Alunite and Crandallite: a Structure Derived from That of Pyrochlore

American Mineralogisl, Volume 65, pages 953-956, 1980

Alunite and crandallite:a structure derivedfrom that of pyrochlore

MIcHEL Gonrauo AND BERNARD RAvEAU Laboratoire de Cristallographie et Chimie du Solide, L.A. 251 Institut des Sciences de la Matiire et du Rayonnement Universiti de Caen, Esplanade de la Paix 14032 Caen CEDEX, France

Abstract

The structures of the minerals alunite KAl3(S04)r(OH)6 and crandallite CaAl3(OH)u[PO3(Or/r(OH)r,r)1,have been investigatedfrom a geometric standpoint. The similarity of this structural type with the pyrochloresA'*"M2O6 or A2M2O7is shown. The framework KAI3(OH)6O6is strongly related to the host lattice M2O6,i.e. MoOr;-it is built up from distorted hexagonaltungsten bronze layersAl3(OH)uO, similar to the M3Oelayers ob- servedfor pyrochlores,and from KO, layerswhich correspondto the MO3 layersof pyrochlores.The stackingsequence of the Alr(OH)uO, and M3Oelayers is howeverdifferent in both structures;it yields, due to the tilting of the octahedra,a coordination number 12 for K* in the KOr layers,while M of the MO, layers is characterizedby an octahedralcoordination.

The crystal structure of the mineral alunite &MoO,o for pyrochlores, where A is a large cation KAl3(SO4)r(OH)uhas formed the subject of several (R > lA) and M is a metal in octahedral coordina- publications (Hendricks, 1937; Pabst, 1947; Rong tion. Wang et al., 1965). Crandallite CaAI.(OH).[PO, The atomic parameters of pyrochlores AM2O., re- (O,/r(OH),/r)1,(Blount, 1974)has a structureanalo- ferred to the corresponding hexagonal cell, are listed gous to alunite. Recently Moore and Araki (1977) in Table 1. They demonstrate that there is a sinilar- have shown that the structure of mitridatite ity between the frameworks KAlr(OH)uOu and Ca.(H,O)u[Fef'Ou(PO.)n] .3H,O is topologicallydis- MoO,r, where Al and M atoms both have the same tinct from, but related to, alunite. Although this octahedral coordination. Previous studies on non- structure is now definitely established,no relation stoichiometric pyrochlores Ar*,M2Ou (Babel et al., with other structural types has been observed.The 1967; Michel et al., 1973) have shown that the sev- present paper describesthe relations between this enth oxygen atom of the ArMrO, compounds is not structure and that of pyrochlore, A2M2O7,which indispensable to the stability of the structure. This forms an important group of mineralswhose classifi- can be considered as built up from a host lattice cation and nomenclaturehas recently been revised MrOu, i.e. MoO,, (M : W, Ta, or Nb) based on octa- by Hogarth (1977). hedra, where the A cations (K*, Rb*, Tl*) are in- The comparison of the hexagonalcell of alunite serted. This host framework can also be described as (a, = 74. and cH = l7A) with the cubic cell of py- built up from two sorts of layers, MrOn and MOr, rochlore (a. = l0A) admits the relations: which are normal to the ( I I I ) direction of the cubic cell. The MrO, layers, made from corner-sharing oc- ar= a"/J2 = 7A and cu= a"Jj = l7A tahedra, are in fact distorted hexagonal tungsten The symmetry of alunite is in fact rhombohedral bronze sheets previously described by Magneli Riz with d closeto 60". Note that the cubic pyro- (1953). The examination of the (001) planes of the chlores (Fd3m) can be equivalently describedin a hexagonal cell of alunite shows that the latter type of rhombohedralcell with the samedimensions as those layer is also observed in alunite and crandallite (Fig. of alunite and characterizedby the samespace group la) and has the composition Al3(OH)uOr. The MOu Riz. From theseconsiderations, it appearsthat the and Al(OH)oO, octahedra are almost regular in both formula KAl3(OH)6(SOn)r,related to the rhombo- the pyrochlore and alunite structures; they are tilted hedral cell of alunite, should be compared with about c" or (lll). with approximately the same in- 0003-004x/80/09I M953$02.00 953 GOREAUD AND RAVEAU: ALUNITE AND CRANDALLITE

Table I Atomic parameters for alunite (Rong Wang et al., 1965) and pyrochlor€ AB2O5 (Michel, 1974) referred to the hexagonal cell of the R3nr spac€ group

Alunite (a = 6.97 ^ - rf t? l\ Pyrochlore (6 = 7 .28 A, c = 17.84 A)

Pos i tion Atom Atom

3 (a) K 0. 0. o. M 0. 0. 0,

e (d) A1 0.5 0. 0.5 M 0.5 0. o.5

6 (c) s o. 0. 0.3030 A o. o, 0.375

6 (c) o(1) o. o. 0 .3844

18 (h) 0(2 ) 0.2180 -0.2180 -o.0588 o.2067 -o.2067 -O.2717

18 (h) oH o.t247 -O.r24l o.1416 o.t26t -o,t267 0.1883

clination with respect to that direction (l8o for alu- the HTB layers at the same level z are identical in nite, l7o for crandallite, and 15" for pyrochlore), dif- both structuresbut the tilting$ of the octahedraabout fering from the hexagonal tungsten bronzes layers the Z, axis are in the oppositedirection, so that both (Fig. lb), whose octahedra are not (or only slightly) host latticescannot be superimposed.Another way of tilted about the c axis. The hexagonal tungsten comparisonby meansof HTB layer types(a, B, or y); bronze layers (HTB) are lying at levels z : l/6,3/6, with the same tilting direction, shows a di-fferent and 5/6 of the hexagonal cell and, in a particular stacking order along i.: a B y sequencein the py- structure, correspond one with another by the trans- rochlore and y B d sequencein alunite type. lation vectors of the R3z space group. Therefore, in Thus, the stackingof the Alr(OH)uO, or MrO" lay- each compound, there are three different states a, B, ers along Z" or ( I I I ). is comparable:two consecutive and y for the HTB layers, according to their configu- layers are derived one from the other by a gliding of ration with respect to the Oi and Of reference axes ar/ Ji; the gliding directionsare howeverdifferent in of the hexagonal cell. The three states are shown in both structures(Fig. 2): they are orientedat l80o one Figure la. from the other. Starting from a HTB layer, chosenas Thus, the structural analysis and comparison of a reference,a pyrochlore type structure is obtained the structures of alunite type and pyrochlore type can from the stackingHTB; HTB (+ t); HTB 1- i;, while be achieved in two different ways. The projections of the alunite type structure agr€eswith HTB; HTB

(b)

Fig. l. Comparison of the M3O" and AI3(OH)6O3 octahedral layers of (a) alunite and pyrochlore with that of (b) hexagonal tungsten bronze (HTB). The references a, B, and 7 show the three possible positions of the distorted HTB layers in the alunite and pyrochlore structures. GOREAUD AND RAVEAU: ALUNITE AND CRANDALLITE

tot (b)

Fig. 2. Stacking of two adjacent distorted HTB layers (a) in pyrochlore (b) in alunite and crandallite.

(-i); HTB (+l) whereT: 0/l)i" + (2ft)6,. As a AI3(OH)"O. layers along c" for the alunite framework consequence,the main differenceappears on the KO. Gig. ab). and MO, layersconaecting two consecutiveMrO, or These frameworks delimit cavities which are dif- A13(OH)6O3layers. The tilting about Z" in the blocks ferent in each structure. In the case of pyrochlore, a "M3O15"of three corner-sharingoctahedra defines large triangle B is sited directly above and below the two so(s of triangleswhose apices are oxygen atoms hexagon formed by six octahedra (Fig. 2a), while for located at the surface of a layer-small triangles alunite such a hexagon is surrounded on both sides noted A and large trianglesnoted B (Fig. la); this is by small triangles A (Fig. 2b). Hence the cavities of in contrast to the ideal hexagonalbronze structure, the pyrochlore structure, bounded by 18 oxygen where all the triangles are equivalent (Fig. lb). The atoms, are limited by the hexagonal crowns of six oc- different stacking modes generatetwo sorts of sites tahedra and the "M3O,5" blocks of two successive which are availablefor M and K cations.In pyroch- MrO, layers (Fig. 5a) and share their distorted hexag- lore, two A trianglesbelonging to neighboring MrOn onal faces, forming tunnels parallel to the ( I l0). di- sheetsare one above the other, forming an octahe- rection. In the non-stoichiometric pyrochlores dron (Fig. 3a) whose ternary axis is parallel to the Ar*"B2Ou, the large A cations (K, Rb, Cs, Tl, etc. . . .) (lll)" direction, where the M ion is located.An are located inside these cavities, whereas they are lo- analogousdisposition is obtained for the B triangles cated on the common faces in A2M2O' pyrochlores, in alunite, forming trigonal antiprisms (Fig. 3b) where the K* ions (or Ca2* in crandallite) are 1o- cated.In fact, the oppositedirections of the tilting of octahedrain Figures 3a and 3b lead to a six-coordi- Mrors nated M cation in pyrochlore,while K* (or Ca'*) is twelve-coordinatedin alunite, since the oxygen MOr atoms which are common to the octahedra of a "M3O,r" group are not at the same z level in both structures:they are neighborsfor K*, but not for M. Msots The polyhedra KO,, can be consideredas built from flattened octahedra KO. (antiprisms) having the samedisposition in KO, layersas the MOu octahedra (o) (b) in MO3 layers. Both structural families can thus be regarded as forming strongly related host lattices Fig. 3. Coordination of M and K* respectively in M03 and KO3 (a) pyrochlore; (b) which are built up from the stacking MO, layers: the MO6 octahedron of the KOu of and antiprism of alunite; the K* ion has however six additional close MrOn layersalong (lll). for the I\[oO,,framework neighbors at equal distances, belonging to the "M3O15" blocks (Fig. 4a) and from the stacking of KO, and above and under the antiprism. 9s6 GOREAUD AND RAVEAU: ALUNITE AND CRANDALLITE

cH Kog

At3(oH)603

{o) tbt

Fig. 4. Stacking of (a) the MO3 and M3Oe layers in pyrochlore as seen along I l0]g, (b) the KO3 and AI3(OH)5O3 layers in the alunite as seen along [10]g.

of intergrowthsof alunite and pyrochlore structures, corresponding to different stackings of the MrO" and Al3(OH)"Oj layers,should be considered.

References Babel, D,, G. Pausewangand W. Wiebahn (1967) Die Struktur einiger Fluoride, Oxide und Oxidfluoride AMe2)Q: der RbNiCrF5-Typ. Z. N aturforsch.,B 22, l2I9-122O. (1974) of crandallite.Am. Min- (o) (b) Blount, A. M. The crystalstructure eral.,59, 4147. Fig. 5. The cavities bounded by 18 oxygen atoms and (or) Hendricks,S. B. (1937)The crystalstructure of alunite and the jar- hydroxyl groups (a) in pyrochlore (b) in alunite. osites.lzr Mineral., 22,'173-784. Hogarth, D. D. (1977)Classification and nomenclatureof the pyrochlore group.Am- Mineral., 62,403410. the seventhoxygen atom being situatedat the center Magneli, A. (1953)Studies on the hexagonaltungsten bronzes of ofthe cavities. potassium,rubidium and cesium.Acta Chem.Scand., 7,315- The cavities of alunite are less open so that the 324. tunnelsno longer exist.Among the 18 oxygenatoms, Michel, C. (1974')Etude cristallochirnique, dvolution thermique et propridt€s d'6changes d'ions de quelques oxydes ternaires de we can consider that t have nearly the same dis- type pyrochlore . Thdse,Caen (France). position as in pyrocholore (Fig. 5b). The other nine - and B. Raveau (1973) Etude de la distribution des ions atoms belong to the A and B triangles which are per- thallium dans les pyrochlores non-stoechiom6triques muted from one structure type to the other. Each Tlr*.(Tat*"Wr --)06. Mater. Res.Bull., 8,451458. cavity contains one "SO" or "PO" group. The oxy- Moore, P. B. and T. Araki (1977)Mitridatite, Car(HzO)o[FelrIOe (POo)sl . 3H2O. A noteworthy octahedral sheet structure. lnorg. gen of group is atom this directed towards the dis- Chem.,16, 1096-1106. torted hexagonal face so that it forms, with the three Pabst,A. (1947)Some computations on svanbergite,woodhouscitc oxygen atoms of the A triangle, a tetrahedronwhere and alunite. Am. Mineral., 32, 16-30. the sulfur or phosphorus atom is located. Rong Wang, W. F. Bradley and H. Steinfink (1965)The crystal These structural relations show the importance of structureof alunite. Acta Crystallogr.,18,249-252. the hexagonal tungsten bronze framework, which can be consideredas an elementary part of the struc- Manuscripl received, December 20, 1979; ture of a greatnumber of compounds.The possibility acceptedfor publication, Apfl 3, 1980.