THE CRYSTAL CHEMISTRY OF WELSHITE, A NON-CENTROSYMMETRIC (P1) AENIGMATITE-SAPPHIRINE-SURINAMITE 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, a = 105.953(4)°, β = 96.294(4)°, γ = 124.948(3)°, V = 738.8(3) Å3, Z = 1. The refined formula, Ca3.78 Mg7.87Sb3.00Mn1.35Si5.73 Be3.33Al1.7Fe0.963+ As0.27O40, is in reasonable agreement with a formula determined by electron microprobe and Mössbauer spectroscopy (Be from the refinement), Ca3.81Mg7.84 Sb3.03Mn1.17Zn0.04 Fe0.052+Si5.57Be3.24 Al1.54Fe1.253+As0.37 O40. 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 electro-static 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 Ca4 Mg9Sb3O4[Si6Be3 Al3O36] and Ca4Mg9 Sb3O4[Si6Be3 AlFe2O36], 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 a crystal from another lie between the end-members ca4Mg8.55 Fe0.45Sb3.0O4[Si5.15 As0.45Be3.45Al0.5Fe2.45 O36] and Ca4Mg7.05Fe3.45 Sb1.5O4[Si6.65As0.45 Be3.45Al0.5Fe0.95O36], 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.