Alunite-jarosite crystallography, thermodynamics, and geochronology

Abstract

The alunite supergroup consists of more than 40 mineral species that have in common the general formula DG 3 ( T O 4 ) 2 (OH,H 2 O) 6 . The D sites are occupied by monovalent (e.g. K, Na, NH 4 , Ag, Tl, H 3 O), divalent (e.g. Ca, Sr, Ba, Pb), trivalent (e.g. Bi, REE) or more rarely quadrivalent (Th) ions; G is Al or Fe 3+ or rarely Ga or V; T is S 6+ , As 5+ , or P 5+ , and may include subordinate amounts of Cr 6+ or Si 4+ . Many of the minerals in this supergroup are exotic, having been described from relatively few localities worldwide, generally in association with ore deposits. Rarely are end-member compositions attained in these natural occurrences, and extensive solid solution is typical for one or more of the D , G , and T sites. In this chapter, the two solid-solution series considered in detail are alunite-natroalunite [KAl 3 (SO 4 ) 2 (OH) 6 – NaAl 3 (SO 4 ) 2 (OH) 6 ] and jarosite-natrojarosite [KFe 3 (SO 4 ) 2 (OH) 6 – NaFe 3 (SO 4 ) 2 (OH) 6 ]. These minerals are by far the most abundant naturally occurring species of the alunite supergroup. Minerals with the generalized formula cited above can be variously grouped, but the simplest initial subdivision is on the basis of Fe > Al versus Al > Fe. Further subdivision is generally made on the basis of the predominant cation within the two T O 4 sites. Thus, within the supergroup, the alunite group consists of minerals in which both of the T sites are occupied by sulfur. This leads to a total negative charge of four on the T O 4 sites. In the ideal formulas of some members of the supergroup [e.g. woodhouseite, CaAl 3 (PO 4 )(SO 4 )(OH) 6 ], half of the T sites are occupied by sulfur, and the other half by arsenic or phosphorus, which produces a total negative charge of five on the T O 4 sites. In still other end-members of the supergroup [e.g. crandallite, CaAl 3 (PO 4 ) 2 (OH) 5 (H 2 O), and arsenocrandallite, CaAl 3 (AsO 4 ) 2 (OH) 5 (H 2 O)], both of the T sites are occupied solely by phosphorus or arsenic, thus producing a total negative charge of six on the T O 4 sites (see Table 1 1 of Dutrizac and Jambor, this volume). In this chapter, however, the primary concern is with those minerals for which T O 4 is represented by SO 4 2− (Table 1 1 ). Precipitates with compositions near those of the end-members in the system alunite-natroalunite and jarosite-natrojarosite are readily prepared using sulfate salts. The products, however, almost invariably have a slight to appreciable deficiency in G 3+ , and have an apparent non-stoichiometry for D . The latter may reflect incorporation a H 3 O + , a solid solution that is difficult to prove because H 3 O + cannot be determined directly by wet-chemistry or microprobe methods. Nevertheless, the existence of two minerals in the alunite supergroup is dependent solely on their D -site predominance of H 3 O + , namely, hydronium jarosite [(H 3 O)Fe 3 (SO 4 ) 2 (OH) 6 ] and schlossmacherite [(H 3 O,Ca)Al 3 (SO 4 ) 2 (OH,H 2 O) 6 ]. This chapter is organized into four sections. In the first section, crystallographic data for alunite-natroalunite and jarosite-natrojarosite are presented and discussed. The second section describes available thermodynamic data for these two solid-solution series, in terms of properties of the end-members and mixing properties for intermediate compositions. The third section discusses the geochemistry and occurrences of alunite and jarosite, and the last section summarizes the published literature on the use of alunite and jarosite in geochronology.