American Mineralogist, Volume60, pages460465, 1975

Palermoite, SrLir[Al4(OH)4(POn)o]: Its Atomic Arrangementand Relationshipto Carminite, Pbr[Fen(OH)n(AsOn)r]

Peul BnIeNMoone, nNo Tlre,g,qnuAnerI Departmentof the GeophysicalSciences, The UniuersityoJ Chicago, Chicago,Illinois 60637

Abstract

Palermoite,SrLi,[AL(OH).(PO.).], Imcb, a I L556(5),b 15.847(7),c 7.315(4) A, -- Z : 4, is structurallyrelated to carminite,Pb,[Fe'(OH)o(AsOnL]. R(hkl) 0.090for l47l non- equivalentreflections. Both contain the same [Ms+.(OH)r(fOr)r]'- octahedraland tetra- hedralslabs oriented parallel to {010}.They are distinguishedby the link to symmetry- equivalentslabs across the glidesat b -- l/4. The spacegroup Amaa for carminiteshares the samesubgroup (Pmaa) with palermoite.In both structures,the octahedraform chainsof edge-linkeddimers which are corner-linkedto symmetry-equivalentdimers resulting in the compositionIf+r(O)?(OH)r. One-eighthof the tetrahedraloxygens are not bondedto theoc- tahedra. IvLi-O Polyhedralinteratomic averages are vIrrSr-O 2.62A, vIAl-O l.9l A, 2.8 A, IVP(1)-O1.53 A andIVP(2)-O 1.54 A. Localisomorphism of all atomsexcepting Li andPb(2) occurs:the Li atomsare split into twicethe equipoint rank number as Pb(2) and possess lower point symmetry.

Introduction cell sincethe spacegroups are neither isomorphic nor is one a subgroupof the other. Palermoiteoccurs locally in moderateabundance Experimental assmall striated prismatic colorless crystals at its type locality,the PalermoNo. I pegmatite,near North Palermoitesingle crystals collected at the type lo- Groton, New Hampshire.It wasoriginally described cality by P.B.M.were submitted to singlecrystal X- by Mrose (1953),who proposedthe formula (Li, ray study.In addition,a qualitativeelectron probe Na)oSrAlr(PO4)s(OH)r,Z : 2, and the unit cell scandetected Sr, Al, P, and only minor Ca (1 l%o). parametersa 7.31A., b t5.19A, c 11.53A with the The extinctioncriteria, from films and singlecrystal spacegroup Immm. Another chemicalanalysis by diffractometer,suggested the spacegroups l2cb ot Frondeland Ito (1965)led to the proposedformula Imcb, in disagreementwith Immm proposed by (Li, Na),(Sr, Ca) AL(PO.)'(OH)0,Z : 4, with the Mrose (1953).Doubly terminatedcrystals and the refined cell parametersc 7.315(4),b 15.849(9),c three-dimensionalcrystal structureanalysis support 11.556(6)A. Meanwhile,Strunz (1960) proposed an the centrosymmetricspace group Imcb. isotypic relationship betweenpalermoite and car- Refinementof the cell parameterson a PIcrnn minite,PbFes+,(AsO.L(OHL. To reconcilethe rather automated diffractometer afforded a 11.556(5),b complex formula of Mrose and the similarity in 15.847(7),c 7.315(4) A. We selectedthe standardax- the crystal cell parametersbetween palermoite and ial conventionfor the orthorhombicsystem to which carminite, he proposed the formula SrAlz(POo), the spacegroup Imcb conformsand acceptedthe cell (oH),. contents 4[SrLi,Alo(OH).(PO4)4].Other salient Despite similarities in the cell dimensions, we were details:graphite monochromatized MoKat radiation puzzled by the difference between the body-centered (I : 0.7093A); maximumsin d/tr : 0.80;twenty sec- cell for palermoite and the end-centered cell for car- ond backgroundcounting times; scan rate l.0olmin- minite. Atomic positions basedon the crystal struc- ute; half anglescan 1.8'. The thick prismaticcrys- ture analysisof carminite by Finney (1963)could not tal, of maximum dimension0. 12 mm, was not be isomorphically transformed into the palermoite corrected for absorption. The equivalent reflec- 460 ATOMIC ARRANGEMENT OF PALERMOITE 461

Teslg. l. Palermoite:Atomic Coordinate and lsotropic Thermal Vibration Parameters*

Point Point group Muft . rd2) Atom group Mrlt. r (.[ 2)

Sr 222 4 0.2500 0.5000 0.5000 0.80 (2) P(2) m 8 .0000 .4s76(r) .773r(2) Pb(r) 222 4 .2500 .5000 .5000 As(l) m I .000 .457 .760

Li m8 .5000 .285r(11) .2723(2r) o(3) m 8 .0000 .3e93(3) .6107(6) . e8(7) Pb (2) 2/n 4 .5000 .2500 .2500 0(3) m 8 .000 .386 .606

AI l16 .1307(1) -3727(L) .1374(2) .s6(2) 0(4) m 8 .0000 .4014(3) .es02(6) .76(6) Fe It6 .136 .378 .13r 0(2) n I .000 .r+05 .968

P(I) 2A .2500 .2922(L) .5000 .4s(2) 0(s) r 16 .1083(3) .48s7(2) .227s(tt') .69 (r+) As(2) 2a .2\O .289 .500 0(r) I 16 .r15 ,qa2 .260

o (1) 116 .1438(3) .2632(2) .0387(4) .7s (5) 0H(1) m L0000 .3363(3) .2606(6) .62(6) o(s) I16 .145 .272 -.013 oH(r) m 8 .000 .3 3r+ .zt+I

o (2) 116 .22s7(3) .3srs(2) .338s(r+) .se (4) oH(2) 2 8 .2500 .4117(3) .0000 .87(6) o (r+) l16 .24I .348 ,326 oH(z) 2 8 .250 .430 .000

* Estimteil standadl etrors refer to the fast diqit. IIE axomic parareters of catminite in Einneg (f963), appropriateLg are shom fot cm@rison.

tions (ftkl) and,(hkD were averaged,yielding l47l diagrams of the space groups Imcb (palermoite) and independentF (obs),which wereobtained through Amaa (carminite) down the c axis in the conventional standardcomputational procedures. b ) a ) c orthorhombic setting. The essential difference between the two is a c elide at b : l/4 in Determinationand Refinementof the Structure Three-dimensionalPatterson synthesis indicated strongvector densities at 0 u 0; l/2 u 0;0 u l/2; and I/4 u l/4. Symmetryrestrictions led to rapid deter- minationof the Sr, Al, P(l), and P(2)positions. The B- and7'syntheses of Ramachandranand Srinivasan (1970)led to unambiguousresolution of all non- hydrogenatoms. Four cyclesof full-matrix,least-squares refinement basedon isotropicthermal vibration parameters,full site occupancies,secondary extinction correction with co : 0.428 X l0-o (Zachariasen,1968) and anomalousdispersion correction for Sr, Al, and P led to Amoo R(hkt): z-1lrc-!gl-l-{es]lXl:0.090 > lF(obs)l for all l47l reflections.The final atomiccoordinates and the isotropic thermal vibration parametersare presentedin Tablel. Table2 liststhe structurefactor data.l

Descriptionof the Structures The formulaSrLi,IAL(OHXPOJ,] (here idealized) proposedby Frondel and Ito (1965)is confirmed. Althoughthe structuresof palermoiteand carminite F are closelyrelated, the two crystalsdo not exhibit an isotypic relation. Figure I featuresthe symmetry ' To obtain a copy of Table 2, order document number AM-75- Ftc. I. Symmetry diagrams of the space groups Imcb, Amaa, 003-8 from the BusinessOffice, Mineralogical Society of America, and Pmaa in standard cell orientation (down the c axis). The first 1909 K Street, N.W., Washington, D. C. 20006. Pleaseremit $1.00 two groups apply to palermoite and carminite respectively. The for the microfiche. third group is the subgroup common to the first two. P, B. MOORE. AND T. ARAKI palermoitewhich is replacedby a n glideat b : | /4 in freedomin the atomic coordinates.Pb(2) possesses carminite.Accordingly, the screwaxis at (0 l/4 z) rank number4, point symmetry2/m, andno degrees and the inversion at (l/4 l/4 l/4) in Imcb is of freedom. translatedby Q /a 0 0) to createAmaa. The two-fold Palermoiteand carminitecontain the sametype of rotationsat(l/4 y0);(/aO z);(x0 0);the a-glideatc M-O octahedralchain. In palermoiteit consistsof an = 0; the a-glideat b : 0; the mirror planeat a : 0; edge-linkeddimer which is corner-linkedat the same andthe inversion at (000)remain invariant in thetwo levelto symmetryequivalent dimers (see Fig. 2). The spacegroups. This correspondsto the mutual sub- chains,which run parallelto the a axis,have com- groupPmaa (D'rn), shownat the bottom in Figure l. positionAldOp)z(OH)r, where Op are oxygensthat The palermoiteand carminitestructures (Fig. 2) belong to PO. tetrahedra.One-eighth of the both contain the same [M3+n(OH)4(TOn)ofo-oc- tetrahedraloxygens, namely O(3) in both structures, tahedral (luj and tetrahedral(O slab oriented do not bondto thetrivalent octahedrally coordinated parallelto {010}.The structuresare distinguished by cations.The pointsof condensationof theoctahedral the links to symmetryequivalent slabs across the c chainsinclude OH(l), OH(2),and O(4) in palermoite, glideat b : l/4 in the formerand the n glideat b = eachof equipointrank number8. l/4 in the latter. In fact, taking the symmetry Remainingin the structuresare pockets al (l/4 l/2 operationswhich are invariant in the two space l/2); and (l/2 l/4 l/4), eachof equipoint rank groups,the pairsof linkedslabs between l/4 I b I number4. Both non-equivalentpolyhedra are dis- 3/4 areisomorphous in the two structures.For this torted cubesand accommodatePb(l) and Pb(2)in reason,the atomic coordinatesfor palermoiteand carminite.Although Sr in palermoiteis isomorphicto carminitein TableI arecompared within thisbound. Pb(l) in carminite, the Li in palermoite It is seenthat all the parametersand their equipoint are split into two equivalences,the coordination rank numbersand point symmetriesare similar, with polyhedron being a distorted tetrahedron.The the exceptionof Li in palermoiteand Pb(2)in car- polyhedralenvironments about Pb(2) and Li are minite. The Li atom possessesan equipointrank shownas projectionsdown the a axis in Figure3. numberof 8, point symmetrym, andtwo degreesof Theseregions are non-isomorphicas they do not

:

Frc. 2. Polyhedral diagrams of palermoite (left) and carminite (right) structures down the c axis, The outline a x D/2 is shown. Heights are glven as fractional coordinates in z. The Pb-, Sr-, and Li-O bonds are dashed in' ATOMIC ARRANCEMENT OF PALERMOITE 463 possessthe samepoint symmetryin the two struc- oil) 0(rl -0.03 tures. +o-47 ? Oneof the curiousfeatures of the two structuresis 2.28 2.69 theobservation that the"X" positionat(l/2l/4 l/4) 0(4)+0.55 2.81 0(41+0.05 in palermoitepossesses polyhedron a coordination Jfl- . a similar to that in carminite,both being distorted 0(31-0.11 0(3)+0.39 cubes.Figure 4 providesthe bond distancesfor "X' x +0.25 in palermoiteand Pb(2)in carminite.The position 2. 2.69 "X", however,still has equipointrank number 8 to(t) oil) sinceits point symmetryremains m and to afford a +o-47 -0.03 carminite-likecomposition would requiredisordered half-occupiedsites over the Pb(2) positions. Since the environmentsabout the X and Pb(2)sites are non- 0 (5) 0(5) isomorphic,the energeticrelationships between the +0.51 two structuresare probably dictatedby the charges 2. 2-56 andionic radii of thecations competing for theX and Pb(2)environments, as the distinctionsin the I otlt.. 0(3)+0.59 bond- a ing over the restof the structuresare small. m-! . 0(2)+ 3.05 0(21+0.03

*oo.!1) -o.t.tJ, Ftc. 4. The ",f'-O bonds and Pb(2)-O bonds in palermoite (top) and carminite (bottom). The symmetry elements(m and 2/m respectively)are shown on the left and apply to the loci of ".f' and Pb(2) Bond distances are specified.

We proposethat palermoiteand carminitestruc- ture ideals with compositionsX(I)X(2)Mr(OH)t (TOn), are combinatorialpolymorphs in the sense definedby Moore (1975). Bond Distances Table 3 lists the anions and the coordinating cationsin rows and columns.In this manner,the valencebond strengthscan be convenientlytabulated and deviationsfrom bond distanceaverages can be relatedto deviationsin bond strengthsums. One problem immediatelyarises regarding the coordination of lithium. Although its nearest neighborhooddefines a distortedtrigonal bipyramid, the valencebalances suggest that the true coordina- tion is distortedtetrahedral, the polyhedralvertices includingone OH(l), oneO(3), and two O(1)atoms. Interatomicdistances are given in Table 4. The Li-O tetrahedraldistances range from 1.94Io 2.29A, the fifth distanceLi-O(4) : 2.46 A suggestingthat the tetrahedralcoordination is morelikely. We note in additionthat a distortedtrigonal bipyramid would Frc. 3. The polyhedralenvironments of Pb(2)in carminite(top) and Li in palermoite(bottom) down the a axis.The Pb(2)atom requirethat the O(3)-O(4)edge be sharedbetween Li residesat (l/2 l/4 l/4). In palermoite,the locus at Q/a l/a l/4)is andP(2). The additionaldistances which obtain from drawn as a circle. the distortedtrigonal bipyramidalmodel are listed 464 P. B, MOORE, AND T. ARAKI

Tlst-t 3 Palermoite:Electrostatic Bond Strenethsand Their Sumsabout the Anions*

^2+ - .1+ ?r q+ Anion 5r L1 A.L P"' H(d) H(a) Sr Li AI

0(r) v4 3/6 5/4 2.00 0 o(2) 2/e 3/6 s/q 2.00 0

o(3) v4 5/4 V6 L.67 0 (4) 3/6+3/6 5/4 2.25 + + 0(s) 2/e 3/6 5/4 'e(vo) 2.08 + 0

oH(r) vq 3/6+3/6 5/6 2.08 oH(2) 3/6+3/6 s/6 r.83

Me-O refers to distances which are greater than (+), less than (-), or within (0) 2o of the interatomic error referred to the polyhedral average. H(d) = hYdrogen donor, H(a) = hydrogen bond acceptor. parentheticallyin Table 4. The Sr-O polyhedron is a and 1.54A averagesfor P(l)-O and P(2)-O respec- distorted cube with distancesbetween 2.59 and 2.65 tively. A. tne remaining polyhedral distance averagesare Table 4 also revealsthe effectsof polyhedraledge- typical, with the l.9l A averagefor Al-O and 1.53 sharing.The SrOr cube, for example,shares two

Tlst-e 4. Palermoite: lnteratomic Distances

AI P(1) Sr

Ar- -oH(2) 1.8ls(s)A 2 P(r) -o(2) r.s3r+(4) 4Sr - o(s)' 2.sso(3) - -oH(1) . 1.8s0(s) 2 -o(r) 1.s3s (4) + 0(2) 2.646(3) - o(2)a I .868 (4) average I q?u average 2 .618 - 0(r) 1.884(4) - 0(s) r.e28(4) + - r+23 + - 0(4) 2 .08e(s) r o(2) -o(2)+++2.423 (4) 2 o(2) o(2)1ii 2 . (4) -o(r)iil 2. s17(t+) u 0(2)..- o(s)1 2.65I(t+)'' averlage 2 o(2) 1.906 r o(r) -o(1)ttt 2 . slg (r+) r+ o(s)11-o(2)rY. 3.3r7 (4) -o(2) - 4.012 (4) * 2 o(1) 2.s26(4) 2 o(s)a o(S)tr1 oH(1) - 2 r+94(7 o(4) . average 2.505 average 3.062 oH(r) - 0(r) 2.se6(6++ 0H(2) .- 0(s) 2.613(6 o(2 '- o(s) ) 6\1rU P(2) oH(2.- 0(r) 2.669(6 o(2 i-oH1z1 2.67L(6 I P(2) -o(3). r.s06(s) 1 Li -oH(r)aa 1.94(2) o(2 '-oH(1) 2.68r(6 2 -O(5) ]v r.s40(4) I - o(3).. 2.oo(2) oH(1 - o(s) 2.688(6 I -o(4) r.s72 (s) z - o(l)" 2.2e(2) o(4 o(s) 2.732(6 (r - o(4) 2.46(2)) '-.- aver?age I.539 0(2 o(r) 2.77L(4 average 2.I3 0(r - o(4) 2.82s(6 oH(2 - 0(4) 2.sr6(7) I o(3) . -o(4) . 2.484(7)** ]v-o(s) 2.48r+(7) **) 1 o(s) y-_ 2 "s02 (4) average 2.692 2 0(3) -O(s)i:. 2 . s07(6) 2.596(6).r 2 0(r+) -0(s)-" 2 . s4o(6) 3.r10(6) Hydrogen Bonds 3.323(4) average 2 .513 3.8e1(7) oH(r)...0(3) 2.74e(7) 4.0r4(7) ) oH(2).,.o(s) 2.84s(6) 4.316(6) )

*-Edge -between r' sharing between AI-AI- octahedra; Sr:-P polyhednal between Sr-AI polyhedna; **between ffbetween Li-P polyhedna; Li-Al polyhed.ra. i = UZ -x, U2 -y, V2 -23 ii = xo U2 -y, j-ii = -x, -!t = -x, U2 +21 = I/2 -X, v, -23 iv = Xr -v, -zi v = -xo -!s -z; vi lt/Z zi uii 1u/2 UZ + y, J,/2 + zi viii = x, U2 + y, V2 -z; ix = -x, VZ -y, U2 + z. These equivalences refer to the d.esignated atoms in Fig. 2. ATOMIC ARRANGEMENT OF PALERMO]TE 465 edges with the P(l)O. tetrahedron and four edges Acknowledgments with the AlO. octahedra.These smaller more tightly Financial support by the N.S.F. Grant GA-40543 and a Sloan bound polyhedra geometrically restrict the O-O' Foundation Grant-in-Aid BR-1489 awarded to P.B.M., and an shared edges such that the shortest SrO. edge dis- N.S F. Materials ResearchGrant awarded to The Universitv of tances are associatedwith P(l)O4 and AlO, respec- Chicago, is appreciated. tively. The OH(l)-O(4) shared edge between AlOu References octahedra is 2.49 A and can be compared with the O(9)-O(9)i shared edge distances(: 2.40 A) in the FINNrv, J. J. (1963) The of carminite. Am. Mineral. 4E, l-13. structurally related bjarebyite (Moore and Araki, FnoNnrr, C., eNo J. Iro (1965) Composition of palermoite. Am re74). Mineral. 50,77'7-779. Finney (1963)noted a short OH(2)-OH(2) : 2.44 Moonp, P. B. ( 1975) Laueite, pseudolaueite,stewartite, and + 0.13 distancealthough that distancedoes not cor- metavauxite:a study in combinatorial polymorphism. Neues respond to any cation- polyhedral edge. In Jahrb. Mineral ,40ft. in press. -, ANDT. Anert (1974) palermoite,the OH(2)-OH(2)"I : 2.80 A distanceis Bjarebyite,Ba(Mn, Fe)'+,Alr(OH). its atomic arrangement. Am. Mineral. 59, 567-5'/2. considerablylonger. [POoh: Mnosr, M. E. (1953) Palermoite and goyazite, two strontium The proposed hydrogen bonds involve OH(l) . . . mineralsfrom the Palermo mine, North Groton, N H. (abstr.). O(3) : 2.75 L and OH(2)-O(5) : 2.84 A distances. Am. Mineral 38. 354 SinceO(5) residesin a generalposition, the hydrogen RnurcHeNoneN, G. N., lNo R. SnlNrveseN (1970) Fourier bond to it is Methods in Crystallograpfty. Wiley-lnterscience, New York, p. on the averagehalf-occupied. Deviations 96-rr9 from averagebond lengthscan be roughly correlated SrnuNz, H. (1960) Isotypie Palermoit, SrAlr(PO.),(OH),,-Car- with degreeof undersaturationor oversaturationof minit PbFedAsO.)r(OHL. Neues Jahrb. Mineral. Monalsh. 3, cationsabout anions (Table 3). Thus, O(3), with 2 : 49-52. 2.25, are all longer than average.OH(l), with ) : ZecHentnssN,W H. (1968) Extinction and Borrmann-effectin mosaic crystals Acta Crystallogr A24, 425-427. 2.08, has shorter than averagebonds, however. At present, we cannot offer a satisfactoryexplanation Manuscript receiued, December 5, 1974; accepted for this discrepancy. for publication, January 30, 1975