The crystal chemistry of welshite, a non-centrosymmetric (P1) aenigmatite-sapphirinesurinamite group mineral

Abstract

Previous attempts to refine the crystal structure of welshite, known only from Långban, Sweden, have been foiled by its extensive polysynthetic twinning, and without a structure determination, derivation of a reasonable formula has been difficult due its eclectic chemical composition. We report a successful refinement [wR(F²) = 0.0566 for 8048 unique reflections] in the non-centrosymmetric space group P1 of a relatively little twinned crystal (4% chiral) from sample NRM040068: a = 10.394(3) Å, b = 10.777(3) Å, c = 8.896(2) Å, α = 105.953(4)°, β = 96.294(4)°, γ = 124.948(3)°, V = 738.8(3) ų, Z = 1. The refined formula, Ca_3.78Mg_7.87Sb_3.00Mn_1.35Si_5.73Be_3.33Al_1.71Fe³⁺_0.96As_0.27O₄₀, is in reasonable agreement with a formula determined by electron microprobe and Mössbauer spectroscopy (Be from the refinement), Ca_3.81Mg_7.84Sb_3.03Mn_1.17Zn_0.04Fe²⁺ _0.05Si_5.57Be_3.24Al_1.54Fe³⁺_1.25As_0.37O₄₀. The P1 symmetry of welshite 040068 is mainly a result of its cation distribution, which is driven by charge ordering on both the octahedral and tetrahedral sites as supported by calculation of electrostatic site potentials. Welshite can accommodate up to 3.46 Be per 40 O atoms by including several T sites with 100% Be occupancies without the formation of unfavorable tetrahedral Be-O-Be linkages that would result in the related centrosymmetric structures of sapphirine, khmaralite, makarochkinite, and høgtuvaite, none of which contain >2.1 Be per 40 O. A generalized formula for welshite is (Ca,Mn)₄(Mg,Mn,Fe²⁺,Fe³⁺)₉(Sb,Fe³⁺)₃O₄[(Si,As)₆(Be,Al)₄(Al,Fe³⁺)₂O₃₆]. Compositions of most of the samples, including NRM040068, can be expressed in terms of two idealized Al and Fe end-members Ca₄Mg₉Sb₃O₄[Si₆Be₃Al₃O₃₆] and Ca₄Mg₉Sb₃O₄[Si₆Be₃AlFe₂O₃₆], respectively; the latter is the most representative for welshite and could be used in databases. One sample is far more heterogeneous and is irregularly zoned on scales from <1 to 10 μm. Compositional variation in this sample and in a crystal from another lie between the end-members Ca₄Mg_8.55Fe_0.45Sb_3.0O₄[Si_5.15As_0.45Be_3.45Al_0.5Fe_2.45O₃₆] and Ca₄Mg_7.05Fe_3.45Sb_1.5O₄[Si_6.65As_0.45Be_3.45Al_0.5Fe_0.95O₃₆], which are related by the coupled substitution MSb⁵⁺ + ^VIMg²⁺ + ^IVFe³⁺ = ^MFe³⁺ + ^VIFe³⁺ + ^IVSi⁴⁺, where M refers specifically to the M7, M3A, and M4A octahedral sites. The lowest measured Sb is 1.454/40 O, which may be close to the limit imposed by the need to maintain local charge balance.