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New data on welshite, e.g. Ca₂Mg_3.8Mn²⁺_0.6Fe²⁺_0.1Sb⁵⁺_1.5O₂[Si_2.8Be_1.7Fe³⁺_0.65Al_0.7As_0.17O₁₈], an aenigmatite-group mineral

¹ Department of Geological Sciences, University of Maine, 5790 Bryand Center, Orono, Maine 04469-5790, USA
² Department of Mineralogy, Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden
³ Division of Structural Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91, Stockholm, Sweden
⁴ Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico 87131, USA

^* E-mail: esgrew{at}maine.maine.edu

Electron and ion microprobe data on two samples of welshite from the type locality of Långban, Sweden, gave analytical totals of 99.38–99.57 wt.% and BeO contents of 4.82–5.11 wt.%, corresponding to 1.692–1.773 Be/20 O. Mössbauer and optical spectra of one of these samples gave ^[iv]Fe³⁺/Fe = 0.91, ^[vi]Fe²⁺/Fe = 0.09, and no evidence of Mn³⁺. The resulting formula for this sample is Ca₂Mg_3.8Mn²⁺_0.6Fe²⁺_0.1Sb⁵⁺_1.5O₂[Si_2.8Be_1.7Fe³⁺_0.65Al_0.7As_0.17O₁₈], and that for the second sample, Ca₂Mg_3.8Mn²⁺_0.1Fe²⁺_0.1Fe³⁺_0.8Sb⁵⁺_1.2O₂[Si_2.8Be_1.8Fe³⁺_0.65Al_0.5As_0.25O₁₈], is related by the substitution involving tetrahedral and octahedral sites: 0.59^[vi,iv](Fe,Al)³⁺ 0.42^[vi](Mg,Mn,Fe)²⁺ + 0.21(^[vi]Sb,^[iv]As)⁵⁺, i.e. 3^[vi,iv]M³⁺ = 2^[vi]M²⁺ + ^[vi,iv]M⁵⁺. Welshite is distinctive among aenigmatite-group minerals in the high proportion of Fe³⁺ in tetrahedral coordination and is unique in its Be content, substantially exceeding 1Be per formula unit. Given the cation distributions in other minerals related to aenigmatite, we think it is reasonable to assume that at least one tetrahedral site is >50% occupied by Be and that one octahedral site is >50% occupied by Sb, so that welshite should be retained as a distinct species with its own name in the aenigmatite group.

KEYWORDS: welshite, aenigmatite group, Långban deposit, Sweden

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