Polytypism of cronstedtite from Pohled, Czech Republic

Cronstedtite (Fe2+3− x Fe3+ x )(Si2− x Fe3+ 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 -2 hhl, hhl, -h 2 hl, h 0 l , 0 kl, -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 -21 l /11 l /-12 l and 10 l /01 l /-11 l rows (hexagonal indices), respectively. Selected crystals were further studied by electron-microprobe analysis (EPMA). Three types of crystals were distinguished: 1) mixed 2 H 1 + 2 H 2 crystals (group D) with the 2 H 1 polytype dominant in most specimens [lattice parameters of both polytypes a = 5.5002(4), c = 14.195(1) A, space groups: P 63 cm (2 H 1), P 63 (2 H 2)]; 2) the new non-MDO polytype 6 T 2 (group A) [ a = 5.4976(3), c = 42.601(1) A, space group P 31]; 3) mixed 3 T + 1 M (group A) crystals, [ a = 5.4975(4), c = 21.262(1) A, P 31 (3 T ), a = 5.4967(6), b = 9.5214(9) c = 7.3216(8) A, β = 104.52(1)°, Cm (1 M )]. Rare twins by reticular merohedry were identified in one 6 T 2 and one 3 T + 1 M crystal. Possible accessories of the 2 M 1 and some unknown polytypes were detected in some of the 6 T 2 and 2 H 1 + 2 H 2 crystals, respectively. The EPMA studies revealed that the 6 T 2 polytype is relatively Fe-poor (in the formula above, average x = 0.73), 3 T +1 M polytypes Fe-rich (average x = 0.77), and 2 H 1 (+ 2 H 2) polytypes have intermediate composition ( x = 0.72–0.78). Minor contents of chlorine were determined for 2 H 1 (+ 2 H 2) polytypes (up to 0.015 apfu ) and 3 T +1 M polytypes (up to 0.009 apfu ).