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Polytypism of cronstedtite from Pohled, Czech Republic

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Cronstedtite (Fe2+ 3-xFe3+ 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 2H 1+2H 2 crystals (group D) with the 2H 1 polytype dominant in most specimens [lattice parameters of both polytyps a=5.5002(4), c=14.195(1) Å, space groups: P63 cm (2H 1), P63 (2H 2)]; 2) the new non-MDO polytype 6T 2 (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 6T 2 and one 3T+1M crystal. Possible accessories of the 2M 1 and some unknown polytypes were detected in some of the 6T 2 and 2H 1+2H2 crystals, respectively. The EPMA studies revealed that the 6T 2 polytype is relatively Fe-poor (in the formula above, average x=0.73), 3T+1M polytypes Fe-rich (average x=0.77), and 2H 1 (+2H 2) polytypes have intermediate composition (x=0.72–0.78). Minor contents of chlorine were determined for 2H 1 (+2H 2) polytypes (up to 0.015 apfu) and 3T+1M polytypes (up to 0.009 apfu).
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Keywords: 1:1 LAYER SILICATE; 1M; 2H2; 2M1; 3T; CRONSTEDTITE; MDO POLYTYPES 2H1; NON-MDO POLYTYPE 6T2; TWINNING

Document Type: Research Article

Publication date: 2016-11-01

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  • The European Journal of Mineralogy publishes original papers, review articles and letters dealing with the mineralogical sciences s.l. These include primarily mineralogy, petrology, geochemistry, crystallography and ore deposits, as well as environmental, applied and technical mineralogy. Nevertheless, papers in any related field, including cultural heritage, will be considered.
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