Klajite (IMA no. 2010-004) was found in ore samples from the Lahóca Hill, Recsk, Mátra Mountains (northern Hungary). It belongs to the Lahóca epithermal high-sulphidation Cu-Au-As ore deposit, a classical occurrence of enargite. The primary ore consists of enargite, luzonite, pyrite, tennantite, chalcopyrite, galena, and sphalerite. Klajite occurs in the cavities of enargite and quartz, in close association with other secondary minerals: gypsum, jarosite, and an unknown Ca-Cu arsenate. Klajite is a secondary mineral formed by the decomposition of enargite. It forms irregular or sheaf-like aggregates, up to 0.5 mm in diameter, made up of lath-like to thin tabular crystals, typically 0.05–0.2 mm in length. The mineral is translucent, greenish yellow to yellowish green in colour with white streak and vitreous lustre. Its Mohs hardness is about 2–3, the calculated density is 3.213 g/cm3. Klajite has a perfect cleavage parallel to {010}, and is extremely brittle; the fracture is uneven. Optically it is biaxial negative or positive with α = 1.595(30), β = n.d. and γ= 1.665(20). It is weakly pleochroic, from colourless to pale green. The chemical composition obtained after correction of electron-microprobe analysis is: MnO 5.67, CuO 32.03, CaO 0.41, As2O5 44.40, H2O (calc.) 17.49, total 100.00 wt%. The empirical formula is (Mn0.82Cu0.10Ca0.08) ∑ = 1.00 Cu4.05As3.98O14(OH)2 · 9H2O, and the simplified formula is MnCu4(AsO4)2(AsO3OH)2 · 9H2O. The strongest seven lines in the X-ray powder diffraction pattern are [dhkl in Å(Iobs %, hkl)] 10.39 (100, 001), 2.916 (64, 202), 2.708 (29, 1̄3), 3.616 (28, 02̄1), 3.050 (28, 02̄2 and 211), 3.956 (27, 020) and 3.110 (24, 122). According to X-ray powder diffraction, klajite is structurally analogous to lindackerite-group minerals; it is triclinic, space group P1̄, a = 6.441(3), b = 7.983(4), c = 10.562(3) Å, α = 85.28(4)°, β = 80.63(5)°, γ = 84.80(4)°, V = 532.4(3) Å3, Z = 1. Klajite is named after Sándor Klaj (bom. 1948), a Hungarian mineral collector.

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