Sándor Szakáll

A new occurrence of katoite and re-examination of the hydrogrossular group

A new occurrence of katoite (Ca2.95Fe0.03A12.03(SiO4)1.12(OH)7.51; a = 12.286) found near Dunabogdany (Hungary), along with its structural refinement is reported here. The crystal-chemical data, the physical properties, and the structure refinement of this new member of the Ca3Al2(SiO4)3-Ca3Al2(O4H4)3 series indicate the presence of about 37% grossular. The positional disorder of the oxygen atom, a peculiar feature of the hydrogarnet structure, is described here in terms of both anisotropy along the d-O vector, and presence of two distinct oxygen sites (unsplit- and split-O models, respectively). The hydrogen atom was localised for both models in the difference Fourier map. The structural features of the katoite from Dunabogdany are compared with those of other hydrogrossulars and with the two end-members “anhydrous” grossular, and Si-free katoite. The results of the unsplit-O model refinement confirm the increase of the tetrahedron volume with the substitution of Si by 4H+. The geometry of the coordination polyhedra in the two configurations resulting from the split-O model are also discussed. This second model better describes from the crystallochemical point of view the substitution of Si4+ with 4H+ in the tetrahedra. However, the tetrahedra of the two dimensions are randomly distributed in the unit cell and hence domains of the two end-member configurations are not expected in katoite structure. This conclusion is also strengthened by the TEM study of katoite from Dunabogdany.**

Klajite from Recsk (Hungary), the first Mn-Cu arsenate mineral

Klajite (IMA no. 2010-004) was found in ore samples from the Lahoca Hill, Recsk, Matra Mountains (northern Hungary). It belongs to the Lahoca 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/cm 3 . 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, As 2 O 5 44.40, H 2 O (calc.) 17.49, total 100.00 wt%. The empirical formula is (Mn 0.82 Cu 0.10 Ca 0.08) ∑ = 1.00 Cu 4.05 As 3.98 O 14 (OH) 2 · 9H 2 O, and the simplified formula is MnCu 4 (AsO 4 ) 2 (AsO 3 OH) 2 · 9H 2 O. The strongest seven lines in the X-ray powder diffraction pattern are [ d hkl in A( I obs %, hkl )] 10.39 (100, 001), 2.916 (64, 202), 2.708 (29, 13), 3.616 (28, 021), 3.050 (28, 022 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 P 1, a = 6.441(3), b = 7.983(4), c = 10.562(3) A, α = 85.28(4)°, β = 80.63(5)°, γ = 84.80(4)°, V = 532.4(3) A 3 , Z = 1. Klajite is named after Sandor Klaj (bom. 1948), a Hungarian mineral collector.

Chabazite-Mg: A new natural zeolite of the chabazite series

Abstract Chabazite-Mg, (Mg0.67K0.52Ca0.48Na0.08Sr0.03)Σ1.78[(Al3.16Si8.89)Σ12.05O24]·9.68H2O, is a new zeolite species of the chabazite series, occurring in basalts of the Karikás-tető area of Prága Hill, Veszprém County, Balaton Highland, Transdanubia, West Hungary. It crystallizes as single, colorless rhombohedra up to 0.4 mm in size. The streak is white and the luster is strong vitreous. Mohs’ hardness is about 4. The observed density is 1.98(1) g/cm3 and the calculated density is 1.964(7) g/cm3. Chabazite-Mg is anisotropic, uniaxial (+), ω = 1.465(5), ε = 1.469(5) (546 nm). In its chemical composition, a predominance of Mg is observed among the extraframework cations. However, K and Ca are also very abundant, while Na and Sr levels are very low. The ratio Si/(Si+Al) is among the highest found in chabazite of hydrothermal genesis. Chabazite-Mg is rhombohedral, R̅3̅m space group, a = 9.3433(5) Å, α = 94.894(4)°. The six strongest X-ray lines measured in the powder pattern [d in Å (I) (hkl)] are: 9.306 (60) (100), 5.537 (37) (11̅ 1̅), 4.958 (25) (111), 4.315 (100) (201̅), 2.924 (78) (31̅ 1̅), 2.869 (41) (310). Single-crystal structure refinement of chabazite-Mg indicated that the extraframework occupation is distinct from other chabazite-series minerals. In particular, the Mg site (C3a) and one water site (W6a) are displaced from the threefold axis parallel to [111]