Gerstmannite, a New Zinc Silicate Mineral and a Novel Cubic Close

A merican M ineralogist, Volume62, pages5l-59, 1977

Gerstmannite,a new zinc silicate mineral and a novel cubic close-packedoxide structure

Pnul B. MooRr ANDTAKAHARU ARAKI

Department of the Geophysical Sciences The Uniuersityof Chicago,Chicago,lllinois 60637

Abstract

Gerstmannite,(Mn,Mg)Mg(OH),[ZnSiOn], orthorhombic, a 8.185(7),b 18.65(2),c 6.256n4, Z : 8, spacegroup Bbcm, occurs as white to pale-pinkmats and spraysof bunched prismaticcrystals associated with calcite,manganpyrosmalite, and sphaleritein a hydrothermalvein paragenesiswhich cuts normal franklinite-willemite"buckshot" ore from the SterlingHill Mine,Ogdensburg, Sussex County, New Jersey.Hardness : 4l; spgr : 3.68(2) s [3.66g cm calculatedfor (Mgo uruMno"rr)rZn(OH)r(SiO,)];cleavage good parallelto {010}. The prismsare composedof {100},{010} and {110}with no terminations.Biaxial (-), a 1.66s(2),P 1.675(2),7 r.678(2),2v- 50-600,xllb, Yllc,zlla. R = 0.042for l09l independentreflections. The structureis basedon oxygencubic close- packing, the close-packedlayers parallel to {130}. The structure is comprisedof -'z[MnMgOr(OH)r]octahedral sheets linked to -'z[ZnSiOn]tetrahedral sheets which are parallel to{0t0}.Averagebonddistancesare6rMn-r3"tO=2.21A,rsrMg-ratrO:2.084, ltzn-,O: 1.95Aand LrSi-r33rO : 1.644,where the left superscriptsrefer to the averagecoordination numbersof the atomsinvolved.

Introduction Zipco (Franklin, New Jersey), one by Mr. Steve Sandford(Franklin), and two in the collectionof Mr. During the summerof 1975,Mr. Ewald Gerst- Gerstmann(cotype and paratype).There is someevi- mannof Franklin,New Jersey,brought to the senior dence that other specimenshave been dispersedto author'sattention two fist-sizedsamples of a curious buyersof mineral specimens. pink prismaticmineral constituting the greaterfrac- The type specimenshows ore consistingof "buck- tion of a well-layeredvein cutting normal frank- shot" franklinite and pale flesh-pink to yellowish linite-willemiteore. According to Mr. JohnL. Baum willemite in equal amount dispersed through a (personalcommunication), the sampleswere gath- gangueof manganoancalcite. The ore mineral grains eredabout five yearsago, but the peculiarnature of range from I mm to 3 mm in diameterand comprise the mineralwas not then recognized,and but few about half the volume of the rock. The vein sharply specimenswere preserved.The materialoriginated cuts the ore and is clearly well-layered.The base is from the ll20' stopeabove the ll00' level in the comprised of a thin layer of magnesiancalcite upon SterlingHill Mine,Ogdensburg, Sussex County, New which was depositeda mixture of waxy brown man- Jersey.More specifically,it occurredin the "west ganpyrosmaliteand yellow-brown lustrous cleavage vein," just north of the east branch of that vein. surfacesof sphalerite.Upon theseminerals rest cal- Quoting Mr. Baum, "The ore was a pale banded cite and the new mineral, hereafter named gerstman- willemite-franklinitecompact ore, more yellowthan nite, the thicker portions of the latter ranging up to 3 greenand certainlynot red or brown,with secondary cm. Occasionalsolution cavitiesdot the gerstmannite willemite veinletscontaining in this casethe un- within which minor amounts of unidentified species known.The veinwall-rock is whitemarble. The vein reside. generallyshows some zincite on its footwalland some Mr. Gerstmann'ssecond specimen, the paratypein black willemiteon its hangingwall." Preservedspeci- this study, affords sufficient information to establish mensinclude one fragmentat the SmithsonianIn- the new mineral's paragenesis.It measuresabout 7 X stitution,one at HarvardUniversity, two in the col- 9 X 5 cm and consistsof a thick vein, the greater lection of Mr. Baum, two preservedby Mr. Nick portion of which is translucentapple-green willemite

5l 52 MOORE AND ARAKI: GERSTMANNITE exhibiting bright-greenfluorescence and dull-green Descriptionand properties phosphorescenceunder short-waveultraviolet ex- Physicalproperties citation.Gerstmannite occurs in generousamount on Gerstmanniteis trinslucentto opaque,and white one side of the vein as coarserosettes of pale-pink to palestpink in color.The streak is white.The luster crystals.It is not fluorescentunder short- or long- is vitreousto subadamantinealong cleavage surfaces, wave ultraviolet excitation. Remnant grains of a althoughthick mats tend to exhibita silky appear- bright pink mineraloccur embeddedin the adjacent ance. The speciesoccurs as mats and spraysof willemiteand exhibit partial replacementby the new bunchedprismatic crystals, the individuals measuring mineral. Single-crystalstudy establishedthe pink up to 2 X 2 X 10mm. Individualsare not terminated, mineral as alleghanyite.Dr. Brian Mason kindly but consistof well-developedprisms including {100}, communicatedan uncorrectedelectron probe analy- and The hardnessis 4t/zon Mohsscale, ( ( {010}, {110}. siswhich afforded KrO 0.1,CaO 0.1,MnO 48.2, specificgravity 3.68(2)determined on the Berman FeO0.2, MgO 14.8,ZnO 3.6,Alros < 0.1,SiOr 26.7 torsion bafance(T : 21.0'C), with tolueneas the percent or approximately (Mno.u2Mgo.rnZno.oo)udisplaced fluid. The density, calculated from (SiO{)r(OH)2,a magnesianalleghanyite. Remnant (M go.ezuMno.sr,)zZn(OH )r(SiO. ) and thestructure cell brown patchesof superficiallyoxidized pyrochroite criteria,is 3.66g cm-3.The cleavageis goodparallel werealso noted in the vein. to {010}. The paragenesis,not unusualfor the SterlingHill Specimensof gerstmanniteclosely resemble pecto- deposit,is interpretedas a hydrothermalvein assem- lite, miserite,thomsonite, and scoecitein appear- blage formed at moderatetemperature under basic ance. The indicesof refraction,which are much and reducingconditions. The gerstmanniteis the lat- higherfor gerstmannitethan theseother species,are est product in the sequenceas it replacesthe will- perhapsthe most reliablesimple means of identi- : emite,alleghanyite, and pyrochroite.Setting IuI2+ fication.The mineraldissolves slowly in dilute HCI octahedralcations, a univariantreaction can bewrit- solution at room temperature,and the solutionis ten, uiz: clearand colorless. 3W*(OH)2 + M3+(OH),(SiO'),t 2Zn2SiOn- pyrochrolte alleghanytte wlllemlte X-ray crystallography aMe.!9llJ*,"'llio'l'Powder,single-crystal rotation, Weissenberg, and acceptingas the componentsM(OH),, MzSiOn,and precessionphotographs were obtained. In addition,a ZnrSiOo.When establishingthe amount of each powder diffractogram (Cul(a radiation, graphite phaseparticipating in the equation,it is moreconve- monochromator,chart speed7zo min-1, corrections nientto determineits ratio based on oxygen,since the for absorption,sample shape, and chartsetting) was structuresare all close-packed.The ratio of the reac- obtainedand completelyindexed (Table l). Least- tants is 3:5:4 with respectto oxygenand suggestssquaresrefinement of thesedata led to a 8.185(7)4,b that the greater portion of alleghanyitein the vein 18.65(2)A,c 6.256$)4,within the limitsof error for wasdestroyed in the formationof the gerstmannite.measurementson calibratedprecession films utilizing A similar reactioncan be written for other mansa- MoKa radiationla 8.176(7)4,6 18.65(1)4,c nesehumites, for example 6.251(5)Al.The successof a detailedthree-dimen- sionalcrystal structure analysis and the singlecrystal M(OH)2 + M3(OH),(SiO1)+ Zn2SiO.* studiesforce us to concludethat the spacegroup is pyrochrolte "Mn-norberglte" wlllemlte orthorhombicBbcm. "'l2l)z?l]!io'r A structureanalysis affords the opportunity for The model is admittedlysimple sincethe phases obtaininga calculatedpowder pattern, the resultsof involving (Mn,Mg) solution are probably sub- which appearin Table l. Thereare definiteadvan- stantiallyordered with respectto thesecations. We tages in calculatingsuch a pattern, since unam- shall provide evidencethat Mn and Mg are ordered biguousMiller indicesare obtainedand effectssuch in the gerstmannitestructure, but no resultsare as yet as preferredorientation can be assessed.Taking this availablefor the degreeof Mn and Mg orderingin into account,the agreementbetween calculated and the humite structures.The simple model suggests, observeddata is good. however,that gerstmanniteis confined to a basic It is notedthat gerstmannite possesses a good {010} hydrothermal vein assemblagewhere pyrochroite, cleavage,since the observedreflections of the type willemite,and a manganesehumite are present. (0k0) are enhanced. MOORE AND ARAKI: GERSTMANNITE

Tablel. Gerstmannite.Observed and calculated powder dataf

I@bs) IGarc) d(obs) d(calc) hkl I Obs)I(carcJ d(obs) d(calc) hkl

85 49* 9.326 9.325 020 )A 1.9982 1.9986 420 50 38 4.806 4.803 lr1 t.9436 430 25 t0* 4,661 4.663 040 3 1.92L9 r33 40 23 4.388 4.386 L2l 35 l5* 1.9t24 r.9127 191 l0 ll 4.088 4,092 200 4 r.8737 440 15 19 3.994 3.997 2IO 2S IJ 1.8692 1.8693 082 52 3.881 3.882 I3t 1.8488 290 15 14 3.747 3.747 220 l0 11 1.8168 1.8174 272 80 62 3.4r8 3,418 230 6 r.7763 r.7769 r53 16 3. 401 14l i r.7124 402 20 3. 108 3. 108 060 ( I .7053 1.7053 381 20 14 s.078 3.076 240 4 1.7001 1.7003 282 60 29 2.983 2.983 l5I 5 7 1.6842 r.6842 422 16 2.966 022 4 1.6509 432 75 58 2.758 2,757 250 5 r.6s02 1.6504 313 r00 100 2.598 2.598 042 4 I . 6019 0. 10.2 7 2 .485 202 l0 o l .6010 1.6010 333 25 l9 2.480 2.479 5lI 10 1t 1.5912 1.s916 292 50 43 2.466 2.464 2t2 3 1.5780 511 25 20 2.403 2.401 222 ZJ 2l r.5644 1.5641 004 104 2.348 2.348 t7l a l. ))oz +J z 75 17* 2.332 2.331 080 4J 10" r. ss48 r.SS42 0.12.0 2.320 33r 3 1.5142 3s3 I5 T6 2.308 2.308 232 J) 47 1.5002 r.4999 462 30 19 2.232 2.233 270 10 l2 t.4920 1.4917 2.10.2 105 2.204 2.205 062 5 5 r.440s 1.4405 472 50 2.t93 2,r93 242 5 7 1.4223 r.4223 234 l0 ll 2.076 2.077 7 1.4003 I.4006 2.rr.2 9 2.068 252 l0 r.3920 r.5919 0.r2.2 2.046 2.046 400 IJ 10 r.3787 r.s784 4.10.0 105 2.025 2.026 280 t0 I r.3603 1.3604 254 4 2.009 IIJ tcuKq radiation, graphite monochrornator, 0.5o nin-I in 20' The data were corrected for absorpiion and sarnple shape (two variable paraneters). Peak heights were neasured. Reflections enhanced by preferred orientation are starred.

Chemical composition The compositionfor gerstmanniteis close to Theresults of onewet-chemical analysis, one deter- Mno.zsMgr.zsZn(OH)dSiOr),The crystal structure minationof water(by Penfieldtube), and one elec- analysisconverged close to this composition,reveal- tron probeanalysis are summarizedin Table2. All ing Mno rcMg,.z,Zn(OH)r(SiO,).That studyalso estab- materialwas gatheredfrom the type specimen.Dr. lisheda highlyordered structure from whichthe formula Jun Ito informed us that emissionspectrographic (Mn,Mg)MgZn(OH)r(SiOn)is proposed.Thus, the analysisrevealed trace quantities of Ti, Ag, Al, Cu, typegerstmannite is magnesian,its idealend-member Cd(?), and As(?).The original batch of separated compositionbeing MnMeZn(OH)r(SiOr). crystalswas known to contain Although gerstmannitecompositionally resem- somecalcite, estimated totl4ptar(Zn2Si)O,(OH)r, to be 3.93percent by weight.Since Ca was not de- bleshodgkinsonite, a rather tectedin the probeanalysis, its amountdetermined abundantvein mineral from the abandonedFranklin the calcitepresent. A qualitativeprobe scan revealed depositsnearby, the structureanalysis reveals a new thatthe calcite contained only minor amounts of Mg, structuretype basedon cubic close-packedoxygens. Mn, and'Zn. Dr. A. J. Irving informedus that the We proposethat the mineralbe classedwith stauro- ("B-spinel") probe grain was homogeneousthroughout. Stand- lite, kyanite,and MgnO[SirO'] owingto ards included syntheticdiopside (Mg,Si), Minas the principleof cubicclose-packing. Geraisrhodonite (Mn), and Franklin, New Jersey, Optical properties willemite(Zn), the last a fragmentof materialana- lyzed by the late L. H. Bauer.No other elements Gerstmanniteis biaxial(- ), o 1.665(2),P 1.675(2), betweenNa and U weredetected. 7 r.678(2),2V- 50-60'(observed), Xllb, Yllc,Zlla. MOORE AND ARAKI, GERSTMANNITE

T able2. Gerstmannite.Chemical analyses Crystal structure Experimental 3 A singlecrystal of thetype material measuring 0.13 (llc) was Na2O 0.04 0.04 nil mm (lla), 0.14mm (llb), and 0.30mm axisparallel to therotation axis of CaO 2.2 nil mountedwith its c a Pailred diffractometer. With graphite mono- Mgo 16.0 16.6 - r9.2 chromatizedMoKa (I : 0.7093A)radiation, auto- - Mn0 2r.2 22.r 27.r 20.2 matedfull scanwidths ranging from 2.4oto 6.4",20 0.06 0.06 niI scanof lo min-', reflectionsof (hkl) and (ft[/) were ZnO 30.5 3t.7 )aA - 30.9 collectedup to sin0/tr : 0.80 and the levels/ : 0 SiO2 23.0 23.9 23.2 - 22.8 through / : 9. Reflectionswhere 1o were unobserved Hzo n.d. n.d. n.d. 4.r2 6.9 were set lo : 2olo. Severallow-angle reflections withinthe blind region of themachine were manually 94.4 93.4 100.0 scanned.The crystalshape, which wasdescribed by IJ. Ito, analyst, on 90 rng. sarnple. The presence sevenfaces, was correctedfor anisotropyin absorp- of calcite was detected, Enission spectrographic tion by the Gaussianintegration method (Burnham, analysis revealed trace quantities of F, Ti, Ag, werethen av- A1, Cu, Cd?, As?. 1966).Symmetry equivalent reflections eragedto yield l09l independentreflections. Individ- 2From (1), after deducting 3.93% CaCO3(calcite). ual reflectionswere assigned weights based on long- intensityand counting 3A. J. Irving, analyst. By electron probe. term fluctuationsof source statistics. +J. Ito, analyst, on 121.0 mg. sample, by Penfield tube, Solutionand refinementof the struclure scornputed for composition (Mgg.6z sMng,37 ) 2Zn(OH) 2 (sio4) . A full Pattersonsynthesis, P(uuw), confirmed the suspicionthat the structurewas basedon oxygen dense-packing,the layers stacked parallel to the {130} plane.Search for octahedraland tetrahedraloccu- pancieswhich satisfiedthe interatomicvector set pro- The mineral is not discernablypleochroic. The calcu- videdcations and anions as trial input for B synthesis mean refractive index is 1.714, adopting the lated (Ramachandranand Srinivasan,1970). A final 7' formula and density in this study and the specific synthesissufficiently sharpened all positionsso that refractiveenergies of Larsen and Berman (1934). least-squaresrefinement could be inaugurated. Refinementproceeded from scatteringfactors for Name Mn'*, Mg2+, Zn'+, Sin+,and 01- from Cromerand Mann (1968),anomalous dispersion corrections, /, It is a pleasureto christenthe new speciesin honor of Mr. Ewald Gerstmannfrom Franklin, New Jersey, who first brought the material to the senior author's Table 3. Gerstmannite.Atomic coordinateparameters* attention. Mr. Gerstmann has preservedsome of the X yz finest specimens of nearly every species from these Mn' 8n 0.00721(8) o.39s94 (4) E famous zinc deposits,and his private museum has 81 been one of the centers of activity for those who Zn 82 . 40053 (6) journey to the Franklin-Ogdensburg mineralogical Si 8n .1.4422(r2) . 34s80(s) 0 mecca. 0H(1) 82 0 !2 0.2822(5) The new speciesand name have received prior 0H(2) 8n .2s01(4) .4306(1) z approval from the InternationalCommission on New o(1) 8n .2484(4) .420s(1) 0 0(2) 8n .2775(4) .27e3(r) 0 Minerals and New Mineral Names, International o(3) 16 I .0294(3) .3373(L) .2rr2t3) Association.The type specimenis pre- *Estimated Mineralogical standard errors refer to the last digit. Included in servedin the National Museum of Natural History, the Iist are the equivalent point ranl numbers and the point 5)metr1es. Smithsonian Institution and a cotype in the private tRefined collection of Mr. Gerstmann. to 0.786(g)I'ln + 0. 21.4M9. MOORE AND ARAKI: GERSTMANNITE

Table4. Gerstmannite.Anisotropic thermal vibration parameters (X 101)*

Btt Bzz B:s Bp 8ra Bzs

Mn 28.8(11) 6.2(2) 47.4(rs) 0.0(2) 0 0 Ms 43.9(2r) 4.e(3) 48.2(s2) 0.1(7) -10.7(16) 0.s(s)

zn 3e.6(e) 4.7(2) 4s.7(16) 0 0 0.s(r) si 27.4(L4) 3.9(3) 4s.2(22) -0.6(4) 0 0

0H(1) 1e.2(31) s.2(6) 62.0(s3) 0.s(10) 0 0 oH(2) 42.4(33) s.7(6) 3s.4(4s) -0.s(r2) 0 0

0(1) 36.7(3r) 2.8(s) ss.2(47) -2.e(10) 0 0 o(2) 43.0(s4) 6.3(6) 48.3(44) e.2(rr) 0 0 o(3) 41.2(24) s.0(4) 7r.r(40) -1.4(8) 21.3(26) -3.s(11)

Coefficients-' '- in the exnraccinn ^*-_ra. -t"2 . a22k2 * BZIL2 + 2}tZhk + 2Btghl + ,B;;k;t.

2 from Cromer and Liberman (1970), and refinement w: o of lf,l . Refinementminimized2w(F"- F"t2. for the coefficientfor secondaryextinction (Zacharia- The secondaryextinction coemcient,c', is 0.59(19)X sen, 1968). The chemical analysis suggestedthat l0 6 and the overall scale factor, s, is 2.13(l). The (Mn,Mg) solid solution may exist in the crystal,and goodness-of-fit>w(f'" - F")"/2(n - m), where n: of thetwoindependentoctahedralsites,oneafforded number of independentdata and m: number of pure Mg2+ and the other exhibited (Mn,Mg) solu- parameters: 0.709. tion. The site distribution for the latter position was The successof the refinementrequires that Mg2+ assessedby a local modification of a least-squares completely occupy the inversion center at (Ve VzVe), program reported by Finger (1963). The final cycle etc., andthat the octahedralpopulation on the mirror included as variable parametersone scalefactor, one plane is in fact 0.786(9)Mn'+ + 0.2l4Mg'+. This is coefficient of secondary extinction, one site popu- further substantiatedby the excellentconvergence in lation parameter,fifteen atomic coordinate parame- thermal vibration parametersto expectedvalues; and ters, and forty anisotropicthermal vibration parame- the chemical analyses, whose average suggests : - gerstmannite ters.The finalcycle led to R (> | l4l l4l I )/Ql F"l ) 0.75Mn'z+* 0.25Mg'+. Thus, type is a : 0.042for all l09l independentreflections and 0.033 magnesian variant of the ideal composition (Rw : 0.049) for those non-zero766 reflections,with MnMgZn(OH)r(SiO.). Atomic coordinate parame-

Table 5. Gerstmannite.Parameters for the ellipsoidsof vibration*

!i oia oib oic B(fr2) Atom i pi oia oib oic B(12)

.os7(2) eo e0 o .7s(2) oH(2) 1 .os8(s) e0 eo o .urto; .099(2) 779 89 90 2 .100(6) 99 t77 90 .LO4(2) 89 l 90 3 .120(5) 9 99 90

Mg .089(s) 72 727 43 .87(3) 0(r) .06s(6) 74 1s 90 .77(3) .095(6) 105 143 123 .108(11) 90 90 180 .127(3) rs7 89 67 .114(11) 1s los 90

0.089(1) 90 744 54 0.80 (2) O(2) .07s(s) s2 r42 90 .93(4) 2 .09s(1) 90 726 L44 .098(8) 90 90 180 3 .116(1) 0 90 90 .142(6) 38 52 90

si1 .082(3) 79 11 e0 .66(2) 0(3) .088(19) 11r 42 s6 .97(3) 2 .09s(7) 90 90 180 .09s(23) sl s0 116 3 .097(7) 11 101 90 . r41(s) 47 101 4s

oH(r) r .080(6) 13 103 90 .74(4) 2 .097(s ) r03 167 90 3 .lri (5) 90 90 0

i = ith principal axis; Ui = r.n.s. anplitude; oia, oib, oic = angles between ith principal axis and the cel1 axes, a, b and c. Est.imated standard errors refer to the last digit. The equivalent isotropic therml vibration paraneter (B) is also listed. MOORE AND ARAKI, GERSTMANNITE

Fig. I Spoke diagram of the gerstmannitestructure down [001].The atoms are labelledto conform with Table 3 Note the distortions of the Mn-O and Me-O octahedra ters appear in Table 3, anisotropicthermal vibration among the kinds of atoms presentbut only whether parameters in Table 4, the root mean square dis- the sites are occupied or not, has the same space placementsof theseparameters and their orientations group as the real structure.It is immediatelyseen that in Table 5, and the structure factor list in Table 6.1 gerstmannitecan be describedas the linkage of octahedral (M\ and tetrahedral (I) sheetswhich are Description of the structure oriented parallelto {010}.The octahedralsheets have Gerstmanniteis an elegantexample of a new kind formal compositioniIMnMgOs(OH)z] and represent of structure type basedon oxygen cubic close-pack- a sliceof the NaCl structurealong {110}. In this slice, ing. The cell is metricallythe sameas that of man- the central MgO. octahedralink to form edge-shar- ganostibite,4Lu(MnuSb)'ar(MnrAs)O,r, a 8.734, b ing chains which run parallel to [001]. The lateral 18.85A c 6.06A, Ibmm. Both structurespossess 48 MnO. octahedralink by edge-sharingabove and be- oxygens in the cell, the maximum allowable in a low to the MgO. octahedra,and link each other at a close-packedcell of this volume and shape,and the common edge to form a dimeric arrangementalong Unlike NaCl, however,alternate levels of oc- close-packedlayers are stackedparallel to {130}. A [010]. (in diagram of the structure is shown in Figure l. It is tahedralpositions along [001]are empty NaCl the more convenient, however, to discuss the ideal ar- slab would have formal compositionM"6u).The oc- rangementwhich is based on perfect octahedraand tahedral sheet is featured as an ideal arrangement tetrahedra.To achievethis, appropriate shading af- down [010] in Figure 3a. Tetrahedral sheetsof com- fords a diagram (Fig. 2) which can be comparedwith position -'z[ZnSiOrllink two octahedral sheetsto- similar diagrams for other cubic close-packedoxide gether to form a framework. The idealized sheets arrangementsdiscussed by Moore and Smith (1970). (Fig. 3b) exploit cubic close-packingthrough voids at positions (in This ideal arranqement.which does not discriminate alternate levels along [001] for the Si c.c.p., the tetrahedral slab would have formal composition T"Q). These observations I To obtain a copy of this table, order Document AM-76-029 compel us to conclude that gerstmanniteis a zinco- from the BusinessOffice, MineralogicalSociety of America, 1909 K Street, N.W., Washington, D.C. 20006. Pleaseremit S1.00in silicate whose formula should be written advance for the microfiche. MnMg(OH),[ZnSiO.]. MOORE AND ARAKI: GERSTMANNITE 57

Fig 2 ldealized polyhedral and spoke diagram for gerstmannite down [001]. The central Mg-O odahedra are stippled and form an : z = 7: (ruled NW-SE). The edge-sharingchain parallel to [001]. The lateral Mn-O octahedra are populated at z 0 (ruled N E-SW) and tetrahedra,drawnasT-Ospokes,arelocatedatz:0andz=72(Si);andz=l/4,3/4(Zn).

Fig. 3a. An idealized octahedral sheet of gerstmannite structure Fig. 3b.An idealizedtetrahedral sheet ofgerstmannite structure down [010]. The Mg-O octahedra are ruled and form the basis for down [010].The Zn-O tetrahedraare in the planeand the Si-O the sheet.The Mn-O octahedra are stippled and share edgeswith tetrahedraare alternately above and belowthe plane.Note that the the Me-O octahedraboth above and below the plane. Zn-O tetrahedraform corner-linkedchains parallel to [001]. 58 MOORE AND ARAKI: GERSTMANNITE

Table7. Gerstmannite.Polyhedral interatomic distances and anglest

I Mn -0H(2) 2.090(4) Ms -oH(2) 2.030(2) 2 -o(3).. 2.1Ie(3) -0H(1) 2.0s6 (2) 1 - o(l)111 2.L67(4) - o(1) 2.1ss(3) -0H(1) 2 2.372(3) average 2.080 average 2.206 H!te-{-(") G+ln-{-(") 0(1) -0H(2)v 2.777(4)*. 83.I (1) 1 oH(l) -oH(l)i 2'726(7)* 70.r (1) 2 0H(l)-0H(2) 2.779(4)* 8s.7 (1) 2 OH(1)-oH(2)... 2.779(4)*. 76.7(\) 2 O(r) -OH(1)r11 2.880(4)' 86.3 (1) 2 on[rj - oi1;iii 2.880(4)* 78.6(1) 2 OH(2)-OH(l)1r1 2.996(4) s4.3(1) 2 oH(l) - 0(3) 3.076(4) 86.2(r) 2 oH(l)- 0(1) 3. 07s (4) 93.7(r) 2 0H(2)- 0(3). 3.0s1(4) e4.s (1) 2 0(1) -0H(2) 3.734(4) 96.s(r) - 101.I (r) 2 o(3) 0(1)Ial 3.511(4) average 2.940 90,0 1 0(3) - o(3)t 3'613(s) tr'?.o(2) average 3.051 88.4 si 2 si - o(3) 7.629(3) r - 0(I) 1.633(3) - (3) 2 Zn - o(2)... I .938 (2) 1 o(2) 1.6s2 - 2 o1s;rtt 1.9s7(3) average L 636 average 1,948 o-si--o'(') o-zn-{- (') 1 0(3) - o(3)1 2.643(s) 108.4(2) - 2 O(2) -0(3)141 2.942(4) e8.4(1) 1 0(1) 0(2) 2.64s(4) r07.2(2) - (1) 2 o(2). -o(3)tv. 3.27s(4) 114.s(1) 2 o(2) o(3) 2.6s3($ 107.9 - I O(3)rv-0(3)111 3.2s2(4) 114.6(1) 2 o(1) 0(3) 2.7r4(4) 112.6(1) -0(2)11 r 0(2) 3.313(4) rr7.s(2) average 2.670 109.4 average 109.6 tEstinated standard errors tefer to the last digit and were cotrlputed fron errors in cell Paraneters = and atomj.c coordinates. Equivalent positions, which refer to Table 3, include i x' y, = -y, x, +-y, +-z; iil = j+x, Y, Z-z: iv L+x, E-y, z; and v l-x, $-2. *Shared octahedral edges.

Gerstmanniteis yet another exampleof a structure multiple of an octahedraledge. The simplestunit for type based on populations of octahpdra in anion oxygen cubic close-packinghas metrical properties cubic close-packing and is most easily visualized @ x 4.2A,n x 3.04, p X 3.04). For gerstmannite : : : as a projection down I l0] direction of the and manganostibite,m 2, fr 6, and P 2. 1n NaCl structure. It is allied to a growing list of addition, the units contain 2mnp anions. Crystals structure types such as staurolite, kyanite, spinel, which fulfill these criteria should be examined in MgoOISizOz],manganostibite, and more recently more detail for they may indeed represent cubic Ni10aAlrl.4sir rOrr-I, the last reported by Ma et al. close-packing.One complex example may be hold- (1975).All of thesestructures involve one axis which enite,ca. 16Mn'(OH)'[ZnrO(AsOr)], a 8.58,b 31.15, is someintegral multiple of twice the octahedralM-O c 11.97,Bmcm (Prewitt-Hopkins, 1949).Here m : 2, distance,a second axis which is some integral mul- n : 10,p : 4, suggestingyet another exampleof an tiple of an octahedraledge, and a third which is also a exotic population of octahedral and tetrahedral cations in an oxide cubic close-packedenvironment. cations Table8. Gerstmannite. tiT5fi:*_""lence balanceof Bond distancesand angles Bond distancesand angles are given in Table 7. 16lMn-135lO Coordinat in s The average bond distances are mron AMg-{ AZn-{ ASi-{ cations Px 2.2lA,t"tMg-rs?rO: 2.0gA, tltZn-tstO= 1.954, and 0(I) Mn+2Mg+Si 2.00 -0.04 +0.07 - +0.00 r4rSi-13'3rO: 1.644 (where the left superscriptgives 0 (2) 2z^+si 2.00 -0.01 +0.02 0(5) Mn+Zn+si+zaHa I.92 -0.09 -0.09 +0.01 -0.01 the average coordination number). These are all within 0.024 of the distancesestimated from the OH(I) 2Mn+2Mg+Hd+Ha2.33 +0.17 -0.02 0H(21 Mn+2Mg+Hd 1.83 -o.12 -0.05 tables of effectiveionic radii (Shannon and Prewitt,

Al,ln-{ etc., refer to differences between the individual bond 1969). Since about 20 percent Mg substitutesfor distances and the polyhedral averages, Hd = hydrogen bond donor, gerstmannite,it is predictedthat the Mn-O Ha = hydrogen bond acceptor, px = sum of electrostatic bond Mn2+ in streneths. distance should be 2.17A. It is noted that MOORE AND ARAKI: GERSTMANNITE

2Mn-OH(l ) : 2.37A is an unusuallylong distance,a viations in bond distancesto the coordinatedcations result of the combined effectof repulsion of the Mn for O(l) and O(2) are very nearly balancedsince cations away from this anion owing to the shared theseanions are electrostaticallybalanced. Slightly edge and the net electrostatic oversaturation of undersaturatedO(3) exhibits net shorteningof OH( I ) by cations.This effectis reminiscentof the one bonds, and for the more severelyundersaturated reported by Moore and Araki (1975) for the crystal OH(2),this effect is morepronounced. OH(l), which structuresof wightmanite and gageite,where certain is the only oversaturatedanion, reveals an unusually local cationic arrangementsabout an anion lead to fong Mn-OH(l ) distanceof 2.372A. observed distance averageslonger than those pre- dicted. Acknowledgments The MnO. octahedronshares five of its edges,four We thank the National Science Foundation for continued sup- port through the grants NSF GA-40543 (Geochemistry) and with MgOu and one with a symmetry equivalent through the Materials ResearchLaboratory grant administered to MnOu. The MgO, octahedron sharessix edges,two The University of Chicago. with equivalentMgO. and four with MnO.. All these Electron probe analysis by Dr A. J. Irving, wet chemical analy- distancesare the shortestfor their polyhedra, as in- sis by Dr. Jun Ito and assistancein the optical study by Mr. A. R. Kampf are also appreciated. spection of Table 7 will show. Owing to the rather uneven distribution of shared edgesfor the MnO. References octahedron, the angle distortions are considerable, Burnham C. W. (1966) Computation of absorption corrections, ranging from 70o to 117o.The widest angle (and and the significanceof the end effect.Am. Mineral, 5I, 159-16'1. longest edge distance)refers to O(3)-O(3)' which is Cromer, D. T. and D. Liberman (1970) Los Alamos Scientifc the edgeopposite to the five sharededges. The angu- Laboratory, Uniu. of Calif. Rep LA-4403, UC-34. lar distortion of the MgO. octahedron is much less, - and J. B. Mann (1968) X-ray scatteringfactors computed from numerical Hartree-Fock wave-functions. ranging from 83o to 97". This arisesfrom the smaller Acta Crystallogr., A24,321-324. radius of Mg2+ and more balanced distribution of Finger,L. W. (1968)Determination of cation distributionby least- shared edgeswhich are related pairwise by an in- squares refinement of single crystal x-ray data Carnegie Inst. verstoncenter. Wash. Year Book. 67. 216-217. Larsen, E. S. and H. Berman (1934)The microscopicdetermina- tion of the nonopaque minerals.U.S. Geol Suru. Bull., 848 Hydrogen bonds and electrostatic bond strengths Ma, C.-8., K. Sahl and E Tillmanns (1975) Nickel aluminosili- cate, phase l. Acta Crystallogr 83l,2137-2139 Symmetry and steric restrictionslimit the possible , Moore, P. B and T. Araki (1974) Pinakiolite,warwickite, and hydrogen bond acceptors.OH(l), which is situated wightmanite:crystal chemistryof complex 3A wallpaper struc- on the two-fold rotor parallelto the c axis,is shielded tures.Am. Mineral .59.985-1004. - by MgOu octahedraledges and consequentlycan only nni J. V. Smith (1970) Crystal structure of B-Mg,SiO.: bond to the equivalentOH(l) related by the mirror crystal-chemical and geophysical implications. Phys. Earth Planer lnter . 3. 166-177. plane.Thus, OH(l) .OH(l)' : 3.53(l)A,is a very Prewitt-Hopkins,J. (1949)X-ray study of holdenite,mooreite and weak bond. OH(2), which is on the mirror plane, is torreyite. A m. M ineral, 34, 589-595. further shielded by MgO. and MnO. octahedral Ramachandran, G. N. and R. Srinivasan (1970) Fourier Methods edges.It bonds to O(3)"t, which accordinglyreceives in Crystallograpfty Wiley-lnterscience, New York, 96-119. an averageof one-half a bond owing to its general Shannon, R D. and C. T. Prewitt (1969) Effectiveionic radii in oxides and fluorides. Acta Crystallogr., 825,925-946. position.Thus, OH(2)...O131"' : 3.162(5)4,with Zachariasen, W. H. (1968) Experimental tests of the general for- the hydrogen on the mirror plane. mula for the integrated intensity of a real crystal. Acta Crystal- These assignmentsallow a complete list of elec- Iogr , A24,212-214 trostatic bond strength sums of cations coordinated to eachof the anions(Table 8). Deviationsin individual bond distances from the polyhedral averages Manuscript receiued, February 5, 1976; accepted closelyfollow the deviationsfrom neutrality. Net de- Jbr publication, June 29, 1976