The high-pressure behaviour of inderborite [ideally CaMg[B3O3(OH)5]2(H2O)4⋅2H2O, space group C2/c with a ≈ 12.14, b ≈ 7.43, c ≈ 19.23 Å and β ≈ 90.3° at room conditions] has been studied by two in situ single-crystal synchrotron X-ray diffraction experiments up to ~10 GPa, using He as pressure-transmitting fluid. Between 8.11(5) and 8.80(5) GPa, inderborite undergoes a first-order phase transition to its high-pressure polymorph, inderborite-II (with a ≈ 11.37, b ≈ 6.96, c ≈ 17.67 Å, β ≈ 96.8° and ΔV ≈ 7.0%, space group unknown). The isothermal bulk modulus (KV0 = β−1P0,T0, where βP0,T0 is the volume compressibility coefficient) of inderborite was found to be KV0 = 41(1) GPa. The destructive nature of the phase transition prevented any structure resolution of inderborite-II or even the continuation of the experiments at pressures higher than 10.10(5) GPa. In the pressure range 0–8.11(5) GPa, the compressional anisotropy of inderborite, indicated by the ratio between the principal components of the Eulerian finite unit-strain ellipsoid, is ɛ1:ɛ2:ɛ3 = 1.4:1.05:1. The deformation mechanisms at the atomic scale in inderborite are here described. Our findings support the hypothesis of a quasi-linear correlation between the total H2O content and P-stability range in hydrated borates, as the pressure at which inderborite undergoes the phase transition falls in line with most of the hydrate borates studied at high-pressure so far.

Triclinic IIb-4 and monoclinic IIb-2 clinochlore polytypes from Kenya are intergrown laterally in (001) platelets and are separated optically in some places by sharp lines of demarcation and in other places by embayed boundaries. Electron microprobe analyses show no compositional differences between the two polytypes. Structural refinements of each show that the 2:1 layer is identical in the two structures with no tetrahedral or 2:1 octahedral cation ordering in either ideal or subgroup symmetries. The two interlayer sheets dilfer only in the distribution of A1 and Mg over interlayer sites M3 and M4, but not in the distribution of Fe3+, Fe2+, or vacancies. The monoclinic IIb-2 polytype is less ordered than the triclinic IIb-4 polytype. The mean M-O,OH bond length of 1.990 Å indicates significantly less of the smaller A1 present in the M4 site of IIb-2 than for the mean M-O,OH value of 1.957 Å for the M4 site of IIb-4. The textural relations and the structural refinements suggest that the monoclinic IIb-2 structure is inherently less stable than the triclinic form. This is in accord with the observed lesser abundance in nature of the monoclinic form.

Classification and identification criteria of maximum-degree-of-order (MDO) polytypes of homo- and meso-octahedral micas based on the distribution of the intensities of 20l (13l) and 02l reflections are proposed. Calculated | F(20l) |2 and | F(02l) |2 values for single crystals of micas with different compositions are given for one-, two-, and three-layer polytypes. Transmission powder diffractometry is proposed as a suitable method for the identification of the different groups of mica polytypes from polycrystalline specimens. Calculated powder patterns and the characteristic properties of the diffraction patterns of random and highly oriented aggregates are employed for identification purposes. The individual MDO polytypes are designated by generalized Ramsdell symbols which also contain information about their position in the classification system.

Factors controlling the crystal structure of phlogopite have been widely investigated; but the role of electrostatic interactions, for example, has received much less attention than other factors. The purpose of the present study was to peform a single-crystal refinement of an Al-saturated phlogopite and to use that refinement to supplement crystal-chemical analyses. The phlogopite investigated was from the Rumford quadrangle, Maine, and has the following chemistry: ${\left({\text{K}}_{0.81}{\text{Na}}_{0.03}\right)}_{\Sigma =0.84}{\left({\text{Mg}}_{1.84}{\text{Fe}}_{0.52}^{2+}{\text{Al}}_{0.45}{\text{Mn}}_{0.02}{\text{Ti}}_{0.07}^{4+}\right)}_{\Sigma =2.90}{\left({\text{Si}}_{2.78}{\text{Al}}_{1.22}\right)}_{\Sigma =4}{\text{O}}_{10}{\left({\text{OH}}_{1.81},{\text{F}}_{0.05}\right)}_{\Sigma =1.86}$\$\end{document}. The sample is a 1M polytype with C2/m symmetry and cell dimensions of a = 5.3220(4), b = 9.2170(7), c = 10.2511(8) Å, and β = 100.081(1)°. Hydrogen atoms were located and the crystal structure was refined to give parameters R1 = 0.0301 and weighted R2 = 0.0887. The octahedral M1 site was larger than the M2 (average M1—O: 2.079 Å, average M2—O: 2.062 Å) and the electron counts were equal (M1 = M2 = 14.8 e−); based on bond distances, which are more accurate than electron counts in determining occupancy; this result is consistent with a slight preference of Mg for M2 and Fe2+ for M1.

Thirty-five Al-rich, natural phlogopite-1M samples that are of (1) high metamorphic grade, and that have (2) total Al contents ⩾ 1.27 atoms per formula unit (a.p.f.u.), (3) Fe3+ contents ⩽ 0.11 a.p.f.u., and (4) Mn contents ⩽0.10 a.p.f.u. along with the newly described phlogopite, exhibited crystal chemical trends related to increasing Al content. Octahedral substitutions of smaller, high-charge cations (i.e. Al) apparently decrease distortions in the octahedral sites and produce longer M2—O4 distances. In addition, VIFe-F avoidance apparently occurs in high Al-content samples, which are generally high in VIFe. The data set also shows that these samples have limited ordering among M sites (Fe2+ in M1 and Al in M2), an increase in β (99.96° to 100.32°) possibly caused by cation ordering and therefore size differences of M1 and M2, and interlayer (A) sites with A—Oouter distances that increase and A—Oinner distances that decrease with increasing Ti content.

Computer models were used to simulate electrostatic interactions in phlogopite structures with variable Al concentrations utilizing Pauling’s electrostatic valency principle, which considers first-coordination electrostatic interactions. The model results were compared to the maximum Al concentrations in natural and synthetic phlogopite samples. Model results revealed no indications (e.g. a limit reached or a sudden change occurred) that charge saturation/undersaturation of the apical oxygen atoms at Al contents equal to the maximum in natural and/or synthetic samples causes instability that could not be balanced by bond-length variation. However, a cation of higher charge substituting at M1 (or M2) may result in higher electrostatic repulsions between the other octahedral sites. Thus, the Al3+ content in the octahedral sites may reach a maximum, with Fe2+ for Mg substitutions favored.

Using type material from the Österby pegmatite in Dalarna, Sweden, the chemical composition and structural parameters of thalénite-(Y) [ideally Y3Si3O10(OH)] were examined by wavelength dispersive spectroscopy electron microprobe (WDS EMP) analysis and single-crystal X-ray diffraction. High contrast back-scatter electron images of the Österby material show at least two generations of thalénite-(Y). The formula of the primary thalénite-(Y) normalized to 11 anions is (Y2.58Dy0.11Yb0.09Gd0.06Er0.06Ho0.02Sm0.02Tb0.02Lu0.02Nd0.01Tm0.01)Σ3.00Si3.01O10F0.97OH0.03. The secondary thalénite-(Y), replacing the primary material, is weakly enhanced in Y and depleted in the lightest and the heaviest rare-earth elements, yielding the formula (Y2.63Dy0.12Yb0.06Gd0.06Er0.05Ho0.02Sm0.02Tb0.02Tm0.01Nd0.01Lu0.01)Σ3.00Si3.01O10F0.98OH0.02. Structural data for thalénite-(Y) from Österby clearly indicate the monoclinic space group P21/n, with a = 7.3464(4), b = 11.1726(5), c = 10.4180(5) Å, β = 97.318(4)°, V = 848.13(7) Å3, Z = 4, which is consistent with previous investigations. The structure was refined from single-crystal X-ray diffraction data to R1 = 0.0371 for 1503 unique observed reflections, and the final chemical composition obtained from the refinement, (Y2.64Dy0.36)Σ3.00F0.987[Si3O10], Z = 4, is in good agreement with the empirical formula resulting from electron microprobe (EMP) analysis. Both techniques reveal a strong dominance of F over OH, which means that the type material actually corresponds to the fluorine analogue. Moreover, new EMP analyses of samples of thalénite-(Y) from an additional seven localities (Åskagen and Reunavare in Sweden; White Cloud and Snow Flake in Colorado, USA; the Guy Hazel claim in Arizona, USA; Suishoyama and Souri in Japan) clearly show the prevalence of F over OH as well. Based on these observations, the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association has recommended a redefinition of the chemical composition of thalénite-(Y) to represent the F-dominant species with the ideal formula Y3Si3O10F, as it has historical priority. Consequently, the later described fluorthalénite-(Y) has to be discredited.