American Mineralogist, Volume 72, pages 178-187, 1987 Solid solution ino and classificationof' gossan-deriYed membersof the alunite-jarositefamily, northwest Queensland,Australia Knrrn M. Scorr CSIRO Division of Mineral Physicsand Mineralogy, P.O. Box 136, North Ryde, NSW 2113, Australia Ansrnlcr Minerals of the alunite-jarosite family with the generalformula AB3(XO4)r(OH). occur in gossansrelated to Pb-Zn mineralization in the Mount Isa region of northwest Queens- land. Examination of 226 electron-microprobeanalyses indicates that extensive solid so- Iution occurswithin the A, B and (XOo) sites,with the variations in the A and (XOo) sites often related. Substitution of divalent cations for monovalent cations in the A sites of alunite or jarosite can be balanced by (l) replacing two monovalent cations by one divalent cation and leaving A-site vacancies,as in plumbojarosite, (2) incorporating divalent ions in the B sites, as in osarizawaite and beaverite, or (3) replacing divalent anions with trivalent anions, as in beudantite. A secondtrivalent anion can still maintain chargebalance pro- vided it is protonated, as in plumbogummite. Complex interaction of the mechanisms often occurs. Complete solid solution betweenAl and Fe in B sites occurs at least in the more phos- phate-rich alunite-jarosites.Nevertheless, because ofthe probable stability differencesand easeof practical subdivision, separation into alunite and jarosite supergroupsaccording to greaterAl or Fe occupancyis consideredappropriate. A classification that prevents proliferation of named minerals of the alunite-jarosite family has been approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association. fNrnoructtou Minerals of the alunite-jarosite family have the general betweenA and (XOo) occupancy.Substitution of one tri- formula AB.(XOo)r(OH)u,where A is a large cation, such valent for one divalent anion as in the beudantite group as Na*, K*, Ag*, NH4+,H3O*, Ca2+,Pb2+, Ba2+, Sr2+, is accompaniedby a concurrent changein the A-site cat- Ce3+,and other rare earth elements in l2-fold coordi- ion from monovalent to divalent. Complete replacement nation. B sites are occupiedby the cations Al3*, Fe3*, by a trivalent anion is accompaniedby substitution of a Cu2*, and Zn2+ it octahedral coordination. The anion trivalent cation on the A site as in the caseofflorencite, (Xoo)" may be SO? , PO?-, AsOl-, Co3-, SbOi , CeAlr(POo)r(OH)u.Alternatively, if the A-site cation re- CrO?-, or SiO? . Al or Fe is the major occupant of the B mains divalent, protonation of one of the trivalent anions sites, but with the plethora of possible combinations in also results in a stable compound, as in the caseof gor- the A and (XOo) sites,the minerals are usually considered ceixite, BaAl,H(POo),(OH)u(Radoslovich, 1982). In this in three groups based on the nature of the anions (e.g., paper it is argued that these alternatives are sufficiently Palacheet al., 1951;Strunz, 1978;Ramdohr and Strunz, different to warrant separateclassification as "florencite" 1978):(l) alunite group,with two divalent (XOo)anions and "plumbogummite" groups. and usually monovalent A cations; (2) beudantite or Although solid solution has been frequently reported woodhouseitegroup, with one divalent and one trivalent in the A sites,similar substitution within the B and (XOo) (XOo) anion and usually divalent A cations; (3) plum- sites of minerals is poorly documented. This paper pre- bogummite, crandallite, or goyazitegroup, with two tri- sents compositional data for 67 minerals of the alunite- valent (XOo) anions and either divalent or trivalent A jarosite family found in 46 gossan samples derived by cations. oxidation of mid-Proterozoic Pb-Zn mineralization in As seenabove, different nameshave been proposedfor northwest Queensland, Australia. It shows that phos- the same groups. In this paper the nomenclature of Pa- phate-sulfatesolid solution from POo/SOo: 0 to at least lache et al. (1951) has been followed, with some slight I is related to the occupancyofthe A sites.Extensive Fe- modification. Al substitution, especiallyin volcanogenicdeposits to the The subdivisions above reflect the strong relationship east of Mount Isa, is also reported. 0003-o04x/87l0102-0 1 78$02.00 178 SCOTT: ALUNITE-JAROSITE FAMILY r79 SunsrrrurroN AND cLASSrFrcATroN-A REVIEw Table 1. lonic radiifor A- and B-sitecations and bond lengths for (XOo)anions (in A) In addition to monovalent ions, divalent Ca2+and Pb2+ B sites A sites XO4sites* may substitute in the A sites. Two different modes of substitution are known: some A sitesare left unoccupied Al3* 0.51 Na* 097 H"Ot 1.24 c-o 1.22 Fe3* 0.64 Ca2* noo Ag- 1.26 s-o 1.43 to maintain the total A-site charge at one, as in mina- Ti4- 0.68 La3* 1.02 K+ 1.33 -t-u 1.50 miite, (Nao.uKo,oCaorr)Al3(SO4)r(OH)6(Ossaka et al., Cu" 0.72 Ce3* 1.03 Ba'?* 1.34 P-O 1.56 As-O 1.78 I 982), or plumbojarosite,Pbo,Fer(SO4)r(OH)6 (Szyman- Zn2* 0.74 Sr'?* 1.12 NHf 143 Pbr* 1.20 ski, 1985),in which 270/oar'd 500/0,respectively, of the A (1976). sites are unoccupied. Alternatively, Note; Data from Weast (1968, p. F152-F1 57) and Botinelly in beaverite, - Average X-O bond lengths. Pb(Fe,Cu)r(SOo)r(OH)6,and its Al analogue, osariza- waite, Pb(Al,Cu)r(SOo)r(OH)u,the chargebalance is maintained by a combination of partial substitution of (CrOo)oo(SOo)o,o(OH),,, (Walenta eI al., 1982),where in- the divalent ions, Cu and Zn, for trivalent ions in the B tersubstitutions occur in A (and B) sites as well. Never- sites and by some deficiencyin the occupancyofthese B theless,except for minor variation about the named stoi- sites(Jambor and Dutrizac, 1983). chiometric endmembers,large-scale substitution in (XOo) Within the plumbogummite group, where A is gener- sites is not well documented. ally divalent, the chargebalance was originally assumed Substantialsolid solution betweenAl and Fe in B sites to be restoredby protonationofone ofthe OH radicals, is also poorly documented, although intermediate com- giving plumbogummitethe formula PbAl3eO4)r(OH)s. positions betweenalunite, KAl3(SOo)r(OH)u,and jarosite, HrO. However, structural work (Blount, 1974; Radoslov- KFe.(SOo)r(OH)u,do exist (Brophy er al., 1962). Similarly ich, 1982) indicates that the extra proton is actually at- a seriesbetween osarizawaite, Pb(Al,Cu)r(SOo)r(OH)u, and tached to one of the XOo anions so that the generalized beaverite,Pb(Fe,Cu)r(SO1)r(OH)6, is known (Paaret al., formula is better written AB.(XO4XXO.OH)(OH). or 1980),but in this casethere is the addedcomplication of AB3H(XO4)r(OH)6.With one divalent and one trivalent divalent ion incorporation in B sites. Alunite may form anion, such minerals could be consideredas members of, under higher pH conditions than jarosite (Hladky and or a subgroupwithin, the beudantitegroup. However, the Slansky, 1981). Stability differencesbetween otherwise protonation of a trivalent anion modifies the P-O bond similar Al- and Fe-rich members also occur generally lengths to make its space group Cm rather than R3z (Hladky,pers. comm., 1982).Furthermore, because ofthe (Radoslovich, 1982), thereby justifying retention of the size difference between Al3* and Fe3+ (Table 1), B-site classification"plumbogummite group." Members with a occupancy influences ao and is readily differentiated by trivalent A-site cation and two "true" trivalent anions X-ray diffractometry and optical methods. Therefore Bo- need to be distinguished from the "plumbogummite tinelly's (1976)suggestion ofinitial subdivisioninto sep- group" as defined above. Therefore they are designated arate Al- and Fe-rich serieshas much in its favor despite members of the "florencite group." the degreeof intersubstitution illustrated below. Solid solution in the A sitesis well documentedwithin In such a scheme,initial subdivision would be into the groups, e.9., among jarosite, natrojarosite,and hydro- "alunite supergroup" or'Jarosite supergroup" depending nium jarosite (K,Na,H.O)Fer(SOo)r(OH)u(Brophy and on the dominant B-site cation. Logically, the valency of Sheridan, 1965). However, substitution involving differ- the (XOo) anions would then be usedto further subdivide ent valencies in the A sites has also been documented the seriesinto groups as in Table 2. Three factors could between the endmembers florencite, CeAl,(POo)r(OH)u, be usedto argueagainst the proposedclassification, name- crandallite, CaAl.H(PO.)r(OH)u, gorceixite, Ba- ly, possibleconfusion in nomenclature(i.e., "alunite" and 'Jarosite" Al3H(PO4)r(OH)u,and Eayazire,SrAlrH(PO.)r(OH)u refer both to a supergroupand a specificgroup (McKie, 1962).Here substitutionof divalent Ca, Ba, or within that supergroup),the comparative rarity of mem- Sr for trivalent Ce could be balanced by either substitu- bers of the lusungitegroup, and the absenceof known Fe tion of SOo2for POi- or protonation of phosphate.Al- equivalentsofthe florencitegroup. However, thesefactors though McKie (1962) found some minor sulfate substi- are outweighedby the practical considerationsofthe ease tution, protonation of the phosphate appears to be the of the initial subdivision into Al- and Fe-rich seriesand major factor in balancing the charge. However, in the the differencesin formation conditions. system alunite [(Na,K)Alr(SO.),(OH)6]-woodhouseite Geor,ocrc sETTTNG [CaAl,(PO. )(SO*)(OH)u ]-goyazite [SrAl.H(pOo),(OH)u ] (Wise, 1975), variable A-site valencies are balanced by Minerals of the alunite-jarosite family occur in gossans SO.-PO4 intersubstitution. Similar large-scaleintersub- at Mount Isa (in the Bernborough,BSD, DDH 2625, and stitution ofdivalent and trivalent anions has recentlybeen Black Star areas), Copalot, Mount Novit, Hilton, Lake reported in the casesof schlossmacherite,(HrOo urCao rr- Moondarra, Kamarga, Dugald River, Jolimont, Pegmont, NaoouKo ooSro o,Bao o,)(A1, ,oCuo o.Feo or)(SOo), ou- and Fairmile (Fig.