Subfamily D non-MDO six-layer polytypes of cronstedtite

J. Hybler

Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2,

182 21 Praha 8, Czech Republic

hybler@fzu.cz

Keywords: cronstedtite; 1:1 layer silicates; 6T₁, 6T₃, 6R₁, 6R₂; twinning.

The layered 1:1 silicate cronstedtite (Fe²⁺_3-x Fe³⁺_x)(Si_2-xFe³⁺_x)O₅(OH)₄, (0.5< x< 0.85) belongs to the serpentine-kaoline group. It forms relatively numerous polytypes generated by stacking 1:1 structure building layers – equivalents of OD packets with the trigonal protocell a = 5.5, c = 7.1 Å. Polytypes are subdivided into four OD subfamilies, or Bailey’s groups A, B, C, D according to different stacking rules.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 single-crystal X-ray diffractometer with area detector processed by an appropriate software provide precession-like reciprocal space sections (RS sections in the following). Similar RS sections are obtained by electron diffraction tomography (EDT), for small crystals [1]. Distributions of so called subfamily reflections along the reciprocal lattice rows [2l]* / [11l]* / [2l]* in (l_hex)* / (hhl_hex)* / (2hl_hex)* RS planes is used for subfamily determination. Similarly, distributions of characteristic reflections along [10l]* / [01l]* / [1l]* rows in (h0l_hex)* / (0kl_hex)* / (hl_hex) planes allow determination of particular polytypes. For this purpose, graphical identification diagrams simulating distribution of reflections along named rows are used [1]. Modern diffractometers allow checking of many specimens and generation of RS section in a reasonable time.

Lot of specimens of cronstedtite from various terrestrial localities and synthetic run products were studied by the author [1, 4, 5, 6]. RS sections were recorded, and selected ones were published.

This contribution is focused on the polytypes of the OD subfamily D. Its stacking rule is characterized by alternating 180º rotations of consecutive layers, combined by ±b/3 (of the orthohexagonal cell) or zero shifts. The sample studied originate from the locality Ouedi Beht, El Hammam, Morocco, about 80 km SEE from Rabat (GPS 33°33'15.19"N, 5°49'53.68"W). The most common polytypes in the occurrence however, are quite common two-layer 2H₁ and 2H₂, occurring either isolated or in mixed crystals. Much more rarely, six-layer polytypes were found. They usually occur in complex mixed crystals containing more polytypes, up to six! Diffraction patterns of such crystals are thus confusing. Fortunately, in many cases polytypes were isolated mechanically by cleaving crystals into smaller fragments, later studied separately. In some cases, the cleaving procedure was repeated until the fragment containing one polytype was isolated.

Hall et all. [7] theoretically derived 24 possible sequences of layer stacking for six-layer polytypes of the subfamily D serpentine minerals, valid also for cronstedtite. Their diffraction patterns were modelled by the author, identification diagrams were constructed, and compared with real RS sections obtained from the experiments. This simulation revealed, that five pairs of sequences (No. 4+6, 7+18, 8+10, 9+13, 11+12) provided identical theoretical diffraction patterns. Polytypes really found in the Ouedi Beht occurrence correspond to following sequences: 1 (Hall’s 6T₁), 5 (proposed 6T₃), 8+10 (6T₅), 11+12 (6T₄) (trigonal polytypes), 22 (Hall’s 6R₁), 23 (Hall’s 6R₂) (rhombohedral polytypes). The sequence 24 was declared by Hall et all. [7] as rhombohedral (6R₃). Modelling of the structure, however, excluded the rhombohedral cell, thus the real symmetry is also trigonal and proposed symbol is 6T₆. This polytype was also discovered in the occurrence.

The hexagonal polytype 6H₂ corresponding to the sequence 14 was found, too. However, the identical diffraction pattern can be produced by the obverse-reverse twin of the rhombohedral polytype 6R₂ (sequence 23).

With exception of 6R₁, all six-layer polytypes mentioned above are so-called non-MDO (Maximum Degree of Order), or non-standard ones. In these polytypes, all triples, quadruples, ….n-tuples of consecutive layers are not equivalent.

The study presents a nice example, how different diffraction patterns can be produced by cleaved fragments of one complex crystal.

1. J. Hybler, M. Klementová, M. Jarošová, I. Pignatelli, R. Mosser-Ruck, S. Ďurovič, Clay. Clay Miner., 66, (2018), 379–402. DOI: 10.1346/CCMN.2018.064106.

2. I. Pignatelli, E. Mugnaioli, Y. Marrocchi, Eur. J. Mineral., (2018), DOI: 10.1127/ejm/2018/0030-2713.

3. I. Pignatelli, E. Mugnaioli, J. Hybler, R. Mosser-Ruck, M. Cathelineau, N. Michau, Clay. Clay Miner., 61, (2013), 277.

4. J. Hybler, J. Sejkora, V. Venclík, Eur. J. Mineral., (2016), DOI: 10.1127/ejm/2016/0028-2532.

5. J. Hybler, Eur. J. Mineral., (2016), DOI: 10.1127/ejm/2016/0028-2541.

6. J. Hybler, J. Sejkora, Z. Dolníček, M. Števko, accepted in Clay. Clay Miner., DOI: 10.1007/s42860-020-00102-9 CLAY-D-20-00085R1

7. S. H. Hall, S. Guggenheim, P. Moore, S.W. Bailey, Can. Mineral. 14, (1976), 314-321.

The study was supported by the project No. LO1603 under the Ministry of Education, Youth and Sports National sustainability programme I of Czech Republic. Author also thanks Martin Števko for providing samples from Morocco.