from the recent structural refinementsof jadeite (Prewitt and Burnham, 1966), NazSiOa(McDonald and Cruickshank, 1967), and an ordered hyper- 858 ]OAN R. CLARK AND J. J. PAPIKE

Teerr 9. BoNl ANcms rN run Stuceru Trrrenrnna Am Cnerns ol P2 Oupnecrrn

Oxygen atoms O-Si-O Angles,atetrahedron of O-Si-O angles SiIA Si2A SilC Si2C

o1-o2 1lgo 112" 119' o1-o3(1) r09' 109" 108" 108' o1-o3(2) 111" 112" r07" 106' o2-o3(1) 109. t07' r07" 110' o2-o3(2) 103" 113" 102" ro7" 03(1)-O3(2) 106' 1040 109" 103'

si-o-siAnglesa Oxygen atom Chain A Chain C

03(1) r32" r37" 03(2) 13go 130"

" Angles* 10. sthene (Ghose, 1965) shows that the observeddifferences are indeed significant. For bridging Si-O, the following average values are found: omphacite,chain A, 1.67A; chain C, 1.65A; jadeite l.$2 h, somewhat lower than the others; Na2SiO3,1.672 h; and hypersthene1.66 A. tte average non-bridging Si-O values are appreciably smaller: omphacite, chain A, 1.62A, ch,ainC, 1.59A; jadeite i.Ot+ A; Na2SiO3,1.592 A; and hypersthene 1.60 A. The differences between the two kinds of bonds range from about 0.02 A in ladeite to 0.0g A in omphaciteand NazSiOg. For the oxygen-oxygendistances within the tetrahedra (Table g), the difierence is smaller and in the reversedirection, probabry as a result of the angular distortions describedbelow. The averageo-o distancesin omphacite are 2.64 A for the bridging pairs and 2.7s A for the external pairs. The variation among average o-si-o angles also becomessignificant when consideredin the sameway. All the o-Si-o angresfor omphacite are given in Table 9, and the averagefor the sixteen anglescontaining both a bridging and a non-bridging oxygen is 108o,close to the tetrahedral value as might be expected.However, consideringthe o-Si-o anglescontaining bridging oxygen atoms only, we find in the various structures the following values: omphacite 105o,jadeite 106.3o,NarSiOa 103.1o, and hyper_ sthene108.2o. The averagevalues for the non-bridgingo-si-o anglesare higher by about 10 to 13o:omphacite 11go, jadeite 11g.5o,NarSiOa 116.9",

860 JOAN R CLARKAND J. J. PAPIKE

satisfactory. rn fact, sucha modelprovides the externalo1 oxygenswith a charge +2'00 received from two M1 neighbors, one M2 neigibor, and si, but leaves the external02 oxygens,each having oneMr.r.ilghbu., o.r. M2 neighbor, and one Si, deficient by about 0.4, whereasthe biidging 03 oxygens,with two M2 neighborsand two Si, receive an excess0.4 charge. The variations in the bond distancesare in the right direction to offset this apparent imbalance:long Si-Odistances for the bridging oxygensand short si-o distancesfor the more erectronegativee*te-ai oz J*yg"rrr. Although the simple ionic moderfails to explain the details of the bond variations, it does provide an indication of the direction in which the variations will occur.

Cnysrar CnBlrrsrny ol CuNopynoxENEs Prewitt and Peacor (1964) have reviewed the generarstructural features of pyroxenes and pyroxenoids;a[ clinopyroxeneshave thesefea- tures, whatever the individual symmetry. rn particular, alr clinopy- roxenescontain the metasilicatechains (zweierketten),arranged so that eight locations per unit cell are available for cations other than those in the tetrahedral chains. Some of the distinctions that may affect the symmetry were pointed out by Morimoto et al. (1960).The resultsof re_ cent studies suggestthat theseand other distinctionsare causedby variations in the chemicalcompositions. In the C2f c structttes, the metasilicate chain is formed by repetition of one tetrahedron by the c glide plane (the bridging o*yg.rr, O5, lyirrg very nearly in this plane), a layer of like chainspu.ull.r io (roo) is built up by the operation of 21 axes,and other identicarlayers are formed by t.he C-centering. This symmetrical arrangement leaves spacesof two different sizes, surrounding twofold axes. Small cations such as Mg fit into the regular, octahedraily coordinated Mr spaces,and larger ones such as Ca fit into the eight-cornered,polyhed,ral M2 spaces.Tie maxi_ mum symmetry possibleto clinopyroxenesis thus achieved,and com_ pounds of the following compositions are known to have this symmetry: CaMgSLOo,diopside; CaFe2+SizOo,hedenbergite; CaMnSirOu, johurrrr_ senitel NaAlSLOo, jadeite; NaFe3+SizOe,acmite, and NaCrr+Sirou, ureyite (Kosmochlor). Mean cation-oxygendistances for most of these cationsare given in Table 11, and the differencebetween pairs in ail of the above compoundsis about 0.4 to 0.6 A. When the size difference diminishes, as in LiAlSirO6, spodumene, to about 0'2 A' or vanishes,as in MgSio3, clinoenstatite,and FeSio3,clino- ferrosilite (Burnham, 1967), the larger polyhedra can no longer b. ud._ quately filled. The structure must shift somewhat to obtain smalrer spaces for the M2 cations,and this is accomplishedeither by rearranging OMPHACITE.S 861

Te,lr,n 10. CeT roN-OxvcaN DISTANCESUrN P2 OMPHAcITE

M1 Cations M2 Cations

M2H Mr M1(1)H Mr(r) M1H M2 M2(1)H M2(1) Na036 Na00'i Oxygenatoms Mg 0.81 Mg 0.80 Al 0 95 Al 0 82 Na064 Na064 Ca097 Fe019 Fe020 Fe005 Fe0'18 Ca036 Ca036 Ca064

o1(1)A 2 12l\ 1.91.E 239L 2 324 o1(2)A 2 09 2.O3 2 464 o1(1)c 207L t.97A o1(2)c 216 1.99 02(1)A 191 2+4 02(2)L 206 233 02(1)c 201 02(2)c 188 2.3t) 2 49 03(1)A 2.67 2.79 03(2)A 2 43 276 03(1)c 2.5r 2.73 2 -48 o3(2)c 2.47A' 2 ss,& Average 201;\ 2.104 1.944 1.9sA 2 s0,q. 2.+4]\

polyhedron' a Distaqcesl 0 02 A; each distance occurs twice in the same 862 JOAN R, CLARK AND J. J. PAPIKE

Terln 11.MraN Canon-Oxycrx DtsraNcrs ron Ocreuronar,CoorurNerroN

Atoms Average distances (A) References

Al-o 1.93 Prewitt and Burnham (1966) Fe3+-O 203 Graeber et al. (1965); Moore (1965); Blake et aI (1966) Fe2+-O 214 Burnham (1967) Mg-o 2.09 Ghose (1965)

Difierences in average distances (A) Compositional joins

(Fe'z+-O)-(Mg-O) 0.05 He-Di (Mg-O)-(Fer+-Q) 0.06 Di-Ac

(Ps3+-O)-(Al-O) 0.10 Ac-Jd (Fe'z+-O)-(Fd+-Q) 0.11 He-Ac

(Mg-o)-(Al-o) 0.16 Di-Jd (Fe'?+-O)-(Al-O) 0.21 He-Jd

PnrnorocrcAr, IMpLrcATroNSor. p2 Ol,rpnacrrBs OMPIIACITES 863

Frc. 5. View alotg a of selected portions of the omphacite structure. The octahedral and the larger chains near r:0 are shown, together with the A and C tetrahedral chains, cation' cations at *:0. Dashed jines indicate the oxygen coordination around an M2

Fyfe and Turner, 1966;Coleman and Clark, 1968;Coleman and Papike, 1668;Essene and Fyfe, 1967)' Experimental studiesof the reaction '*NaAlSizOo CaMgSizOo* rNaAlSiaOe : CaMgSizOe * rSiOz diopside albite omPhacite

highly ordered compounds at low temperatures. Nevertheless,synthetic 864 JOAN R. CLARK AND T. J. PAPIKE

studies at low temperature in the natural omphacite compositional range are necessaryif the phasediagrams are to be correctly related to the natural occurrences.Meanwhile, the occurrenceof p2 omphacitesis estab- lished in Type c eclogites along the pacific from california through Guatemalainto Colombia, and again in New Caledonia;the omphacite from Japan (Hashimoto,1964) is expectedto have p2 symmetry, accord_ ing to its composition. These occurrencesthus provide additional evi- dence that physical conditions of metamorphism and eclogite genesis have been similar throughout the Circumpacificbelt. Suulranv The principal conclusionsof this study may be summarized as fonows. First, omphacite is defined within the system Jd_Ac_Di_He_Tschas having the composition (M2)(M1)(Si,Al)rb6, whereM2 representsNa f Ca in the range0.2 < Na/(Na* Ca)( 0.g, and Ml is divided amongtne octahedralcations Mg, Fe2+,AI and FeB+such that Al/(Allp.a+))0.5. The amount of tetrahedrally coordinatedAI is expectedto be small. Ac- cording to this definition, omphacites may occur within any kind of rocks and are no longerrestricted to eclogites.Second, those omphacites having space group P2 are called P2 omphacites and are expectedto lie within a narrow compositionalrange, having Na/(Naf Ca) =0.5, and octahe_ drally coordinated AI contents greater than those of Fe3+.Third, the crystal structuresof P2 omphacitescontain orderedcation distributions, with (Mg,Fe2+)-octahedraalternating with (Al,FeB+)_octahedrato form Ml octahedral chains. Fourth, the bonding within the p2 omphacite structure accordswith the distinctionsproposed by cruickshank (1961) betweenbridging and nonbridging oxygen atoms, the averageSi-O dis_ tance for the bridging oxygensbeing 1.66 A comparedwitfr t.OOA io, the nonbridging oxygens.Fifth, p2 omphaciteshave so far been found only in environmentswhere low temperature-highpressure formation conditions are otherwiseindicated, so these omphacitesmay be useful indicators of these formation conditions.

AcxNowre oceffiNrts This study would not have been possible without the cooperation of many scientists, all of whom cannot be acknowledged here, but *'e are most grateful to alr for their assistance Dr. R. G. Coleman, U. S. Geological Survey, Menlo park, California, and prof. Brian Skinner, J' formerly u s.G.s. and now at yale university, New Haven, connecti- cut, suggested the study and encouraged its progress; Dr. coleman suppried numerous samples, gave geological information, and participated in many discussions of the problems. Prof. Charies W Burnham, Harvard University, Cambridge, Massachusetts, gurr" -u_ terial assistance with the site refinement carculations, and he and Dr. c. T. p.e*itt, _o.t. duPont de Nemours Corp., Wilmington, Delaware, made available their computer pro_ OMPHACITES 865

Larry grams and information about the jadeite structure in advance of pubiication' Mr. calculations iV. Fing.r, University of Minnesota, Minneapolis, Minnesota, kindly arranged

sponsibility for the definition proposed here.

RBlrrlxcBs

of the hematite structure. Amer. Mineral'.,51,123-129' Bnrrruaurr, Aucusr (1865) Omphazit. Berg- und. Huettenmiinni,schezeitung,24, 365. Bunwnmr, crrnar-ns w. (1967) Ferrosilite. cornegi.elnst.Wash. Year Book,65,285-290. (1967) A proposed Bunxuan, C. W., JolN R. Cr.anr<, J. J. Peuro e'Nn C. T' Pownt 109- crystallographic nomenclature for clinopyroxene structures. z. Kr'istal,l,ogr.,125, 119. Stones. Introd'uction, Carnv, E.mr,r R. ,lwt JonN F. C. Rrcn,q,nos (1956) Theophrastus On Greek tert, Englishtranslation, anil commentary. ohio state university Press, columbus, Ohio, pp. 51, 120-121. of Cnuncn, Wir.r.rau R. (1966) Comparative mineralogical and chemical composition CatriJ' ariegite and eclogite labstt.l. Geol'. Soc.Amer' Ann. Meet', San Franci'sco, ordered with P2 symmetry' Cr-enr, Joex R. exo J. J. Parrxn (1966) Eclogitic pyroxenes' Science, 154' 1003-1004. Teetono- cor-nulN, R. G., (1966) Glaucophane schists from california and New Caledonia. fh)'si.cs, 4, 479498' facies of Cali|ornia. Amer' --- AND JoaN R. Cl.qnr (1968) Pyroxenes in the biueschist J. Sci'. 266, 43-59. D.E'LBr(1963)Glaucophane-bearingmetamorphicrocktypesoftheCaza- dero area, California' J. P etrol'ogy,4, 2ffi-301. (1965) Eclogites and eclogites: , L. B' Br.ttrv, axo W. W. Bnlxrocr their difierencesand similarities. GeoI.Soc' Amer. Bull',76t 483-508' California, blueschists. J. J Pmrra (1968) Alkali amphiboles from Cazadero, I. PetrologY 9t 105-122. 866 JOAN R. CLARK AND T. J. PAPIKE

CnutcrsneNx, D. W. J. (1961) The role of 3d-orbitals in r_bonds between (a) silicon. phosphorus, sulphur, or chlorine and (b) oxygen or nitrogen. f. Chem. Soc., lg6i 5486-5504. DeNa, Eoweno Sar.rseunv (1892) The system oJ Minerarogy oJ James Dwight Dana. De- scriptiae Mineralogy, 6th ed. John Wiley and Sons. New york. p. 352. Dnrn, w. A., R. A. Howre ann J. Zuss'eN (1963) Roch Forming Minerars.yor.2. chain Silicates. John Wiley and Sons, Inc, New york. EtNst, W. G. (1963) Petrogenesisof glaucophaneschists. -I. petrology 4,7_30. Esxor.a, PnNrrr (1921) on the ecrogites of Norway. Norsk vidmsh. Skr. I, Mot.-Natura., 1{1., No. 8, 118 p. EssnNn, E. J' .lwn w. S. Fvrn (1967) omphacite in carifornian metamorphic rocks. conrr. Minerol. Petrology, 15, | 23. EvlNs, Howem T., Jn., D.lNrnr, E. Appr_auan .lnn De,vro S. HaNnwnnrcn (1963) The least-squares refinement of crystal unit cells with powder diffraction data by an auto- matic computer indexing method [abs. E-10]. prog. Absr,r.,Amer. cryst. Ass.Meet. Canbr,i.dge,Moss. p. 42. Foners, Ro*nr B. (1965) The comparative chemical composition of eclogite and basart. J. Geophys. Res.7O, 1515-1521. Fosnac, w' F. (1955) chalchihuitr-a study in jade.Amer. MineraJ.40, 1c6z-1070. - (1957) Mineralogical studies on Guatemalan jade. Smithsonian Mi.sc. Collections 135,no. 5 (Publ.4307),60p. Fneno, R' L pre,con aNn DoNero R. (1967) Refinement of the crystal structure of johannsenite. Amer. Mi.neral., SZ. 709,720 Fvlr, W. S. eNo F. J. Turrrn (1966) Reappraisal of the metamorphic facies concept. Contr. M iner oJ. P etrolo gy 12, 3 5+-364. Grrosn, Sulna.ra (1965) Mgz+-p"z+ order in an orthopyroxene, Mgo saFerozsizoo. Z. K r i stallo gr. 122, 9l-9[. Gnarnnn, Eowano J., Bnuwo MonosrN, eNn Aen^HaM RoseuzwBrc (1965) The crystal structure of krausite, KFe(SOr)z.HzO. Amer. Mi.neral,.SO, 1929_1936. GnnnN, D' H' ,rNo A. E. Rrncwooo (1966) An experimentar investigation of the gabbro to eclogite transforrnation and its petrological apprications. chapier r in pet"ology of the upper Mantle. Dept. Geophys. Geochem., Austrarian Nat univ , pubrication No. aaa. GnuNr, A. .lNo M. Pr'u (1952) structure cristalrine du mdtas|icate de sodium anhydre, NazSiOs.Acta Crystallogr. 5, 837-840. Hasnruoro, Mrrsuo (1964) omphacite veins in meta-diabase from Asahine in the Kant6 Mountains, J apan. Pr oc. J apan A cad..,40, 3 1-3S. Haiiv, R. I. G822) Trait| tle n in6r,a)ogie,2nded., vol. 2. Bachelier, paris. p. 576, 456'7. K'ux, Convnr,rs, Jn. (1966) Mineratogy and petrology of the metamorphosed Wabush Iron Formation, Southwestern Labrador. J. p etrol,ost, 7. 246_305. Kusurno, (1965) r. clinopyroxene solid solutions at hi!-h pr"ssures.carnegie Inst. wash. YearBook 64,112-115. McDorer'n, w. s. ano D. w. J. cnurcrsnervr (1967) Lreinvestigation of the structure of sodium metasilicate, Na2SiO3.Acta Crystallogr,22, 3Z-43. Moonn, Paur. Bnrlx (1965) The crystal structure of laueite, Mn2+Fe23+(eH)r(pOe)r (HrO)u.2HrO. Amer. Mineral., SO, 188+-1892. Monruoro, Noruo (1956) The existence of monocrinic pyroxenes with the space group Czn6-P2r/c.Proc. J opan Acail., 3?, 7 SO-752. ---, DaNru, E. Appr,ruaw eNn Howenn T. EveNs, Jn. (1960) The crystal structures of clinoenstatite and pigeonite. Z. Kr,istallogr., ll1, 120_147. Ntcrror-as, A. ,ll+o GacNv, Cr,. (1964) Donndes min6ralogiques sur l,omphacite et la OMPHACITES 6()/

glaucophane d,un schiste d glaucophane des Alpes pidmontaises Bul,l. soc. FranE. Min1ral Cristollogr 87' 105-108. Min' Norr-, W. (1961) Zv Elektronentheorie der silikatischen Bindung [abstr.]. Fortschr. (1963)]' eralog.39,354 fTransl' Angew- Chem.Intern. Ed' 2,73-80 in the crystal structure of PAprKE, J. J AND JolN R. Cr-,mr (1966) Cation distribution gtu,r.opttutt" fi, the high-pressurepolymorph [abstr']' Geol' SocAmer' Ann' Meel'' San Francisco, Coli'.f. jadeite, NaAl- Rrwrrr, C. T. .q.NoCnenlrs W. BunNn.rlr (1966) The crystal structure of SizOa.A mer.M ineral',51' 956 975. --- nNl DoNALDR. PBlcon (1964) Crystal chemistry of the pyroxenes and pyroxenoids Amer. Mineral, . 49' 1527 1542' Arbor' Rrcn, C. M (1954) Dictionary oJ Geologicol Terms' Edwatds Bros', Inc , Ann Michigan. Scnrrrrr, H H. (1963) Petrol'ogy ond Structure of the tiihsundsdal EctrogiteComPlex, Hareidlond', Sunmfre, Norway. Ph D. Thesis, Harvard University' Srr,wnuaN,J.N,r.NnS.H.SruoNsBN(1960)Ausefulanalyticapproximationtoatomtc and unitary atomic scattering f actors A cta Cry stallogr', 13, 50-54' and Tnropnn.qsrus (circa 315 B.C') De Lapidibus Edited with introduction, translation commentary by D. E. Eichholz. Oxford: Clarendon Press' 1965,pp' 69, 109' (Loire- vnr-on, Bnucr (1966) Etude min6ralogique d'une 6clogite de Fay-de-Bretagne Atlantique). BuIl. Soc.Fronq. Mineral'. Cristallogr.,89, 385-393' Amer ' Minerol', 49' 146l- WenNun, Jamxrv (1964) X-ray crystallography of omphacite' 1467. wannny, B. .q.NnW. LawnoNcr Bnecc (1928)XIL The structure of diopside,caMg(sio:)2. Z. Kr'istallogr., 69' 168-193. W.tnnrN,B.E.,ANDJ.BIScoE(1931)Thecrystalstructureofthemonoclinicpyroxenes. Z. Kristallogr., 80' 391-401. granulites' Amer Mineral'' Wnrrn, A. J. R. (1964) Clinopyroxenes from eclogitesand basic 49, 883-888. Wrr-rr,trus,Ar,pnBusF.(1932) TheGmesisoJ theDiomond" Vol' I,p' 329' London:Ernest Benn, Ltd. J' Sci',248,225-248' Yorrn, H,q.rrrN S., Jr. (1950)The jadeiteproblem, PartI' Amer' natural C. E. Trr.r-nv (1962) Origin of basalt magmas: An experimentai study of and synthetic rock systems. J. Petrology,3,3+2-532'

ArpBNux oN CoMPUTATIoNS

written by Dr. c. w. Burnham, Geophysical Laboratory, carnegie Institution, washing- Tabl'es ton D. C.r Atomic scattering factors are reproduced from thetables inlnternational (The Press: Birmingham' England); Jor X-ray Crystal'lography,VoI. III, 1962 Kynoch were values for neutral atoms were selected. Dispersion corlections for MoKa radiation normal taken from this same source. The least-squares program uses the full matrix of the

I Present address: Dept. of Geology, Harvard Univ , Cambridge, Mass' 868 JOAN R. CLARK AND J. T, PAPIKE