The Crystal Structure of Bermanite, a Hydrated Manganese Phosphate

American Mineralogist, Volume 61, pages I24l-1248, 1976

Thecrystal structure of bermanite,a hydratedmanganese phosphate

ANruoNy R. Keupp ANDPAUL B. Moonp Department of the Geophysical Sciences The Uniuersity of Chicago Chicago, I llinois 60637

Abstract

Bermanite,Mn'z+(H,O).[Mn!+(OH),(PO,)r], a 5.446(3),b 19.25(l\, c 5.425(3)A,A 110.29(4)',monoclinic, space group P2r,Z :2, is a novelstructure type based on dense- packed[Mn!+(OH)r(POn)r]'9- slabs parallel to the {010}plane linked by tetrahedralcorner- sharingto insulartrans-fMnz+(Op)r(HrO)J octahedra. Chains of edge-sharingMn3+-O octahedra,expressed asJ[Mns+(OH)(Op)s], run parallelto the [l0l] directionand are corner- linked by the (PO.) tetrahedrato form the slabs. A compact slab approximateslocal IC'A'B.C] anionpacking wherethe A andB layersare fully occupiedand the C layersonly one-quarteroccupied. R(hkl) : 0.059for 1254independent reflections. The averageinteratomic distances are Mns+(l)-O 2.O2OA,Mns+(2)-O 2.023A, Mn'+(3)-O 2.t86A, F1r;O 1.550Aand p(2)-O L5484. Local dense-packingand pseudo-orthogonalcharacter of the structureallow several kinds of twinning,which are explicableon the basisof local structuregeometry.

Introduction sequentlyconfirmed from restudyof the type mate- Bermaniteoccurs in moderateabundance at sev- rial. The twin laws observedas well as non- eral pegmatites(Moore, 1973)as a late-stagehydro- orthorhombicorientation of the optical indicatrix thermally reworked product of primary manga- forced them to concludethat bermaniteis sensibly nese-iron phosphates,in particular triplite, monoclinicwith the cellconstants a : c : 5.4264,b (Mn,Fe)!+(F,OHXPO{).Originally describedby : 19.206A,9: I l0'30'(on crystalsfrom Argentina) Hurlbut (1936) from a pegmatiteon the 7-U-7 andthe spacegroup P2t. A newanalysis of bermanite Ranch,west of Hillside,Bagdad district, Arizona, it from the type locality confirmed the ideal formula occurredin veinletsand cavitieswhich cut and line proposedby Leavenswith Z : 2. triplite. Hurlbut establishedorthorhombic ho- Recently,Mr. J. E. Johnsonof the SouthAustra- losymmetryfrom single-crystalgoniometric and X- lian Museumsent us severalspecimens of bermanite ray studywhich afforded a = 6.254,b : 8.92A,c = crystalsfrom the McMahon pegmatitenear Olary, l9.6lA, and he proposed the compositionSouthAustralia. The mineralreplaces triplite along (Mn,Mg)!+(Mn,Fe)!+(PO,)3(OH),9 . l5HrO based fracture surfacesand occurs in associationwith on the analysisof F. A. Gonyer.-Aspace group was phosphosiderite,strengite, siderite, hureaulite, rock- not proposed.Leavens (1967), upon reexaminingber- bridgeite,manganese oxides, and an unknownyellow manite from the type locality as well as from other fibrous mineral.The bermanitecrystals are of ex- hithertounrecorded locations, confirmed the ortho- ceptionalquality and up to I mm in maximumdi- rhombicsymmetry and determined the cellconstants mension.Twinning is prevalent;however, a small a = 6.204,b : 8.92A,c : 19.20Awirh spacegroup untwinnedindividual, eminently suitable for crystal C222r.From space-grouprestrictions and an electron structureanalysis, was recovered with little difficulty. microprobeanalysis, he proposedthe ideal formula Knowledgeof the crystal structurewould establish Mn'+Mn!+(POo)r(OH),. 4H2O, Z : 4. Hurlbut and the true symmetry and ideal chemicalformula of Aristarain(1968) studied bermanite from the triplite- bermaniteand would explainthe varioustwin laws bearingEl Criollo pegmatitenear C6rdoba,Argen- determinedby Hurlbut and Aristarain. tina, anddetected persistent twinning which they sub- Our principalmotivation for determiningcrystal t24l t242 A. R. KAMPF AND P, B. MOORE structuresof aquatedtransition-metal phosphates, of ensuing structure analysis. Errors assignedto the re- which bermanite is a member, is to establish the flections followed the procedure adopted by Moore nature of polyhedral clusteringof the octahedraand and Araki (1976). Least-squaresrefinement of 30 ref- to define a sharp link between these structures and erence reflections establishedthe cell criteria. their parageneticsettings. Bermanite is of particular interest becauseit is one of the few mineral species Solution and refinementof the structure which contains essentialMn'*, an ion expectedto Despite the relatively simple composition for ber- evince pronouncedJahn-Teller tetragonal distortion manite, strong homometric character was encoun- in an octahedral oxygen environment for the high- tered, and the structure determination was not a tri- spin I state.The similarity betweenthe formulae of vial task. The three-dimensionalPatterson synthesis, bermanite and whitmoreite, Fe2+(HrO)1[Feg+(OH),P(uuw), revealed several pairs of vectors (urvrw; (PO.)rl, adds interest to the species,although the two t/z-urvrlz-w) and a prominent vector at (Vz,0,Yz). are in no way isotypic.We suspectedthat Jahn-Teller Such relationships are not inherent in the space distortion and the subsequentsevere anisotropy in groups P2, or P2r/m, and we concludedthat a pro- electrondensity about Mn8+ compared with spherically nounced substructure relation prevailed for atoms in symmetric Fe8+ may furnish an arrangement of independentpositions. The vector pairs and the vec- octahedra quite distinct from that found in whit- tor at (t/2,0,t/z)were judged to result from two non- morelte. equivalent manganeseatoms, Mn(1) and Mn(2), in general positions which were related to each other by Experimental : (x,y,t) of Mn(l )-(x,y,r) of Mn(2) (t/z,O,Vz)for the A crystal of bermanite,prismatic parallel to the a space group P2r. All vectors between equivalent axis and averaging0.07 mm in thicknessand 0. 18 mm atoms are given by 2x, Vz,22. Thus, the Mn(l)- in lengthwas selectedfor study. Precessionand Weis- Mn(l)' and Mn(2)-Mn(2)' vectors almost exactly senberg photographs showed the crystal to be un- overlap owing to their pseudo-symmetricrelationship twinned. Monoclinic symmetry was suggestedby a to each other. The Mn(3), P(l) and P(2) posi- small inequality in the a- and c-crystallographicaxes tions were established through a search of the and by the intensities of the diffraction spots. The vectors remaining in P(uuw). Trial positions were crystal was mounted parallel to the a axis, and data submitted to a B-general synthesis, described by were collected on an automated four-circle diffrac- Ramachandranand Srinivasan(1970). Although the tometer, utilizing graphite monochromatizedMoKa oxygen atoms associated with the phosphate tetra- radiation, N^,* : 55o, scanspeed 2o min-t, with 20 hedra and the OH- groups were clearly resolved, the secondstationary background measurements on each coordination about Mn(3) was puzzling and resisted side of the peak. A total of 2697 reflectionswere easy interpretation. It was then discovered that recorded. Since a relatively wide peak scanwas neces- Mn(3) exhibited a pseudo-symmetricrelationship sary for our crystal and since the reflections along b* and a choice of one of three possiblepositions was are closely spaced, significant overlap was encoun- required; a split residual peak proved to be the cor- tered for many strong reflections and this was at- rect position. Redefining Mn(3) resulted in unam- tested by asymmetric background counts. These re- biguous location of all non-hydrogen atoms. A trial flections were eliminated from further data reduction. bond-distance calculation proved in good agreement ln all,273 reflections were rejected, and in every case with anticipated Mn2+-O, Mne+-O and Pu+-O dis- but one (1.21.0)their symmetry equivalentreflection tances. was recovered and preserved for data reduction. Despite the pronounced homometric character of Twenty-six additional reflections were eliminated by the structure, refinement of the atomic coordinates space-groupextinction. Absorption correction was and isotropic thermal vibration parameters pro- not applied owing to the relativelylow linear absorp- ceeded without difficulty. We employed a highly tion coefficient(p : 40.0 cm-1), the small sizeof the modified version of the familiar ORFLS program of crystal and its approximately cylindrical shape. Re- Busing et al. (1962), scattering curves for Mn2+, duction of the raw data to obtain lFol was achieved Mn3+, P3+,and O'- from Cromer and Mann (1968) through averaging symmetry equivalent reflections and anomalousdispersion corrections for the cations and accepting only those whose backgrounds were (Cromer and Liberman, 1970). Five cycles of full- symmetrical. After correction for Lorentz and polari- matrix scale factor, atomic parameters and isotropic zation effects,1254 independent llc,l remained for the thermal vibration parameter refinement convergedto CRYSTAL STRUCTURE OF BERMANITE 1243

R : 0.059and Ra : 0.065,where R : >ll4l - T,lsI-r l. Bermanite:atomic coordinatesand isotropic thermal vibration Darameters lr"lll>lFoland R, : [>,(lr,l - l4l),/Z.Fo,f,,, for all 1254 l4l . The final cycle minimized -l4l Atom B(12) >,1lF,l l2where w : op2.The"goodness offit," S : >.llf'l-l4ll"/(n - n) wheren : numberof : Mn(1) 0 .42s0(4) 0.0000 0.?368(4) 0.s3(3) independentF. and z numberof parametersis Mn(2) .s207(4) .00r3(2) .7367(4) 0. ss(5) 4.06.The scalefactor, s, refinedto 1.016(3).Atomic Mn(3) . s386(s) .2s0e(2) .s082 (s) I .s8 (4) coordinatesand isotropicthermal vibration parame- P(1) .44s0(6) .oes8(3) .7116(7) 0.s3(6) tersare presented in Table l, andthe structurefactors P(2) .39sr(6) .s046(2) .760s(6) 0.44(6) arelisted in Table2.1 0(1) .327(2) .074e(s) .4rs(2') 0.s(2) o (2) .sr4(2) .92ss(s) .0s7(2) 0.6(1) Descriptionof the structure 0 (3) .2s2(2) .060s(s) .870(2) 0.7(2) o (4) .s4s(2) .93s8(s) .60s(2) 0.7(2) The structureof bermaniteis basedon linearedge- sharingchains of Mn3+-Ooctahedra which run par- 0(s) .7ss (2) .0743(s) .82s(2) 0.8(2) alleltothe Eachshared edge is linked o (6) .101(2) .9260(s) .644(2) 0.6(2) [01]direction. 0 (7) .40s(2) . 1742 (6) .726(2) 0.e(2) by one phosphateoxygen (Op) and one hydroxyl 0 (8) .s97(2) .Ja)+lr ) .2s7(2) o.s(2) group (OH) and the remainingvertices receive Op. 0H(1) .786 (2) .0344(s) .377(2) 0.8(2) Thus, the chain formula can be expressedas oH(2) .0ss(2) .97r5(s) .101(2) 0.s(r) l[Mn3+(OHXOp),].The phosphatetetrahedra link in such olv(1) .838(21 . I 863(6) .443(2) r.7 Q) a way to form a dense-packedslab parallel to 0l{(2) .24r(2) .31s6(6) .sss(2) r.6(2) {010} with formal compositionIMn[+(OH), 0l,\](3) .2ss(2) .2042(6) .r3s(2) r.4(2) (PO.)rl'-. This compactslab is basedon a local olv(4) .82r(2) .290s(6) .877(2) r.8(2) IC.A.B.CI packingwhere the A andB layersare fully occupiedand the C layersare only one quarter I a copyof Table2, orderDocument AM-76-030 from occupied.The C layers,which correspond To obtain to the ter- the BusinessOffice, Mineralogical Society of America, 1909K minal Op pointingaway from the slabs,link to the Street,N.W., Washington,D.C. 20006.Please remit $1.00 in ad- insularMn2+-O octahedra which bridgeslabs sepa- vancefor the microfiche.

Taslr 3. Bermanite:interatomic distances

prt I'{rt(1) I'h(2) I'{n(3) \ P(2)

I\frr(1)- o(2) 1.892 I\h(2)- 0(s) I .887 I\,h(3)-0(8) 2.079 P(1)-o(7) 1.s28 P(2)-o(8) 1.s28 - o(1) 1.914 - 0(6) I.9T2 - 0(7) 2.160 -0(3) 1.s4s -0(4) 1.s36 -oH(2) I .949 -0H(1) 1.959 -oltl(r) 2.r77 -o(s) 1.ss8 -o(6) 1.s60 -0H(1) 1.960 -0H(2) 1.949 -0l,ll(4) 2.rs6 -o(l) r. s63 -o(2) 1.s66 - - -oI,|I(z) 2.23L 0(31 2.200 o(3) 2.211 average 1.550 average 1.548 - o(4) 2.203 - 0(4) 2.239 -ol,ll(3) 2.27r average 2.020 average ) n)7 average 2.186 o(3)-o(7) 2.48 0(6)-o (8) 2.48 o(1)-o(7) 2.49 0(2)-0(8) 2.s0 o(3)-0H(2) 2.69 0(s) -oH(1) )aa o(7)-ol4l(2) 2.88 o(3)-o(s) 2.53 0(4)-o (6) 2.s3 o(1)-oH(r) 2.69 o(3)-oH(2) 2.69 0(8)-oI,|I(3) 2.s2 0(1)-o(3) 2.s6 o(4)-0 (8) 2.s4 o(1)-oH(2) a a6 o(6)-oH(2) ) al o(8)-0l^'(1) 3.00 o(s)-o(7) 2.s6 o(2)-o(4) 2.s4 0(2)-oH(2) 2.72 o(s) -oH(2) ) '7) o!1l(2)-0l\'(4) 3.03 o(1)-o(s) 2.s6 o(2)-0(61 2.s7 -oH(r) -0H(1) 3.06 o(4) z. to o(4) 2.76 ol|i(1)-oI{'(3) average 2.53 average 2.53 o (2)-oH(1) )70 o (6) -oH( 1) 2.77 0(7)-oI4'(4) 3.08 0(2)- o(3) 2.90 0(4)- 0(s) ?Rq 0 (7) -ohl(3) 3. 09 o(1)- o(4) 2.90 o(3)- o(6) 2.90 o(8)-olv(2) s.r2 0(1)- o(3) 2.91 0(4)- o(6) ow(1)-0l,\'(4) s.r3 o(2)- 0(4) 0(3)- 0(s) ?.95 o(7)-oI4I(1) 3.22 o (4) -0H( 2) 3.74 o(3)-oH(1) ? tq 0(8)-0I^I(4) 3.23 o(3)-oH(r) o(4)-oH(2) 0l{(2)-0l4I(3) 3.34 average 2.86 average )94 average 3.09

Standarderrors: Mn-O,P-O t 0.009 - O.OfSi,;O-O'l 0.010 - 0.0191 1244 A R, KAMPF AND P. B, MOORE

Frc. L Polyhedraldiagram ofthe berrnanitestructure down [0I0]. The Mne*-O octahedraare stippled, the Mn'+-O octahedraunshaded, and the P-O tetrahedraare ruled. The bridging Mnzt-O octahedron above the [Mnir (OH), (PO.L]' sheetsare linked via two tetrahedral vertices,the remaining four octahedrallycoordinating ligandsconsisting of water molecules rated by b/2. The Mnz*-O octahedron is further Mn'+(H2O)a[Mn|+(OH),(POn),J,which expressesthe ligatedby four water moleculesand can, therefore,be compact region of the structure. Figure I shows a written trans-fMn2+(Op)r(HzO)nl.The dense slabs fragment of one such slab viewed down the b axis parallel to {010} with relatively few bonds between with the bridging Mn'+-O octahedronabove it. Fig- them is consistentwith the perfectcleavage parallel to ure 2 is a view down fl0l]showing the linkage be- {010}. With the structural information at hand, we tween adjacent slabs, and Figure 3 is an idealized propose to write the bermanite formula as sketch of the close-packedslab. It is seen that the a- and c-crystallographicaxes are each the sum of one octahedral plus one tetrahedral ed3e. Although the

Ftc 3. The bermanite [Mn!* (OH)r(PO.),]'z sheet idealized according to close-packing. Note the pronounced pseudo- FIc 2. A view of the bermanite structure down the ll0ll axis hexagonalcharacter. The (OH)- ligands are representedas solid Note the role of the bridging Mn2+-O octahedron disks. CRYSTAL STRUCTURE OF BERMANITE 1245 idealB-angle would be 120o,Jahn-Teller distortion, which closelycorresponds to the 2.024 averageob- discussedfurther on, reducesthis to I10.3". servedfor Mn(l)-O and Mn(2)-O in bermanite. Bermaniteis yet anotherexample where insular Md+-O octahedrabridge dense, compact Me3+-O * Hydrogenbonds P5+-Osheets in phosphateminerals. Other examples The locationsof possiblehydrogen bonds were include the laueitegroup, Mn'+(HrO)n[Fe3+(OH),inferred from geometricaland electrostaticvalence (HrO)r(PO4)21. 2H2O (Moore, I 975),and whitmoreite, balancearguments. The potentialdonors of hydro- Fe'+(H,O)n[Fe!+(OH)r(POa)r](Moore et al., 1974). genbonds are OH(l), OH(2),OW(l), OW(2),OW(3) This patternwill aid in interpretationof as yet un- and OW(4). We proposethe following hydrogen known structures,but in additionit emphasizesthe bonds:Ow(l)...OH(1), 2.954; Ow(l).'.O(7), tendencyfor Md+-O octahedrato denselycluster 2.96A;ow(2).'.o(2), 2.83A; ow(2).'.0(6), with POo tetrahedrain coexistencewith insular 2.83A;ow(3). ..o(l), 2.88A;ow(3).'.o(7), Me2+-Ospecies. This point is of considerableimpor- 2.68A;ow(4). .'O(8), 2.82A; ow(4)...Ow(3), tancetoward unravellingthe complexparageneses of 2.83A; OH(1)".OH(2), 2.734. The angles coexistingtransition-metal phosphates, and suggests O-OW-O' computeto OH(l)-Ow(l)-O(7) 91.4'; that insulardivalent octahedral species may coexist in o(2)-ow(2)-o(6)80.2'; o(l)-ow(3)-o(7) 103.1'; the fluid with more complexclusters of trivalent oc- and O(8)-OW(4)-OW(3)106.1'. According to this tahedralspecies in late-stageassemblages. model,O(l), O(2),0(6), O(8),OH(l), OH(2),and OW(3) eachreceives one hydrogenbond and O(7) Polyhedralinteratomic distances recelvestwo. Both Mn3+-O octahedra are elongated(Table 3). This is not the only reasonablehydrogen bonding Mn(1) has two oxygens in trans-configuration at an model based on geometrical arguments; however, it is averagedistance of 2.20A with four planar oxygensat the one which yields the best fit between electrostatic an averagedistance of 1.93A. Mn(2) has two at an valence balance and variations in individual bond averageof 2,23A and four at an averageof 1.92A. distances,as discussedin the next section. This 4 * 2 coordination is typical of transition-metal ions with dn or d" high-spin configurations, since their Discussion electron clouds are subjectedto pronounced Jahn- Electrostatic ualencebalance Teller distortion in octahedral fields. shannron elt al. (1975) found that the averageMn3+-O distancesfor The principle of electroneutralityhas becomeone Jahn-Tellerdistorted octahedra increase in a predict- of our more important interpretive tools in crystal able fashion with the degreeof distortion. The aver- structure analysis.In its simplest form, we are pro- age distancesof Mn(l)-O and Mn(2)-O calculated vided with a means of discriminating oxo-anions, from their equation are2.02 and 2.03A respectively, hydroxyl groups and water moleculeswhen the struc-

Tlsl-n 4. Bermanite:electrostatic bond strengths(po) and their sums about the anions

Anion I'kr( 1) I{n(3) P(1) P(2) H(d) H(a)

0(1) 3/6(7/r2) s/4 L/6 L.92 (2 .00) 0(2) 3/6(7/12) L/O 1.92(2.00) 0(5) 3/6(4/t2) 3/6(4 /12) s/4 2.2s (7 .e2) 0(4) 3/6(4/12) 3/6(4/L2) 2.25(7.92)

0(s) 3/6(7 /r2) s/4 r 7( r1 R?\ 0 (6) s/ 6(7/r2) s/4 t/6 1.92(2.00) 0 (7) 2/6 s/4 2/6 1 0, 0(8) 2/6 s/4 L/6 1.75

oH(1) 3/6(7/12) 3/6(7/72) s/6 r/6 a inl) 1a\ oH(2) 3/6(7/12) 3/6(7/12) s/6 t/6 2.00(2.r7)

ow(r) 2/6 Lo/6 2.00 0w(2) 2/6 r0/6 2.00 ol|i(3) 2/6 10/6 7/6 2.I7 0I4I(4) 2/6 t0/6 2.00 t246 A R. KAMPF AND P B. MOORE ture determinationdoes not provideresolution of the pair and undersaturatedanions distributed about the hydrogenatom locations. remainingfour meridionalapices. This argument Table 4 lists the valencebond strengthsfor the leads to the hypothesisthat stable structuresin- oxygenatoms in bermaniteon the basisof the num- volving octahedralMn3+ in mineraloxide crystals are ber and kinds of cationscoordinating to them. In- those whose trans oxygensare oversaturatedand cludedare the hydrogenbonds proposed earlier. A whosemeridional oxygensare undersaturatedif an bond strengthof s : 5/6 is ascribedto a hydrogen isotropicIW+ cationis placedin that sile. Thus, pro- donor(Hd) ands : l/6 for a hydrogenacceptor (Ha) ceedingfrom idealizedstructure types, those fulfilling as suggestedby Baur (1970)on empiricalgrounds. this criterionwill be likely candidatesfor Mn3+oxy- The electrostaticbond-strength sums all fall within salts.Although sphericallysymmetrical cations like the acceptablerange for a stablecrystal structure. Al3+and Fe3+form isomorphicsalts for a largenum- Baur(1970) correlated the variations in thesums of ber of structuretypes, Mn3+ substitution is probably bond strengths,po, received by anionsto their indi- limited only to thosewhich fulfill the criteriaof trans- vidual distancesto coordinated cations. Over- oxygenoversaturation and meridionaloxygen under- saturatedoxygen atoms, that is, thosewhich receive saturation. Since structural isomorphism between bond-strengthsums from cationsin excessof 2.00, Mn3+and Als+ and Fd+ appearslimited in oxysalt generallyshow longer metal-oxygen distances, while chemistry,we favor the Jahn-Tellerdistortion itself shorterthan averagedistances are found for under- as the basisfor the selectionof thosestructure types saturatedoxygens. This correlationis borneout by whereMn3+ will be stablein an octahedralfield. the MnS+-Oand Mn2+-Odistances in bermaniteas This suggeststhat bond strengthsshould be re- illustratedin Table 5. It shouldbe noted,however, distributedin an unsymmetricalfield as inducedby that ambiguitiesin assignmentof hydrogenbonds Mn8+ so that the assignmentsare lessfor the two wereresolved in favor of this correlation. longerbonds and greaterfor the four shorterbonds. The questionarises as to the origin of the elon- Confirmationof this viewis seenin the poor correla- gationin the Mna+-Ooctahedra. Is it theresult of the tion betweenthe bond strengthwith the P-O bond oversaturationof two transoxygens or is it the result distancesand the obviousimprovement in this corre- of the Jahn-Tellerdistortion? Moore and Araki lation with the suggestedredistribution. We propose (1974)emphasized the orderingof Mn3+in the crys- an additionof As : +l/12 to be appliedto eachof tal structureof pinakiolite,MgrMns+Oz[BOs], which the four shortmeridional bonds and a subtractionof was consistentwith electrostaticallyoversaturated 6t : '2/12 from the bond strengthsof the longer anionsdistributed at an opposingoctahedral apical apicalbonds and recommends : 7/ 12for meridional

Trnlp 5. Bermanite: variation in Me-O distances(dd) compared to variation in electrostatic bond strength sums of the anions (6)o")

Mn(1) Mn(2) l4n(3) P(l) or P(2)

Anion 6d 6lpo 6d 6lpo 6d 6lpo 6d 6lpo

o(1) -. 11 - .L4 +.01 - ,05(+.08) 0(2) -.13 -.L4 +.02 -.04(+.08) 0(3) +.18 +.19 +.22 .00 +.28(.00) o(4) +.18 +.19 +.22 +.22 -.0I +.29(.00) 0(s) -.74 -.28 +.01 - .22(+.191 0(6) -.11 -.11 +.01 - .04(+.08) o (71 - .03 - .05 -.02 -.06(.00) 0(8) 11 11 -.02 -.27(-.17)

oH(l) -.06 -.06 - .08 - .05 oH(2) -.07 -.06 - .07 -.03

0l^I(1) -.01 +,03 olll (2) +.05 +.03 ol4r(3) +.09 +.20 oI\I(4) +.01 +.03 CRYSTAL STRUCTURE OF BERMANITE 1247 bondsand s : 4/ 12 for apicalbonds. Further sup- port of theseassignments comes flrom pinakiolite (Mooreand Araki, 1974)where all four non-equivalent anionsreceiving apical Mn8+-O bonds are over- saturatedby Apo: +2/12 whenthe isotropicmodel (s : 3/6) is adopted.

Relationshipof bermaniteto whitmoreite With the substitutionof Fe for Mn, theformula of bermanitebecomes identical with that of whitmo- TypeI Type2 reite (Moore et ql. 1974).The grossstructural fea- turesof theseminerals are alsorather similar. Both possesssheets of octahedrawhich can be ideally written [Me!+(OHD(Oph].In both instances,the (PO.) tetrahedrashare three of their four verticesrvith octahedrain the sheetto form a slabwith formal composition[Me!+(OH)r(PO4)r]'z-. The tetrahedra point up and down away from the slab, and sharetheir remaining oxygenswith an insular Me2+-O octahedron. Each Md+ bondsto one tetrahedraloxygen from eachof the two adjacentslabs, thereby linking slabstogether; and eachMd+ is furthercoordinated Type3 Type4 by four water moleculesso that the octahedral Fro 4. Twinning schemesfor bermanite down the [010] formulacan be written trans-fM*+(Op)z(HrO).1. The direction.The compositionplane is shown bold. Composition distancebetween the slabsin eachis similar,9.63A planeand twin axis are: Type I {101},[01]; Type2,{101}, {l0l}; for bermaniteand 9.984 for whitmoreite. Type3, {001},l00l; Type4, {301},il001. The linkageof the octahedrain the sheets,however,differs markedly. In whitmoreite,the Fe3+-O relative to one another with a reorientation of the octahedraform edge-sharingdimers which further insular octahedronwould result in orthorhombic link togetherby sharingcorners. We submit these symmetryof the structure,even though the structural differencesas further prooffor thestabilization ofthe topologydoes not change.The reorientationof the bermanitestructure type by Jahn-Tellerdistortion. insularoctahedra would bringthe water molecules in An "Fe3+-bermanite"or "Mn3+-whitmoreite"would adjacentoctahedra too closetogether, and it appears leadto seriousdeviations from electrostaticneutral- that stericrestrictions are responsible for the mono- rty. clinic symmetry. On structuralgrounds, therefore, the substitutions Twinningis the rule,rather than the exception,for of Fd+ andMne+ for eachother in thesestructures is bermanite.Hurlbut and Aristarain(1968) note four predictedto be quite limited.There is no structural commontypes of twinningin bermanite.Specified in reason.however. to limit to substitutionof Fe2+and termsof compositionplane and twin axis,these are: Mn2+at the insularoctahedral sites. On groundsof (l) {101},[10r]; (2) {l0l}, [101]; (3) {00r}, u00l; (a) oxidationpotential, Mn3+ and Fd+ areincompatible. {301},[00]. All of thesecan be explainedstruc- As expected,Moore et al. (1974)note nearlyequal turally, and Figure4 showshow the four typesof amountsof Fe2+and Mn'+ in whitmoreitewith no twinningcould occur in idealizedclose-packed ber- evidenceof Mne+,and Hurlbutand Aristarain (1968) maniteslabs. Type 3 is the simplestand merelyre- and Leavens(1967) report small amountsof Fe8+, versesdirection of the octahedralchains. Type 4 re- but no Fe'+,in bermanite. sultsin a curiouswedge-like arrangement reminiscent of the octahedralwedges found in the crystalstruc- Symmetryand twinning ture of synadelphite,Mn!+(OH)r(HrOD (AsOs) The pseudo-orthorhombicmorphology and cell ge- (AsOo),(Moore, 1970).Type 2 createslocal arrange- ometryof bermaniteis the resultof an internalstruc- mentssimilar to a sheetin hematite,and Type I ture which doesnot deviategreatly from orthorhom- createsan octahedraldouble band. Twinning on the bic symmetry. In fact, a small shift in the slabs scaleof unit-celldimensions. of course.could lead to t248 A. R. KAMPF AND P, B. MOORE a novelstructure type, and it is conceivablethat some HURLBUT,C. S.,Jn. (1936)A newphosphate, bermanite, occurring bermanite-likephases might exist as a result of this with tripfite in Arizona.Am. Mtneral.21, 656-661. - "chemicaltwinning." We suggestthat bermaniteof ANDL. F. AnlsrenetN(1968) Bermanite and its occurrence in C6rdoba,Argentina. Am. Mineral. 53,416431. unusualdevelopment be carefullychecked by single- LnrvrNs, P. B (1967)Reexamination of bermanite.Am. Mineral. crystalstudy in the possibleevent that a related,but 52.1060-1066. distinct,structure type may exist. Moonr, P. B. (1970)Crystal chemistryof the basic manganese arsenates:IV. Mixed arsenicvalences in the crystalstructure of Acknowledgments synadelphite.Am. M ineral. 55, 2023-2037. We thank Mr. J. E. Johnsonfor the fine bermanitespecimen, - (1973)Pegmatite phosphates: descriptive mineralogy and which concludedthe yearsof searchfor a suitablesingle crystal. crystalchemistry. Mineral. Rec. 4, 103-130. Gratefulappreciation is alsoextended Professor C. S.Hurlbut, Jr., - (1975)Laueite, pseudolaueite, stewartite and metavauxite: a ProfessorP. Leavensand Mr. W. L. Robertswho donatedsam- study in combinatorialpolymorphism, Neus Jahrb. Mineral. ples. Dr. Takaharu Araki assistedin the various stagesof the Abh. 123,148-159. crystalstructure analysis. - ANDT. Annrr (1974)Pinakiolite, warwickite and wightma- The National ScienceFoundation supported this work through nite: crystalchemistry of complex3A wallpaperstructures. ,4m. the grant NSF GA 40543and through the MaterialsResearch Mineral.59,985-1004. Laboratorygrant administeredto The Universityof Chicago. - ANDT. Anext (1976)A mixedvalence solid solution series: crystal structuresof phosphoferrite,Fe?+(HrO)s[PO.]r; and References kryzhanovskite,F{+(OH )s[POrf". I norg. Chem. 15, 316-321. -, A. R. Keupn, .tNo A. J. InvrNc (1974) Whitmoreite, Bnun,W. H. (1970)Bond lengthvariation and distortedcoordina- Fd+FeB+(OH)r(HrO){[PO.]r, a newspecies: its descriptionand tion polyhedrain inorganiccrystals. Trans. Am. Crystailogr. atomicarrangement. Am. Mineral.59, 900-905. Assoc.6, 129-155. RnuncH,rNoneN,G. N. luo R. SnrNrves.lN(1970) Fourier Meth- BusrNG,W. A., K. O. MnnrrNluo H. A. (1962) Lrvv ORFLS,a odsin Crystallography.Wiley-lnterscience, New York, p. 96-119. Fortran crystallographicleast-squares program. S. Natl. U. SulNNoN,R. D., P. S.GuMsn[leN AND J. CHENAves(1975) Effect Tech.Inf Seru.ORNL-TM-305. of octahedraldistortion on meanMn8+-O distances.Am. Min- CRoMER,D. T. eNo D. LrsrR.N,hN(1970) Los AlamosScientifc eral 60.714-716. Laboratory, Uniu. of Calif. Report LA-4403, UC-34, - ANDJ. B. M.,rNN(1968) X-ray scatteringfactors computed from numericalHartree-Fock wave-functions. Acta Crystallogr. Manuscript receiued, February 6, 1976; accepted A24.32r-324. for publication, July 2, 1976.