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The Forsterite-Tephroite Series:I

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The Forsterite-Tephroite Series:I American Mineralogist, Volume 65, pages 1263-1269, 1980 The forsterite-tephroite series:I. Crystal structure refinements Cenr A. FReNcIs Mineralogical Museum,Hamard University Cambridge,M assachusetts02 I 38 AND PAUL H. RIBBE Department of Geological Sciences Virginia Polytechnic Institute and State University B lac ksbur g, Virginia 2 4 06 I Abstract The crystal structures of the following metamorphic olivines, M;*[SiO4], in the forsterite- tephroite (Fo-Te) serieswere refned in space group Pbnm: Cation compositionM Cell parameters(A) Mg Mn Ca Fe a b c Rfactor Fol 1.028 0964 0.006 0.002 4.794 10.491 6.t23 0.029 Ten, 0.181 1.780 0.013 0.026 4.879 10.589 6.234 0.039 Treating minql ps and Ca as though they were Mn, the refined cation distributions (esds < 0.01) and mean bond lengths are: M(1) site M(2) site Mg Mn (M(lFo) Mg Mn (M(2)-o) (si-o) For, 0.92 0.08 2.116I^ 0.1I 0.89 2.185A r.6374 Te", O.l7 0.83 2.0294 0.00 1.00 2.2274 l.6,t0A By comparison, a previously refned synthetic ForrT€o, specimen "heat-treated at l(X)0oC" is significantlymore disordered(KD : 0.196)than the naturally occurring For' (Ko: 0.011). As expected,mean M(l)-O and M(2)-O distancescorrelate linearly with Mgl(Mg+Mn) oc- cupancy. Introduction However, the crystal structures of only two Mg- With the widespread availability of automated X- Mn olivines have previously been refined. One of ray di-ffractometeruand least-squaresrefinement pro- them contains I I mole percent of Zn SiOo, thereby grams, order-disorder has become a principal theme complicating the octahedral site refinement (Brown, of contemporary crystal chemical investigations 1970). The other is synthetic ForrTeo, which was (Burnham, 1973).Both natural and synthetic com- "heat treatedat l000oc" (Ghoseand Weidneg 1974; pounds with the olivine structure have been in- Ghoseet al., 1976)and thus doesnot representa nat- tensivelystudied (for a review,see Ganguli,1977). At urally equilibrated Mg/Mn distribution. The present least 25 different room-temperature refinements of refinementswere undertakento examineMg/Mn or- the geologically-importantMg-Fe olivineshave been dering in natural olivines uncomplicated by addi- carried out and 13 additional refinements extend the tional substituents. observationalconditions to temperaturesof - l000oC Experimental proceduresand structure refinenent and pressuresof -50 kbar @assoet al., 1979;Birle er al., 1968; Brown and Prewitt, 1973; Finger, 1970; The crystals used for refinement were selected Finger and Virgo, l97l; Hazen, 1976;Smyth, 1975; from the suite of specimensdescribed in Part II of Smyth and Hazen, 1973;Wenk and Raymond,1973). this study (in preparation). The manganoan forsterite w03-.004X/80/l I 12-1263$02.00 t264 FRANCIS AND KIBBE: FORSTERITE-TEPHROITE S.ERIES from Lingban, Sweden (Harvard University Te' were carried out using the program RFINE 4 #116463) has the compositioo Mg,orrMno"u4 (Finger and Prince, 1975).Scattering factors for neu- Ca"*uFeooouSio4.It occurs with fine-grained haus- tral atoms with corrections for anomalous dispersion mannite in a calcite skarn. In hausmannite-free were taken frort the International Tablesfor Crystal- bands anhedralforsterite crystalsrange from 0.1-1.0 lography(1974, p.99, 149).The refinementswere ini- cm in size.The tephroite, from an unspecifiedloca- tiated using the positional parametersof forsterite tion in Madagascar(Harvard University #108206) (Hazen, 1976),fully orderedcation distributions [Mg hasthe compositionMn, rroMgo,r,Feoo""Ca.o,rsiO4.It concentrated on M(l)], and reasonableisotropic tem- occurs as discrete rounded crystals l-2 mm in diame- perature factors. After several cycles varying the ter associatedwith rhodonite, yellow spinel, and blue scale factor and positional parameters, the conven- apatite in a calcite marble. For the purposesof site tional R factor dropped to -0.10. Site occupancyre- occupancy refinement these compositionswere ap- finements were undertaken by varying the Mg con- proximated as Fo'Teon and Te",Fonand are denoted tent of M(l) while constraining the total cation below as Fo' and Ter,. chemistriesto agreewith the observedcompositions. Refinements were carried out in the conventional Site occupancies were refined alternately with iso- but nonstandard spacegroup Pbnm. Unit-c.ell dinen- tropic temperature factors, and the R factors de- sions (Table l) were calculatedfrom the orientation creasedto R : 0.038for Fo' and R : 0.056for Ten,; matrices and are identical to within three estimated all positional parameters were identical within thlee standard deviations of those obtained by least- estimated standard deviations to those of the final squaresrefinement of powder data as reported in anisotropic model. Anisotropic thermal parameters Part II. The crystalsused for data collectionare tabu- and an isotropic extinction correction were applied in lar with dimensions0.24 x 0.24 x 0.07 mm for Fo' the final rycles of both refinements, leading to un- and 0.30 x 0.30 x 0.20 mm for Ter,. Intensity data weighted R factors for all data of 0.029for Fo' and were collectedin two octants(20 < 70") at l SoCon a 0.038for Ten,. Picker FAcs-l four-circle di-ffractometer,using Nb- Structure factors for which F"o"< 2oF"* were con- filtered MoKc radiation (I : 0.70926A)and ttLeo:-20 sidered unobserved and were excluded from the re- scanning technique at a rate of l" 20 per rrinute. finement. These are indicated in the structure factor Twenty-second background measurementswere tables(Tables 2a,b)' by asterisks.Atomic coordinates made at either end of dispersion-corrected scan are listed in Table 3, anisotropic temperature factors, ranges. Two standard diffractions were monitored equivalent isotropic temperature factors, and their during data collection and used to calibrate the in- root-mean-squareequivalents are listed in Table 4, tensities by interpolation. The data were corrected and interatomic distanc.esand interbond angles are for background, Lorentz, and polarization effects. recordedin Table 5. Crystal shapeswere approximatedby polyhedral en- velopesin the absorption correction. Linear absorp- The olivine structure tion coefficients(p) for MoKa are 47.7 cm-' for Fo' The olivine structure is so well-known that only a and 77.2 cm-' for Ten,.Finally, the data were aver- brief description is given here. [For structure dia- aged to yield sets of 731 and 773 unique structure grams the reader is referred to Birle et al. (1968) and factors for For, and Ten,respectively. Hazen (1976).1It consistsof a slightly distorted hex- Full-matrix least-squaresrefinements of Fo' and agonal closest-packedarray of oxygen anions with one-half of the octahedral sites filled with divalent Table L Crystal data cations and one-eighth of the tetrahedral sites occu- pied by silicon. The key structural feature is the ser- to5, rated chain, parallel to c, of two symmetrically-inde- YI Langban, Sweden Madagascar pendent, edge-sharing octahedra. The M(l) 4.7 94(2)*A 4.879(2) A octahedron is at the interior of the chain and the b 10.491(4) 10.s89 (4 ) M(2) octahedron forms the "elbows" of the chain. c 6.r23(2) 6.234(3) -aa 'zar .3 Vol 3oa.z(z) n3 )zz. L\z) A Density (ca1c. ) 3.67 glcc 4.UO g/cc I To obtain a copy of Table 2, order Document AM-80-141 from the BusinessOffice, Mineralogical Society of America, 2000 ^Numbers ln parentheses represent es!imated standard Florida Avenue, NW, Washington" DC 20009. Pleaserenit $1.00 deviation (14) and refer to the last decimal olace. in advance for ttre microfiche. FRANCIS AND RIBBE: FORS TERI TE-TEP H ROI TE SERI'S 1265 Table 3. Atomic coordinates mentsby Brown (1970)failed to detectMglFe order. Subsequentrefinements by Finger (1970),Finger and t", )l t- Virgo (1971),Brown and Prewitt (1972),Wenk and Raymond (1973),Ghose et aI. (1976),and Basseer a/. Atom x v (1979) have shown small but signfficant degreesof M(1) ordering of Fe onto M(l) rather than M(2). This un- M(2) 0.9870(1) o.27 90 (r) , t4 b1 o. 4226(2) 0.0910(1) expected result, termed anti-ordering by Brown and 0(1) 0.7s8s (s) 0.0867(2) , Prewitt (1973),has generally been attributed to the 0(2) 0.2301 (s ) 0.4489 (2) ,4 0(3) 0.27 82 (3) 0.1590(2) o.037 4 (2) greater distortion from ideal Oo symmetry of the [M(l)O6] octahedron which results in a slightly Atom v z greater crystal field stabilization energy for Fe2* on M(1) M(l) (Walsh et al.,1974) [althoughfor someMg-rich M(2) 0.9877(2) 0.2801(1) ,4 t4 olivines, Fe'* may show a slight preferencefor M(2) 5a o . 4265(4) 0.0951(2) 0(1) o .7s7 4 (LO) 0.0914(5) ,4 (Wenk and Rayrrond, 1973). In general,site prefer- 0(2) 0.2162(rr) o. 452s(5) \ ence of divalent transition-metal cations relative to 0(3) 0.28s7 (8) 0.1625(3) 0.0401 (s) Mg in olivine is determined by two competing fac- tors: cation sizeand crystal field etrects(Rajamani et al., 1975). The [SiO.] tetrahedron sharesits basal edgeswith Divalent manganesetras t high spin d' 6snfigura- two M(l) and one M(2) octahedra.Its apex links to tion which is unaffected by crystal field effects.Thus the adjacentchain, which is related to the first by D- cation ordering in Mg-Mn olivines is predicted glide planes. (Brown, 1970;Burns, 1970,p.118) to be governedby the size criterion with Mg (r : 0.72L) preferring Cation ordering M(l) and Mn (r : 0.834) preferring M(2). The cat- Consideration of cation ordering in non-calcium ion distributions determined in the present refine- olivines began with the prediction of Ghose (1962) ments and an earlier refinement of a synthetic man- that becausethe effective ionic radius of Fe (r : ganoanforsterite (ForrTen ) "heat treatedat 1000"C" 0.78A) is larger than that of Mg (r : 0.72A) it should by Ghose and Weidner (1974)genfirm the predicted be preferentially ordered into the larger M(2) site.
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