Tokyoite, As-rich Tokyoite, and NoÉlbensonite: New Occurrences from the Postmasburg Manganese Field, Northern Cape Province, South Africa Available to Purchase

Samples of manganese-rich rock containing two compositional varieties of tokyoite in association with noélbensonite 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 noélbensonite 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 μm in size. The outer walls of the vugs are lined with microcrystalline K-feldspar, witherite, and/or sérandite. 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 (Ba_1.92Sr_0.05Pb_0.03)_∑2.00(Mn³⁺_0.98Fe³⁺_0.02)_∑1.00[(As_1.050V_0.950)_∑2.00O₈)](OH). Electron backscatter diffraction analysis results point to a monoclinic, P2₁/m space group, with cell parameters a 9.121 Å, b 6.142 Å, c 7.838 Å, α = γ = 90°, β = 112°, 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 noélbensonite 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 ~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.