The Crystal Structure of Metastrengite and Its Relationship to Strengite Ani) Phosphophyllite

THE AMERICAN MINERALOGIST, VOL 51, JANLTARY FEBRUARY, 1966

THE CRYSTAL STRUCTURE OF METASTRENGITE AND ITS RELATIONSHIP TO STRENGITE ANI) PHOSPHOPHYLLITE

Pa.ur B. Moonr, DepartmenloJ Geophysicol Sc,iences, Un,irersity oJ Chicago,Chicago, Illinois.

ABSTRACT 'I'he crystal structure of metastrengit., pg:+(PO)(HzO)2, was solved by interpretation of heavy atom vectors on three Patterson projections, by electron density projections, and by three-dimensionalleast-squares refi nements. 'fhe structure consists of iinkages of octahedra and PO43- tetrahedra. The octahedra are insular and are held together to form a three-dimensional structure by the tetrahedra. As in strengite, the water moleculesappear at the ends of an octahedral edge,hence the octahedral structural formula can be n-ritten cls-Fe(Oo)a(O6)2,where Oo is oxygen belonging also to the POn:- tetrahedral group and Or, is an oxygen also belonging to a water molecule. Dach of the On oxygens is associatedwith one phosphorus center and one iron center. 'I'he major difierences in the strengite and metastrengite structures are attibuted to the "tilt" of the tetrahedra. It is further noted that metastrengite-like subcells in the phosphophyllite structure confirm Strunz's suggestion of structural similarities bett'een metastrengite and phosphophyllite.

INtnooucrroN Metastrengite, Fe3+(PO+)(HrO)r,is a common product of hydrother- mallv reworked triphylite occurringin Li-Fe phosphatebearing pegmatites, a rare mineral in limonite beds,and a constituentof certain soils.It has one known isotype, metavariscite,Al(PO4)(HzO)2, and one known dimorph, strengite.Strengite, in turn, has three known isotypes-variscite, Al(POn)(HrO)r; scorodite, Fe3+(AsOr)(HzO)z;and mansfieldite, Al(AsOq)(HzO)2. Metastrengite and strengite commoni)' occur together in pegmatites and are often difficult to distinguish,as both usually occur as rose-pink aggregatesor reniform crusts.Metastrengite is monoclinicand strengite is orthorhombic; pertinent data of their structure cells are given in Table 1. Some reasonsfor undertaking solution of the metastrengitecrystal structure are

(1) to compare it with the structure of strengite, which is knorvn, (2) to test the suggestion of Strunz (1942) that there may exist a structural relationship with phosphophyllite, ZnzFe(POa)r(HzO)a,a recently determined structure, and (3) to add another species to the structural classification scheme of Fe-Mn orthophosphate hydrates as proposed in a prevlous paper.

168 STRUCTURE OF MDTASTRLNGITIi 169

Teer-n 1. Cor,r,Der,c, or MerestnnNctrr, SrnnNGtrEAND PrrospnopnyllrrE

Phosphophyllite Metastrengite Strengite Kleber Cell Strunz Cell a s.30A 10.05 10 23 t0.32 b 977 9.80 508 5.08 c 8.73 8.65 t0 49 1797 p 90036' 120"15' 88"21', Z 4 8 2 SG ph/n Pcab P21/c B21ld Ref McConnell McConnell Wolfe (1940) Strunz (1942); (1e3e) (1e40) this study

'I'Buppnerunr Te,ntn 2. Coonnrxerts eno Isornolrc Fec:rons lon Mr:reslnnNcrtn; Fe-O eNo P-O lxrrn,lrourc DrsraNcns

Fe .0916+.0005 .8270+.0007 . 1919+ .0003 0.59 + .03 P - 0867+ 0008 .1506 + .0013 .1836+.0005 0.70+.05 *or .3874+.0022 .9494+.0028 .1799+.0014 2.83+.23 *o: 0871+.0022 .8730+.0028 .4275+ 0014 1.60+.18 Or 3324+.0022 6814+.0027 .2383+.0014 1.29+ 16 Or -.1152+.0022 - 0066+.0035 .1708+.0014 1 17+.16 o; - 2010+.0022 .7050+0028 .2178+.0014 0.86+.15 oc .0939+.0022 .7873+.0028 .0250+.0014 1.63+.18 * Water nrolecules. Octahedral distancesl Fe-Or 2 00 A Fe-O: 2.1.O f e-Or 1.95 Fe-O+ 2.14 Fe-Or 2.08 F e-Oo 1.91

Average 2ffi4

Tetrahedral distancesr P-O: 1.58 A PO+ 1.58 P-O; 1.52 P-Oo t.52

. -. ? Average I ..').) A

t All distances+.03 A. t70 PAUL B. MOORE

Drrenutx.q.uoN oF rnn SrnuctunB l'he data of McConnell (1939)were confirmedby examiningWeissen- berg and precessionphotographs. Nletastrengite has four molecules of Fe3+(PO+)(HzO):in a unit cell of spacegroup P2r/n. Since it is most likely a structureof isolatedoctahedra (since ) Fe:) 0:1:6) linked to tetradentate PO+3-tetrahedra, there must be one iron, one phosphorus and six oxygen independent coordinates. The Patterson projections P (uv), P (urv), and P (vw) confirmed the suspicionthat all atoms are in generalpositions. The o- and c-axisprecession photographs were used for deriving P (uv) and P (vw). Weissenbergphotographs of D-axis0-,l-, 2-,3-, and 4-levels in three setseach (48, 24 and 8 hour exposures)were preparedusing Zr- liltered Mo radiation and a singlecrystal of dimensions0.24 X 0.43X 0.43 mm from Pleystein, Bavalia. From these, 678 independentintensities were collectedand estimatedvisually using a spot scaleof 25 units pre- pared from the cr1-stal.Absorption and (Lp)-l correctionswere made using the GNABS program of Burnham (1963). Scattering tables were usedfor ps2+-,p2*, and O 1,derived from self-consistentfield calculations and listedin International'Iables,Vol. 3. All computationswere done on the IBtr{ 7094facilities at the Institute of Computer Research,Univer- sitl'of Chicago. The equivaient set of generalpositions for P21fn is x,,t-,2;*l*, t-\', triz and their inversions.llhus, on P (uv), there will exist a prominent vector somewherealong the line v:], related to the Fe atom x-coordi- -t nate by | 2x: u. Likewise,on the line u: ], we have a peak occurringat |*2y:v, whereu and v are coordinatesin Pattersonspace. The same -l holdsf or P (vw) i at v: |, a peak at | 2z definesthe possibleFe z-coordinates.The possiblesolutions thus derivedfrom the Pattersonprojections arex: +.090,y: *.161,z: * .183. Concentratingnow on the xz-projection,the two possiblecombinations of symmetrv independent Fe coordinates are (.090, .183) or (.090, -.183). A rough trial set of structure factor calculationsfavored the former. The P (uw) map (Fig. 1) reveals the vector Fe*,-Fe;-*,;-, (labelled"B") and the phosphoruscoordinates rvere derived from vectors ttA" - - (Fe*, P 6 x,l") and "C" (F.*, P*,). Rough structure factor calculations confirmed the phosphorus coordinates (-.085, .189) and the signs thus derived from the Fe and P coordinateswere applied to the observedstructure factors. Only four oxvgen atoms appeared on the electrondensity map, but the other two could be locatedb1'assuming an octahedraloxygen configurationabout the iron atom. Three more elec- STRUCTURITOF M|I:,TASTRENGITE t7r tron densitr-maps resultedin R6or:9.20. The label "D " on P (uw) corre- spondsto the contributionsof the Fe*,- (O*,t, O*,u)vectors. The possibleFe coordinatesare (.090, .161, .183) or ('090, -.161' .183).The latter was chosenafter sometrial structurefactor calculations.

c------. 4

2 I

Frc. 1. P (uw) of metastrengite.

Assuminginteratomic distances of Fe- O-2.00 A and P - O-1.54 A, the y-coordinatesof the remaining atoms were calculated,two structural configurations being possible. A three-dimensionalthree-cycle least squaresrefinement (Busing et at. 1963)on the favored configuration with fixed temperature factors (B set to 1.0) and using all collected reflections resulted in R6ur:9.22. Inspection of the individual structure factors 172 7AUL B. MOORT,

- showedalmost consistentll,-I F,o" I I F".1"I iessthan 0 f or the stronglow- angie reflections.This could be due either to extinction errors or non- iinearitiesin the intensitl' s.u1"or both. Omissionfrom refinement oI 2l of the strongestintensities resulted in Rnu,:9.20 after one more c1,cle. Further reirnement,now with fixed coordinatesand varying temperature factors resulted in R6ur:9.17. After three more cvcles,va^-ing coordinatesand isotropictemperature factors, Rr,tr:0.156. Final inspectionof all the employed structure factors showedno grossdisparities between those observedand those calculated.The Fos"and F"u1"structure factor tables can be obtained upon request.

Drscussrowor, THE Srnucrunr The metastrengitestlucture consistsof isolatedmetal-centered ox.n-gen octahedralinked together via POna-tetradentate tetrahedra to form a three-dimensionaledifice. Two water ligands are associatedwith each octahedron,the octahedral configuration being cas-Fe(O,,)+(Ou)2,where Ouis oxygenalso associatedwith the PO+3-groups and Or,is oxygen also associatedwith a water molecule.Each pO+3 oxygen is associatedwith only one P atom and one Fe atom. Figure 2 is a 6-axisprojected poh.- hedral diagram showing one octahedron (the as."-mmetricunit of structure), its f our associatedPO+3- tetrahedra, and the remainingpoaa- tetra- hedra in the unit cell.

Relation to the strengite Dimorph. The metastrengitestructure is very similar to the strengite structure. Both are cas-Fe(oo)+(or,)zstructures (Fig. 3). The coordinatesof the strengitestructure are from Hirivana and sakurai (1949).The notable differencesare the more "open" disposition of the tetrahedra about the octahedronin metastrengite(expiaining the lower densitl' of this mineral) and the order of the tetrahedralgroupings about the octahedron.rn each of the cls-structures,there are two octahedral faces which have three PO+a-ox),gens at the vertices. Further_ more, in both structures,for the projectionschosen, the tetrahedrapoint either up (soiid iines) or down (dashedlines). For the two facesof con- cern on the metastrengiteoctahedron, the associatedtetrahedra point d,own-doun-up.For strengite, they point d.own-down-wpand doun-d,own_ dowm. rt is now evident that strengiteand metastrengiteare stereoisomersof eachother, even though the order of the ligand speciesis the sameabout the octahedralasymmetric unit. The difference,however. is in the "tilt', or "twist" of the PO+3-ligands. rt is interestingto speculateas to whether a trans- arrav of tetrahedr.a could existabout an octahedronwith the possibilitv of a third hypotheti- S T R(I CT I] R I1:,O F M Ii-,TA S T ]T IljNG I 7'I' 173

Frc. 2. Metastrengite crystal structure projected on ac-plane. one octahedron (an asymmetric unit of structure), the Poas- ,",.un"oral environment, and the remaining tetraheclra in the cell are shown"r'he HzO oxygens are specified; the circles represent Fe atoms.

-c- o -b I I

Irrc.3. Environment of tetrahedra and H:O groups about octahedron in metastrengite{left) andstrengite (right). 174 PAUL B. I,IOORL,

,,girdle', cal polymorph. The tetrahedra would form a about the octahedron and crude models show that such tetrahedra and octahedracan fit niceiy. However, with two molecuiesof water on oppos,ingoctahedral vertices,there results a more open structure. This is becausethe tetrahedra stericaily hinder any reasonablepacking of the ends occupiedby water molecules.To better facilitate packing of such octahedra,the open ends(the positionsof the water molecules)would have to becomefused to form chainsof octahedralstructure as in laueite (Moore, 1965).rn that

o @ d @ @\.o o ,/ e'| D>\,D

rOr @ V ,,. \_/

Frc. 4. The phosphophyllite structure. Four Kleber cells each of dimensions axc are shor'vn.The long- and short-dash cell is the B-centered cell of strunz r,vith dimensions a'xc'. Tl'e t\a'o metastrengiteJike cells A and B are denoted by short dashes. Large circlesare iron atoms and small circlesare zinc atoms. Hatch marks denote operation of the inversion center. arrangement,the tetrahedralgroups wourdzigzag up this chain,connect- ing neighboringoctahedral links and cross-linkingto other similar chains. Laueite often occursin closeassociation with strengiteand metastrengite indicating that configurationsof chains of octahedraas rvell as isolated octahedralgroups can arisein very similar environments. The name redondite for ferrian "variscite" was resurrectedby eech et al. (1962).This mineral was originallv named bv shepard (1g69) and work bv eech et ol. suggests redondite to be anothei polirmorph of AIPOI(HzO)2.Further studies on this material would be most infornra- tive.

Relation to Phosphophyllite,Zn2Fe2+(pOr)r(HrO)r. We turn now to the possibility of a structural relationshipbetween metastrengite and phos- STRLICT(IREOF METASTRENGITN, 175 phoph.vtlite. Figure 4 shows a 6-axis projection of the phosphophyllite structure,2lZnrFe(PO) r(HrO)r], P21f c, a:10.23, D:5.08, c:10.49 A, A:120"15'as determined by Wolfe (19a0) and used in the structure anall'sis of Kleber et ol. (.t961). Four cells of the Kleber structure are shown, and the B-face centeredcell of twice the area (Strunz, 1941)is sketchedin. This cell has the dimensionso': 10.32and c' : L7.97A, each nearly twice the dimensionsof the metastrengite a- and c-axes.The Strunz cell is further partitioned into sub-cells A and B, each approxi- matell' of metastrengitedimensions. Cell A contains two POa3 tetrahedra whosecenters closely resemble x, z andx, Z coordinatesassociated with the phosphorus atoms and the On ox.vgenatoms in metastrengite. - Cell B contains the remaining atoms with ]-x,f z and |!x,l-lz coordinates, and, furthermore, contains zinc atoms strung along the cell diagonal.In metastrengite,these zinc coordinatesresemble those of iron, which are somewhatdisplaced from the diagonal.The zinc in phospho- ph1-lliteis in a distorted Oo tetrahedron. Further analogy breaks down when water and iron in phosphophylliteare considered,as these atoms are associatedwith octahedracentered at 00, !!, and || positionsin the Strunz cell and are not presentin metastrengite.It is interestingto note that the 1.-coordinatesof P, the Oo oxlrgens,and Zn, as determinedby Kleber et al. (1961)bear similarity to thosefor P, On oxygensand Fe in metastrengite.The similarity in coordinatesas well as the existenceof pseudoceilsin phosphophyllitemade up of piecesof metastrengite-like structure explain the similarity in powder data of the two minerals' as noledby Slrunz. CoNcr,usroNs Metastrengite is a cis-structure as is strengite, though more open. Though the ligands in the two speciesare in the same order, they tilt differently, thus afiectingpacking of the tetrahedra and octahedra.The similarity of the metastrengite structure to that of phosphoph-u"lliteis due to similaritiesin the packing of the PO+3-tetrahedra as well as the posi- tion of theZn atoms (in phosphophyllite)and Fe atoms (in metastrengite). The arrangementis suchthat subcellsof phosphophyllitebear strik- ing similarit,vto piecesof the metastrengitestructure.

AcrNowr-noGEMENTS This work was undertaken while I was a NSF Predoctoral Feliow at the Department of GeophysicalSciences, University of Chicago.I wish to also acknowledgeDr. J. V. Smith who supplied funds for computation through the NSF GP-443grant. 176 PAUL B. MOORE

Rrnpnrmces

BunNu.Llr, c. w. (1963) An rBM computer program for computing transmission factors for crystals of arbitrary shape (Author's copy). BusrNc, W. I{ , K. O. MenuN amo H. A. Lnvu (1962) ORFLS, A fortran crystallographic least - squares program. Oak Rid.ge NoLional Laboratory Documenl. crcn, Ir , P. PovoNnne ano E. sr.eNs

Manuscript receiled, March 11, 1965; accepted.for .publi.cation,May 20, 1965.