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The alkali-deficient tourmaline, foitite [□(Fe2+2Al Al6Si6O18(BO3)3(OH 3(OH)], and associated hematite occur in quartz veins that cut the geon 17 Baraboo Quartzite in south-central Wisconsin. The bluish green prismatic crystals of tourmaline are chemically zoned from core to rim, with the cores being very aluminous, highly alkali-deficient and, in one sample, relatively magnesian. Electron-microprobe analyses demonstrate that the tourmaline has a prevailing alkali-deficient in the X site, which ranges from 49 to 87%, with a mean of 73%, making this the most alkali-deficient tourmaline reported to date. In one sample, high contents of Al (up to 7.7 Al apfu) and high cation-charge excess demonstrate the likely existence of a dominant "oxy-foitite" component [□ (Fe2+ Al2) Al6Si6O18(BO33(OH)3(OH)3(O)], which is the first recognition of such in a natural occurrence. The wide range of chemical zoning in the tourmaline is most consistent with substitutions represented by the □Al(NaR)-1. AlO[R(OH)]-1, FeAl-1 and MgFe-1 exchanges, where R symbolizes Fe + Mg. The alkali-deficient character of the Baraboo tourmaline largely reflects the alkali-depleted and chemically mature composition of the host Baraboo Quartzite, but core-to-rim compositional variation in the tourmaline records the evolving nature of the attendant hydrothermal fluid, from a Na-poor, relatively alkaline early stage to a more sodic, acidic later stage.

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Canadian Mineralogist

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