Crystal chemistry of danalite from Daba Shabeli Complex (Somalia)

DANALITE is the Fe 2+ end-member of the minerals of the helvite group, which have the general formula M8(BeSiO4)6S2, with M = (Mn,Fe2+,Zn). These minerals are relatively uncommon, although limited amounts are known at many localities round the world (e.g. Ragu, 1994; Larsen, 1988; Kwak and Jackson, 1986, and references therein). Their typical host rocks are skams, but occurrences in mineralized veins pegmatites, and altered alkali granites have also been reported. Danalite is rarer than helvite (Mn endmember) and genthelvite (Zn end-member) and, unlike the other two end-members, has never been found nor synthesized as a pure mineral (Mel'nikov et al., 1968). Hassan and Grundy (1985) demonstrated that there is no crystal-chemical reason for pure danalite not being stable and that complete miscibility should exist between the Mn, Fe, and Zn end-members. The apparently restricted stability of danalite in natural environments is probably controlled by sulphur and oxygen fugacities, danalite being replaced by pyrite- or pyrrhotite-bearing assemblages at high fs2 and by magnetite-bearing assemblages at high fo2 (Burt, 1980; Kwa k and Jackson, 1986). It follows that the association of danalite with magnetite, quartz, fayalite and/or pyrite (or pyrrhotite) may help to constrain petrogenetic conditions. Several works have addressed the chemical and physical properties of danalite (e.g. Kwak and Jackson, 1986, and references therein), but few crystal-chemical data have so far been produced. Pauling (1930) first recognized helvite-group minerals to be isotypic with sodalite, the (Mn,Fe,Zn), Be, and S atoms of helvite corresponding to the Na, A1, and C1 atoms of sodalite, respectively. Holloway et al. (1972) were the first to refne the structure of a helvite crystal. More recently, Hassan and Grundy (1985) reported structural data for six members of the helvite group, including two danalite samples from Western Australia and New Hampshire. In the present work we report the first finding of danalite from the Daba Shabeli Complex, Somalia, and provide new crystal-chemical data on a Fe65Mn23Zn12 danalite sample from this locality.