Crystal structure of a new mineral lahnsteinite Zn4(SO4)(OH)6 · 3H2O

A sample of a new mineral from the oxidation zone of the hydrothermal Pb, Zn, Ag Friedrichssegen deposit (Rhineland-Palatinate, Germany) was studied by single-crystal X-ray diffraction. The parameters of the triclinic (pseudoorthorhombic) unit cell are found to be a = 8.312(1) ?, b = 14.545(1) ?, c = 18.504(2) ?, ?? = 89.71 (1)°, ?? = 90.05(1)°, ?? = 90.13(1)°, and V = 2237.3(3) ?³. The structure is solved by direct methods into the sp. gr. P1 and refined to an R factor of 10.7% using 3788 reflections with |F| > 3??(F) in the isotropic-anisotropic approximation. The crystallochemical formula of lahnsteinite (Z = 8) is [(Zn_2.6Fe_0.3Cu_0.1)^VI(OH)₃][Zn^IV(OH)₃(H₂O)][SO₄] · 2H₂O, where the compositions of the layer composed of Zn octahedra and isolated Zn and S tetrahedra are given in square brackets. The mineral under study is chemically and structurally similar to namuwite and is a natural analog of synthetic zinc hydroxide sulfate trihydrate.     

Determination of the crystal structure of diopside (Ca0.8Fe0.2)(Mg0.8Fe0.2)Si2O6 from the Gazakh Trough (Azerbaijan)

The crystal structure of a clinopyroxene is determined by X-ray diffraction. The unit cell parameters are as follows: a = 9.732(7) ?, b = 8.896(6) ?, c = 5.280(4) ?, and ?? = 106.160(11)°; V = 439.1(5) ?³; space group C2/c; and Z = 4. The structure is determined by the direct method and refined in the anisotropic approximation by the full-matrix least-squares method (R[F ² > 2??(F ²)] = 0.027, wR(F ²) = 0.072). According to the results of the X-ray structure analysis, the crystal chemical formula of the compound is (Ca_0.8Fe_0.2)(Mg_0.8Fe_0.2)Si₂O₆. In the structure, columns of (Mg, Fe)-octahedra are connected on two sides with Si₂O₆ chains. The resulting structural units have the [(Mg,Fe)₂(Si₂O₆)₂]^4? composition. These units form a heterogeneous framework, which is typical of all pyroxene structures. The [(Mg,Fe)₂(Si₂O₆)₂]^2? composition of the framework corresponds to the ratio [(Mg,Fe) + Si]: O = 3: 6 (1: 2). The (Ca, Fe) atoms are located in the holes of the framework.     

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 (Al_1.68Mg_0.31Fe_0.01)(Be_2.68Si_0.02Al_0.26??_0.04)Si_6.00O₁₈ · Cs_0.07 (a = 9.2359(9) ?, c = 9.204(1)?) and the Cs variety of beryllian indialite (Mg_1.90Fe_0.10)(Be_1.02Al_1.98)(Al_0.30Si_5.70)O₁₈ · Na_0.02Cs_0.16 (a = 9.598(3) ?, c = 9.284(3) ?) ?? 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 Be_T2 ?? 2(Cs+) _R + ??_T2, whereas the complex heterovalent substitution 9Si_T1 + Al_T2 ?? 9Al_T1 + Be_T2 + 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 ??^T1 value suggests that the change in the composition caused by the incorporation of Cs⁺ cations leads to the incongruent melting of beryllian indialite.     

Hydrogen bonds in the crystal structures of vanadyl phosphates Rb(VO2)[HPO4] and CsAl(VO)[PO4]2(H2O)

The crystal structures of two vanadyl phosphates, Rb(VO₂)[HPO₄] (a = 7.029(4) ?, b = 9.513(6) ?, c = 18.049(10) ?, sp. gr. Pbca, Z = 8, ??_calcd = 2.910(3) g/cm³) and CsAl(VO)[PO₄]₂(H₂O) (a = 8.0128(13) ?, b = 8.0714(13) ?, c = 14.118(2) ?, ?? = 106.302(2)°, sp. gr. I2₁/c, Z = 4, ??_calcd = 3.295(1) g/cm³), obtained by hydrothermal synthesis, have been refined using X-ray diffraction (Bruker Smart diffractometer, T = 100 K). Hydrogen atoms are located, and their coordinates and thermal parameters are refined. The geometric characteristics of hydrogen bonds are calculated and their crystallochemical function is analyzed. The topological and genetic relationships of the vanadyl phosphates under study with the compounds of derivative structural types are revealed.     

Crystal structure of magnesioferrikatophorite

The crystal structure of the Na,Ca-amphibole magnesioferrikatophorite found in carbonatites from the Turiy Cape (Kola Peninsula) was refined (Siemens P4 diffractometer, λMoK _α radiation, 1481 independent reflections with |F|>4σ(F), anisotropic refinement, R(F) = 0.039). The parameters of the monoclinic unit cell are a = 9.875(5) Å, b = 18.010(8) Å, c = 5.309(3) Å, β = 104.39(5)°, sp. gr. C2/m, Z = 2. The distribution of the cations over the crystallographically nonequivalent M(1–4)-positions was revealed by Mössbauer spectroscopy and X-ray diffraction analysis. The character of splitting of the A-position correlates with the characteristic features of the magnesioferrikatophorite composition. The resulting structural formula (Na_0.87K_0.13)_{Σ = 1} · (Na_1.18Ca_0.82)_{Σ = 2}(Mg_1.41Fe _0.42 ³⁺ Ti _0.17 ⁴⁺ )_{Σ= 2} Fe _1.31 ³⁺ Mg_0.69)_{Σ = 2}(Mg_0.60Fe _0.38 ²⁺ Mn_0.02)_{Σ = 1}(Si_3.16Al_0.84)_{Σ = 4} · Si₄O₂₂(O_1.05OH_0.66F_0.29)_{Σ= 2} agrees well with the electron microprobe analysis data. Based on the zonal character of the crystal and high Fe ³⁺ content, the conditions of crystallogenesis are defined as oxidative against the background of a decrease in the Na potential in the course of the evolution of a mineral-forming system.     

Crystal structure of new synthetic Ca,Na carbonate-borate Ca2Na(Na x Ca0.5 ? x )[B 3 t B 2 δ O8(OH)(O1 ? x OH x )](CO3)

New Ca,Na carbonate-borate Ca₂Na(Na _x Ca_{0.5 ? x} ) [B ₃ ^t B ₂ ^?? O₈(OH)(O_{1 ? x} OH _x )](CO₃) crystals (x ?? 0.4) have been synthesized by the hydrothermal method in the Ca(OH)₂-H₃BO₃-Na₂CO₃-NaCl-system at t = 250°C and P = 70?C80 atm; the structure parameters are found to be a = 11.1848(3) ?, b = 6.4727(2) ?, c = 25.8181(7) ?, ?? = 96.364(3)°, V = 1857.60(9) ?³, sp. gr. C2/c, Z = 8, and ??_calcd = 2.801 g/cm³ (Xcalibur S autodiffractometer (CCD), 2663 reflections with I > 2?? (I), direct solution, refinement by the least-squares method in the anisotropic approximation of thermal atomic vibrations, hydrogen localization, R ₁ = 0.0387). The structure is based on boron-oxygen layers of pentaborate radicals 5(2?? + 3T). Ca and Na polyhedra and CO₃ triangles are located between the layers. A crystallochemical analysis of the new Ca,Na carbonate-borate has established its similarity to natural Na,Ca pentaborates (heidornite and tuzlaite) and synthetic Na,Ba-decaborate.     

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 (Mg_0.5)(Mg_1.0)(Mg_0.338Fe _0.162 ²⁺ )(Fe _0.47 ³⁺ Ti _0.21 ⁴⁺ Mg _0.15 ²⁺ Al _0.10 ³⁺ Fe _0.07 ²⁺ (BO₃)O₂. Highly charged titanium ions in the M4 position are balanced mainly with magnesium and not with divalent iron ions.     

Hole defects in the crystal structure of synthetic lipscombite (Fe 4.7 3+ Fe 2.3 2+ )[PO4]4O2.7(OH)1.3 and genetic crystal chemistry of minerals of the lipscombite-barbosalite series

The crystal structure of a synthetic analog of the mineral lipscombite (Fe _2.3 ²⁺ Fe _4.7 ³⁺ )[PO₄]₄O_2.7(OH)_1.3 obtained under hydrothermal conditions in the LiF-Fe₂O₃-(NH₄)₂HPO₄-H₂O system is resolved (R = 0.040) by X-ray diffraction analysis (Bruker Smart diffractometer with a highly sensitive CCD detector, MoK _α radiation): a = 14.776(3) Å, b = 14.959(3) Å, c = 7.394(1) Å, β = 119.188(4)°, sp. gr. C2/c, Z = 4, ρ_exp = 3.8 g/cm³, ρ_calcd = 3.9 g/cm³. Fe²⁺ and Fe³⁺ cations are statistically distributed in each of four crystallographically independent positions, while occupying the corresponding octahedra with probabilities of 60, 90, 100, and 91%. The ratio Fe²⁺/Fe³⁺ in the composition of the crystals was established by Mössbauer spectroscopy. Lipscombite is interpreted as a mineral of variable composition described by the formula (Fe _x ²⁺ Fe _n?x ³⁺ )[PO₄]₄O_y(OH)_4?y . The field of stability is determined as a function of the iron content and the ratio Fe²⁺/Fe³⁺. It is shown that at n = 6 iron cations are ordered in octahedra and barbosalite structure is formed. An interpretation of genetically and structurally related members of the lipscombite family within a unified polysomatic series is proposed.     

Crystal structure of the mineral (Na,Ca,K)2(Ca,Na)4(Mg,Fe)5(Mg,Fe,Ti)5[Si12Al4O44](F,O)4: a triclinic representative of the amphibole family

A mineral belonging to the amphibole family found at the Rothenberg paleovolcano (Eifel, Germany) was studied by single-crystal X-ray diffraction. The triclinic pseudomonoclinic unit-cell parameters are a = 5.3113(1) ?, b = 18.0457(3) ?; c = 9.8684(2) ?, ?? = 90.016(2)°, ?? = 105.543(4)°, ?? = 89.985(2)°. The structure was solved by direct methods in sp. gr. P1 and refined to the R factor of 2.7% based on 6432 reflections with |F| > 3??(F) taking into account twinning. The mineral with the idealized formula (Na,Ca,K)₂(Ca,Na)₄(Mg,Fe)₅(Mg,Fe,Ti)₅[Si₁₂Al₄O₄₄](F,O)₄ has some symmetry and structural features that distinguish it from other minerals of this family.     

Crystal structure of britvinite [Pb7(OH)3F(BO3)2(CO3)][Mg4.5(OH)3(Si5O14)]: A new layered silicate with an original type of silicon-oxygen networks

The crystal structure of a new mineral britvinite Pb_7.1Mg_4.5(Si_4.8Al_0.2O₁₄)(BO₃)(CO₃)[(BO₃)_0.7(SiO₄)_0.3](OH, F)_6.7 from the Lángban iron-manganese skarn deposit (V?rmland, Sweden) is determined at T = 173 K using X-ray diffraction (Stoe IPDS diffractometer, λMoKα, graphite monochromator, 2θ_max = 58.43°, R = 0.052 for 6262 reflections). The main crystal data are as follows: a = 9.3409(8) ?, b = 9.3579(7) ?, c = 18.8333(14) ?, α = 80.365(6)°, β = 75.816(6)°, γ = 59.870(5)°, V = 1378.7(2) ?³, space group P1, Z = 2, and ρ_calcd = 5.42 g/cm³. The idealized structural formula of the mineral is represented as [Pb₇(OH)₃F(BO₃)₂(CO₃)][Mg_4.5(OH)₃(Si₅O₁₄)]. It is demonstrated that the mineral britvinite is a new representative of the group of mica-like layered silicates with structures in which three-layer (2: 1) “sandwiches” composed of tetrahedra and octahedra alternate with blocks of other compositions, such as oxide, oxide-carbonate, oxide-carbonate-sulfate, and other blocks. The tetrahedral networks (Si₅O₁₄)_∞∞ consisting of twelve-membered rings are fragments of the britvinite structure. Similar networks also form crystal structures of the mineral zeophyllite and the synthetic phase Rb₆Si₁₀O₂₃. In the crystal structures under consideration, the tetrahedral networks differ in the rotation of tetrahedra with respect to the layer plane. Original Russian Text ? O.V. Yakubovich, W. Massa, N.V. Chukanov, 2008, published in Kristallografiya, 2008, Vol. 53, No. 2, pp. 233–242.     

Synthesis and structures of Na4P2Se6, Cs3PSe4, and Rb4P2Se9

Na₄P₂Se₆ (1) crystallizes in the orthorhombic space group Cmca (No. 64) with a = 11.836(3) ?, b = 13.311(4) ?, c = 8.061(2) ?, V = 1270.0(6) ?³, Z = 4, and, D _c = 3.283 g/cm³. Na₄P₂Se₆ belongs to the family of compounds with the general formula where A = Na⁺ and Q = Se²⁻. The crystal structure consists of isolated ethane-like P₂Se anions surrounded by Na⁺ cations. Cs₃PSe₄ (2) crystallizes in the orthorhombic space group Pnma (No. 62) with a = 10.0146(9) ?, b = 11.9899(10) ?, c = 9.9286(9) ?, V = 1192.17(18) ?3, Z = 4, and, Dc = 4.154 g/cm³. Cs₃PSe₄ belongs to the family of compounds with the general formula [(A^{+ x} ) _{z / x} (P ^v Q₄)^{− z} ] where A = Cs⁺ and Q = Se²⁻. The crystal structure consists of isolated methane-like PSe anions surrounded by Cs⁺ cations. Rb₄P₂Se₉ (3) crystallizes in the monoclinic space group C2/c (No. 15), a = 9.725(2) ?, b = 10.468(3) ?, c = 19.155(5) ?, β = 93.627(5)°, V = 1946.1(8) ?³, Z = 4, D _c = 3.804 g/cm³. Rb₄P₂Se₉ belongs to the family of compounds with the general formula [(A^{+ x} )_{(2 z −2)/ x} (Q₃PQ′—Q″—Q′PQ₃)^{−(2 z −2)}] where A = Cs⁺ and Q = Se²⁻, Q′ = Se¹⁻, Q″ = Se⁰. The crystal structure consists of isolated P₂Se anions surrounded by Rb⁺ cations.     

Distribution of Cations and the Proton Location in Kaersutite

The crystal of Kaersutite, (Na_0.63K_0.37Ca_1.99) (Mg_3.10Fe_0.98Al_0.51Mn_0.01Ti₀₅₅) (Si_6.07Al_1.93) (O, OH)₂₄ from lecality Vlčí Hora – Bohemia was studied by powder neutron diffraction. The refinement of the crystal structure within space group C2/m gave cell dimensions a = 0.9863, b = 1.8040 and c = 0.5306 nm, β = 105° 28′, Z = 2. Protons were found to form bridges between two 03 ioxygens. (This location allows for the existence of proton per f.u. in maximum). The actual population was refined to 0.8. The occupation of the five eation sites was determined as follows: M 1 (0.60 Mg + 0.25 Ti + 0.10 Al + 0.05 Fe), M 2 (0.50 Mg + 0.35 Fe + 0.15 Al), M 3 (0.8 Mg + p.2 Fe), M 4 (Ca) and A (Na, K).     

Direct and outer-sphere coordination of the magnesium ions in the crystal structures of complexes with 2-furancarboxylic acid (I) and 3-furancarboxylic acid (II)

(I) Mg₂C₂₀H₂₄O₁₈ monoclinic,PT,a=10.760(2) Å,b=11.052(2) Å,c=12.822(3) Å, α=105.31(3)^o, β=98.18(3)^o, γ=91.59(3)^o,Z=2. (II) MgC₁₀H₁₄O₁₀, monoclinic,C2/c,a=30.817(6)Å,b=10.499(2)Å,c=9.000(2)Å, β=91.31(3)^o,Z=8. Magnesium in complexes with furoic acids reveals two ways of coordination: direct, when furoic anions are bonded to Mg²⁺ in an ionic fashion and outer-sphere, when cations bind water in the first coordination sphere and furancarboxylic ligands are hydrogen bonded to the water molecules. This results in the formation of three bridging systems: ?Mg?O_carboxyl?C?O_carboxyl?Mg?, ?Mg?O_water ?O_carboxyl?C?O_carboxyl?C?O_carboxyl?Mg?, and ?Mg?O_water?O_carboxyl?C?O_carboxyl?O_water?Mg?. Magnesium 2-furancarboxylate (I) is dimeric, while magnesium 3-furancarboxylate (II) exhibits a polymeric structure.     

Molecular and crystal structure of tris-(2-hydroxyethyl)ammonium 4-chlorophenoxy acetate

Tris-(2-hydroxyethyl)ammonium 4-chlorophenoxy acetate has been synthesized and characterized by ¹H NMR spectra. The molecular structure of this compound has been established by single-crystal X-ray diffractometry (a = 7.5298(23) ?, b = 22.8112(46) ?, c = 10.0921(16) ?, ?? = 111.42(2)°, V = 1613.7(7) ?³, sp. gr. P2₁/c, d _calcd = 1.382 g/cm³). In a cation, the hydrogen atom, in the presence of a nitrogen atom, forms three intramolecular hydrogen bonds with the O atoms of hydroxyl groups [2.29(3)?C2.31(3) ?]. At the same time, all three cation hydroxyl groups are involved in the formation of three intermolecular H bonds with oxygen atoms of the carboxyl anion group [H-O = 1.81(5)?2.06(4) ?].     

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: (Ca_0.12?_0.88)(Al_1.69Ni _0.81 ²⁺ Fe _0.50 ²⁺ )(Al_5.40Fe _0.60 ³⁺ )(Si_5.82Al_0.18O₁₈)(BO₃)₃(OH)_3.25O_0.75 I, a = 15.897(5), c = 7.145(2) Å, V = 1564(1) Å; Na_0.91(Ni _1.20 ²⁺ Cr _0.96 ³⁺ Al_0.63Fe _0.18 ²⁺ Mg_0.03)(Al_4.26Ni _1.20 ²⁺ Cr _0.48 ³⁺ Ti_0.06)(Si_5.82Al_0.18)O₁₈(BO₃)₃(OH)_3.73O_0.27 II, a = 15.945(5), c = 7.208(2) Å, V = 1587(1) ų and Na_0.35(Al_1.80Co _1.20 ²⁺ )(Al_5.28Co _0.66 ²⁺ Ti_0.06)(Si_5.64B_0.36)O₁₈(BO₃)₃(OH)_3.81O_0.19 III, a = 15.753(8), c = 7.053(3) Å, V = 1516(2) ų. The reliability factors are R ₁ = 0.038?0.057 and wR ₂ = 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.     

Alkali metals in beryl and their role in the formation of derivative structural motifs: Comparative crystal chemistry of vorobyevite and pezzottaite

The crystal structures of high-alkali beryl, i.e., vorobyevite Cs_0.08Na_0.42(H₂O)_{0.18 + y} × [Al₂(Be_2.35Li_0.65)Si₆O₁₈], (a = 9.2102(14) Å, c = 9.2179(14) Å, space group P6/mcc, Z = 2, ρ_calcd= 2.74 g/cm³) and pezzottaite Cs_0.75Na_0.23(H₂O)_0.24[Al₂Be₂Li(Si₆O₁₈)] (a = 15.955(3) Å, c = 27.810(8) Å, space group, R $ \bar 3 The crystal structures of high-alkali beryl, i.e., vorobyevite Cs_0.08Na_0.42(H₂O)_{0.18 + y} × [Al₂(Be_2.35Li_0.65)Si₆O₁₈], (a = 9.2102(14) ?, c = 9.2179(14) ?, space group P6/mcc, Z = 2, ρ_calcd= 2.74 g/cm³) and pezzottaite Cs_0.75Na_0.23(H₂O)_0.24[Al₂Be₂Li(Si₆O₁₈)] (a = 15.955(3) ?, c = 27.810(8) ?, space group, R c, Z = 18, ρ_calcd= 3.13 g/cm³), are determined at a temperature of 100 K. It is confirmed that, at a high lithium content in minerals of the beryl group, lithium is selectively incorporated into Be tetrahedra. The positive charge deficit due to the replacement of Be²⁺ cations by Li⁺ cations is compensated by incorporating large alkali cations into the “zeolite” channel. It is shown that, when the lithium content becomes close to unity per the corresponding formula, the Li and Be atoms are ordered and the rhombohedral structure of pezzottaite is formed. It is proposed to retain the historical name vorobyevite for the lithium- and cesium-containing variety of beryl with a disordered distribution of Be and Li atoms. Original Russian Text ? O.V. Yakubovich, I.V. Pekov, I.M. Steele, W. Massa, N.V. Chukanov, 2009, published in Kristallografiya, 2009, Vol. 54, No. 3, pp. 432–445.     

Crystal structure of tveitite-(Y): Fractionation of rare-earth elements between positions and the variety of defects

The crystal structure of the mineral tveitite-(Y) (Y_0.883Na_0.106) (Ca_0.841 LREE _0.159)(Ca_0.716Na_0.204 HREE _0.080)(Ca_0.092Na_0.074)F_6.952 from amazonite pegmatites of Rov-Gora Mountain (Keivy, Kola Peninsula) is determined using X-ray diffraction (Stoe IPDS diffractometer, λMoK _α, graphite monochromator, 2θ_max = 63.5°, R = 0.051 for 1542 reflections). The main crystal data are as follows: a = 17.020(2) Å, c = 9.679(2) Å, V = 2428.2(4) ų, space group $R\bar 3$ , Z = 18, and ρ_calcd = 4.00 g/cm³. The idealized structural formula of the mineral is represented as (Y,Na)₆(Ca,LREE)₆(Ca,Na,HREE)₆(Ca,Na)F₄₂ (Z = 3). The defect structure of the mineral manifests itself in a mixed occupation of all four independent cation positions and in a randomly disordered distribution of fluorine atoms over the majority of anion positions. It is shown that the crystal structure of tveitite-(Y) fulfills the function of an “Eratosthenes sieve” for yttrium cations and two groups of lanthanide cations, so that these cations are distributed over three different positions.     

Investigation of the crystallization features,atomic structure,and microstructure of chromium-doped monticellite

A series of Cr⁴⁺:CaMgSiO₄ single crystals is grown using floating zone melting, and their microstructure, composition, and crystal structure are investigated. It is shown that regions with inclusions of second phases, such as forsterite, akermanite, MgO, and Ca₄Mg₂Si₃O₁₂, can form over the length of the sample. The composition of the single-phase regions of the single crystals varies from the stoichiometric monticellite CaMgSiO₄ to the solid solution Ca_{(1 ? x)}Mg_{(1 + x)}SiO₄(x = 0.22). The Cr:(Ca_0.88Mg_0.12)MgSiO₄ crystal is studied using X-ray diffraction. It is revealed that, in this case, the olivine-like orthorhombic crystal lattice is distorted to the monoclinic lattice with the parameters a = 6.3574(5) Å, b = 4.8164(4) Å, c = 11.0387(8) Å, β = 90.30(1)^o, Z = 4, V = 337.98 ų, and space group P2₁/c. In the monoclinic lattice, the M(1) position of the initial olivine structure is split into two nonequivalent positions with the center of symmetry, which are occupied only by Mg²⁺ cations with the average length of the Mg-O bond R _av = 2.128 Å. The overstoichiometric Mg²⁺ cations partially replace Ca²⁺ cations (in the M(2) position of the orthorhombic prastructure) with the average bond length of 2.347 Å in the [(Ca,Mg)-O₆] octahedron. The average distance in SiO₄ distorted tetrahedra is 1.541 Å.     

Crystal structure of the NaCa(Fe<Superscript>2+</Superscript>, Al,Mn)<Subscript>5</Subscript>[Si<Subscript>8</Subscript>O<Subscript>19</Subscript>(OH)](OH)<Subscript>7</Subscript> · 5H<Subscript>2</Subscript>O mineral: A new representative of the palygorskite group

A specimen of a new representative of the palygorskite-sepiolite family from Aris phonolite (Namibia) is studied by single-crystal X-ray diffraction. The parameters of the triclinic (pseudomonoclinic) unit cell are as follows: a = 5.2527(2) Å, b = 17.901(1) Å, c = 13.727(1) Å, α = 90.018(3)°, β = 97.278(4)°, and γ = 89.952(3)°. The structure is solved by the direct methods in space group P \(\bar 1\) and refined to R = 5.5% for 4168 |F| > 7σ(F) with consideration for twinning by the plane perpendicular to y (the ratio of the twin components is 0.52: 0.48). The crystal chemical formula (Z = 1) is (Na_1.6K_0.2Ca_0.2)[Ca₂(Fe _3.6 ²⁺ Al_1.6Mn_0.8)(OH)₉(H₂O)₂][(Fe _3.9 ²⁺ Ti_0.1)(OH)₅(H₂O)₂][Si₁₆O₃₈(OH)₂] · 6H₂O, where the compositions of two ribbons of octahedra and a layer of Si tetrahedra are enclosed in brackets. A number of specific chemical, symmetrical, and structural features distinguish this mineral from other minerals of this family, in particular, from tuperssuatsiaite and kalifersite, which are iron-containing representatives with close unit cell parameters.     

Crystal structure of the Na-, Ca-, Be-cordierite and crystallochemical regularities in the cordierite—sekaninaite series

The crystal structure of the beryllium-rich cordierite [(Na_0.28Ca_0.19)(H₂O)_0.92](Mg_1.86Fe _0.14 ^22? ) · { Al_3.39Be_0.61Si₅O₁₈} was established by X-ray diffraction analysis (IPDS Stoe diffractometer, λMoKα radiation, graphite monochromator, 2θ_max = 65°, R = 0.0186 for 1378 reflections): a = 16.850(3), b = 9.729(2), and c = 9.298(2) Å; V = 1524.3(4) ų, sp. gr. Cccm, Z = 4, and ρ_calcd = 2.655 g/cm³. The numbers of Mg and Fe atoms isomorphously occupying the octahedral positions, Al and Be atoms in tetrahedra, and Na and Ca atoms and H₂O molecules in the framework cavities are refined. The hydrogen atom is localized and its coordinates and thermal displacement parameters are refined. Based on the comparative crystallochemical analysis of cordierites from different locations and origins, the regularities relating the unit-cell parameters b and c and the distortion index to the features of the chemical composition of the minerals under study are shown.