Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2,
CZ-182 21 Praha 8, Czech Republic
hybler@fzu.cz
Keywords: cronstedtite; 1:1layer silicate; Polytypes 1T, 3T, 1M, 2H1, 2H2, 2M1, 6T1, 6T2, 6R, 3A; twinning.
The layered silicate cronstedtite (Fe2+3-x Fe3+x)(Si2-xFe3+x)O5(OH)4, (0.5< x< 0.8) provides relatively numerous polytypes generated by stacking 1:1 structure layers - OD packets with the trigonal protocell a = 5.5, c = 7.1 Å. Cronstedtite occurs rarely in low temperature hydrothermal deposits [1], in certain meteorites (CM chondrites) [2], and presumably on asteroids. Synthetic micrometer-size crystals were prepared by Pignatelli and her co-workers [1,3]. The data collected by four circle X-ray diffractometer with area detector processed by an appropriate software provide precession-like reciprocal space sections (RS sections) allowing for the determination of OD subfamilies (A, B, C, D), and particular polytypes. Similar RS sections are obtained by electron diffraction tomography (EDT), for small crystals [1].
The synthetic material contains mainly 1M and 2M1 polytypes (subfamily A), sometimes twinned by the 120° rotation around chex axis. The 3T polytype (subfamily A) is less common. Only one rare mixed crystal of the 1M and 1T polytypes of A and C subfamilies, respectively was identified, too. Another, apparently ninetuple polytype with metrically R-centred lattice is in fact triclinic, because of the lack of a threefold axis. It is thus denoted as 3A. Common diagrams for determination of polytypes were generalized in order to include non-trigonal and non-hexagonal polytypes [1].
Several new occurrences were studied. Cronstedtite crystals from Nagybörzsöny, Hungary, contain nice examples of the well-ordered 1M polytype, pure or in mixed crystals with 3T. In some crystals, the 1M, in another the 3T polytype is dominant. The twinning by the 120° rotation around chex axis is also present in some crystals. Moreover, totally or almost disordered crystals with diffuse streaks instead of characteristic reflections were found.
Very promising appeared the new occurrence Ouedi Beht, El Hammam, Morocco, about 80 km SEE from Rabat (GPS 33°33'15.19"N, 5°49'53.68"W). The locality is represented by a small pit in the mountains. Of the A subfamily: polytypes 3T, 1M, 2M1, 6T2 (previously described from Pohled, ČR) [4,5] were identified. Monoclinic polytypes occur not separately, but always in mixed crystals, like 1M+3T, 2M1+3T, 1M+2M1, 2M1+6T2. Again, monoclinic polytypes are often twinned by the 120° rotation around chex axis. 2H1, 2H2 polytypes of the subfamily D are also common. Moreover, a sextuple polytype was identified in one separate crystal and in many mixed crystals with 2H1. It is not yet clear, whether it is identical with the 6T1, described by Hall et al. [6]. The polytype 6R was found as dominant in one mixed crystal with 2H1. Another sextuple R-centred polytype was found in one rare crystal, twinned by 60° rotation around chex axis (the so-called obverse-reverse twin).
It is worth noting, that the participants of the colloquium, who used a train from Prague to Žďár nad Sázavou, passed closely near two localities of cronstedtite: Rejské Lode at Kaňk (the hill right from the line, just before approaching the station Kutná Hora), and Pohled (the quarry at the right side, after passing the station Pohled). Other two localities Chvaletice (indicated by a chimney of the power station) and Litošice (approximately in the same direction, hidden in the forest) are somewhat distant from the line.
Author thanks Martin Števko for providing samples from Nagybörzsöny and Morocco.