Tourmaline-bearing Quartz Veins in the Baraboo Quartzite, Wisconsin: Occurrence and Significance of Foitite and “oxy-foitite”

The alkali-deficient tourmaline, foitite [ (Fe2Al)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-deficiency 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 [ (FeAl2)Al6Si6O18(BO3)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.