THE AMERIC.\N MINERALOGIST. VOL. 56. SI]PTI'MBER_OCTOBER, 1971

A NOVEL FACE-SHARING OCTAHEDRAL TRIMER IN THE OF SEAMANITE Paur B. Moonn, Departmentof the GeophysicalSciences, The Uniaersitlt of Chicago,Chicago, Illi.nois 60637

AND

Susnare Guosr, Planetologlt Branch, Goddard Space Fl,ight Center, G'reenbelt,Maryland 2077 1.

Assrnacr The atomic arrangementof seamanire,Mnl+(oI{)dB(oH)4][Po{1, a 7.811 (5). 6 15.114(10), r 6 691(5) L, Z:+, spacegroup Pbnm,was deciphered b1. vector set tech- niques.Utilizing 1217independent reflections, full-matrix refinementconverged to Rau :0.062. The structure is basedon a novel face-sharingMn-O octahedraltrimer which links bv edge-sharingwith symmetry equivalent trimers to form infinite chains running parallel to the c-axis Each chain is weakly linked to an equivalent chain through common hy- droxylgroups to form a band.The ligandsinclude (OH)- anions,and tetrahedral[B(OH){l and [POa]groups The tetrahedral groups bridge octahedralbands to form a fairly rigid three-dimensionaledifice. One oxygenatom associatedrvith P doesnot bond to the octa- hedralcations but acceptsfour possiblehydrogen bonds which, with the P5+cation, define a distorted trigonal bipyramid. The peculiartrimer is apparentlystabilized by the shortO-O'edge associated rvith the borate tetrahedron. Of the three trimeric face-sharingoctahedral isomers, two are no$ known to exist in natural crystals

ItTrnooucrror.I

Seamanite, a rare hydrated borate-phosphate of manganese, was first described by Kraus, Seaman, and Slawson (1930). It occurred in frac- tures cutting siliceous rock at the Chicagon Mine, Iron River, Michigan, as pale pink to yellow acicular crystals associated with , man- ganic oxyhydroxides, and . Few specimens were preserved, most of which appear in the A. E. Seaman Mineralogical Museum at Michigan Technological University. McConnell and Pondrom (1941), by X-ray study and etch tests, establisheda 7.83 (2), b 15.I4 (2), c 6.71 (2), space group Pbnrn On the basis of powder photographs, they further con- cluded that seamanite, believed to be Mq(BOB) (POD.3H2O, was prob- ably not isostructural with reddingite, Mq(PO4)2'3H2O. As part of a general program concerning octahedral clustering in manganese anisodesmic oxysalts, we report a detailed study of the seamanite atomic arrangement. We wish to remind readers that the rarity of a specieshas little bearing on the significance of formal crystal structure analysis directed toward furthering our knowledge of crvstal chemistry. As a point of fact, some of the most unexpected clusters have t527 ls28 MOORE AND GEOSE been revealed only through the study of more exotic species.Seamanite proved no exception; not only have we found a novel octahedral face- sharing cluster hitherto believed impossible on geometrical grounds, but we are also committed to interpret the seamanite crystal-chemical for- mula as a basic compound with composirion Mry2+(OH)r[B(OH).][POI].

ExprnnmNur-

A nearly perfectly symmetrical rectangular prism (U.S. National Museum No. 96282) of 0.005 mmt volume was selected for study. 3000 reflections with c as the rotation axis were collected on a PATLRED automated difiractometer using a graphite monochromator and Mo radiation to 20:650. The cell parameters in Table 1 were obtained from con- ventional least-squares refinement of the indexed powder data in Table 2. These powder data were obtained from a spherical powder mount utilizing a 114.6 mm diameter Buerger camera and Mn-filtered Fe radiation, and the corresponding Miller indices were un- ambiguously selected by comparison with the strong intensities of the single crystal study. Individual single crystal integrated intensities were obtained using a 1.8o half-angle scan, with background counting time on each side of the peak of.20 seconds. Reflection pairs of the kind I (hkl) , I (ilkl) were averaged since o-scans about the basal reflections revealed that absorption anisotropy correction amounted to only 6 percent of the mean peak value. A total of 1217 s)'nmetry independent non-zero reflections were processed and were the only ones used in the ensuing study. Among the remaining 515 zero-reflections were the space group absences as well as weak data which were within the error of the background difierences and these data were excluded from further considera- tion.

TABI,E 1. SEAIANITE: CRYSTAI, CELL DATA

ed) 7.8rr (s) !. (8) Is.1r4 (10) s. (8) 6.6er (s) v 63) 7e0.0 (7)

6pace group B!lg, . tr 1 fomula le'3(olD z [B(o]D nl LnouJ z 4

specific gravityl 3.128

calc 3.L32 gry'cn3 I

-Kraus,I Seaman and SLawson (1930). STRUCTURE OF SEAMANITE

TABLE 2. SEAI'AMTE: PO{DER DATAa r_/r_o d(obs) d (calc) lEl

6 7.551 7 .557 020 10 6.917 6:939 ll0 5 5 .420 5 .43r 120 5 4.807 4.817 u1 6 4.204 4.2L7 l2r 5 3.899 3.906 200 3.777 3.781 210 6 3.467 3 .469 220 5 3.394 3 .401 140 3 3.313 3 .345 oo2 3 3.286 3 .290 04r 5 3.057 3.059 o22 8 2.843 2.848 L22 2 2.7L3 2.7L5 240 3 2.6L7 2.625 132 ? 2.58r 2.566 3r0 2 2.555 2.5r9 060 2.502 2.506 2L2 6 2.393 2.397 160 I 2 .3sl 2.357 06I 2 2.309 2.310 32L 3 2.263 2.269 232 2 2.184 2 .186 331 4 2.III 2.LI7 260 2.108 242 I 2.073 2.08I L70 2 2.061 2.063 L23 I 2.O25 2.036 3L2 2 2.0r5 2.018 26I 2.OL? 062 I L.974 I.987 t7r I r.9 45 1.945 252 5 1.901 1.903 332 I I.818 1.8I8 27L 2 1.769 L.77L 181 1530 MOORL:,AND GHOSIi

TABLE 2. (contintled)'

T,/T g (cale) --o $(obs) ILl I t_.736 L.735 440 t I.687 1.687 402 l+ L.672 r.6 76 4L2 r.673 004 r.643 I.645 082

a r.6 36 1.640 450 3 1.610 1 .6r0 182 1.594 1.592 362 z r.5r+2 1.543 290 3 r.528 r.529 380 I.529 520

I I.502 1.501 144 1.488 r.488 372 I I.4I9 I.4I8 183 z 1.402 1.401 311+ 1.401 292 L"372 I.372 164 I .371 254

r.330 r.329 472

1.325 L.326 542

L.322 193

I 1"304 1.302 600

2 L"262 1 .260 630

l_ r .25r+ L.252 084

t F"K* ; 114.6 mrncamera dianeter. Filrn conrected fon

shrinkage. When morb than one reflection contributes

to d(obs), all are listed in decreasing value of spacing. ST'RUCTURE OF SI.:.A M A N IT li 1531

Sor,urron ol tun Srnucrunp

Patterson projections, P(ua) and P(z'ru), combined with polyhedral packing models assisted in the structure determination. The only hindrance in the experiment was the prejudice on the part of the investigators that the clusters revealed by vector set analysis were crystallochemically impossible and were rejected in the initial trial parameter refine- ments, thus rendering the problem more costly in time than justified. The correct model, consisting of two s)zmmetry independent Mn atoms and one P atom, yielded a heavy atorn least-squares refinement of Iiir*,"1- Roo,: _ =:2 la","ll:0.28 p_

Difierence synthesis of the phases thus produced unambiguousiy revealed all atoms in the asymmetric unit of the crystal structure. Inclusion of the anions in the refinement followed by further difference synthesis revealed the position of the atom.

RerrntlmN:r

The study by McConnell and Pondrom (1941) indicated a centrosynmetric crystal and we did not find it necessary to consider lower s],'rnmetry throughout this study. AII atomic free parameters in the asymmetric unit were refined according to full-matrix least-squares procedures using a local version of the familiar program of Busing, Martin, and Levy (1962). Scattering curves for Mnl+, P3+, B2+, and Or- r,r'ereobtained from MacGillavry and Rieck (1962). The reliability index, R7,47,converged to 0.O62 Ior alI 1277 non-zero reflections which were each ascribed unit weight. The atomic coordinates and isotropic -f'",r" thermal vibration parameters are listed in Table 3 and the lF.o"l data appear in Table 4.1A ratio of independent data to variable parameters of 30:1, Iow estimated stan- dard errors for the atomic parameters and thermal vibration parameters typical for their ionic speciesadd confidence to the interatomic distances discussed further on.

DnscnrprroN oF THE Srnucrune Seamanite is certainly a candidate for one of the most exotic of min- eral structures. The Mn-O octahedral arrangement is one of the most peculiar and unexpectedon record,and we shall discussit in detail. Figure 1 features a polyhedral diagram of the octahedral backbone and its surrounding tetrahedral [B(OH)4] and [PO4]ligands, projected down the ,r-axis.We isolate the octahedral backbone and show it in Figure 2. It consistsof an octahedralface-sharing trimer which jcins to identical trimers by reflection across shared edges.The resulting chain runs parallel to the prismatic c-axis of the crystal. The face-sharing trimer is the most peculiarfeature of the structure sinceits arrangement on geometrical grounds utilizing regular octahedra with oxygen atoms

I To obtain a copy of Table 4, order NAPS Documdnt 101519from National Auxiliary Publications Service of the A.S.I.S., c/o CCM Information Corporation, 909 Third Avenue, New York, New York 10022; remitting $2.00 for microfiche or $5.00 for photo- copiespayable to CCMIC-NAPS. 1532 MOORE AND GIIOSE

TABLE 3. SEAIANITE: ATOMIC CO0RDINATES AND ISOtROPIC TEMPERATUREFACTORS.

Gstimlgg g@da4 errors in parentheses refer to the last dieit.)

x z qt82l

l4n(I) o .?783 (2) 0.47s8(1) 0.2500 o.s3(2)

},fu'(2) .3s07(r) .6406 (r_) .s016(r) . s8(r)

P .693r (2) .ssrl2(I) .2500 .30(2)

B .o07 2 (r2) .3146(6) .2500 .63(1r)

o(1) = 6s- . r+6ls(8) .3688(r+) .2500 .e1(8)

o (2) = oH- .1862(8) .6165(r+) .2500 .80(8)

0(3) = oH-(B) .03ee(e) trnaartr\ .2500 1.01(e)

o(a) = sg1s; - .r-8+6(8) .3027(4) .2500 .6e(8) = (7) o(s) 611-131 " 08r-0(6) .273e (3) .07r+2(81 L .r2 0(6) = oz-(P) .7246(6) . r+961 (3) .0648(7) . e1(6) 2- o(7) = 0- (P) . s042(8) qRtrq rtrf .2500 . e0(e)

o(8) = 62-qP; .8ltz(8) .6340(r+) .2500 .e6 (e)

at the verticeswould be impossible.The Mn(1)-O octahedron,situated on a mirror plane,shares two adjacentfaces with the Mn(2) -O octahedra' Closer inspection of octahedral clustersin general reveals that only three kinds of topologically and geometrically distinct f ace-sharing trimers can exist and all three possessthe stoichiometryMagrz, where M is the octahedralcenter and Q a vertex. Figure 3 featuresthe three trimers projected down the octahedraledge and arrangedto most effec- tively show their point symmetries. One arrangement has point sym- metry 2f m and the other two possessmm2 The centro- ('5.1A"_sYmnretry. symmetric arrangementpersists in the fibrous basic ferrous- ferric phosphates,as discussed in detail by Moore (1970).In the seamanite arrangement,the trimer is the least favored geometricallysince the gap between the two free vertices across the mirror plane is only 1.5A, referring to a regular octahedraledge of 3.0A. The third arrangement' which so far has not beenfound in crystals,is favorable on geometrical grounds. This peculiar trimer is apparently stabilized by the B(OH)4 ligand group,since the O(5)-O(5)' tetrahedral ed^gewhich bridgesthe free ver' tices is 2.35A. The dilation of cc. *0.8A away from the undistorted octahedral model is indicative of forced ligancl'' In seamanite' the STRUCTURE OF SEAMANITE 1533

t- I I b =llt b=Vq OH c=0 Ott c:0

L b=l b=l c=0 c=l

Frc. 1. Polyhedral diagram of the seamanite atomic arrangement. The octahedral Lackbone is stippled and the surrounding P-O and B-O tetrahedra are featured.TheO(1) hydroxyl anion connects to two octahedral backbones related by the inversion operation. The O(1) position is shown on a black disk. The O(2) hydroxyl anion is situated under the stippled backbone and is bonded to 2 Mn(2) f Mn(1). Its position is near the symbol "OH." r^ I I b.h u- t4

L I b=l b=l c=0 c=l

F'rc. 2. The octahedral backbone in seamanite. Note the edge-sharing of the octahedral face-sharing trimers which results in the chain running parallel to the c-axis. IJ.'4 MOORE AND GHOSE

mm2 mm2 Ftc. 3. The three possible topologically distinct octahedral face-sharing trimers. They are projected parallel to octahedral edges and oriented to fully show their point symmetry trimers share their terminal edgeswith each other by reflection. Here, the bridging O(6)-0(6)' :2.48 L atoms belong to the [POo]tetrahedral edge.The size differencebetween the two different tetrahedra must be acceptedas a stabilizins feature for the octahedralchains and, for this reason,the B and P atoms are ordered in the non-equivalentoxygen tetrahedra. The crystal structure analysis suggeststhat the deviation from the 1: 1 B: P ratio found in the seamanitechemical analysis of Ktaus, Seaman,.andSlawson (1930) is largely an analytical error. Figure 1 reveals that the octahedralbackbone is not strictly insular with respectto other octahedrabut is weakly condensedwith the sym- metry equivalent chain related by the inversionoperation. As shown in Figure 1, the two chainslink by the O(1) hydroxyl ions toforma band. Individual bands are further joined by the tetrahedral ligand groups which also link to similar bands generated by the glide operations to form a three-dimensionalarray of corner-linkedpolyhedra.

Cnvsrar CnBursrnv Table 5 presentselectrostatic valence balances of the cationic charge contributionsto the anions.Itis readily apparent that O(1), O(2), O(3), ST RUCT UR]J OF SEA M A N I'I' E. 1535

O(4), and O(5) are hvdroxyl groups,and O(6) and O(7) are oxide anions. Despite the fact that it is considerablyoversaturated with respect to cations,O(4), with 2:1.42, has to be acceptedas an hydroxyl anion. Thus, the ligand groupsare (OH)-, [B(OH)4],and [POr]and the seaman- ite crystal formula is Mn32+(OH)r[B(OH)4][PO+].No water molecules are present either as ligands or as water of hydration and the stoichio- metrically similar formula Mne[BOa][PO4].3HrOis crystallochemically meaningless.Seamanite is an interestingexample of a coordinationcom- plex where a neutral acid-[POrl-and a basic fraction-(OH)--are present in the same crystal which includes the amphoteric [B(OH)a] group. This would suggestthat the compound forms under rather re- stricted pH conditions which, in combination with the mr.rtualpresence of boron, ,and manganese,may be a contributing factor in the restrictedoccunence of the soecies.

INrBnn torrrc DrsrANcES The relativelv low errors in atomic positions and the unusual octa- hedral clustering in seamanite warrant a detailed discussion of the interatomic distances.Table 6 lists Me-O and O-O' distancesin order of increasingvalues for eachpolyhedron. Most interestingare the distances associatedwith O(7) since this anion is considerably over-saturated electrostaticallywith respect to cations and since the anion occurs at the vertex common to the two Mn(1)-O sharedfaces. The Mn(1)-O(7) :2.11 h is unusually long-in excessof +O.20 A for typical Mn2+-O octahedral averages. In addition to its cation oversaturation, the Mn(2)=+Mn( 1)e-Mn(2) cation-cationrepulsions across the shared face

TABLE 5. SEAIANITE: ELECTROSTATIC VALENCE BAIANCES 0F CATIONS AB0llT ANIONS

x Ax' o(1) = s6 Ifrt(1) +ltr(2) +lftt(2) U6+U6+U6 r.00 +0.00 o(2) = 6Y- lftt (1) +t'ln (2) {+h (2) U6+U6+UE r.00 +0.00

0(3) = 66 r.tl(1)+B U6+3/4 rno +0.09 o (a) = 611- }&r(2) +I,ln(2) +B U6+U6+3/4 r.42 +0.42

0(s) = oH- r.h(2) +B z/6+3/4 l_.09 +0.09

0(6) = e2- ltr (I) +l'h (2) +P U6+2/6+s/+ L.92 -0.08 o(7) = 62- l'h(1) +D'ln(2)+l"tr(2) +P U6+U6+U6+5/4 +0.25 o(8) = 62- P 5/+ 1.25 -0.754 aExclutles the plesence of four BossibLe bonds. 1s36 MOORD AND GHOSD

TAIU 6. SLASNITE: POLYI[iDmI ]NTtl0lOMrC DISTANC|So, (istifrated standard errors: Mn-o, P-O +o.006 I, B'o +0.0r3 8, o-o +0.00S 8)

ll h(l) Mn(2)

IP-o(8) 1.517A I nr(I) - o(3) 2.fff r h(2) - o(s) "' 2.1'rrl ? - 0(6) r.538 2 - 0(6)' 2.150 - o(2) 2.150 ' r - o(7) r.5q5 | - 0(r) 2.r59 - 0(6) 2,188 r.s:l I r - o(2) 2.2\q - 0(r) 2.222 r - o(7) 2.q11 - o(7) 2-23\ ' r 0(6) - 0(6) " 2.\79 average 2.20q - o(c) 2.277

2 0(6) - o(7) 2.507 averagc 2.203 ur r o(8) - o(7) 2.5L2 olzy - oq;1 2.530 bz ur 2 0(6) - o(8) 2.5t7 o1z1 - oqel' 2.796 olzy"',o121 2.530 2.506 "2 o(7) - 0(6) 3.020 "r 0(6) - o(2) 2.796 'r z o1:1 - o1o1' 3.L37 oqrl' - o1+1' 2.939 B 2 o(r) - 0(6)' 3.27q "r 0(7) - 0(6) 3.020 ' I o(r) - o(7) 3.276 r o1u1 - o1s;"' 3.C66 2B-o(s) l.qq7 r o(2) - o(3) 3.32q r 0(r)' - 0(6)' 3.O76 r - o(3) l.q6l r o(r) - 0(3) 3.3s2 r ogsl"'-o1r1 3.165 r - o(q) 1.509 average 3.078 r o(r)' - o(5)"' 3.199 t.q66 r o(2) - o(s)"' 3.2r7 ' ' h(r) - b(2) 3.058 r o1r; - oloy 3 -252 1 o(s) - o(s) 2.351 t o('r) - o(3) 2.347 m(2) - un(2)"" 3.126 r o(1)' - o(7) 3.432 2 O(3) - o(s) 2.389 2 o(4) - o(s) 2 -qzq h(2) - h(2) " 3.369 r 0(2) - o(4) 3.56q 2.394 3.I05

uun(l) - m(z) *a"ed face and h(2) - Mn(Z)" shared edse.

om(r) - m(z) shared face onfy.

"m(z) - w(z) " " $u""a edse onf y.

dSuperscripts reresent swetry equivalent positions of Table 3:

' = -x, -Y, V2 +zi

', = x, y, 1/2 -";

ttt = V2 -x, V2 +y, V2 -zi

tt,, = x, y,3/2 -2. are so profound that the Mn(1)-O(7) distanceis affectedalong with the foreshorteningof the octahedraledges associated with the sharedfaces. The O(2)-O(7):2.534 edge is common to the two Mn(l)-O shared faces and a shared edge between the Mn(2)-O octahedra.This edge is foreshortenedby co. -0.55 A with respect to O-O' averagesassociated with Mn-O octahedra, indicative of particularly violent cation-cation repulsioneffects. As shown in Table 6, all O-O' distancesassociated with sharededges are among the shortestfor their polyhedra.Thus, the un- usual face-sharingtrimer cannot be explainedsolely on the presenceof Mn-Mn metallic bonds since the edge distance relationships follow the electrostatic model. Although a weak metallic bond may be present, distortion of the octahedral clusters conform to a model with predomi- nant ionic character. As interpreted,the O(4):(OH)- anion is considerablyoversaturated with respectto cationssince it coordinatesto two Mn-O octahedraand STRUCTURE AF SEAMANITE r.).t/ one B atom. Consequently,the Mn(2)-O(a) and B-O(4) distancesare the longestfor their polyhedra.This correlationbetween charge balance and interatomic distancesadds support to the interpretation that O(4) is an hydroxyl group. But this meansthat O(S) mav be severelyundersaturated with respect to cations sinceit coordinatesonly to the P atom. The P-O(8) distance is the shortestfor the tetrahedron but in addition O(8) potentially re- ceivesfour hydrogen bonds.These bonds incl,ude2 O(8) ' ' 'O(5) 2.71, O(8) . . .O(+) 2.73,and O(8) ' ''O(2) 2.944. They may be described as two lateral bonds from hydroxyl groups associatedwith boron atoms relatedby the mirror plane,one bond in the *a direction to an hydroxyl group and one in the -a direction to a boron hydroxyl group. This net- work of bonds through the unit cell is shown in Figure 4. Such an arrangementadds further confidenceto the interpretation that O(8) is an oxide anion whereasO(4) is an hydroxyl anion. Indeed, Baur and Rama Rao (1968) have found a similar example in metavauxite, Fe(HzO)e'Alr(OH)r(HrO)z(POr)2, where one phosphorus oxvgen does not coordinateto octahedralcations but receivesthree hydrogen bonds instead.In the crystal structure of Nar(HrO)i(PO3OH),Baur and Khan (1970) have found that each of the three tetrahedral oxYgen atoms receivesfour hydrogen bonds which, including the P5+ cation, d,efinea tetragonal pyramid. The range of these bonds are 2.72-3.06A. The

L-l b=| b=| c:o c=l

Frc. 4. The possible O-H' ' ' O(8) bonds in seamanite. The tails of the arrows are located at the hydroxyl donor and the heads point toward the O(8) acceptol. Heights are siven in fractional coordinates. 1538 MOORI':,AND GHOSE

polyhedron in seamanite is a highly distorted trigonal bipyramid includ- ing the P5+ cation in the coordination sphere. We have performed a three-dimensionaldifference synthesis as an attempt to locate hvdrogen atom positions, but the peak heights proved to have little physical meaning,doubtless arising from the presenceof the heavy atoms in the structure and the feeble contribution of hydrogen atoms to the struc- ture factors.

Acxxowr,nocrlmltls

We thank Dr. George Switzer of the U.S. National Museum for the donation of the seamanitecrystals used in this study. one of us (P.B.M.) acknowledgesthe NSF GA10g32 research grant and a Dreyfus Foundation grant for financial support.

Rnlnnrncns

Baun, W. H., Arrt B. Reua Reo (1967) The crystal structure of metavauxite. Notur- wissenschaJtm21, 561'562. eNo A. A. Ku,u (1970) On the crystal chemistry of salt hydrates. VI. The crystal structures of disodium hydrogen orthoarsenate heptahydrate and disodium hydrogen orthophosphate heptahydrate. ,4 cta Crystdlogr. 826, 1584-1596. BusrNc, W. R., K. O. ManrrN, lNo H. A. Lrw (1962) ORFLS, a Fortran crystallographic least-squares program. U .5. Oak Riilge Nat,. Lab. (U .5. Cleari.nghouseFd. Sci. Tech. InJo.) Rep. ORNLTM-30S. Kneus, E. H., W. A. Snaunx, eNp C. B. Sr,awsor (1930) Seamanite, a new manganese phospho-borate from fron County, Michigan. Amu. MineraJ. lSr 220-225. MncGrrr,avnv, C. H., axo G. D. Rmcr, EDS. (1962) International Tables Jor X-ro-t. Crystallography, VoL 3,TheKynoch Press, Birmingham, England. McCoxNnrr., D., exo W L. Poxonou, Jr. (1941) X-ray crystailography of seamanite. Amer. Mineral. 26, MGM7. Moonr, P. B (1970) Crystal chemistry of the basic iron phosphates. Amer. Mineral. SS, 135-169.

M anu,seripl recehed.,f anuary 12, 197I ; acceptedfor publicolion, M arch 23, 1971 .