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 structures of two ancylite modifications

The crystal structures of two ancylite specimens from Khibiny massif (the Kola Peninsula, Russia)—ancylite-(Ce) from alkali hydrothermalites (Sr_1.01Ca_0.02Ba_0.01)_Σ1.04(Ce_0.52La_0.28Nd_0.11Pr_0.04 Sm_0.01)_Σ0.96(CO₃)₂(OH_0.83F_0.13)_Σ0.96 · 0.9H₂O and ancylite-(Ce) from carbonatites—have (Sr_0.80Ca_0.05Ba_0.01)_Σ0.86(Ce_0.62La_0.40Nd_0.09Pr_0.03) _Σ1.14(CO₃)₂(OH_0.99F_0.15)_Σ1.14 · 1.0H₂O been refined by the Rietveld method. A focusing STOE-STADIP diffractometer with a bent Ge(111) primary monochromator was used (λ MoK _{α 1} radiation, 2.16° < 2θ < 54.98°; reflection number 237–437). All the computations for ancylite from alkali hydrothermalites were performed within the sp. gr. Pmc2₁, a = 5.0634(1) Å, b = 8.5898(1) Å, c = 7.2781(1) Å, V = 316.55(1) ų, R _wp = 1.90; the computations for ancylite from carbonatites were performed within the sp. gr. Pmcn, a = 5.0577(1) Å, b = 8.5665(2) Å, c = 7.3151(2) Å, V = 316.94(1) ų, R _wp = 2.38 in the anisotropic approximation of thermal vibrations of cations and oxygen atoms.     

Crystal structures and luminescence properties of novel compounds K4M2[Al2Si8O24] (M = Ce, Gd)

Two novel potassium rare earth silicates, obtained by hydrothermal synthesis, have been investigated by X-ray diffraction and described by the general formula K₄ M ₂[Al₂Si₈O₂₄] (M = Ce, Gd). The parameters of the monoclinic K₄Ce₂[Al₂Si₈O₂₄] and K₄Gd₂[Al₂Si₈O₂₄] cells are, respectively, as follows: a = 26.867(1), 26.6520(2) Å; b = 7.4150(2), 7.2854(1) Å; c = 14.910(1), 14.8182(1) Å; β = 123.52(1)°, 123.46(1)°; and sp. gr. P2₁/n. The structures are solved by the charge flipping method and refined in the anisotropic approximation of thermal vibrations for atoms to R = 5.2 and 2.5%, respectively. The compounds under study are crystallized into a new structural type, which is based on two-level [Al₂Si₈O₂₄] layers, combined into a three-dimensional framework by columns of edge-sharing seven-vertex REE polyhedra. Potassium atoms are located in the framework channels. The spectral luminescence characteristics are determined. The luminescence bands are typical of Gd³⁺ and Ce³⁺ ions. Upon excitation by light with λ _exc = 246 nm, a band due to the ² D → ⁸ F _5/2 transition with λ_max = 430 nm is observed in the spectrum of K₄Ce₂[Al₂Si₈O₂₄] and a band related to the ⁶ P _7/2 → ⁸ S _7/2 transition with λ_max = 311 nm is observed in the spectrum of K₄Gd₂[Al₂Si₈O₂₄].     

Refinement of the crystal structure of calcioancylite-(Ce) by the Rietveld method

The crystal structure of calcioancylite-(Ce) of the (Ca_0.30Sr_0.22)_Σ0.52(Ce_0.78La_0.47Nd_0.16Pr_0.05Sm_0.02)_Σ1.48(CO₃)₂((OH)_1.20F_0.28)_Σ1.48 · 1.97H₂O composition from alkali hydrothermalites of Mont Saint-Hilaire, Canada, has been refined by the Rietveld method. The refinement details are as follows: ADP-2 diffractometer, λCuK _α radiation, Ni filter, 10.50° < 2θ < 140.00°, and the number of reflections (α₁ + α₂) 652. All calculations have been performed within the sp. gr. Pmcn (a = 5.0095(1) Å, b = 8.5006(1) Å, c = 7.2670(1) Å, V = 309.46(1) ų, R _wp = 3.45) in the anisotropic approximation of thermal vibrations for cations.     

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.     

X-ray diffraction study of 2,2′,4,4′,6,8,8′,10,10′,12-decamethylbicyclo[5.5.1]hexasiloxane and 2,2′,4,4′,6,8,8′,10,10′,12-decamethylbicyclo[5.5.1]-9-carbahexasiloxane

The crystal structures of two organosilicon compounds are studied by X-ray diffraction at 200 K. Crystals of 2,2′,4,4′,6,8,8′,10,10′,12-decamethylbicyclo[5.5.1]hexasiloxane (C₁₀H₃₀O₇Si₆) are monoclinic, a = 12.260(1) Å, b = 11.716(1) Å, c = 32.169(2) Å, β = 92.802(2)°, space group C2/c, Z = 8, wR2 = 0.132, and R1 = 0.053 for 3989 reflections with F > 4σ(F). Crystals of 2,2′,4,4′,6,8,8′,10,10′, 12-decamethylbicyclo[5.5.1]-9-carbahexasiloxane (C₁₁H₃₂O₆Si₆) are triclinic, a = 10.709(1) Å, b = 11.046(1) Å, c = 12.111(1) Å, α = 117.059(1)°, β = 95.566(1)°, γ = 108.088(1)°, space group $P\bar 1$ , Z = 2, wR2 = 0.115, and R1 = 0.048 for 3948 reflections with F > 4σ(F). It is found that the substitution of the methylene group for the oxygen atom is accompanied by a minor conformational change in the bicyclic fragment and the SiCSi angle changes from the tetrahedral angle to 121.0(1)°. The high variability of the SiOSi bond angles is confirmed.     

Mixed tetrahedral anionic framework in the K3Ga2(PO4)3 crystal structure

The crystal structure of a new synthetic potassium gallophosphate K₃Ga₂(PO₄)₃ grown from a solution in the melt of a mixture of GaPO₄ and K₂MoO₄ is determined using X-ray diffraction (Bruker Smart diffractometer, 2θ_max= 56.6°, R = 0.044 for 2931 reflections, T = 100 K). The main crystal data are as follows: a = 8.661(2) Å, b = 17.002(4) Å, c = 8.386(2) Å, space group Pna2₁, Z= 4, and ρ_calcd = 2.91 g/cm³. The synthesized crystals represent the third phase in the structure type previously established for the K₃Al₂[(As,P)O₄]₃ compound. It is shown that the structure consists of a three-dimensional anionic microporous tetrahedral framework of the mixed type, which is formed by PO₄ and GaO₄ tetrahedra shared by vertices. Large-sized cations K⁺ occupy channels of the zeolite-like framework. The crystal chemical features of the formation of structure types of compounds with mixed frameworks described by the general formula A ₃ ⁺ M ₂ ³⁺ (TO₄)₃ (where A = K, Rb, (NH₄), Tl; M = Al, Ga, Fe, Sc, Yb; T = P, As) are analyzed.     

Refined crystal structure of parakeldyshite and the genetic crystal chemistry of zirconium minerals with [Si<Subscript>2</Subscript>O<Subscript>7</Subscript>] diorthogroups

The structure of the mineral parakeldyshite Na_1.93ZrSi₂O_6.93(OH)_0.07 is refined by X-ray diffraction analysis. The main crystallographic data are as follows: space group P \(\overline 1 \), a = 6.617(2) Å, b = 8.813(1) Å, c = 5.426(1) Å, α = 87.26(3)°, β = 85.68(3)°, γ = 71.45(3)°, and R _F = 0.0153. The initial structural model of this mineral is confirmed. Within this model, the structure of parakeldyshite is based on the heteropolyhedral framework formed by [Si₂O₇] diorthogroups, which are linked together through isolated zirconium octahedra. The fundamental difference between the structure under investigation and the initial structural model is associated with the arrangement of the extraframework cations. A comparative crystal chemical analysis of the zirconium silicates with [Si₂O₇] diorthogroups is performed.     

Iron-rich schüllerite from Kahlenberg (Eifel,Germany): Crystal structure and relation to lamprophyllite-group minerals

A single-crystal sample of iron-rich schüllerite found at the Kahlenberg quarry in the Eifel paleovolcanic field (Germany) was studied by X-ray diffraction. The triclinic unit-cell parameters are as follows: a = 5.4061(1) Å, b = 7.0416(6) Å, c = 10.2077(7) Å, α = 99.86(1)°, β = 97.8(1)°, and γ = 89.98(1)°. The structure was solved by direct methods in sp. gr. P1 and refined to the R factor of 7.9% based on 4321 |F| > 6σ(F). The idealized formula is Ba₂Na(Ca,Mn)(Fe²⁺,Fe³⁺)MgTi₂[Si₂O₇]₂O₂(O,F)F. The new mineral differs from schüllerite by a lower sodium content and higher iron and calcium contents and is characterized by some distinguishing structural features. The dependence of the topology of layered HOH modules on the sodium content in schüllerite and lamprophyllite-group minerals and the character of regular intergrowths of these minerals are discussed.     

The crystal structure of vyuntspakhite: A redetermination

The crystal structure of the mineral vyuntspakhite (Y, TR)₆{Al₂(OH)₃[H_1.48Si_1.88O₇][SiO₄][SiO₃(OH)]}₂(a = 5.7551(11) Å, b = 14.752(3) Å, c = 15.906(4) Å, β = 96.046(4)°, sp. gr. P2₁/n, Z = 2), which had been established earlier in the pseudo-unit cell, is redetermined by X-ray diffraction (R = 0.040, T = 100 K). The redetermination of the structure shows that pronounced pseudotranslation along the axis c′ = c/3 is associated with the fact that Y(TR) atoms are related by a 1/3 translation along the [001] direction. Most of the hydrogen atoms are located. The crystal-chemical function of hydrogen bonds is analyzed. In the unit cell of vyuntspakhite, the cationic layers consisting of edge-sharing (Y,TR) eight-vertex polyhedra alternate along the b axis with mixed anionic layers composed of isolated Si tetrahedra (orthotetrahedra), Si₂O₇ double-tetrahedra (diortho) groups, Al five-vertex polyhedra, and Al₂O₈ double-tetrahedra groups linked by shared vertices and through hydrogen bonding.     

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 low-symmetry rondorfite

The crystal structure of an aluminum-rich variety of the mineral rondorfite with the composition Ca₁₆[Mg₂(Si₇Al)(O₃₁OH)]Cl₄ from the skarns of the Verkhne-Chegemskoe plateau (the Kabardino-Balkarian Republic, the Northern Caucasus Region, Russia) was solved in the triclinic space group with the unit-cell parameters a = 15.100(2) Å, b = 15.110(2) Å, c = 15.092(2) Å, α = 90.06(1)°, β = 90.01(1)°, γ = 89.93(1)°, Z = 4, sp. gr. P1. The structural model consisting of 248 independent atoms was determined by the phase-correction method and refined to R = 3.8% with anisotropic displacement parameters based on all 7156 independent reflections with 7156 F > 3σ(F). The crystal structure is based on pentamers consisting of four Si tetrahedra linked by the central Mg tetrahedron. The structure can formally be refined in the cubic space group (a = 15.105 Å, sp. gr. Fd \(\overline 3 \), seven independent positions) with anisotropic displacement parameters to R = 2.74% based on 579 reflections with F > 3σ(F) without accounting for more than 1000 observed reflections, which are inconsistent with the cubic symmetry of the crystal structure.     

X-ray diffraction investigation of trans-2,8-dihydroxy-2,8-diphenyl-4,4′,6,6′,10,10′, 12,12′-octamethylcyclohexasiloxane and trans-2,8-dihydroxy-2,4,4′,6,6′,8,10,10′, 12,12′-decamethyl-5-carbahexacyclosiloxane

The crystal structures of two organosilicon compounds are studied by X-ray diffraction. Crystals of trans-2,8-dihydroxy-2,8-diphenyl-4,4′,6,6′,10,10′,12,12′-octamethylcyclohexasiloxane, C₂₀H₃₆O₈Si₆, (I) are triclinic; at 110 K, a = 11.476(1) Å, b = 12.106(1) Å, c = 13.636(1) Å, α = 94.337(1)°, β = 112.669(1)°, γ = 112.216(1)°, space group $P\bar 1$ , Z = 2, and R = 0.057 for 5069 reflections with F > 4σ(F). Crystals of trans-2,8-dihydroxy-2,4,4′,6,6′,8,10,10′,12,12′-decamethyl-5-carbahexacyclosiloxane, C₁₁H₃₄O₇ Si₆, (II) are tetragonal; at 293 K, a = b = 15.487(3) Å, c = 11.364(3) Å, space group P4₂/n, Z = 4, and R = 0.055 for 955 reflections with F > 4σ (F). It is found that the structure of compound I, in which the substituents at the Si(1) atom differ in volume and inductive effect, contains two crystallographically independent molecules with different conformations of the hexasiloxane ring. In structure II, the oxygen atom and the methylene group are statistically disordered as a result of the location of the molecule at the center of symmetry. Although the SiCSi angle [122.1(1)°] differs noticeably from the tetrahedral angle, its value is characteristic of cyclohexasiloxanes and is not related to the disorder.     

Crystal structure of chabazite K

The crystal structure of the chabazite K with the formula (K_1.33Na_1.02Ca_0.84)[Al₄Si₈O₂₄] · 12.17H₂O from late hydrothermalites in the Khibiny alkaline massif (Kola Peninsula) is established by X-ray diffraction analysis (CAD4 four-circle diffractometer, λMoK_α radiation, graphite monochromator, T=193 K, 2θ_max = 70°, R₁ = 0.047 for 4745 reflections) on the basis of experimental data (6265 reflections) obtained from a twin (twinning parameter 0.535(1)): a = 13.831(3) Å, c = 15.023(5) Å, sp. gr. \(R\bar 3m\), Z = 3, ρ_calcd = 2.016 g/cm³. It is shown that cations occupy five independent positions in large cavities of the tetrahedral Al,Si,O anionic framework in potassium-rich chabazite. A comparative crystallochemical analysis of chabazites of different composition and origin is performed.     

Crystal structures of low-symmetry cancrinite and cancrisilite varieties

The high-sodium variety of cancrinite [Si_6.3Al_5.7O₂₄][Na₂(H₂O)₂][Na_5.7(CO₃)_0.9(SO₄)_0.1(H₂O)_0.6] (Kovdor Massif, Kola Peninsula, Russia) and the calcium-containing variety of cancrisilite [Si_6.6Al_5.4O₂₄][(Na_1.2Ca_0.4)(H₂O)_1.6][Na₆(CO₃)_1.3(H₂O)_1.2] (Khibiny Massif, Kola Peninsula, Russia) are studied. The trigonal unit cell parameters of the crystal structures under investigation are as follows: a = 12.727(4) Å, c = 5.186(2) Å, and space group P3 for the former mineral and a = 12.607(4) Å, c = 5.111(1) Å, and space group P3 for the latter mineral. The reduced symmetry of the new varieties as compared to the symmetry of typical cancrinite and typical cancrisilite is associated with the specific features in the arrangement of the carbonate groups and water molecules in channels. This inference is confirmed by the IR spectroscopic data.     

Refined structure of afwillite from the northern Baikal region

The crystal structure of the mineral afwillite [Ca₁₂(H₂O)₈][SiO₄]₄[SiO₂(OH)₂]₄ from the northern Baikal region was refined in a triclinic unit cell with the following parameters: a = 16.330(2) Å, b = 5.6389(6) Å, c = 11.685(1) Å, α = 90.08(1)°, β = 126.446(2)°, γ = 89.95(1)°, and sp. gr. P1. The triclinic unit cell is related to the monoclinic unit cell determined earlier (sp. gr. Cc, a = 16.278(1) Å, b = 5.6321(4) Å, c = 13.236(1) Å, and β = 134.898(3)°) by the transformation matrix [?100/010/101]. The structural model was determined using the phase-correction procedure and refined anisotropically to R = 2.8% based on 3270 independent reflections with F > 3σ(F). The crystal-chemical formula of afwillite was revised. The characteristic features of the IR spectrum of afwillite were explained based on the structural data.     

Pyroxenoids of pyroxmangite–pyroxferroite series from xenoliths of Bellerberg paleovolcano (Eifel,Germany): Chemical variations and specific features of cation distribution

The pyroxferroite and pyroxmangite from xenoliths of aluminous gneisses in the alkaline basalts of Bellerberg paleovulcano (Eifel, Germany) have been studied by electron-probe and X-ray diffraction methods and IR spectroscopy. The parameters of the triclinic unit cells are found to be a = 6.662(1) Å, b = 7.525(1) Å, c = 15.895(2) Å, α = 91.548(3)°, β = 96.258(3)°, and γ = 94.498(3)° for pyroxferroite and a = 6.661(3) Å, b = 7.513(3) Å, c = 15.877(7) Å, α = 91.870(7)°, β = 96.369(7)°, and γ = 94.724(7)° for pyroxmangite; sp. gr. \(P\overline 1 \). The crystallochemical formulas (Z = 2) are, respectively, ^M(1–2)(Mn_0.5Ca_0.4Na_0.1)₂^M(3–6)(Fe, Mn)₄^M7[Mg_0.6(Fe, Mn)_0.4][Si₇O₂₁] and ^M(1–3)(Mn, Fe)₃^M(4–6)[(Fe, Mn)_0.7Mg_0.3]₃^M7[Mg_0.5(Fe, Mn)_0.5][Si₇O₂₁]. For these and previously studied representatives of the pyroxmangite structural type, an analysis of the cation distribution over sites indicates wide isomorphism of Mn²⁺, Fe²⁺, and Mg in all cation M(1–7) sites and the preferred incorporation of Сa and Na into large seven-vertex M1O₇ and M2O₇ polyhedra and Mg into the smallest five-vertex M7O₅ polyhedron.     

Average structure of incommensurately modulated monoclinic lazurite

The average structure of the monoclinic modification of lazurite Ca_1.26Na_6.63K_0.04[Al₆Si₆O₂₄](SO₄)_1.53S_0.99Cl_0.05 (discovered in the Lake Baikal region) incommensurately modulated along the c axis is solved by the single-crystal X-ray diffraction method. The unit-cell parameters are a = 9.069(1) Å, b = 12.868(1) Å, c = 12.872(1) Å, γ = 90.19(1)°, sp. gr. Pa, R = 6.9%, 2057 reflections. The AlO₄ and SiO₄ tetrahedra form a partially ordered framework. The positions in the cavities of the framework are split and randomly occupied by Na and Ca atoms and the SO₄, S₂, S₃, and SO₂ anionic groups. The structure of the mineral is compared with the superstructure of triclinic lazurite. Conclusions are drawn about the causes of the incommensurate modulation in monoclinic and other lazurites.     

Crystal structure and properties of K<Subscript>2</Subscript>[OsO<Subscript>2</Subscript>(C<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript>2</Subscript>] · 2H<Subscript>2</Subscript>O

The synthesis and X-ray diffraction study of the compound K₂[OsO₂(C₂O₄)₂] · 2H₂O are performed. The compound crystallizes in the triclinic crystal system, space group P \(\bar 1\), a = 6.545(1) Å, b = 6.835(2) Å, c = 7.595(2) Å, α = 85.76(2)°, β = 65.33(2)°, γ = 71.14(2)°, and Z = 1. The osmium atom is located at the center of symmetry and has a distorted octahedral coordination formed by oxygen atoms: two oxygen atoms of the osmyl group occupy the apical positions [Os-O, 1.730(2) Å], and four oxygen atoms of the oxalate ions lie in the equatorial plane. The K⁺ cation is surrounded by ten oxygen atoms located at different K-O distances in the range from 2.787(2) to 3.158(2) Å. The assignment of the absorption bands in the IR spectrum of K₂[OsO₂(C₂O₄)₂] · 2H₂O is performed. The electronic absorption spectra of the compound are recorded in different solvents, and the thermal behavior in air is studied.     

Crystal structure of Li2MgSiO4

The crystal structure of Li₂MgSiO₄ was established by single-crystal X-ray diffraction analysis. The crystals are monoclinic, a = 4.9924(7) Å, b = 10.681(2) Å, c = 6.2889(5) Å, β = 90.46(1)°, Z = 4, sp. gr. P2₁/n, V = 335.54 ų, R = 0.062. In a Li₂MgSiO₄ crystal, four types of independent T(1–4) tetrahedra share vertices to form a three-dimensional framework. Three of these tetrahedra are occupied simultaneously by Li and Mg cations, which corresponds to the crystallochemical formula (Li_0.98Mg_0.02)(Li_0.80Mg_0.20) · (Li_0.22Mg_0.78)SiO₄. In slightly distorted SiO₄ tetrahedra denoted as T(1), the average Si-O distance is 1.635(2) Å. The distortions of other tetrahedra and the average (Li _x Mg_{1 ? x} )-O distances increase with an increase in lithium content. These distances in the T(2), T(3), and T(4) tetrahedra are 1.955(2), 1.971(4), and 2.019(6) Å, respectively. The structure of the new compound is compared with the crystal structures of other Li₂ M ²⁺SiO₄ compounds and the luminescence spectra of Cr⁴⁺: Li₂MgSiO₄.