Crystal-chemistry of a unique jadeite-rich acmite-poor ompha¢ite from the Nyb¢ eclog te pod, S rpollen, Nordfjord, Norway DAVID C, SMITH, ANNIBALE MOTTANA & GIUSEPPE ROSS! LITHOS Smith, D, C., Mottana, A, & Ro,~i, G. 1980: Crystal-chemistlT of a unique jadeite-rich acmitc~-po~ omphacite from the Nyb~ evlosite pod, S~rpollen, Nord~lord, N.orwa~,. &ithes 83, 2 2 7 - ~ . Oskb. ISSN 0024-4937, An omphacite from the Nyb~ ecJogite pod, Norway, has a chemical eemposition whi~-hpio~s within the gap, often regarded as a miscibility gap, which is evident in all previous compilations of natural jadeite-rich acmite-poor omphacite compositions, Its cation site polmlations determined by crystal structure refinement accord well with its chemical composition analysed by electron microprobe. Its disordered space Stoup C2/¢ symmetry extends the known compositional range of C~¢ pyroxene. The postulated miscibility gap is reprded as real and wide at low temperatures, but it appoints to narrow with increasing temperature or. ultimately with increasing acmit~ proportion, The ~ent l~fimi|ive symmetry field appears to initially widen with increasing temperature or increasing acnfite propt~ion, at the expense of lhe diminishing miscibility gap, before finally dosing with further increase. David C. Smith, Orae~tInslilut¢ of Geology, Edinburgh University, Wes+' Mains Road, Edinburgh F,H~ 3JW, Scotland, U,g, Aunibale Mouaee. Catwdra di Mineralogia. Univers~td di Rome. 001¢~ Cind Umversiteria. Ro~e, Italy, Giuseppe Rossi. C.N,R. Cenn'o di Studio per la Cristallografia Strunurate. lstituto di MiRer~log~a, Univer+fftd di Pavia, Via Bassi 4, 27100 Pavia, Italy. Composition gaps, cation ordering and phase relationships in jadeite-rich acmite-poor sodic pyroxenes Dobretsov (1962) noted a ,:ompositionai gap in natural sodic pyroxenes, from about 60 to 80 mole % (jadeite+ acmite) with less than 50 mole % acmite in the diopsid¢-jadeiLe--acmite (Di-Jd-Ac) system, and suggested that this is a region of immiscibility, Subsequent compilations of natural sodic pyroxenes (Dobretsov & Ponomareva 1964; Ginzburg & Sidon'enko 1964; Essene & Fyfe 1967; Coleman & Clark 1968: Edgar et al. 1969; Onuki & Ernst 1969; Dobretsov et ai. 1971; Carpenter 1979)all have a compositional gap in the same general region. The compositional gaps from five of these com- pilations are reproduced and superimposed on our XNa vs. XFe ~÷ diagram (Fig. In), where they reveal differences between their boundaries con~ siderably in excess of minor discrepancies which may be expected from the differing analytical techniques and methods of e~d-member calcula- tion employed. Only in the Ac-poor region is there a gap common to all. Analyses of pyrox- 16- Lhhos 3/80 enes from eclogites associated with g|aucophaae schists in California and New CaledonieL(Essene & Fyfe 19~7) constrain the compositional gap to X~,, ~÷ < 0.3 whilst maintaining Dobretsov's (1~2) limits 0.6< XN,< 0.8. A number of pyroxenes from blues~:hist rocks in Greec,v (Carper~ter 1979) further constrain the gap to XFd + < 0.25. Two analyses from Greece have XF+ ~" ~ 0.2, but these differ markedly from all other analyses from the saree suite of rocks; Carpenter (perso comm.) concurs with our suggestion that these two un- corroborated single spot analyses are probably composite analyses of coexisting different pyroxenes, r, common occurrence in these rocks° Low Xv~ ~, values are recorded in five pyroxenes from nodules in kimberlite diatremes on the Colorado Plateau (O'Hara & Mercy 1966; Watson & Morton 1969) (Nos. 1-5 in Figs. la and b) which may have been derived from a subducted blueschist terrain (Helmstaedt & D<fig 1975). 1wo recent analyses further extend the known range of pyroxeae compositions ~o slightly lower Xv~ • values i~1this XNa region (No. 6 (Lappin & Smith 1978) and No. 7 (this paper) in Figs. lla and b) both of which occur in rock sample C413 a clinopyroxenite from a remark-
Transcript
Crystal-chemistry of a unique jadeite-rich acmite-poor ompha¢ite from the Nyb¢ eclog te pod, S rpollen, Nordfjord, Norway DAVID C, SMITH, ANNIBALE MOTTANA & GIUSEPPE ROSS!
LITHOS Smith, D, C., Mottana, A, & Ro,~i, G. 1980: Crystal-chemistlT of a unique jadeite-rich acmitc~-po~ omphacite from the Nyb~ evlosite pod, S~rpollen, Nord~lord, N.orwa~,. &ithes 83, 2 2 7 - ~ . Oskb. ISSN 0024-4937,
An omphacite from the Nyb~ ecJogite pod, Norway, has a chemical eemposition whi~-h pio~s within the gap, often regarded as a miscibility gap, which is evident in all previous compilations of natural jadeite-rich acmite-poor omphacite compositions, Its cation site polmlations determined by crystal structure refinement accord well with its chemical composition analysed by electron microprobe. Its disordered space Stoup C2/¢ symmetry extends the known compositional range of C~¢ pyroxene. The postulated miscibility gap is reprded as real and wide at low temperatures, but it appoints to narrow with increasing temperature or. ultimately with increasing acmit~ proportion, The ~ e n t l~fimi|ive symmetry field appears to initially widen with increasing temperature or increasing acnfite propt~ion, at the expense of lhe diminishing miscibility gap, before finally dosing with further increase.
David C. Smith, Orae~t Inslilut¢ of Geology, Edinburgh University, Wes+' Mains Road, Edinburgh F,H~ 3JW, Scotland, U,g, Aunibale Mouaee. Catwdra di Mineralogia. Univers~td di Rome. 001¢~ Cind Umversiteria. Ro~e, Italy, Giuseppe Rossi. C.N,R. Cenn'o di Studio per la Cristallografia Strunurate. lstituto di MiRer~log~a, Univer+fftd di Pavia, Via Bassi 4, 27100 Pavia, Italy.
Composition gaps, cation ordering and phase relationships in jadeite-rich acmite-poor sodic pyroxenes Dobretsov (1962) noted a ,:ompositionai gap in natural sodic pyroxenes, from about 60 to 80 mole % (jadeite+ acmite) with less than 50 mole % acmite in the diopsid¢-jadeiLe--acmite (Di-Jd-Ac) system, and suggested that this is a region of immiscibility, Subsequent compilations of natural sodic pyroxenes (Dobretsov & Ponomareva 1964; Ginzburg & Sidon'enko 1964; Essene & Fyfe 1967; Coleman & Clark 1968: Edgar et al. 1969; Onuki & Ernst 1969; Dobretsov et ai. 1971; Carpenter 1979)all have a compositional gap in the same general region. The compositional gaps from five of these com- pilations are reproduced and superimposed on our XNa vs. XFe ~÷ diagram (Fig. In), where they reveal differences between their boundaries con~ siderably in excess of minor discrepancies which may be expected from the differing analytical techniques and methods of e~d-member calcula- tion employed. Only in the Ac-poor region is there a gap common to all. Analyses of pyrox-
16 - Lhhos 3/80
enes from eclogites associated with g|aucophaae schists in California and New CaledonieL (Essene & Fyfe 19~7) constrain the compositional gap to X~,, ~÷ < 0.3 whilst maintaining Dobretsov's (1~2) limits 0.6< XN,< 0.8. A number of pyroxenes from blues~:hist rocks in Greec,v (Carper~ter 1979) further constrain the gap to XFd + < 0.25. Two analyses from Greece have XF+ ~" ~ 0.2, but these differ markedly from all other analyses from the saree suite of rocks; Carpenter (perso comm.) concurs with our suggestion that these two un- corroborated single spot analyses are probably composite analyses of coexisting different pyroxenes, r, common occurrence in these rocks° Low Xv~ ~, values are recorded in five pyroxenes from nodules in kimberlite diatremes on the Colorado Plateau (O'Hara & Mercy 1966; Watson & Morton 1969) (Nos. 1-5 in Figs. la and b) which may have been derived from a subducted blueschist terrain (Helmstaedt & D<fig 1975). 1wo recent analyses further extend the known range of pyroxeae compositions ~o slightly lower Xv~ • values i~1 this XNa region (No. 6 (Lappin & Smith 1978) and No. 7 (this paper) in Figs. lla and b) both of which occur in rock sample C413 a clinopyroxenite from a remark-
228 David C. Smith et al. LITHOS 13 (1980)
X / ' 4 / \ ..-'x 'WZ . No.2// " 3+ /
o ' ' ' "
oi Jd XNa >
b / / / X o;' X_\
Di .o\ .; .4\ .6\ .8\ 1.0\.0 Jd
XNa >
Fig. I. Natural composition gaps and inferred compositional limits of primitive symmetry in the Jd-rich Ac-poor part of the sodic pyroxene system. Co-ordinates: XNa = Na/(Na + C~) and XF~ a÷ = Fea+/(Fe a+ + AI) in cation proportions. Note that the XNa = .0 apex includes hedenbergite lHd) and calcium Tschermak's (CaTs) compotaents, but excludes enstatite (En) and orthoferrosilite I Fs), unlike Essene & Fyfe (1967). In. Compt, sition gaps recorded by D (Dobretsov & Ponomareva 1964), E (Essene & Fyfe 1967), Co lColeman & Clark 1968), O
IOnuki & Ernst 1969), and Ca (Carpenter 1979), with the sides of the gaps indicated by the symbols (all reproduced from the original compilations without recalculation to a .~;tandard method because of a lack of published raw data values). Curret~.t natural composition gap re~resented by the dotted area. Omphacite compositional field as defined by Clark & Papike (1968). Single analyses: 1-5 from t,odules in kimberlite pipes, Colorado Plateau, U.S.A. (Watson & Morton 1969); 6 from the Nyb0 eclogite pod, SCrpollen, Nordfjord, Norway (C413e, Lappin & Smith 1978); 7 (C413f-CPI) from the same pod (this paper).
I!b. Compositional limits of the primitive symrnetr~, field as inferred by CI (Clark & Papike 1968), F (Fleet et al. 1978), and observed by Ca (Carpenter 1979) with dashed line denoting minimum limits (amended here to accord with the compositional gap Ca). Analysis 8 (Hold 102) from a metagabbro, Koralpe, Austria (Heritsch 1973). Other notation as in Fig. la.
able eclogite pod at Nyb0, S0rpollen, Nordfjord, Norway ISmith 1976). The current composi- tional gap in natural Jd-rich Ac-poor sodic pyroxenes is thus limited to the small region approximated by the dotted area in Figs. [a and b.
Coexisting pyroxenes with compositior~s plot- ting on each side of this gap occur in meta,,~oma- tic jadeile ~odies in the ultramafic rocks of Western Sayan, Northern Balkhash and the Po- lar Urals (Dobre~tsov etal. 1971), and zoned pyroxenes with a compositional break across this gap occur in jadeite rocks in Guaternal~, and
in blueschist rocks in Gzeece (Ca rp,,~nter 1979). These examples provide support for, though not proof of, Dobretsov's (1962) suggestion tiaat this composition gap represents a miscibility gap since, from the limited petrographic data avail- able in each case, it appears that these coexisting pyroxenes were in equilibrium. Dobretsov et al. (1971) further suggested that the shape and size of this miscibility gap depends upon pressure and temperature conditions and upon the extent of solution of other pyroxene components, prhlcipally hedenbergite; in particular they ar- gued for a widening gap with decreasing tem-
L I T H O S 13 (1980) Omphacite from the Nyb¢ eclogite 229
perature. A two-pyroxene miscibility gap in the pressure(P)-temperature(T) range 30-40 kb, 1200-1450°C, with the temperature of the solvus crest increasing with P, was claimed by the experimental studies of the Di-.~d system by Bell & Davis (1965, 1966) but was not reaffirmed by Bell & Davis (1969), nor corroborated by related experimental work by Bell & Kalb (1969) and Kushiro (1969a, b).. This supposed high T solvus is now generally discredited, but some in- vestigators have also cast doubt upon the ex- istence of a solvus at low T (e.g. Essene & Fyfe 1967:18; Ganguly 1973:151) whilst others have deduced that, if it exists, a soivus at low T must be narrow (e.g. Onuki & Ernst 1969:246; Fleet et al. 1978:1104).
The question of the postulated miscibility gap(s) in the Di-Jd-Ac system is closely tied to considerations of cation ordering and differing space groups since the end-members all show disordered C2/c symmetry (e.g. Brown 1!972) whilst natural pyroxenes close to Di.s0Jd.~o Ac.o have an ordered primitive symmetry. This was originally thought to belong to space group P2 (e.g. Clark & Papike 1966, 1968; Ogniben 1968; Black 1970, 1974; Brown 1972; Heritsch 1973), but space group P2/n has been demonstrated by Ma~tsumoto & Banno (!970), Kanazawa & Ma~Isumoto (1972), Matsurnoto et al. (1975), Curtis et al. (1975) and Rossi et al. (in press). In a ,~chematic model of temperature/composition (T/ ~0 phase relationships in the Di-Jd system sug- gested by Champness (1973), a field of pyroxene with ,primitive symmetry is ~eparated from the C2/c symmetry fields on each side by miscibility gaps which close with increasing temperature, as does the primitive field (Fig. 2). Since 'ideal' ordering in omphacite requires equal proportions of Na and Ca on the M2 sites (Clark et al. 1969), then the field of primitive symmetry ought to lie astride the composition Di.~0Jd.~0Ac.o; hence the virtualtly symmetrical schematic T/X phase diagrams of Champness (1973), Yokoyama et aJ. (1976) and Fleet et al. (1978) (Fig. 2). However the electron diffraction evidence of Carpenter (1979) suggests a widening of the ordered field from Di.~0Jd.~oAc.o with increasing Ac propor- tion, thus extending primitive symmetry beyond the lfic~tits of 1~.e omphacite compositional field a.,; defined by Clark & Papike (1968) towards both the compositions Di.0Jd.~0Ac.~o and Di.~0Jd.0Ac.~o (F~g. lb).
In this pe~per we examine the chemistry. su~cture ant ~ homogeneity of the Jd-rich Ac-
Fig. 2. Schematic T/X phase relationships in the Di-Jd system as inferred by Ch (Champness 1973), Y (Yokoyt~ma et al. 1976) and F (Fleet e't al. 1978), l~ressure unspecified except that tlae P2/n--*C~c transition oc0:urs at T=725°C a: P= 15-18 lib according to F. Solidus arid liquidus at 30 kb from B (BeJ] & Davis 1969).
poor omphacite saNple No. 7 (Figs. la and b) in order to establish basic cr~'stal-che~ical data for this hitherto unrecarded composition, and dis- cuss its be;wing upon phase relationships.
Sample data Field, petrographical ar.~d mineralogical data on the Nyb¢ eclogite pod and its clinopyroxenite layer m'e given by Smith (1976) and Lappin & Snfith (1978) where the location of the pod is described as SOrpollen. In order :o distinguish this unique pod from man} others outcrop~;.g around S0rpolien fjord, the more precise Iccation NybO will be used henceforth (grid reference 017727, w~p sheet I ! 18 ! M~l¢y, series M71 I, Norges Guografiske Opl maling), in brief, ~he pod shows a marked compositional layering with variable amount:; of the :mhydrous minerals garnet, clinopyrox¢ne, orthopyroxene, quartz, ru~ile and ferromagnesite and of the hydrous n ine~ls phlogopite and clinoamphibole, i.e. a typi=al ~hydrous ecloghe facies' paragenesis (Smith 1971). As is typica~ of eclog~te pods in Nordt]ord and neighbouring districts in the Basal Gneiss Region of western South Norway, the ma:gins are completely ~,mphibolitised (transformed into amplfibolite-facies paragene~es dominated by plagioclase + hornblende compati- ble with similar parageneses in the enveloping country-rock gneisses) and certain layers of the pod are partially amphibolitised.
The original hand specimen C413 was about 15 x 7x 5 cm in size. Different fragments were taken for the X-ray fluorescence bulk rock analysis by Smith (1976) (fragment C413d), for the electron microprobe analyses published by Lappin & Smith (1978) (fragment C413e) and for the crystal- lographic studies reported in this paper (fragment C413t"). The bulk rock composition g~ves XN,= 0.77 whilst the mean min- eral analysis of C413e gives )~,~= 0.79 This difference may not be significant in view of the different analytical techniques employed. The particular crystal upon which the single crystal X-ray stnlcture refinement was made (('413f-CPi, plotted as No. 7 in Figs. la and b) was retrieved and analysed by electron microprobe (Table 1). Its XN~ value of 0.73 differs significantly from tha~ in fragments C413~1 and C413e and confirms the necessity to~ ~ this a~,~alytical approach.
230 David C, Smith et al. LITHOS 13 (1980)
Chemical data A~alyses of C413f-CPI were determined with the energy-dispersive system on an electron microprobe ~Table 1). Trace elerne,ats Cr, Mn, Ni and K were not detected; small amounts of these elements were detected by the crystal spectrometric technique used for analysis C413e (Lappin & Smith 1978) and these amounts are assumed to be present in this cry~tal. The max- imam difference between the major element con- tents in the three spot analyses and their mean fie we~l within the expected analytic~fl precision of the energy-dispersive technique such that no significant inhomogeneity is detected in crystal C413f-CPI. Slow (45 #m/min) electron mi- croprobe scans across this and other crystals from sample C413 measuring Na, Ca, AI and Fe with the crystal spectrometric technique did not reveal any aIternating, stepped or gradual in.- homogeneity detectable above the inevitable statistical fluctuations.
One indication of the oxidatior~ state of iron (O.S. = Fe:~+/(Fe :~+ -~ FC +) is O.S. :=0.555 (as de. termined by wet chemical analysi~ for FeO and X-ray fluorescence analysis for total iron) in the bulk rock analysis C413d, whi~zh comprise,,i about 95% primary clinopyroxene. Another in. dication is preliminary M6ssbauer spectroscopic examination of clinopyroxene from sample C413f IG. DeAngelis & F. Sgarlata, pers~ ct~mm.) which sugge,;ts that O.S. = 0.700 + 0.050: the M6ssbauer spectrum was however obtained on 100 rag of pyroxenes from samlple C413t" and thus chemical differences from crystal C413f- CPI may be expected. The compromise value of O.S. =0.666 (Table 1) result~ in the pyroxene Jd ,~4Ac.o~(Di + Hd).,,d En + Fs).o,,CaTs.oo with perfect stoichiometric balance of the X, Y and Z sile occupancies .-f coupling of Na = A! + Fe 3~-, ar,d of the sum of cations all to witt:in better than ± 0.010 cations per 6.000 oxygens.
Optical data The ;ndividuai clinopyroxene grahls in the rock samph: C413 have sub-parallel clear cu:~pate boundaries a!ong the prism, thus forming the well-defined lineation of the rock, and ,;how good triple junctions when viewed across the lineation. The crystals are colourless and apleo- chroic in thin sectio~ and grass-green in mass. Cleavage according to {110} is evident with an
T~:ble 1. Electron microprobe analysis of crystal C413f-CPI.
SiO2 57.62+ .27** Si AlzOa 15.44_.+ .11 A~ Cr2Oa 0.03 Cr FezOa 2.82+ .04 Fe a+ FeO 1.27+ .04 Fe 2+ MnO 0.04 Mn MgO 5.03+ .12 Mg NiO 0.02 Ni TiO2 O. 12 Ti CaO 7.04+. 10 Ca Na20 10.54+ .20 Na K20 0.0~ K
.708] .991 Y
.283/
i ~020 ]
.263 | .994 X
.711/
Sum 99.98 Sum 3.991
Cation proportions
XNa = Na/(Na+ Ca) =0.730 Xve . . . . Fea+/(Fe a+ + AI) = 0. ~04 XF~ 2' = Fe2+/(Fe 2+ + Mg) = O. 124 O . S = Fea+/(Fe a+ + Fe 2+) = 0.666
* Analysed, utilising standard correction procedures, with the 'Harwell' pulse-processor and 'Kevex series 3000' Si(Li) detector of the Link Systems Energy-Dispersive-System attached to the Cambridge Microscan V electron microprobe at Edinburgh University (;e~:logy Department; see text for a description of the derivation of the values fat" Cr~O3, MnO, NiO & K.,O, and O.S. ** Maximum dcvialion from t~e mean of three spot analyses. *** Best fit to t!:e structural formula. **** Excluding orthopyroxen: components, calculated as functions of the cation proportions, e.g. Jd = XNa. ( I -- Xvea~), Ac = XNa" XFo 3+ etc.
angle of 88_+ ]°(110 ̂ 1]0) determined on several crystals in fragment C413f. Twinning according to {010} is present in a few crystals. Except in :sections normal to c, where extinction is slightly anomalous with bluish colours, the extinction is :aomogeneous in all the grain~ with neither pres- rare distortions nor the fibrgus appearance so common in jadeite-rich clincpyroxenes in ecl*~- gites from blueschist facies terrains.
The refractive indices (J~:= 1.669, r~ = 1.678, ny= !.692 (all+0.002)), lthe extinction angle (Z'c = 44+ 1 °) and optic axial angle (2Vz = 74+ 2 °) were determined on severa ~, cryst- als in fragment C413f. The values of the two angular parameters are idendcai' to t;ne values
LITHOS 13 (1980) Omphacite from the Nyb¢ eclog,~te 231
Table 2. Unit cell dimensions of crystals C413f-CPI & CP2.
1 2 3 4
a 9.522(2) 9.513(3) 9.524{5) 9.526(I) A b 8.682(2) 8.687(2) 8.696(4) 8.692(I) A c 5.246(1) 5.243(2) 5.247(3) 5.246(1) A fl 107.29(3) 107.20(5) 107.23(3) 107.21(2) ° V 414.1 413.9 415.1 414.9 ]~a
1 & 2 = measured with the single-crystal PW 1100 dLffracto- meter on two separate grains CPI & CP2; 3 = computed from the powder data of Table 3 using 27 fixed and uniquely- indexed reflections; 4 = compu:ed from the powder data of Table 3 using 28 computer-gc.,,:rated self-indexed reflections. Numbers enclosed in parenth.-ses ~re standard errors of the least-squares programs used for the computations.
( Z ' c = 4 4 °, 2Vz=73 °) indicated by Tr6ger's (1962) compilation of optical data for the Di-Jd-Ac system for the composition C413f-CP 1 with the small Hd component included in Di rather than being ignored. The refractive index value (m = 1.682) indicated by the same compila- tion is slightly higher.
X-ray data Unit cell dimensions were determined upon two crystals with program LAT of the Philips PW 1100 single crystal diffractometer by scanning the rows (h00), (0k0), (001), (h0h) and (h0h) in the range 3°< 0< 50 ° (MoKa) (Rossi et al. in press). Using the circle with the highest precision (to) with coupled 20, the profiles of the four strongest reflections (as weighted by tan 0) in each row were scanned, both in the positive and in the negative 0 region, the centres of gravity of these eight profiles then being used for a least-squares refinement of Xd. The d-spacings of the above- mentioned five - ,vestigated rows were used to obtain the a, b, c and / /ce l l dimensions. Those obtained from the two crystals (Table 2) are not considered to be significantly different. X-ray powder diffraction data collected on a portion of fragment C413f are listed in Table 3. Evidence for a two-pyroxene intergrowth in the form of split peaks is lacking, though coexisting pyrox- enes on opposite limbs of a narrow miscibility gap may not yield detectably different X-rey diffraction pat~terns.
The unit cell dime~,~sions were also contputed by two methods of least-squares refinement of the X-ray powder data. The values (Table 2) for
Table 3. X-ray powder data from cl~nopyro3:enite fragment C413f.
dobs = observed d-spacing, dealt = calculated d-spacing, ob- tained using a Si internal standard and Ni-filtered CuK~ radiation and indexed using the unit cell parameter values listed in column 1 in Table 2; Ilia = intensity data determined by averaging peak-background heights on different tracings; * = peak read on a :;i-free tracing, because of Si intefferer~ce, but Opt used for :he unit cell parameter determinatic, ns; *" = ',broad or poorly-resolved reflection; *** = reflection neglected due to low intensity and uncertainty in indexirtg.
a, b, c and hence V are only very slightly higher than ~hose directly naeasured on the single crys- tals. '~
The indices of all the reflections are compat- ible with the C2/c space group criteria that the only permitted reflectiorcs are (hkl) with
232 David C. Smith et al.
Table 4. Ato~:aic parameters and site occupancies of trysted C413f--CPt.
LITHOS 13 (1980)
Co-ordinates~ isotropic equivalent temperalur¢ factors (Hamilt, m 1959) artd site occupancies. The nomenclature of the atom,q is ~hat adopted by Burnham et al. (1967).
Atom x/a y/b z/c Bn(A ~) Occupancy
Si .2894(1) .0928(1) .2290(2) .38 1.0 S,~ Ol .1112(2) .0783(3) .1319(5) .72 1.0 for all O atoms 02 .3602(3) .2590(3) .3018(5) .72 03 .3524(2) .0110(3) .0034(4) .60 M 1 0 .9050(1) .25 .33 .924(2) AI; .076 Fe M 2 0 .3007(2) .25 .67 .735(4) Na; .265 Ca
Anisotropic temperature factors in the form: e ~p I-(h2/3n +. - . + 2 hl/~n...)1 x 10 ~.
Atom fin f12~ flaa fl!2 /31a fl~a
Si 78(08) 173(10) 313(28) - 1(07) 17(12) -31(13) O I 82(22) 341 ( 28) 760(80) 39(20) 5( 34) -55(38) O 2 250(24) 195( 26J 700( 81 ) -48( 21 ) 79(35) -48(37) 03 86(22) 312(28) 542(73) 22(20) 53(31) -92(37) 'd I 56(12) 159(14) o 28~(41) 0 -4(17) 0 id 2 256(16) 168(17) 436(51) 0 -89(22) 0
Standard deviat';,ms are in parentheses.
h + k = 2n and (h01) with h= 2n and 1= 2n. No v,eak reflec~dons violating these criteria were observed in precession photographs even after 48 hours of X-ray exposure.
Structure refinement ×-ray inteng~ity data were collected with the single crystal automatic diffractometer on the same two crystals, C413f-CPI and -.CP2, using MoKa radiation monochromatised by a flat graphite crystal. The intensities of the reflections with 0< 30 ° were measured with the to scan nmde and the equivalent (hkl) and (hl~l) pairs were scanned. Three standard reflections were monitored at four-hour intervals; no variation greater than 3% was observed. The intensities were corrected for absorption following the method of North et al. (1968) and the values of the equivalent pairs were averaged. The result- ing discreparcy factors:
Rsy,n" = ,~l(lhkl- i)/l~kl i wh,.re | = (lh~,~ + lh~)/2, were less than 0.02. The X-ray data were processed with a version, mod- ified in Pavia, of a program (Hornstra & Stubbe 1972) specifically written for the PW 1100 diffractometer.
The least-squares structure refinements were
carried out using a version, also modified in Pavia, of the program ORFLS (Busing etal. 1962). This program allows the assignment to every site involved in isomorphous replacements of two scattering curves, fl and t'2, and refine- ment of' the site occupancy factors, X(f), with the constraint that X(fl)+ X(f2)= 1. No chemical analysis was used to constrain the site occu- pancy refinements. Isotropic temperature fac- tors were used in the first least-squares cycles; successively all the atoms were treated anisotropically. The different Fourier syntheses, computed at the end of the refinements, were featureless.
A sequence of refinement steps~ was devised in such a way as to avoid correlation effects be- tween variable;i; in each least-squares cycle, atomic co-ordinates and, in turn, thermal parameters or occupancies or scale factor or secondary extinction correction (7.achariasen 1963), were allowed to vary together. The initial atomic parameters were those of jadeite (Prewitt & Burnham 19661! and the scattering curves for neutral atoms were those given by Doyle & Turner (1968). Of the 605 measured reflections only ~ne 561 with lint> 3tr (Int) were introduced, with equal weights, into the refine.merits. The final conventional R factors are 0.022 and 0.023 for the observed reflections of the two crystals
LITHOS 13 (1980)
Table 5. Bond distances (A) and relevant angles (°) of crystai C413f-CPl.
Atoms Distance
Si - e l 1.625(2) 02 1.591(2)
Mean, non-bridging 1.608
Si- 03(!) 1.639(2) 03(2) 1.653(2)
Mean, bridging 1.646
Mean of 4 1.627
AnOe
O! - S i - 02 !18.30(13) O3(1) 108.19(12) 03(2) 107.82(12)
0 2 - S i - O3(I) 110.29(13) 03(2) 105.49(12)
O3(1)- S i - 03(2) 106.05(8) S i - 0 3 - $i 138.51(15) 03(2) - 03(1) - 03(2) 171.70(21)
Distance
M I -- O1(!) *2.039(3) O 1(2) * 1.974(2) 02 "1.915(2)
Mear~ ! .976
M2- e l "2.371(3) 02 *2.390(2) O3(1) *2.429(3) O3(2) *2.74412)
Mean of 8 2.484 Mean of 6 2.397
* Distances which occur twice; standard deviations are in parentheses.
(0.025 for all the 605 reflections). The results of the refinements were identical within one e.s.d. for the two crystals and thus only one set (for CPl) of atomic parameters and site occupancies (Table 4) and of bond distances and angles (Table 5) is published. The other set for CP2, together with the lists of the observed and calcu- lated structure factors, can be obtained from one of the authors (G.R.).
The population of the M2 si~e determined by means of the site occupancy refinement assumed that only Na and Ca were,, present. The resulting XNa value of 0.735 (Table 4) is encouragingly
Omphacite from the Nyb¢ eclogite 233
close to the XNa values of 0.730 determined with the electron microprobe (Table 1) and is also close to the value of (A! + Fe3+)/(AI + Fe a+ + Ms) =0.743 determined in the M I site as de- scribed below.
During the M 1 site refinements (Table ~). the scattering curves of A 1 and Fe were used as it is not possible to distinguish A 1 ~om Mg because the difference in their atomic numbers is too small. Taking into account this fimitation and the fact that the refinements were not constrained to agree with the chemical analysis, it was neces- sary to use M I--O bond distances to determine the M 1 site population. This determination has been accomplished for this crystal (Rossi et al. in press) on the basis of the number of electrons occurring at the M I site (from site occupancy refinements), with the assumptions that Ai, Mg and Fe 3+ (with negligible amour~ts of Fe z+) were present and that the observed M1-0 distances were a linear combination of the bond distances of the mentioned cations in the corresponding end-member pyroxenes. The resulting site popu- lation of MI (0.643 AI, 0.257 Mg, 0.100 Fea+) is also encouragingly close to the chemical analysis (Table I).
Discussion and conclusions Within the limits of resolution of the techniques employed here, i~! is evident that clinopyroxene C413f-CPI is chemically hotaogeneous. The de- termined space group is C2fc, such that if any cation ordering has taken place then its extent must be below the limit of detection by X-ray diffraction of the necessary additional reflec- tions.
The chemical analysis of crystal C413f-CPI by electron microprobe is satisfactorily stoi- chiometric and, excepting the oxidation state of iron, is closely compatible with the cation site populations determined completely indepen- dently by the crystal structure refinement. Its position in the Di-Jd-Ac system constrains the currently-known composition gap in natural sodic pyroxenes to lower Xve ~÷ values than hith- erto.
The observed compositional gaps reproduced in Fig. la arose from compilations of ana|yse[! pyroxenes from blueschist terrains, from eclc,- gites associated with blueschists, and frc~rn jadeite bodies associated with blueschists or greenschists. We find the balance of evidence in
234 David C. Smith et al. LITHOS 13 (1980)
favour of these gaps being real such that we recognise the existence of a wide miscibi~,.'ty gap in the Jd-rich Ac-poor region at relatively low T. The available evidence suggests that ~t the P-T condition 7 kb, 300°C, a typical coradition of formation of blueschis: facies parageneses (e.g. Newton & Fyfe 1976), the primitive and C2/c pyroxene fields are separated by a miscibility gap from around 0.6 to around 0.8 X~a, which extends to 0.3-0.4 Xv,?*. Under the higher P-T conditions deduced for the omphacites in Greece ~- 13 kb, ~ 500°C, Dr. J. E. Dixon, pets. comm. in Carpenter 1979) the Xv,? ÷ limit of this gap diminishes to 0.2-0.25, but the XN, width is essentially unchanged such that this gap would have near-vertical boundaries if projected on to the .,-~v,, ,., d~,E,,,,,,"~'"""" in Fig. 2 between 200--500°C with a concomitant pressure increase necessary to maintain the stabifity of sodic pyroxene. Thus we dispute the 'very narrow' two-pyroxev~e field and lhe consequent locatior~ of C2/c pyroxene at Xx,, values as low as 0.6 at any T as deduced by Fleet et al. (1'978) (Fig. 2).
The pyroxenes from the Nyb¢ eclogite pod formed at ct)n,,,iderably higher P-T conditions (~ 23 kb, ~ 800°C according to Bryhni e tal. (1977) or 15-28 kb, 700-850°C, or possibly 15-45 kb, 700--850°C and previously 30-40 kb, 1200- 1370°C according to Lappin & Smith (1978)). The homogeneous C2/c nature of py;oxene C413f-CPI witl~. Xn~, = 0.73, Xv,.:" = 0.10 n'my be interpreted as f,,)llows. At the P-T conditions of this pyroxene's (re)crystallisation, the high X.~,, C2/c field expands down to at least X x,, = 0.73, but with the continued existence of a primitive field, by retraction of the miscibility gap to either lower ,Xx,, or lower ,Xv,. :,~ values or both, i.e. to a position similar to or smaller than the current compositional gap (Fig. Ibl. Alternatively these P-T conditions may have been above the upper P-T stability limit of the primitive field such that all sodic py."oxenes would have C2[c symmetry. In either case. crystal C413f-CPi is the C2/c pyroxene having the lowest Xx,, value kn~wn to date in the Jd > Di half oi' the Di-Jd-Ac ..;yt, tem with Xv,," < 0.3.
Primitixe symmetry has been determined in pyroxenes with 0.4< Xxa< 0.fl from tw~.~ other Norwegia~l eclogites (Eskola's (1921) Van- elvsdalen eclogite, X.x,,= 0.51, Clark & Papike ( 1968); Bryhni et al. 's (1969) Totiandsv/~g eclo- gite, Xx,,--0.49, Yokoyama et al. (1976~). and al'so from c, ther layers in the Nyb0 eclogite podt with X~,, = 0.41 a~nd = 0.50 (Rossi et al. in press)
However, it is not at all certain that these different eclogites experienced a common P-T trajectory. Furthermore, there i~; the possibility that some, if not all, of these primitive pyroxenes had C2/c symmetry at the time of their eclogite facies equilibration, but became'ordered during retrograde metamorphism whilst kinetic factors prohibited larger scale diffusion necessary for mineral breakdown reactions. The observation of anti-phase domains in some pyroxenes in the Nyb¢ eclogite pod with XNa =0.46-0.49 (Car- penter & Smith, in prep.) provides some support for a previous disordered structure, though this may have formed metastably within a stability field for a primitive structure (Champness 1973; Carpenter 1978, 1979) or may have formed at the higher T igneous generation of eclogite a':~ post- ulated in the case of at least one eclogite by Lappin & Smith (1978).
Fleet et al. (1978) expelimentally disordered two natural P2/n omphacitcs (with 0.51 and 0.54 XsaJ at 725_+ 20°C at 15-IS kb. They deduced that the entire primitive field closed at about this temperature and hence much higher than the 'very low' critical temperature less than 300°C as inferred by Yokoyama etal. (1976:778), but much loweJ: than the near-solidus temperature inferred by Champness (1973) (Fig. 2). Unfortu- nately these experimental data do not assist in distinguishing the two hypotheses since 725°C lies within the range of estima*~d temperatures of recrystailisation of the Nyb¢ eclogite pod. The role of pressure, which has received rela- tive y little attention in the literature on ordering and mi:scibility gaps in the sodic pyroxene system, may be significant. By analogy with the miscibility gap in the Di-En system (e.g. How- ells & O'Hara 1975, Mori & Green 1975), in- creasing pressure may extend the compositional width of the miscibility gaps on each side of the primitive field in the Di-Jd system. The two P2/n omphacites upon which Fleet et al. (1978) ex- perimented show respectively 0.05 and 0.27 Xv,;". Hence there is, in fact, no evidence to date, neither from natural nor experimental pyrox- enes, which may assist in locating the P2/n, miscibility gap and C2/c boundaries at lower Xv~?" values~ in particular along the pure Di-Jd join. It is thus conceivable that primitive sym- metry could extend to T> 725°C in compositions on the Di-Jd join.
Whilst not discussing in detail here the phase relationships on the low X.~a side of the primitive field, it is however relevant to note that primitive
LITHOS 13 (1980) Ompkacite from the Nyb¢ eclogite 235
symmetry is reported in pyroxenes with XNa < 0.4 in certain amphibolite facies rocks from Austria (Heritsch 1973, plotted as No. 8 in Fig. lb) and from France (Dr. W. L. Brown, pers. comm.). Thus the primitive field evidently widens with increasing T on this side of the 'ideal' ordered omphacite composition since it is constrained to about XN~=0.4 in lower T blueschist facies pyroxenes (Fig. Ib). Hence in this respect T and increasing Ac proportion (Fig. lb) have the same effect. With increasing T the primitive field may also widen on the high XNa side at the expense of a diminishing miscibility gap, by analogy with its widening with increasing Ac content (Fig. Ib) as the compositional gap diminishes (Fig. ~ " :a j, and by analogy with the situation on the low XNa side.
Although several aspects of the phase rela- tionships in the Ac-poor Di-Jd-Ac system have yet to be resolved, our assessment of the limited data available allows us to deduce:
(i) a wide miscibility gap in the region 0.6< XNa < 0.8, Xre ~+~ 0.1 at low T, which nar- r o w s with increasing T or ultimately, though not initially, with increasing Ac proportion, and fi- nally closes with further increase in either var- iable;
(ii) a primitive symmetry field around the composition Di.50Jd.5oAc.0 at low J', which in- itially widens towards bo~h Di and Jd with in- c:easing T or increasing Ac proportion before finally closing with further increase in either variable;
Off) a C2/c symmetry field in the region 0.8 < XNa < 1.0 at low T, which widens towards lower XNa values with increasing T or ultimately, though not initially, with increasing Ac propor- tion, and finally unites with the analogous C2/c symmetry field extending from DiL00Jd.oAc.o with further increase in either variable.
Acknowledgements. - The original field and petrographical work was supported by a Research Studentship, and the electron microprobe work was carried out under tenure of a Research Fellowship, both kindly provided for D. C. Smith by the British Natural Environment Resea,'ch Council. The opti- cv.I and X-ray work was supported by the Italian Consiglio Nazionale delle Richerche. Professor F. Mazzi and Drs. M. A. Carp,rater, M. A. Lappin, T. Mori and B. G. J. Upton offered helpfal comments on a manuscript of this paper.
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Accepted for publication September 1979 Printed July 1980
