TOKYOITE, As-RICH TOKYOITE, AND NOELBENSONITE: NEW OCCURRENCES FROM THE POSTMASBURG MANGANESE FIELD, NORTHERN CAPE PROVINCE, SOUTH AFRICA

Samples of manganese-rich rock containing two compositional varieties of tokyoite in association with noelbensonite were retrieved from a drill core obtained from the Postmasburg area in the Northern Cape Province of South Africa. The samples consist of fine-grained braunite, hematite, and hausmannite. Within this material abundant vugs are observed that are filled with witherite, baryte, and barytocalcite. In addition, As-rich tokyoite, tokyoite, and noelbensonite occur in the center of the vugs, in fine aggregates 0.1 to 1 mm in size. Individual As-rich tokyoite grains are typically 20-200 mu m in size. The outer walls of the vugs are lined with microcrystalline K-feldspar, witherite, and/or serandite. Textural evidence of the Ba-rich mineral phases in association with As-rich tokyoite suggest an epigenetic mode of formation of the observed assemblages, caused by V- and As-bearing alkali-rich fluids interacting with pre-existing Mn-rich minerals. Electron microprobe analysis (EMPA), electron backscatter diffraction analysis (EBSD), and Raman spectroscopy show that the As-rich tokyoite is a mineral belonging to the brackebuschite mineral group. Its measured mineral composition corresponds to the formula (Ba1.92Sr0.05Pb0.03)(Sigma 2.00)(Mn0.983+Fe0.023+)(Sigma 1.00)[(As1.050V0.950)(Sigma 2.00)O-8)]OH. Electron backscatter diffraction analysis results point to a monoclinic, P2(1)/m space group, with cell parameters a 9.121 angstrom, b 6.142 angstrom, c 7.838 angstrom, alpha = gamma = 90 degrees, beta = 112 degrees, Z = 2. The structure is therefore similar to gamagarite. Raman spectra of As-rich tokyoite were compared to spectra of arsenbrackebuschite and arsentsumebite, and those of noelbensonite to spectra of hennomartinite and lawsonite. The indexing of Raman peaks in As-rich tokyoite, by similarity with arsentsumebite, suggests a possible ordering of the AsO4 and VO4 tetrahedra. This observation, correlated with mineral chemistry and specifically an As:V ratio of similar to 1:1, suggest that the As-rich tokyoite may in fact represent a possible new mineral with ordering of As and V at tetrahedral positions, or at least an unknown As analogue of tokyoite.