American Mineralogist, Volume 63, pages 874-877, tgZS

Crystalstructures of thehumite : V. Magnesianmanganhumite

Cnnl A. FnaNcrsr,qNo Peul H. Rrssr Department of Geological Scjences Virginia Polytechnic Institute and State Uniuersity B lac ksburg, Vi rginia 2406 I

Abstract

The crystalstructure of typemanganhumite from the Brattlorsmine, Nordmark, Sweden [(Mno,u,MgoroFeoo,Cao.o,),(SiO.)s(OH),,Pbnm,a:4.815(l),b:10.580(2),c:21 .448(5)Al hasbeen refined byconventional least-squaresmethodstoR :0.051 (R- = 0.081),using 1562 structurefactors with F"u" ) 2o(Fo6").Mean Si-O distancesfor the isolatedtetrahedra are (Si(f)-O) : 1.628Aand (Si(2)-O): 1.632A.The apicalSi-O distancesof 1.600and 1.6044 are the shortestyet observedin humite-groupminerals. Site refinement yielded Mgl(Mg+ Mn) ratiosof 0.30in the M(l)Ou octahedron(mean M-O distance:2.1'70A),0.0 in the M(2)O6 and M(2)O,OH octahedra((M-O, : 2.222 and 2.206A), and 0.75 in rhe M(3)O4(OH),octahedron ((M-Ot : 2.117A).The correlationcoefficient of Mgl(Mg+Mn) occupancyand meanoctahedral bond lengthis r : 0.986,and as expected,octahedral sizes correspondalmost exactly to theweighted radii of thelarger Mn andsmaller Mg cations(r : 0.989).

lntroduction been observed in all subsequentrefinements of hu- The humite minerals are a homologous seriesof mite group minerals, including several titanian hu- magnesium orthosilicates based on hexagonal mites(Robinson et a|.,1973;Kocman and Rucklidge, (1971) closest-packedarrays of anions and structurally re- 1973).Ribbe and Gibbs concludedthat dis- lated to forsterite. Microprobe analyses of fifty-five tortion and ligancy rather than sizeof the octahedral govern humite samples(Jones et al., 1969)led to the estab- sites the ordering of Fe in Mg humites, Fe preferring (F,OH) lishment of the generalformula: those octahedra with no ligands, i.e.,M(l)Ou and M(2)O6. nlM al . M F,T|,(OH,F)2-2,O2" The recent discovery of Mn humites containing "SiO where M is Mg,Fe,Mn,Ca,Zn in decreasingorder of considerableMg provides an opportunity for further abundance,0 < x < l,andn : I fornorbergite,2for investigationof cation ordering in humites.This pa- ,3 for humite, and 4 for (see per reports the resultsof a site occupancyrefinement Table l). Manganeseend-members of three of the of magnesianmanganhumite. four homologues are well-known, and unpublished Experimental procedures microprobe analysesby C. Richardson(private com- munication, 1976)indicate extensive, if not complete, Crystalsof manganhumitefrom the type specimen from were gen- solid solution betweenMg and Mn end-members. Brattfors Mine, Nordmark, Sweden, Ordering of the small concentrationsof Fe nor- erously donated by ProfessorP. B. Moore, Univer- mally presentin Mg humiteswas initially detectedby sity of Chicago, and upon completion of this study Ribbe and Gibbs (1969,l97l) in a humitewith com- will be deposited in the Mineralogical Museum of Harvard University. Moore (1978) reports their position MguuFeo osirO,rF(OH). Mg/Fe orderinghas composition to be (Mno..rMgo.roFeo.orCao.or)r(SiOa)g (OH),' ' Presentaddress: Mineralogical Museum, Harvard University, Manganhumite is orthorhombic and was refinedin 24 OxfordStreet, Cambridge, Massachusetts 02138. the nonstandardspace group Pbnm to conform with 0003-004x/7 I /09l 0-0874$0200 FRANCIS AND RIBBE: MAGNESIAN MANGANHUMITE 875

Table l. Nomenclatureof humite group mineralsr[M,SiOo]. assigned,and after two cycles of refining the scale M(OH,F), with referencesto moderncrystal structure refinements factor and several more cycles of refining both the Honologue Mg end-menber Mn end-nenber scale factor and the positional parameters,the con- q= 1 MAN* ventional R factor dropped to 0.22. At this stage, (clbbs and Ribbe, 1959) : \= 2 Chondrodlre Alleghadylte diffractions of the type h + k 2n i I showed good (clbbs er at., 1970) (Rentzeperis, 1970) agreement between .Fou.and F"ut", while diffractions != 3 Hulte ManganhmiEe (Ribbe and Cibbs, 1971) (This 6rudy) of the type h+k : 2n showed poor agreement.This g= 4 Clinohmite Sonolite systematicdiscrepancy was attributed to Mn/Mg or- (Robinson eE a1 , 1973) (Karo, 1n preparaEion) dering, and when Mg was assignedto the four non- *Manganese Analog of Norbergice (},IAN), synrhesized by CAF: !=4.869(2'), b=10.796(2), c=9.179(2) A. equivalent octahedral sites on the basis of average M-O bond distances,the R factor improved suf- previousstudies (Taylor and West,1928; Ribbe and ficiently to releasethe temperaturefactors. Using the Gibbs, l97l) and the recommendationsof Jones temperature factors, atomic coordinates, and scale (1969).Unit cell dimensions,4 : 4.815(l),b : factor generated in this manner, site refinement fol- 10.580(2),c = 21.448(5)A,were determined by least- lowed. squaresrefinement (program of Appleman and Being the chief substituent,Mg was selectedas the Evans,1973) of 22 powderX-ray lines (16-60"U) independentvariable and, subject to the restriction recordedwith monochromatizedCuKa radiation(X that the total magnesium content must equal 2.1 : l.54l8A) on a PhilipsNorelco powder diffrac- atoms, was first distributed randomly over all four tometerusing BaF, (a : 6.198A)as an internalstan- octahedral sites.The trace levelsof Fe and Ca were dard. ignored throughout the site refinement.The concen- A cubicfragment 0.2 mm on an edgewas mounted trations of Mg in M(2), and M(2). dropped to with b nearlyparallel to thephi axisof a PickerFecs- slightly negative values, so these sites were sub- I four-circlediffractometer. Intensity data were col- sequentlyconsidered to be fully occupiedby Mn. The lectedin two octants(20 < 70") usingNb-filtered site occupancies then converged to the values MoKa, radiation(I : 0.709264)and a20 scanrate 0.304(6)Mg in M(l) and 0.746(6)Mg in M(3), irre- of lo/minute.Background measurements were made spectiveofthe sitechosen as the dependentvariable. for 20 secondson eitherside of dispersion-corrected Data for which Fou" I 2o(Fo6.)were considered scanranges (1.2'm basewidth). unobserved.The 238 data rejectedfrom the refine- Symmetricallyrelated standard diffractions mon- ment by this criterion are indicated in the structure itoredafter every 50 datashowed a maximumvaria- factor table (Table 2)'zby asterisks.In the final cycles bility of t5 percent.No interpolationof thedata was made,but an "ignorancefactor" of 0.025was incor- 2To receivea copyof Table2, order DocumentAM-78-086 from poratedin the calculationof the weightingfunction the BusinessOffice, Mineralogical Society of America,Suite 1000 (Fingerand Prince,1975, p.5). The datawere cor- Lower Level, 1909K Street,N.W., Washington,D.C. 20006. rectedfor background,Lorentz,polarization and ab- Pleaseremit $1.00 in advancefor the microfiche. sorptioneffects (p*o*o: 63.1cm-') and thenaver- Table3. Positionalparameters, isotropic temperature factors and agedto yield a setof 1800unique structure factors r.m.s.equivalents, p, for manganhumite usingthe programs DnrnLIs andDnresonr from the qt(A) Atom xla ytb z/c B(A2) World List of CrystallographicComputer Programs M(1) o.oo1s(1)* 0.37s7(1)0.r7s8(r) 0.90(2) o.107(1) (3rd ed. and supplements). M(2)6 .s141(2) .1s83(r) .2s .63(2) .089(l) M(2)s .008s(r) .0s74(r) .10e1(r) .s3(2) .l0s(i) M(3) .4s01(2) .86s7(l) .0270(L) .84(3) .103(2) Refinement si (r) . 07s0(4) .e6s9(r) .2s .63(2) 089(l) A full-matrixleast-squares refinement was carried si (2) . s7s3(3) .2844(r) .1043(1) s7(2) .o8s(r) out usingthe programRnINn4 (Finger and Prince, o(2.3) .71S0(6) .2178(2) .16s9(l) .84(4) .103(2) 0(1.3) .2168(6) .038r(2) .1896(1) 86(4) .104(2) 1975),and atomic scatteringfactors for neutral o(2.4) .71s7(6) .2130(2) .0447(r) . 7s(4) . 098(3) atomstaken from Volume4 of the InternationalTa- o(2.1) .24s2(7) .28s0(3) .1029(1) .89(4) t06(2) bles for Crystallography(1974, p. 99, 149).The re- o(r 2) .278s(8) .32e3(3) .2s .80(s) .101(3) o(r.1) .742r(s) .s680(4) .2s .s7(6) .1rr(3) finementwas initiatedusing the positionalparame- o(2.2) .7778(6) .92s2(2) .1031(l) .72(4) .0s6(5) tersof humite(Ribbe and Gibbs, l97l) andassuming oH .263r(6) .0306(2) .0333(l) .87(4) .los(2) that all octahedralsites were completely occupied by rNunbers in parentheses are estinated standard deviations and Mn. Reasonableisotropic temperature factors were refer to to the last decinal Place. 876 FRANCIS AND RIBBE: MAGNESIAN MANGANHUMITE of refinement,corrections for anomalousdispersion tancesand angles are reported in Table4. Theatomic and extinctionwere includedwithout significantly designationsfollow thoseused by Ribbeand Gibbs alteringthe results.The final unweightedR factoris (re7l). 0.051(R* : 0.081)for the observeddata; R : 0.058 andR* : 0.082for all 1800data. Atomic coordinates Resultsand conclusions and isotropictemperature factors and their r.m.s. Manganhumiteis confirmedto be isotypicwith equivalentsare listed in Table 3. Interatomicdis- humite,and becauseof its closesimilarity to humite, the readeris referredto Ribbeand Gibbs (1971)for Table 4. Si-O, M-O and O-O distances(in angstroms) and O-Si- structurediagrams and detailed discussion of theste- O and O-M-O angles (in degrees)in manganhumite reochemistry.We note,however, that the apicalSi-O bonddistances of 1.600and 1.6044in manganhumite Isio, I rerrahedra st(1)-o(1,1)a 1.604* sr(2)-o(2,1)A 1.600* (Table 4) are the shortestyet observedin humite- o(1,2) r.645 o(2,2) r.648 group minerals. o(r,3) [2] 1.533 o(2,3) r.640 Mean 1.628 o(2,4) r.540 The cationsin manganhumiteare ordered(Table ltean 1,632 Angles at Angles at 5), the magnesiumbeing concentrated in the M(l)06 0...0 distances* S1(1)** 0...odlstances* sl(2)** and M(3)O.(OH),octahedra, each of whichare "in- (r,3)-(1,2) [21 2.5598 r02.7 <2,3)-(2,2) 2,s74r rO3.O (1,3)-(r,3) 2,59L 105.0 (2,4)-(2,2) 2.570x r02.8 terior" sitesin the serratedchain of edge-sharing (1,2)-(1,1) 2.735 LL4.7 (2,3)-(2,4) 2.599t 104.8 octahedra(Ribbe and Gibbs, 1971,Fig. l). These (r,3)-(r,1) [2] 2.731 115.1 (2,r)-(2,2) 2.742 rr5.2 Ilean 2,651 L09.2 (2,4)-(2,L) 2.72L Lr4.2 sites,being constrained by the sharingof polyhedral (2,3)-(2,1) 2.737 r15.3 Mean 2.657 IQ9.2 edgesbounding opposite faces of the octahedra,are lM(O,OH)61octahedra smallerthan the M(2) sitesat the "elbows" of the r.1(1)-o(2,2) 2.rL4 M(2)(-o(1,3)A 2.093 ' o(L,2) 2.L44 o(2,2)^ 2.1,36 chains(see Table 5 for details).The correlationcoef- o(1,1) 2.186 o(2,3) 2,258 o(2,1) 2.r9r o(2,1) 2,285 ficientof Mgl(Mg + Mn) occupancyand meanoc- o(1,3) 2.L7s o(2,4) 2.309 tahedralbond lengthis r : 0.986,and as expected, o(2,3) 2,209 0H 2,755 I,lean 2.L70 llean 2.206 octahedralsizes correspond very closely to theweigh- Angles at Angles at 0..'o dlstance6* M(1)** o"'o distances* M(2) ted radii of the largerMn2+ and smallerMg2+ cat- S** : (2,3)-(2,2) 2.s74t 73.0 Q,3)-(2,4) 2.s99t 69.4 ions,r 0.989(see Fig. I ). Thecorrelation of octahe- (1,3)-(r,2) 2.559E 72,7 (2,4)-(2,L) 2.9r5o 78.7 (1,3)-(1,1) 2.9370 a4.1 (2,3)-(2,1) 2.9670 61.) dral site occupancywith sizerather than ligancyor (1,2)-(r,r) 2.9050 84.3 (r,3)-(2,2) 3.0s5 92.5 octahedraldistortion, as expressedquantitatively by (2,L)-(2,2) 2.9L5o 85.2 (2,3)-(r,3) 7.Lo2 9r,4 (2,3)-(2,7) 2.9670 84.8 (1,3)-(2,r) 1,206 94.O octahedralangle variance (Robinson et al., l97l), <2,L)-(L,2) 3.193 94,9 (2,4)-(2,2) 3.301 95.8 (r,1)-(2,2) 3.186 95,6 (2,3)-(2,2) 3.385 100.7 canbe rationalizedby thefact that Mn2+with its high (2,3)-(1,1) 3.200 93.8 oH-(2,2) 2.968 88,4 (1,3)-(2,1) 3.263 96.7 0H-(2,1) 3.077 spin du electronicconfiguration has no crystalfield (2,3)-G,2) 3.46s los.s 0u-(2,4) 3.192 93.8 (L,3)-(2,2) 3.487 108.8 0n-(1,3) 3.359 I04 .5 stabilizationenergy (Burns, 1970), whereas Fe2+ (d') Mean 3.054 90.0 Mean 3.094 89.9 does.Although Fe is largerthan Mg, it prefersthe M(2)6-o(1,2)A - 2.Li4 M(3)-o(2,4)A 2.ozl o(2,3)Al2l 2.r4L o(2,r) 2.rs5 M(2)O6and M(l)Oe octahedrain the Mg humite 0(1,1) 2.292 o(2,4)' 2.167 0(1,3)t2l 2.37L o(2,2' 2,22s (Ribbeand Gibbs, 1971),whereas in manganhumite ldean 2.222 OH 2,06j Mn prefersM(2)O6, M(2)O'(OH), and, to a lesser 2.069 ltean 2.117 extent,the smallerM(l)06 octahedron.The results Anglesat Aillll(sa! are consistentwith observedcat- 0..'0 dlstances* M(2)6** o'..0 dLsrances* M(3)** for manganhumite (1,3)-(1,3)r 2.s9Lt 68.2 (2,4)-(2,2) 2.s7ot jL.s ion distribution in (Mnt.g.Mg..ruZno.zaFeo.rz)SiOo (1,3)-(r,1)[21 2.9370 79.3 (2,4)-(2,r) 2.sLso 84.8 (1970) and (Mg'.o"Mno.'o)SiOo (2,3)-(r,2) [2] 3.0r0 89.5 Q,D- 2,2) 2.9r5o 83,3 studied by Brown (1,3)-(2,3)[2] 3.101 88.2 oH-oHt 2.9800 84.r studiedby Ghoseand Weidner(1974). (2,3)-(1,1)[2] 3.208 92.4 oH-(2,4) 2.s35 sr.1 (1,3)-(1,2)[2] 3.352 97.8 oH-(2,r) 2.9s4 90.4 (2,3)-(2,3)' 3.608 -89:5'114.8 oH'-(2,2) 2.s9r 87.3 Table 5. Octahedral ligancy and shared edges, octahedral angle Mean tnT o1'-(2,4)t 3.070 s.,a variance,ofi, mean o""o * Mn) ratiosfor oH'-(2,2) 3.102 92.5 oH'-(2,4) 3.191 97.5 H:,Jt;f#.Me/(Me (2,1)-(2,4)', 3.156 98,6 (2,4)-(2,4)' . No. of edses shared wiEh Mean Mg 3.L75 98.6 5IEe LrSanCy /d?\\-e/ * Mean 3.000 89,8 Octahedra Tetrahedra M-(o,0H) (Mg+Mn) * M(1) O. 4 2 L2E 2.170^ 0.304(6) E9tircted etandad e?rcre @e 0.005 A fo! Si.-O @14M-0, Lese M(2). 2 tha 0.006 A fo" 0...O distqrcee. l2l indiutea mltipli,city. O. I 97 2.222 0.0 M(2) 0. (0H) Z ** fhe estiwted etmdail enor in all bond anglee iB 0.1o. _ L 94 2.206 0.0 M(3) 0, (0H)^ 3 1 66 2. t-r7 0.7 46(5') t Edge ehued behteen tetmhedton md, octahedyon. o Edge ehated behMen hno octahedta. * - 6 = apical bond. aftet tlp con entim of Ri,bbe and OLbbs (15?1). ofr Y

O---O Meonlonic Rodius, A Brown, G. E., Jr (1970)Crystal Chemistry of the Oliuines.Ph.D. o a2 0.80 0 78 0.76 0.74 0.72 Thesis, Virginia PolytechnicInstitute and State University, <.225 Blacksburg,Virginia. E Burns, R. G. (1970) Mineralogical Applicationsof Crystal Field CD c Theory.Cambridge University Press, Cambridge, England. o2o 4.20 Finger,L. W. and E. Prince(1975) A systemof Fortran IV com- It c o puter programsfor crystalstructure computations. U.S. Natl. o Bur Stand. Tech.Note 854. 2.15 T Ghose,S. and J. R. Weidner(1974) Site preferenceof transition o metaf ions in olivine (abstr.). Geol.Soc. Am Abstractswilh Programs,6,751. I 2.to = Gibbs,G. V. andP. H. Ribbe(1969) The crystal structures of the C Mongonhumite o humiteminerals: I. Norbergite.Am. .,54, 376-390. o = -'-- and C. P. Anderson(1970) The crystalstructures of oo o',-::*nr,i,l.".i8 ro the humiteminerals: II. Chondrodite.Am. Mineral., 55, ll82- n94. InternationalTables for X-ray Crystallography,Vol. IV (1974)Ky- Fig. l. Plot of mean M-(O, OH) bond distancesas a function of noch Press,Birmingham, England. content of the octahedral site. Lower abscissa:Mg/(Mg + Mn) Jones,N. W. (1969)Crystallographic nomenclature and twinning indicated by * and the solid line. Upper abscissa:mean cation in the humiteminerals, Am. Mineral.,54,309-313. radius determined from site occupancy, using ry" = 0.72A and ryn -, P. H. Ribbeand G. V. Gibbs(1969) Crystal chemistry of : 0.83A (Shannon,1976), indicated by circlesand the dashedline. the humite minerals.Am. Mineral.,54, 391-411. Kocman,V. and J. Rucklidge(1973) The crystalstructure of a titaniferousclinohumite. Can. Mineral., 12, 39-45. Furtherdiscussion of Mg/Mn orderin olivinesand Moore, P. B. (1978)Manganhumite, a newspecies. Mineral. Mag., 42, t33-136. the Mn-analoguesof the humiteminerals is deferred Rentzeperis,P. J. (1970) The crystal structureof , pendingthe completionof otherrefinements now in Mn'[(OH),1(SiOn)'].Z. Kristallogr.,I 32, 1-18. progress. Ribbe,P. H. andG. V. Gibbs(1969) MelFe orderingin humite, MgurFeonSigO,rF(OH)(abstr.). Geol. Soc. Am. Abstractswilh Acknowledgments Programs,I, 188. We gratefully acknowledge the following contributions to this - and- ( l97l ) Crystalstructures of thehumite minerals: study: Dr. Catherine Richardson, formerly at Harvard University, III. Mg/Fe orderingin humiteand its relationto other ferro- kindly communicated the results of her unpublished survey of magnesiansilicates. Am. Mineral.,56, 1155-ll'73. humite group chemistry. The crystals were donated by Professor Robinson,K., G. V. Gibbs and P. H. Ribbe(1971) Quadratic Paul B. Moore of the University of Chicago. The intensity data elongation:a quantitativemeasure of distortionin coordination were collected in the laboratory of Professor Charles W. Burnham polyhedra.Science, 1 72, 567-570. at Harvard University. Drs. R. J. Hill and J B. Higgins assisted and - (1973)The crystalstructures of the generouslywith computational problems.The ResearchDivision humiteminerals. IV. Clinohumiteand titanoclinohumite.Am. of Virginia Polytechnic Institute and State University provided Mineral.. 58.43-49. computer funds; partial financial support was provided under NSF Shannon,R. D. (1976)Revised effective ionic radii and systematic grantsDES75-l4912andEAR.'77-231l4 to PHR and G. V. Gibbs. studiesof interatomicdistances in halidesand chalcogenides. A cta Crystallogr.,A 32, 751-7 67. References Taylor, W. H. and J. West (1928)The crystal structureof the chondroditeseries. Prac. R. Soc.London, All7,5l'l-532. Appleman,D. E.and H. T. Evans(1973) Job 9214: Indexing and leastsquares refinement of powderdiffraction data. Natl. Tech. Inf. Seru., U.S. Dep. Commerce,Springfield, Virginia, PB-216 Manuscript receiued, April 3, 1978; accepted r88 for publication, May 25, 1978.