Crystal chemistry of schoonerite-group minerals
Synchrotron single-crystal structure refinements for five schoonerite-group minerals (SGMs) from the Hagendorf Süd, Bavaria, and Palermo No.1, New Hampshire, pegmatites were combined with results from previous studies on type schoonerite and the SGMs wilhelmgümbelite and schmidite to evaluate the main crystallochemical relations between the minerals. Elements Zn and Fe3+ are essential to the structure and are ordered in specific sites, while Mn and Fe are distributed relatively uniformly over three octahedral sites, M 1, M 2 and M 3. The Mn/(Mn + Fe) atomic ratio is relatively constant for the samples studied, ~0.26–0.31, and is close to this ratio in the primary phosphate, triphylite, from which the SGMs are derived. The main crystallochemical variations are due to different degrees of oxidation of the Fe, which ranges from 45% of the total Fe as Fe3+ in green SGMs, to 98% of the Fe as Fe3+ in red SGMs. The Fe oxidation state is linked to a significant structural change, whereby Zn in a trigonal bipyramidal site, Zn, is partially partitioned into an adjacent tetrahedral site, Zn, when the amount of Fe as Fe3+ increases above 70% There is an apparent correlation between the extent of partitioning of Zn, and cation deficiency in the M 3 site, leading to a general formula for SGMs: [ (Zn, Fe) x (Zn, Fe)1 - x] M 1 M 2 (M 31 - x x ) Fe3+ (PO4)3 (OH) y (H2O)9 - y ·2H2O; where  = vacancy and ≤ 0.3. The sites M 1, M 2 and M 3 contain varying amounts of Fe2+, Fe3+, Mn2+ and Zn, with minor Mg. The different SGMs are distinguished by the dominant-cations and their oxidation states in the M 1, M 2 and M 3 sites. Wilhelmgümbelite has M 1 = Fe3+ M 2 = Fe3+, M 3 = Fe2+, while schmidite has M 1 (Fe3+ 0.5 Mn2+ 0.5)M (Fe3+ 0.5 Mn2+ 0.5) M 3 = Zn and schoonerite has M 1 = Mn2+, M 2 = Fe2+, M 3 = Fe2+. One of the SGMs studied was found to have a new ordering of dominant-cations of dominant-valency, with M 1 (Fe3+ 0.5Mn2+ 0.5), M2 = (Fe3+ 0.5Mn2+ 0.5) M 3 = Mn2+, and it has subsequently been approved as the new mineral wildenauerite (IMA 2017-058).
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Document Type: Research Article
Publication date: June 1, 2018
This article was made available online on June 22, 2018 as a Fast Track article with title: "Crystal chemistry of schoonerite-group minerals".
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