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 between(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). Accordingly, a term f, defined as one-half,the )04 ) 2* .2 7t.4 91.1 and number of fluorine atoms per formula unit times the +o2{t"t1 number of monovalent ligands associatedwith the octa- =.9,1or-tor.+zoy27s hedron,was includedin the regressionanalyses. *, 12 =_0, o'(oct) (0.-90.00)'/ll (M(2)-(O,OH,F)):l.s4l + 0.8185(r'rr) - 0.01649f(r :0.994) (M(3)-(O,OH,F)): 1.502+ 0.8386(rur:) - 0.02r22f(r: o.iSol (M-(O,F,OH)> : 2.2124 are principally occupiedby Mn This permitted data for M(2n), M(2s), and M(2e) to be (r = 0.830A). Site occupanciesand consequentlythe incorporatedinto a single equation and resulted in signifi- effective ionic radii, (r-) (Shannon, 1976), calculated cant improvement in the correlation coefficients. from the occupancies are both highly correlated with Summary of cation ordering in the humites meanoctahedral bond length: Humites may contain Mg, Ca, Ti, Mn, Fe and Zn (M{O,F,OH)):2.216 - 0.r2r7Me/(Ms+Mn+Zn) and Ca form end members.These r : -0.986 although only Mg, Mn cations are ordered into the octahedralsites which vary in (M-(O,F,OH)): 1.384+ 1.001(r-) r = 0.985 distortion, ligancy, size, and symmetry. This and the previous refinements of intermediate Mg-Mn humites Octahedralsize in the humite seriesdecreases in the clearly demonstrate that manganeseis concentrated in order M(26) > M(2s) > M(l) > M(3) due to increasing the larger sites, which decreasein size in the order M(2e) numbers of shared edges and the substitution of three- > M(2s) > M(1) > M(3). Kato (pers.comm.) has refined coordinated monovalent anions (OH and F) for four- the structure of a sonolite (Mn-rich, n : 4 homologue) coordinated oxygens. The difference in size between which is the most calcium-rich natural humite yet report- M(20)and M(3) in humite (Ribbe and Gibbs, 1971)and ed. Its (M(20)-O)and (M(25)-(O,OH))distances are larger clinohumite. (Robinson et al., 1973) averages0'0534 than (M-O) distances of olivine and humite octahedra which is equivalent to the diference in ionic radii be- exclusivelyoccupied by Mn, implying that Ca is concen- tween Mg and Fe2+. Thus, octahedral size must be trated in the largest octahedra' Tia* is believed to be considereda significantfactor affecting cation ordering in preferentially concentratedin M(3) which is the smallest humite group minerals. site(Ribbe, 1979). While this is consistentwith The relationship betweenoctahedral size and the radius octahedral size criteria, the coupled substitution calculatedfrom site occupancyfor the M(1), M(2), and M(3) octahedrain four Mg-Mn olivines and eleven hu- Ti4* + 2o2- : M2* + 2(oH,F)- mites (data in Francis 1980)are described by the follow- factor. Fe2* was observed by ing regressionequations. is probably the decisive Ribbe and Gibbs (1973)to be concentratedin M(2d and : (M(l)-O) : 1.445+ 0.9172(rlnt') (r 0.995) M(l), thus avoiding octahedracoordinated by (OH'F). ordered (M(26)-O): 1.52r+ 0.8459(r'(2u)) (r : 0.997) Thus, octahedral cations in the humites are accordingto size criteria, although ligancy (O,OH,F) may = (r'(2s))(r : 0.983) (M(2s)-(O,OH,F)) l.a9t + 0'8713 be the controlling factor where chargebalance (e.g., Tia*) (M(3){o,OH,F)): 1.414+ 0.9442(rrrr) (r : 0'939) and crystal-fieldeffects (e.g., Fe2*) are involved.

Table 4. Octahedral site occupancies,ligancies and mean M-O distancesof magnesianalleghanyite

No. of edqesshared !ith .. ,f.ln^.,, M-(u t Si te Li gancy Tetrahedra 0ctahedra ' ,ufl,

M(1) 06 0.13s(7)0.865 4 t 101 u(2)s 05(F.oH) o.oe8(4)o.eo2 1 2 2 )12 M(3) 04(F,oH)20.808 o.1oo o'092 1 3 2.178 FRANCIS: STRUCTURE OF MAGNESIAN ALLEGHANYITE 185

Acknowledgments Jones,N. W. (1969)Crystallographic nomenclature and twinning in the humite minerals. I am grateful to ProfessorC. W. Burnham for accessto the X- American Mineralogist, 54, 309-313. Rentzeperis,P. J. (1970) ray diffraction apparatusand to Dr. D. L. Bish for assistancein The crystal structure of alleghanyite, Mn5[(OH)rl(SiOo)r].Zeitschrift data collection and processing.Critical reviews by S. Ghose, G. fiir Kristallographie, 132,l-18. Ribbe, P. H. (1979) Lumpkin, J. Post, P. H. Ribbe, and J. A. Speerimproved the Titanium, fluorine, and hydroxyl in the clarity of the manuscript. humite minerals. American Mineralogist, g, 1027-1035. Ribbe, P. H. (19E2)The humite series and Mn-analogs.In paul References H. Ribbe,Ed., Orthosilicates,Reviews In Mineralogy,v. 5, p. 231-274.Mineralogical Society of America, Washington,D.C. Albee, A. L. and Ray, L. (1970)Correction factors for electron Ribbe, P. H. and Gibbs, G. V. (1971)Crystal structuresof the microprobe microanalysis of silicates, oxides, carbonates, humite minerals: III. Mg/Fe ordering in humite and its relation phosphates, and sulfates. Analytical Chemistry, 42, l4}g_ to other ferromagnesiansilicates. American 1414. Mineralogist, 56, lt55-1173. Bence, A. E. and Albee, A. L. (1968) Empirical correction Robinson,K., Gibbs,G. V., and Ribbe, p. H. (1971) factors for the electron-microanalysisof silicates and oxides. euadratic elongation: A quantitative measureof distortion in coordina- Journalof Geology,76,3EZ-403. tion polyhedra.Science, 172, 567-570. Cook, D. (1969)Sonolite, alleghanyiteand leucophoenicitefrom Robinson,K., Gibbs,G. V., and Ribbe,P. H. (1973)The crystal New Jersey. American Mineralogist, 54, l3g2-139g. structure of the humite minerals. IV. Clinohumite and titanb- Doyle, P. A. and Turner, P. S. (1968)Relativistic Harrree-Fock clinohumite. American Mineralogist, 58,43-49 and 346. X-ray and electron scatteringfactors. Acta Crystallographica, Rogers,A. F. (1935)The chemicalformula 424, 390-397. and crystal systemof alleghanyite.American Mineralogist, 20, 25-35. Finger, L. W. and Prince, E. (1975)A system of Fortran IV Ross,C. S. and Kerr, P. F. (1932)The manganeseminerals of a computer programsfor crystal structure computations. United vein near Bald Knob, North Carolina. American Mineralogist, StatesNational Bureau g54. StandardsTechical Note 17,l-18. Francis, C. A. (1980)Magnesium-manganese solid solution in Shannon,R. D. (1976)Revised efective ionic radii and systemat- the olivine and humite groups. ph.D. Dissertation, Virginia ic studies of interatomic distancesin halides and chalcogen- Polytechnic Institute and State University, Blacksburg, Vir_ ides. Acta Crystallographica, ginia. A32, 7 5l-:767 . Takei, H. (1976) Czochralski growth of Mn2SiOa (tephroite) Francis,C. A. and Ribbe,P. H. (197E)Crystal srructuresof the singlecrystal and its properties.Journal of Crystal Growth, 34, humite minerals: V. Magnesian manganhumite. American r25-t31. Mineralogist, 63, 874-877 . Taylor, W. H. and West, J. (192E)The crystal structure of the Francis, C. A. and Ribbe, P. H. (1980)The forsterite_tephroite chondrodite series. Proceedingsof the Royal Society of Lon- series: I. Crystal structure refinements. American Mineral- don, SeriesA, l17. 517-532. ogist,65, 1263-1269. Yamamoto, K. (1977)Hydroxyl-chondrodite. Acta Crystallogra- Ghose,S. and Weidner,J. R. (1974) preference Site of transition phica,B33, l48l-14E5. metal ions in olivine. GeologicalSociety of America Abstracts with Programs, 6, 751. Gibbs, G. V., Ribbe, P. H., and Anderson, C. p. fl970) The crystal structuresof the humite minerals:'II. Chondrodite. Manuscript received, February 9, 1964; American Mineralogist, 55, 1192-1194. acceptedfor publication, September4, 1984.