Original paper

Polytypism of cronstedtite from Nižná Slaná, Slovakia

Hybler, Jiří; Števko, Martin; Sejkora, Jiří


Cronstedtite, (Fe2+ 3-xFe3+ x )(Si2-x Fe3+ 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 userdefined pre-experiments, the 2 h h l, h h l , h 2 h 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 1 l /1 1 l/ 1 2 l 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 3 T, occurring either as the only one in crystals, or intergrown with 1 M polytype. Most of 3 T 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 2 H 1 and traces of the 2 H 2 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.


2 h1twinning1:1 layer silicate2 h2cronstedtite1 mmdo polytypes 3 t