I. V. Rozhdestvenskaya

Tubular chains in the structures of natural and synthetic silicates

Tubular chains in the structures of natural and synthetic silicates have been described and systematized using the theory of graphs and by unfolding the tube topology onto a plane. Eight types of tubular chains have been considered: seven silicon-oxygen chains and one mixed titanosilicate chain; their topological and geometric characteristics are analyzed. It is shown that the presence of large cavities in tubes is generally related to their occupation by large low-valence cations (K+, Cs+, Ba2+, Sr2+, etc.) and water molecules. The overwhelming majority of minerals containing tubular silicate fragments were found in hydrothermal veins of alkaline massifs in Russia and abroad.

Crystallochemical characteristics of alkali calcium silicates from charoitites

The characteristic features of the crystal structures of alkali calcium silicates from various deposits are considered. The structures of these minerals, which were established by single-crystal X-ray diffraction methods, are described as the combinations of large construction modules, including the alternating layers of alkali cations and tubular silicate radicals (in canasite, frankamenite, miserite, and agrellite) and bent ribbons linked through hydrogen bonds in the layers (in tinaksite and tokkoite). The incorporation of impurities and the different ways of ordering them have different effects on the structures of these minerals and give rise to the formation of superstructures accompanied by a change of the space group (frankamenite-canasite), leading, in turn, to different mutual arrangements of the layers of silicate tubes and the formation of pseudopolytypes (agrellites), structure deformation, and changes in the unit-cell parameters (tinaksite-tokkoite).

Refinement of the crystal structures of synthetic nickel- and cobalt-bearing tourmalines

The crystal structures of synthetic tourmalines with a unique composition containing 3d elements (Ni, Fe, and Co) have been refined: (Ca0.12▭0.88)(Al1.69Ni0.812+Fe0.502+)(Al5.40Fe0.603+)(Si5.82Al0.18O18)(BO3)3(OH)3.25O0.75 I, a = 15.897(5), c = 7.145(2) Å, V = 1564(1) Å; Na0.91(Ni1.202+Cr0.963+Al0.63Fe0.182+Mg0.03)(Al4.26Ni1.202+Cr0.483+Ti0.06)(Si5.82Al0.18)O18(BO3)3(OH)3.73O0.27 II, a = 15.945(5), c = 7.208(2) Å, V = 1587(1) Å3 and Na0.35(Al1.80Co1.202+)(Al5.28Co0.662+Ti0.06)(Si5.64B0.36)O18(BO3)3(OH)3.81O0.19 III, a = 15.753(8), c = 7.053(3) Å, V = 1516(2) Å3. The reliability factors are R1 = 0.038−0.057 and wR2 = 0.041–0.060. It is found that 3d elements occupy both Y- and Z positions in all structures. The excess positive charge is compensated for due to the incorporation of divalent oxygen anions into the O3(V)+O1(W) positions.

Structures and isomorphous substitutions in Cs-Mg-beryl and Cs-rich beryllian indialite formed in a flux medium

The crystal structures of isostructural compounds — Cs-Mg-beryl (Al1.68Mg0.31Fe0.01)(Be2.68Si0.02Al0.26□0.04)Si6.00O18 · Cs0.07 (a = 9.2359(9) Å, c = 9.204(1)Å) and the Cs variety of beryllian indialite (Mg1.90Fe0.10)(Be1.02Al1.98)(Al0.30Si5.70)O18 · Na0.02Cs0.16 (a = 9.598(3) Å, c = 9.284(3) Å) — were refined. These compounds were formed in the Al2Be3Si6O18-Mg,Ca/F,Cl flux system in the presence of cesium chloride. The main structural features of these compounds were determined. It was found that the iso-morphous incorporation of Cs+ cations into anhydrous beryl proceeds according to the simple “vacancy” scheme BeT2 → 2(Cs+)R + □T2, whereas the complex heterovalent substitution 9SiT1 + AlT2 → 9AlT1 + BeT2 + 9(Cs, Na)R is observed in Cs-rich beryllian indialite under anhydrous conditions; i.e., no vacancies are formed in the tetrahedral framework of the latter structure. In Cs-Mg-beryl, an increase in the average bond lengths in the M octahedron and in the interring T2 tetrahedron leads to an increase in the unit-cell parameters a and c. In Cs-rich beryllian indialite, a slight increase in the M-O bond length and a decrease in T2-O bond length cause a slight increase in the parameter a and a decrease in the parameter c. The Cs+ cations are incorporated into the channels of both compounds at the height of the interring M-T2 layer (like K+ cations), whereas the Na+ cation is incorporated inside the Si6O18 ring. The δT1 value suggests that the change in the composition caused by the incorporation of Cs+ cations leads to the incongruent melting of beryllian indialite.

Structural features and isomorphous substitutions in chromium- and magnesium-containing beryls and chromium-containing beryllian indialite grown by the flux method

Beryls and beryllian indialite {the general formula MVI2T(2)IV3T(1)IV6O18} synthesized in magnesium-containing flux systems saturated with chromium are investigated using X-ray diffraction. The isovalent schemes of the isomorphous incorporation of chromium into Moctahedra of these compounds and the simultaneously realized heterovalent schemes with the participation of other components are revealed from the occupancies of the positions. It is demonstrated that an increase in the average bond lengths in the M positions leads predominantly to an increase in the parameter α. In the beryllian indialites, the T(1) tetrahedra are substantially closer to perfect tetrahedra, the T(2) tetrahedra are distorted to a lesser extent, and the M octahedra are distorted to a greater extent than those in beryls. The structural indications of the ability of compounds with a beryl structure to congruently melt are distinguished.

Refinement of the crystal structure of calcium-lithium-aluminum tourmaline from the pegmatite vein in the Sangilen Upland (Tuva Republic)

The crystal structure of a natural calcium-lithium-aluminum tourmaline, which has the unique composition (Ca0.62Na0.32□0.06)(Al1.08Li0.99Fe0.662+ Mg0.24Ti0.03)Al6[Si6O18](BO3)3(OH2.28O0.72) · (F0.84O0.16), is refined (R = 0.019, Rw = 0.022, S = 1.47). It is found that the O(1)(W) site is split into two sites, O(1) and O(11), which are incompletely occupied by fluorine and oxygen anions, respectively, and that the O(3)(V) site contains bivalent oxygen anions. The solid solution studied is close in composition to the liddicoatite mineral species and differs from the latter one by the Li: Al ratio in the Y octahedra and the presence of bivalent oxygen anions in the O(3) site. The tourmaline studied differs from the hypothetical oxyliddicoatite by the population of the O(1)(W) site by fluorine and accommodation of additional oxygen anions in the O(3)(V) site.

Refinement of the crystal structures of three fluorine-bearing elbaites

The crystal structures of three Li-Al natural tourmalines (elbaites) containing 0.88–1.39 wt % F are refined to R= 0.0294, 0.0308, and 0.0417. It is revealed that the W threefold anion site is split into two sites, namely, the W1 threefold site and W2 ninefold site (W1–W2 ∼ 0.4 Å, Y-W1 ≥ 1.94 Å, Y-W2 ≥ 1.75 Å). The following hypothesis is proposed and justified: the W1 and W2 sites are partially occupied by OH groups and fluorine anions, respectively. The ratio of the [YO4(OH)2] octahedra to the [YO4(OH)F] octahedra depends on the fluorine content and varies from structure to structure. The fact that the W site is more than 50% occupied by fluorine in the structures of two tourmalines under investigation allows the conclusion that fluor-elbaite with the ideal formula Na(Li1.5A11.5)A16(Si6O18)(BO3)3(OH)3F is a new mineral species and that elbaite can be considered a superspecies.

Structure of the new mineral paratsepinite-Na and its place in the labuntsovite group

X-ray diffraction study of high-strontium Ti-enadkevichite showed that it is a new mineral, which was given the name paratsepinite-Na, sp. gr. C2/m, a double value of the parameter c, and a different distribution of the non-framework cations in comparison with tsepinite-Na. In particular, strontium is located not only in the hexagonal window of the small channel, but also in three positions of the large channel. The characteristics of the mosaic blocks constituting the crystal are studied. It is assumed that polysynthetic twinning in these crystals and many other representatives of this family is associated with the phase transformation occurring in this hydrothermal mineral during its cooling after formation. The comparison of all the three structures of the vuoriyarvite subgroups of the labuntsovite group allows us to explain the disability of vuoriyarvite to absorb strontium from the aqueous solution by its smaller unit-cell volume and higher framework charge in comparison with those of tsepinites.

Refinement of apatite atomic structure of albid tissue of Late Devon conodont

The crystal structure of carbonate-containing apatite from albid tissue of Late Devon conodonts of Polygnathus species (Ozarkodinida order) has been refined by single-crystal X-ray diffraction with the application of electron-probe X-ray microanalysis and Raman spectroscopy data. This apatite-organic composite yields a diffraction pattern characteristic of a single crystal. The atomic structure of biological apatite is close to that of stoichiometric apatite-(CaF). The content of carbonate ions replacing [PO4]3− anions is very low (∼1 wt %). Channels of the structure contain not only fluorine but also OH ions (in a ratio of 3: 1); the latter are partially replaced with water molecules. The main cationic substitutions occur in the Ca2 position. The specific features of the diffraction pattern of albid tissue indicate that the apatite-organic composite under study is a nanostructured material (a biological mesocrystal).

Cation distribution in the structure of titanium-containing ludwigite

The crystal structure of natural titanium-containing ludwigite has been refined. The unit-cell parameters are a = 9.260 ± 0.002 Å, b = 12.294± 0.002 Å, c = 3.0236± 0.0005 Å, sp. gr. Pbam, and R = 0.0288. The observed cation distribution over the M1-M4 positions corresponds to the structural formula (Mg0.5)(Mg1.0)(Mg0.338Fe0.1622+)(Fe0.473+Ti0.214+Mg0.152+Al0.103+Fe0.072+(BO3)O2. Highly charged titanium ions in the M4 position are balanced mainly with magnesium and not with divalent iron ions.