Schafarzikite from the type locality Pernek (Malé Karpaty Mountains, Slovak Republic) revisited

A rare mineral schafarzikite, an oxide of Fe²⁺ and Sb³⁺, was found after more than 80 years at the type locality near Pernek (Malé Karpaty Mountains, Slovak Republic). Crystals, druses, and crusts of schafarzikite occur on fractures in quartz-carbonate-stibnite hydrothermal ores. The Sb mineralization is bound to black shales and phyllites in a zone of actinolitic rocks. Associated minerals include ankerite, berthierite, stibnite, valentinite, kermesite, senarmontite, and gypsum. Prismatic crystals of schafarzikite are 0.1–1.0 mm, rarely up to 1.5 mm large, with the dominant forms {110}, {121}, {112}, {010}, {221}, {131}, and {231}. The optical properties are: uniaxial, with relatively strong pleochroism in red-brown tints; refraction indices higher than 1.74; the average refraction index, calculated from the Gladstone-Dale equation, is 2.001. The physical properties of schafarzikite from Pernek are: dark brown to black color, adamantine to metallic luster, brown streak; translucent (brown to orange) in very thin fragments; good {100} cleavage and perfect cleavage along unindexed planes parallel to z axis, tenacity-brittle; VHN10g micro-hardness = 251 and 278 kp/mm² (for two cuts with differing orientation), corresponding to Mohs’ hardness of 3.5–4; calculated density D_x = 5.507 g/cm³. The electron microprobe analysis gave FeO 19.38, ZnO 0.02, PbO 0.02, Sb₂O₃ 80.36, As₂O₃ 0.55, Bi₂O₃ 0.16, SO₂ 0.04, and calculated formula Fe_0.97(Sb_1.99As_0.02)S2.01O₄. The XRD pattern was indexed in a tetragonal setting, with refined unit-cell parameters are a = 8.6073(2) c = 5.9093(3) Å, V = 437.80(2) ų, c : a = 0.6865. Thermogravimetric (TG) curve shows mass gain of 1.62 wt. % in the range 20–580 °C, and 5.28 wt. % in the range 580–850 °C caused by Fe²⁺ and Sb³⁺ oxidation, respectively. The product of TG analysis is a phase isostructural with rutile. A tentative assignment of FTIR and Raman spectra of schafarzikite is given. Schafarzikite from Pernek most likely crystallized from late oxidizing hydrothermal fluids. Hence, it is not a weathering product, as assumed previously.