The present study presents a generalized procedure to accurately identify trioctahedral 1:1 layer silicates. The reciprocal space (RS) sections were obtained by the precession method by “unwarping” frames that were recorded using diffractometers with area detectors or from electron diffraction tomography (EDT) patterns. Distributions of subfamily reflections along the reciprocal lattice rows [21̄l]* / [11l]* / [1̄2l]* in (2hh̄lhex)* / (hhlhex)* / (h̄2hlhex)* RS planes were used to determine OD (Ordered — Disordered) subfamilies (Bailey’s groups A, B, C, D). The distributions along the [10l]* / [01l]* / [1̄1l]* rows in the (h0lhex)* / (0klhex)* / (h̄hlhex) RS planes allow determination of the polytypes. The use of traditional identification diagrams for the determination of OD subfamilies and polytypes was generalized in order to identify monoclinic and orthorhombic MDO (Maximum Degree of Order) polytypes. In these polytypes, the distribution of characteristic reflections along rows is different in RS sections that are perpendicular and diagonal to the symmetry plane of the polytype. The identification diagram of non-MDO polytype 6T2 is also presented. The method was applied to the identification of single crystals of cronstedtite that were synthesized during interactions of Fe metal with a natural claystone during experiments over a temperature range of 90-60°C. Conical and pyramidal crystals with a maximum size of a few micrometers were studied using electron diffraction tomography (EDT). The following polytypes were identified: 1M, 2M1, an apparently ninetuple polytype that was interpreted as triclinic 3A (group A), 1T (group C), and disordered crystals of group D. The 1M polytype was the most abundant. Some 1M crystals were twinned by reticular merohedry with a 120° rotation along the chex axis as the twin operation. The 2M1 occurred as isolated crystals as well as in mixed crystals. Intergrown 1T and 1M polytypes of the C and A group, respectively, were identified in one mixed crystal. The possible stacking sequence and the 3A polytype identification diagram was presented and discussed. Example RS sections of all polytypes were identified and demonstrated.

Charoite, ideally (K,Sr,Ba,Mn)15–16(Ca,Na)32[(Si70(O,OH)180)](OH,F)4·nH20, is a rock-forming mineral from the Murun massif in Yakutia, Sakha Republic, Siberia, Russia, where it occurs in a unique alkaline intrusion. Charoite occurs as four different polytypes, which are commonly intergrown in nanocrystallme fibres. We report the structure of charoite-96 (a = 32.11(6), b = 19.77(4), c = 7.23(1) Å, β = 95.85(9)°, V = 4565(24) Å3, space group P21/m), which was solved ab initio by direct methods on the basis of 2676 unique electron diffraction reflections collected by automated diffraction tomography and refined to R1/wR2 = 0.34/0.37. The structure of charoite-96 is related to that of the charoite-90, which was also solved recently. Both structures are composed of three different types of dreier silicate chains running along [001] and separated by ribbons of edge-sharing Ca- and Na-centred octahedra. In the structure of charoite-96, adjacent blocks formed by three different silicate chains and stacked along the x axis, are shifted by a translation of 1/2 c. The shifts involve a hybrid dreier quadruple chain, [Si17O43]18– and a double dreier chain, [Si6O17]10–. In charoite-90 adjacent blocks are stacked without shifts.

The crystal structure of karibibite, Fe33+(As3+O2)4(As23+O5)(OH), from the Urucum mine (Minas Gerais, Brazil), was solved and refined from electron diffraction tomography data [R1 = 18.8% for F > 4σ(F)] and further confirmed by synchrotron X-ray diffraction and density functional theory (DFT) calculations. The mineral is orthorhombic, space group Pnma and unit-cell parameters (synchrotron X-ray diffraction) are a = 7.2558(3), b = 27.992(1), c = 6.5243 (3) Å, V = 1325.10(8) Å3, Z = 4. The crystal structure of karibibbite consists of bands of Fe3+O6 octahedra running along a framed by two chains of AsO3 trigonal pyramids at each side, and along c by As2O5 dimers above and below. Each band is composed of ribbons of three edge-sharing Fe3+O6 octahedra, apex-connected with other ribbons in order to form a kinked band running along a. The atoms As(2) and As(3), each showing trigonal pyramidal coordination by O, share the O(4) atom to form a dimer. In turn, dimers are connected by the O(3) atoms, defining a zig-zag chain of overall (As3+O2)n-n stoichiometry. Each ribbon of (Fe3+O6) octahedra is flanked on both edges by the (As3+O2)n-n chains. The simultaneous presence of arsenite chains and dimers is previously unknown in compounds with As3+. The lone-electron pairs (4s2) of the As(2) and As(3) atoms project into the interlayer located at y = 0 and y = ½, yielding probable weak interactions with the O atoms of the facing (AsO2) chain.

The DFT calculations show that the Fe atoms have maximum spin polarization, consistent with the Fe3+ state.