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Crystal Structure Refinement of Magnesian Alleghanyite

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Crystal Structure Refinement of Magnesian Alleghanyite American Mineralogist, Volume 70, pages 182-185,1985 Crystal structure refinementof magnesianalleghanyite C.cnL A. FneNcrs Harvard Mineralogical Mus eum 24 Oxford Street, Cambridge, Massachusetts02138 Introduction fifteen analyseson two crystals. The rather large amount of zinc and very minor amounts of iron and calcium are typical of Mn- group four homo- The humite group, strictly, is the of rich olivines (Francis, 1980)and humites (Ribbe' 1982)from the logues characterized by the general formula nMg2 Franklin marble. SiOa'Mg(OH,F)2wheren = I fornorbergite,n:2for The crystal was mounted on a Picker r'.q.cs-l four-circle chondrodite. n : 3 for humite and n : 4 for clinohumite. diffractometerwith its diad nearly parallel to the phi axis and was Alleghanyite, the manganese analogue of the n = 2 oriented in terms of the nonstandard space group F21lb to homologue,was first describedfrom Bald Knob, Allegha- conform with previous studies(Taylor and West, 1928;Gibbs et ny County, North Carolinaby Ross and Kerr (1932).Its al.. 19701Yamamoto, 1977)and the recommendationsof Jones (1969). : 4.815(2),b = 10.574(3),c - isotypism with chondrodite was proposed by Rogers The unit cell dimensions:a and a : 108.74(2)"were refined from twelve, indivi- (1935) and subsequently established by Rentzeperis 8.083(3)4, dually-centered,high-angle reflections (20> 40'). Intensity data (1970). Alleghanyite was recognized in specimensfrom were collected in one quadrant (20 = 72") at l8"C using Nb- County, the mines at Franklin and Ogdensburg,Sussex filtered MoKal radiation (I = 0.70926A)and a 20 scan rate of l' New Jerseyby Cook (1969)where it occurs in at least two per minute. Twenty second background measurementswere parageneses-as isolated anhedral crystals in the Frank- made on either side ofdispersion-corrected scan ranges(1.2" 2e lin marble and as euhedral crystals in veinlets cutting the base width). ore. An example of the latter from the Sterling Mine Two reflections monitored after every fifty data were used to (Harvard University #109468')exhibits crystals of magne- calibrate the data set by linear interpolation. The data were polarization effects.The sian alleghanyite among a druse of brilliant black cube- corrected for background, Lorentz and was approximatedby a polyhedral envelope octahedronsoffranklinite on a slickensidedsurface ofred shapeof the crystal in the absorption correction (p = 55'6 cm-r)' Structure factors willemite-franklinite-calcite ore. The crystal structure of for which Fo6. < 2a were consideredunobserved, leaving a data this specimen approximately an untwinned crystal from set of 1873observations. 0.3 x 0.2 x 0.1 mm in size was refined as part of an investigation of Mg/Ir4n ordering in the olivines and humites(Francis, 1980;Francis and Ribbe, 1978'1980). Refinement Full matrix, least-squaresrefinement was carried out using the Experimental procedures program nrlre (Finger and Prince, 1975).Scattering curves for (1968)' Chemicalcomposition was determined on an automatedARL- neutral atoms were taken from Doyle and Turner the eux electron microprobeoperating at 15 kv accelerating Corrections for anomalous dispersion were taken from (194, p' 149)' potentialand 0.02 pA samplecurrent on brass'Intensity data International Tables for Crystallography 9' parameters werereduced on line usingthe techniqueof Benceand Albee Refinement was initiated using the positional of Mn (l%8) andthe correction factors of Albeeand Ray (1970). Simple Gibbs et al. (1970)with the octahedralsites fully occupiedby 0'542 for oxidesand silicates, including synthetic tephroite (Takei' 1976)' and with isotropic temperaturefactors of 0.5, 0.3, and parameters were used as standards.The formula, MnzsoMgr s+eZno tu Mn, Si, and O respectively. After the positional Mg was assignedto the octahedralsites accordingto Feo oBCao 0o5(SiO4)2(OHr.o6Foea) represents the average of converged, 0003-004x85/0l 02-0 I 82$02.00 tE2 FRANCIS: STRUCTURE OF MAGNESIAN ALLEGHANYITE lE3 mean M-O distances(Francis and Ribbe, l97E). Refinementof Table 1. Atomic coordinates and equivalent isotropic site occupancies,and isotropic temperature factors converged temperaturefactors (Beq) of magnesianalleghanyite with a conventional R-factor of 0.057 without constraining the total chemistry. Zinc was then assignedto M(3), the smallest Atom x y z seq.(i)z site, (cf. Ghoseand Weidner, 1974),and the total chemistry was constrainedto agreewith the chemical analysis.The refinement, including anisotropic temperature factors, rapidly converged to produce : positional !,f(r) 4 0 L 0.39 a model with R 0.M8 (Rw = 0.053). H(2)(5) 0.010i(2) 0.i718(1) 0,30i3(1) 0.33 parameters,temperature factors, and interatomic distancesand l,!(3) 0.4897(3) 0.8841(1) 0.0760(1) 0.34 si 0.07s7(2) 0.144s(1) 0.7090(2) 0.27 anglesare listed in Tables I and 2.r 0(1) 0.78?2(7) 0.9946(3) 0.2906(4) 0,50 o(2) 0.7186(7) 0.2446(3) 0.1245(4) 0.5s Stereochemistry o(3) 0.2135(7) 0.1698(3) 0.s374(4) 0.s7 0(4) 0.2632(6) 0.8550(3) 0.2890(4) 0.55 The steric details of the chondrodite-alleghanyite (0H,F) 0.2614(7) 0.0527(3) 0.0919(4) 0.61 structure, like those of the olivine and other humite structures, result from distortions of the close-packed anion array by cation-cation repulsions across shared polyhedral edges.These have been thoroughly discussed for chondrodite by Gibbs et al. (1970) and yamamoto Table 2. Interatomic distances and angles of magnesian alleghanyite (1977) and for alleghanyite by Rentzeperis (1970). For structure diagrams,site nomenclatureand a review of the crystal chemistry of the entire humite group see Ribbe tSi041 tetrahedron (1982).This structure difers from those of others in the Si ... distances(A) 0 ... 0 dlstances(A) Anglesat St (') seriesin that the M(3) octahedron is significantly smaller si-0(4) 1.623(3) o(1t-o(3) 2.551(4): t03.05(z) than the other 0(4) 1.627(3) o(r)-o(?) 2.552(4): 103.47(2) two octahedra resulting in a greater range 0(3) 1.635(3) 0(2)-0(3) 2.s88(4)" 10s.03(2) in (M-O) disrances(0.0944) than in chondrodire(0.038A) 0(1) 1.632(3) 0(1)-0(4) 2.744(4) rL4.s2l2l fean 1.629 0(2)-0(4) 2.741(4) r14.s4(2) and in alleghanyite(0.022A;. polytredral distortions, as 0(3)-0(4) 2.747(4) 114.44(2) measuredquantitatively by the bond angle variance pa- Mean 2.654 L09.24 (d) rameter of Robinsonet al. (1971)are comparedin ltl(1)06octahedron Table 3. The distortion parametersfor the silicate tetrahe- lll ... 0 distances(A) 0 ... 0 distances(A) Anglesat il(l) (.) dron, the M(2), and M(3) sitesare intermediatebetween r.,r(1)-0(1) r2l 2.157(3) o(r)-o(3)rzr z.ssr(s)r 7r.54(1) the respective values -0(3)t2t 2.207(3) 0(l)-0(4) t2t 2.900(3)" 83.24(1) for chondrodite and alleghanyite, -0(4) r?l 0(3)-0(4) t21 2.e37(3\" 83.37(1) _2=!99_LqI ' which is consistentwith the observation(Robinson l,lean 2.191 0(3)-0(4) r21 3.2e8(3) s6.63(1) et al., 0(1)-0(4)| r2r 3.254(3) 96.76(r) l97l) that for the olivines, distortion of the tetrahedron 0(L)-0(3), r2l 3.541(3) 108.45(r) decreaseswith increasing mean M-O while distortion of Range0.052 ilean3.082 90.00 the octahedra increaseswith increasing mean M-O. The l'l(2)05(0H,F) M(l) octahedron octahedron in magnesianalleghanyite, however, is ll ... 0 distances(A) somewhatmore distorted than in alleghanyite.Distortion r.r(2)5-0(3) 2.107(3\ of the 0(l) 2.13e(3) octahedra decreaseswith increasing number of 0(4) 2.258(3) monovalentligands. 0(3)' 2.2s3(31 o(2) 2.342(31 (0H,F) 2.150(3) Cation ordering ilean 2.212 The octahedral cations in magnesianalleghanyite are Range0,235 ordered (seeTable 4 for details). Although Mn-Mg distri- bution cannot be uniquely determined becauseof signifi- l4ean 3.121 89.34 cant zinc content, the refined occupanciesare consistent x(3)04(oH,F)2octahedron ll with the previousresults for olivines(Francis and Ribbe, ... 0 distances(A) 0 ... 0 distances(A) Anglesat il(3) (e) r't(3)-0(2) 2.0u(3) l9E0)and manganhumite(Francis and Ribbe, l97g). The 0(4) 2.141(3) smallest 0(21 2.152(3\ octahedron,M(3) with (M-(O,F,OH)) : 2.1l8A, o(1) ?.248(3\ is principally occupiedby the : (0H,F) 2.063(3) smallestcation, Mg (r (0H,F) 2.075(3) 0.7204),while M(l) with (M-O) = Z.rglland M(25)with ilean 2.118 Range0.231 I To receive a copy of the structure factor and anisotropic filean 3.007 89.82 temperaturefactor tables, order document AM-85-259from the BusinessOffice, Mineralogical Society of America, EEdge 2000Florida shared between -roge tetrahedron and octahedron. Avenue NW. Washington,D.C. shared between trc octahedra. 20009.Please remit $5.00in *llu]tipllcity advancefor microfiche. indicated In square brackets. 184 FRANC/S; STRUCTURE OF MAGNESIAN ALLEGHANYITE Table 3. Polyhedral distortion parameters(l) for chondrodite, For the M(l) and M(26) octahedra,which are exclusively magnesianalleghanyite and alleghanyite coordinatedby oxygen atoms, the correlationsare strong- ly linear. However, the replacemenlof OHIII(1.36 < r < Chondrodite Maqnesian At leqhanvite = Polyhedron cibbs et al, (1970) Atleghanyite Rentze;eri; (1970) 1.45A) (Ribbe, 1979)by FIII(r 1.30A) reducesmean cation-anion distances. For example, the difference be- tween(M(2s)-(O,OH)) in synthetichydroxyl chondrodite sicot 44.6 35.4 29.4 (Yamamoto, 1977) and (M(25)-(O,F,OH)) in a natural f1( 1)0.* 10i.2 156.5 150.9 fluorine-richchondrodite (Gibbs et al., 1970)is 0.0174, 14(2)05 ( F 'oH )* 82.5 106.4 712.8 which is abofi 30Voof the size difference between M(20) r.,r(3 (F'0H 59 M(3).
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