Fibre textures of cronstedtite

J. Hybler

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

182 21 Praha 8, Czech Republic

hybler@fzu.cz

Keywords: cronstedtite; 1:1 layer silicates; fibre textures

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 micron-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 micron-size 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 quick generation of RS sections. These techniques allow identification of various polytypes, twins, as well as allotwins – oriented crystal associations of more polytypes.

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 and interpreted.

Recently cronstedtite from the new locality in Morocco was studied [6]. The sample was originally collected in 2017 by local people digging for mineral specimens from the hydrothermal veins with pyrite and calcite hosted in a skarn body situated at the base of the El Hammam hill (Djebel el Hammam), close to the Wadi (Ouedi) Beht (Beht river). It is located near the El Hammam fluorite deposit, ~45 km SW of Meknès in the northeastern part of the Variscan Moroccan Central Massif in northern Morocco. The sample was purchased from Fabre Minerals by M. Števko for the National Museum, Prague, where is now stored (catalogue No. P1N 114314).

The specimens separated from the sample provided a relatively high number of common as well as unusual (non-standard) polytypes of subfamilies A and D. Many crystals were identified as twins and/or allotwins of more polytypes (up to six). In many cases, the particular polytypes were mechanicaly separated by cleaving of allotwinned crystals. Some polytypes found were not known to date [6].

Several specimens separated from the central part of the sample appeared to be polycrystalline aggregates with a strong fibre texture – (001) preferred orientation and azimuthally misoriented (100) and (010) directions of domains or crystallites. The (l_hex)*/ (hhl_hex)*/ (2hl_hex)* and (h0l_hex)*/ (0kl_hex)* / (hl_hex)* RS sections indicating the subfamily D and 2H₂ polytype were superimposed (Fig. 1a). In order to further examine this peculiar kind of intergrowths, the series of RS sections (hk0_hex)*, (hk1_hex)*, (hk2_hex)*, (hk2_hex)*, (hk4_hex)*, etc., perpendicular to c_hex was generated. In these sections, concentric rings around c_hex were recorded instead of discrete reflections. The apparent reflections visible in (l_hex)*/ (hhl_hex)*/ (2hl_hex)* and (h0l_hex)*/ (0kl_hex)* / (hl_hex)* RS sections represented in fact intersections of named planes with these rings rather than discrete points. The nature of rings varied from sample to sample, from coarse-grained to quite smooth (Figs. 1b-d). However, in addition of [00l]* row with discrete maxima, some reflections and/or denser maxima on rings were usually present in the reciprocal space, so that indexing of diffraction patterns and generation of RS section became possible. For few crystals, however, the indexing procedure failed, possibly due to ’too perfect’ rings in the reciprocal space.

Figure 1. Examples of RS sections of strongly textured polycrystalline samples. a The apparent superposition of (hhl_hex)* and (h0l_hex)* sections of the well-ordered 2H₂ polytype of the subfamily D. b-d Examples of diffraction rings at the level of (hk2_hex)* RS section of several specimens varying from coarse-grained to almost smooth.

The back-scattering electrons (BSE) photograph of one of these specimens revealed existence of domains elongated in the c direction, probably azimuthally misoriented. (Fig. 2a). For comparison, a BSE image of an ordinary single crystal of the subfamily D is added (Fig. 2b). The electron microprobe analysis revealed partial substitution of Mn and Mg for Fe (0.09-0.10 and 0.19-0.25 a.p.f.u., respectively).

Figure 2. a The BSE image of the polycrystalline aggregate with fibre texture. Crystals are elongated about c_hex, and are parallel to the section in clusters above and below the centre of the image. In the central cluster, the orientation of crystals is somewhat inclined, so that they are cut obliquely. Borders of individual crystals are recognizable. b The BSE image of the common single crystal of the subfamily D. Note the same degree of grey throughout the surfaces of both specimens due to the homogeneity of chemical composition. Photo Z. Dolníček.

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., 30, (2018), 349-354, 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), 277-289, DOI: 10.1346/CCMN.2013.0610408.

4. J. Hybler, J. Sejkora, V. Venclík, Eur. J. Mineral., 28, (2016), 765-775, DOI: 10.1127/ejm/2016/0028-2532

5. J. Hybler, Eur. J. Mineral., 28, (2016), 777-788, DOI: 10.1127/ejm/2016/0028-2541

6.  J. Hybler, J. Sejkora, Z. Dolníček, M. Števko, Clay. Clay Miner., 69, (2016), 702-734, DOI: 10.1007/s42860-021-00157-2

The research was supported by project 18-10504S of the Czech Science Foundation, and by project CZ.02.1.01/0.0/0.0/16_019/0000760 Solid 21 under the Ministry of Education, Youth and Sports. Author also thanks National Museum for allowing of taking of specimens for the study.