Abstract

Cronstedtite, (Fe2+3−xFe3+x)(Si2xFe3+x)O5(OH)4, was found in the Manó metasomatic siderite deposit near Nižná Slaná, Slovakia. The deposit is hosted in Early Palaeozoic rocks of the Gemeric Superunit. Cronstedtite occurs mostly as slim, up to 2.5-mm trigonal pyramids in drusy cavities in the octahedral or cubo-octahedral crystals of pyrite in quartz veins. Separated crystals (21 altogether, including fragments of larger crystals) were checked on the four-circle X-ray diffractometer with area detector. From user-defined pre-experiments, the −2h h l, h h l, −h 2h l, h 0 l, 0 k l, −h h l (in hexagonal indices) precession-like images of reciprocal lattice planes were constructed using the diffractometer software. The OD (ordered–disordered) subfamilies (Bailey’s groups) and polytypes were identified from the reflection distribution along −2 1l/1 1 l/12l and 1 0 l/0 1 l/−1 1 l rows (hexagonal indices), respectively. Some selected crystals were further studied by electron probe micro-analysis (EPMA). Almost all crystals studied belong to the A group. The most common polytype is 3T, occurring either as the only one in crystals, or intergrown with 1M polytype. Most of 3T crystals are twinned by reticular merohedry interchanging the obverse/reverse settings of the rhombohedral subfamily structure. This twinning affects predominantly bottom and central parts, but not apices of crystals. In a rare, rounded tabular, and strongly disordered D-group crystal, the 2H1 and traces of the 2H2 polytypes were recognized. Besides the major Si and Fe contents, small amounts of Cl (0.02–0.06 apfu) and mainly of S (up to 0.11 apfu) were detected by EPMA.

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