Abstract

Cronstedtite (Fe2+3−x Fe3+x)(Si2−xFe3+x)O5(OH)4, was found in an active quarry near Pohled, Czech Republic, in the Monotone Series of Moravian Moldanubicum near the eastern edge of the Moldanubian pluton. Rare, up to 3 mm large crystals occur in a vuggy, 3 cm thick hydrothermal pyrite vein, penetrating a migmatized paragneiss near the contact with two-mica granite of the Eisgarn type. Separated crystals (43 altogether) were studied using a four-circle X-ray diffractometer with a CCD area detector. From 217 frames per user-defined pre-experiment, the −2hhl, hhl, -h2hl, h0l, 0kl, -hhl (in hexagonal indices) precession-like reciprocal lattice sections were constructed using the diffractometer software. The OD subfamilies (Bailey’s groups) and polytypes were identified from the reflection distribution along −21l/11l/−12l and 10l/01l/−11l rows (hexagonal indices), respectively. Selected crystals were further studied by electron-microprobe analysis (EPMA). Three types of crystals were distinguished: 1) mixed 2H1 + 2H2 crystals (group D) with the 2H1 polytype dominant in most specimens [lattice parameters of both polytypes a = 5.5002(4), c = 14.195(1) Å, space groups: P63cm (2H1), P63 (2H2)]; 2) the new non-MDO polytype 6T2 (group A) [a = 5.4976(3), c = 42.601(1) Å, space group P31]; 3) mixed 3T + 1M (group A) crystals, [a = 5.4975(4), c = 21.262(1) Å, P31 (3T), a = 5.4967(6), b = 9.5214(9) c = 7.3216(8) Å, β = 104.52(1)°, Cm (1M)]. Rare twins by reticular merohedry were identified in one 6T2 and one 3T + 1M crystal. Possible accessories of the 2M1 and some unknown polytypes were detected in some of the 6T2 and 2H1 + 2H2 crystals, respectively. The EPMA studies revealed that the 6T2 polytype is relatively Fe-poor (in the formula above, average x = 0.73), 3T +1M polytypes Fe-rich (average x = 0.77), and 2H1 (+ 2H2) polytypes have intermediate composition (x = 0.72–0.78). Minor contents of chlorine were determined for 2H1 (+ 2H2) polytypes (up to 0.015 apfu) and 3T +1M polytypes (up to 0.009 apfu).

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