Structural relations and pseudosymmetnes in the andorite homologous series

摘要

The structure of quatrandorite is reported for the first time from an untwinned sample from Oura mine, San Jose, Bolivia. The mineral crystallizes in P2_1/c, a = 19.1686 (19) A, b = 17.160 (3) A, c = 13.042 (2) A, β = 90.008 (12)°, V = 4289.9 (11) A~3, Z = 4. Refinement to R_(obs) = 5.66% was obtained with Jana2006. Quatrandorite belongs to the andorite series, whose members share two cell parameters while the third can be expressed as n × 4.3 A, with n = 2, 4 and 6 for ramdohrite (uchucchacuaite, fizelyite), quatrandorite and senandorite, respectively. Both quatrandorite and senandorite are strongly pseudosymmetric up to Cmcm with one parameter corresponding to n = 1 (～ 4.3 A). The hypothetical structure corresponding to Cmcm is also the aristotype common to both minerals. The strong structural similarity of quatrandorite and senandorite may explain their co-existence in some samples, which has in the past led to hypothesize the existence of a further member of the series, nakaseite, which was however later shown to consist of a random stacking of the two minerals. The Cmcm aristotype is not common to the n = 2 minerals (uchucchacuaite, ramdohrite, fizelyite), which are thus structurally less closely related to the two other members. A common aristotype to all three minerals can nevertheless be obtained via a different path, which leads to Cmme with the same cell parameters as Cmcm; the degree of pseudo-symmetry in this supergroup is however lower and there remain a difference in one sulfur position in this aristotype. It nevertheless confirms previous reports in the literature stating that the bulk of the structure of the minerals of this series can be reduced to a common principle, essentially a distorted galena; the departures from it are however crucial for the realization of the individual structures.