Fahlore thermochemistry: Gaps inside the (Cu,Ag)<Subscript>10</Subscript>(Fe,Zn)<Subscript>2</Subscript>(Sb,As)<Subscript>4</Subscript>S<Subscript>13</Subscript> cube

Possible topologies of miscibility gaps in arsenian (Cu,Ag)₁₀(Fe,Zn)₂(Sb,As)₄S₁₃ fahlores are examined. These topologies are based on a thermodynamic model for fahlores whose calibration has been verified for (Cu,Ag)₁₀(Fe,Zn)₂Sb₄S₁₃ fahlores, and conform with experimental constraints on the incompatibility between As and Ag in (Cu,Ag)₁₀(Fe,Zn)₂(Sb,As)₄S₁₃ fahlores, and with experimental and natural constraints on the incompatibility between As and Zn and the nonideality of the As for Sb substitution in Cu₁₀(Fe,Zn)₂(Sb,As)₄S₁₃ fahlores. It is inferred that miscibility gaps in (Cu,Ag)₁₀(Fe,Zn)₂As₄S₁₃ fahlores have critical temperatures several °C below those established for their Sb counterparts (170 to 185°C). Depending on the structural role of Ag in arsenian fahlores, critical temperatures for (Cu,Ag)₁₀(Fe,Zn)₂(Sb,As)₄S₁₃ fahlores may vary from comparable to those inferred for (Cu,Ag)₁₀(Fe,Zn)₂As₄S₁₃ fahlores, if the As for Sb substitution stabilizes Ag in tetrahedral metal sites, to temperatures approaching 370°C, if the As for Sb substitution results in an increase in the site preference of Ag for trigonal-planar metal sites. The latter topology is more likely based on comparison of calculated miscibility gaps with compositions of fahlores from nature exhibiting the greatest departure from the Cu₁₀(Fe,Zn)₂(Sb,As)₄S₁₃ and (Cu,Ag)₁₀(Fe,Zn)₂Sb₄S₁₃ planes of the (Cu,Ag)₁₀(Fe,Zn)₂(Sb,As)₄S₁₃ fahlore cube.     

The crystal structure of alumoklyuchevskite,K<Subscript>3</Subscript>Cu<Subscript>3</Subscript>AlO<Subscript>2</Subscript>(SO<Subscript>4</Subscript>)<Subscript>4</Subscript>

The crystal structure of the unstable mineral alumoklyuchevskite K₃Cu₃AlO₂(SO₄)₄ [monoclinic, I2, a = 18.772(7), b = 4.967(2), c = 18.468(7) Å, β = 101.66(1)°, V = 1686(1) Å] was refined to R ₁ = 0.131 for 2450 unique reflections with F ≥ 4σF _hkl. The structure is based on oxocentered tetrahedrons (OAlCu ₃ ⁷⁺ ) linked into chains via edges. Each chain is surrounded by SO₄ tetrahedrons forming a structural complex. Each complex is elongated along the b axis. This type of crystal structure was also found in other fumarole minerals of the Great Tolbachik Fissure Eruption (GTFE, Kamchatka Peninsula, Russia, 1975–1976), klyuchevskite, K₃Cu₃Fe³⁺O₂(SO₄)₄; and piypite, K₂Cu₂O(SO₄)₂.     

Thermal expansion of new arsenate minerals,bradaczekite, NaCu<Subscript>4</Subscript>(AsO<Subscript>4</Subscript>)<Subscript>3</Subscript>, and urusovite,Cu(AsAlO<Subscript>5</Subscript>)

Thermal behavior of two new exhalation copper-bearing minerals, bradaczekite and urusovite, from the Great Tolbachik Fissure Eruption (1975–1976, Kamchatka Peninsula, Russia) has been studied by X-ray thermal analysis within the range 20–700°C in air. The following major values of the thermal expansion tensor have been calculated for urusovite: α₁₁ = 10, α₂₂ = α_b = 7, α₃₃ = 4, α_V = 21 × 10⁻⁶°C⁻¹, μ = c∧α₃₃ = 49° and bradaczekite: α_11aver = 23, α₂₂ = 8, α_33aver = 6 × 10⁻⁶°C⁻¹, μ(c∧α₃₃) = 73°. The sharp anisotropy of thermal deformations of these minerals, absences of phase transitions, and stability of the minerals in the selected temperature range corresponding to conditions of their formation and alteration during the posteruption period of the volcanic activity are established.     

Crystal chemistry of selenates with mineral-like structures. III. Heteropolyhedral chains in the crystal structure of [Mg(H<Subscript>2</Subscript>O)<Subscript>4</Subscript>(SeO<Subscript>4</Subscript>)]<Subscript>2</Subscript>(H<Subscript>2</Subscript>O)

The crystal structure of a new compound [Mg(H₂O)₄(SeO₄)]₂(H₂O) (monoclinic, P2 ₁/a, a = 7.2549(12), b = 20.059(5), c = 10.3934(17) Å, β = 101.989(13), V = 1479.5(5) ų) has been solved by direct methods and refined to R ₁ = 0.059 for 2577 observed reflections with |F _hkl | ≥ 4σ|F _hkl |. The structure consists of [Mg(H₂O)₄(SeO₄)]⁰ chains formed by alternating corner-sharing Mg octahedrons and (SeO₄)^2? tetrahedrons. O atoms of Mg octahedrons that are shared with selenate tetrahedrons are in a trans orientation. The heteropoly-hedral octahedral-tetrahedral chains are parallel to the c axis and undulate within the (010) plane. The adjacent chains are linked by hydrogen bonds involving H₂O molecules not bound with M²⁺ cations.     

Magnetic properties and neutron diffraction study of two manganese sulfosalts: monoclinic MnSb<Subscript>2</Subscript>S<Subscript>4</Subscript> and benavidesite (MnPb<Subscript>4</Subscript>Sb<Subscript>6</Subscript>S<Subscript>14</Subscript>)

Mn²⁺Sb₂S₄, a monoclinic dimorph of clerite, and benavidesite (Mn²⁺Pb₄Sb₆S₁₄) show well-individualized single chains of manganese atoms in octahedral coordination. Their magnetic structures are presented and compared with those of iron derivatives, berthierite (Fe²⁺Sb₂S₄) and jamesonite (Fe²⁺Pb₄Sb₆S₁₄). Within chains, interactions are antiferromagnetic. Like berthierite, MnSb₂S₄ shows a spiral magnetic structure with an incommensurate 1D propagation vector [0, 0.369, 0], unchanged with temperature. In berthierite, the interactions between identical chains are antiferromagnetic, whereas in MnSb₂S₄ interactions between chains are ferromagnetic along c-axis. Below 6 K, jamesonite and benavidesite have commensurate magnetic structures with the same propagation vector [0.5, 0, 0]: jamesonite is a canted ferromagnet and iron magnetic moments are mainly oriented along the a-axis, whereas for benavidesite, no angle of canting is detected, and manganese magnetic moments are oriented along b-axis. Below 30 K, for both compounds, one-dimensional magnetic ordering or correlations are visible in the neutron diagrams and persist down to 1.4 K.     

Thermodynamic properties,low-temperature heat-capacity anomalies,and single-crystal X-ray refinement of hydronium jarosite, (H<Subscript>3</Subscript>O)Fe<Subscript>3</Subscript>(SO<Subscript>4</Subscript>)<Subscript>2</Subscript>(OH)<Subscript>6</Subscript>

Crystals of hydronium jarosite were synthesized by hydrothermal treatment of Fe(III)–SO₄ solutions. Single-crystal XRD refinement with R₁=0.0232 for the unique observed reflections (|F_o| > 4_F) and wR₂=0.0451 for all data gave a=7.3559(8) Å, c=17.019(3) Å, V^o=160.11(4) cm³, and fractional positions for all atoms except the H in the H₃O groups. The chemical composition of this sample is described by the formula (H₃O)_0.91Fe_2.91(SO₄)₂[(OH)_5.64(H₂O)_0.18]. The enthalpy of formation (H^o_f) is –3694.5 ± 4.6 kJ mol^–1, calculated from acid (5.0 N HCl) solution calorimetry data for hydronium jarosite, -FeOOH, MgO, H₂O, and -MgSO₄. The entropy at standard temperature and pressure (S^o) is 438.9±0.7 J mol^–1 K^–1, calculated from adiabatic and semi-adiabatic calorimetry data. The heat capacity (C_p) data between 273 and 400 K were fitted to a Maier-Kelley polynomial C_p(T in K)=280.6 + 0.6149T–3199700T^–2. The Gibbs free energy of formation is –3162.2 ± 4.6 kJ mol^–1. Speciation and activity calculations for Fe(III)–SO₄ solutions show that these new thermodynamic data reproduce the results of solubility experiments with hydronium jarosite. A spin-glass freezing transition was manifested as a broad anomaly in the C_p data, and as a broad maximum in the zero-field-cooled magnetic susceptibility data at 16.5 K. Another anomaly in C_p, below 0.7 K, has been tentatively attributed to spin cluster tunneling. A set of thermodynamic values for an ideal composition end member (H₃O)Fe₃(SO₄)₂(OH)₆ was estimated: G^o_f= –3226.4 ± 4.6 kJ mol^–1, H^o_f=–3770.2 ± 4.6 kJ mol^–1, S^o=448.2 ± 0.7 J mol^–1 K^–1, C_p (T in K)=287.2 + 0.6281T–3286000T^–2 (between 273 and 400 K).     

Hydrostatic compression of galenobismutite (PbBi<Subscript>2</Subscript>S<Subscript>4</Subscript>): elastic properties and high-pressure crystal chemistry

A single-crystal sample of galenobismutite was subjected to hydrostatic pressures in the range of 0.0001 and 9 GPa at room temperature using the diamond-anvil cell technique. A series of X-ray diffraction intensities were collected at ten distinct pressures using a CCD equipped 4-circle diffractometer. The crystal structure was refined to R1(|F₀| > 4σ) values of approximately 0.05 at all pressures. By fitting a third-order Birch-Murnaghan equation of state to the unit-cell volumes V ₀ = 700.6(2) ų, K ₀ = 43.9(7) GPa and dK/dP = 6.9(3) could be determined for the lattice compression. Both types of cations in galenobismutite have stereochemically active lone electron pairs, which distort the cation polyhedra at room pressure. The cation eccentricities decrease at higher pressure but are still pronounced at 9 GPa. Galenobismutite is isotypic with CaFe₂O₄ (CF) but moves away from the idealised CF-type structure during compression. Instead of the two octahedral cation sites and one bi-capped trigonal-prismatic site, PbBi₂S₄ attains a new high-pressure structure characterised by one octahedral site and two mono-capped trigonal-prismatic sites. Analyses of the crystal structure at high pressure confirm the preference of Bi for the octahedral site and the smaller one of the two trigonal-prismatic sites.     

Zinclipscombite,ZnFe<Stack><Subscript>2</Subscript><Superscript>3+</Superscript></Stack>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>(OH)<Subscript>2</Subscript>, a new mineral species

Zinclipscombite, a new mineral species, has been found together with apophyllite, quartz, barite, jarosite, plumbojarosite, turquoise, and calcite at the Silver Coin mine, Edna Mountains, Valmy, Humboldt County, Nevada, United States. The new mineral forms spheroidal, fibrous segregations; the thickness of the fibers, which extend along the c axis, reaches 20 μm, and the diameter of spherulites is up to 2.5 mm. The color is dark green to brown with a light green to beige streak and a vitreous luster. The mineral is translucent. The Mohs hardness is 5. Zinclipscombite is brittle; cleavage is not observed; fracture is uneven. The density is 3.65(4) g/cm³ measured by hydrostatic weighing and 3.727 g/cm³ calculated from X-ray powder data. The frequencies of absorption bands in the infrared spectrum of zinclipscombite are (cm^?1; the frequencies of the strongest bands are underlined; sh, shoulder; w, weak band) 3535, 3330sh, 3260, 1625w, 1530w, 1068, 1047, 1022, 970sh, 768w, 684w, 609, 502, and 460. The Mössbauer spectrum of zinclipscombite contains only a doublet corresponding to Fe³⁺ with sixfold coordination and a quadrupole splitting of 0.562 mm/s; Fe²⁺ is absent. The mineral is optically uniaxial and positive, ω = 1.755(5), ? = 1.795(5). Zinclipscombite is pleochroic, from bright green to blue-green on X and light greenish brown on Z (X > Z). Chemical composition (electron microprobe, average of five point analyses, wt %): CaO 0.30, ZnO 15.90, Al₂O₃ 4.77, Fe₂O₃ 35.14, P₂O₅ 33.86, As₂O₅ 4.05, H₂O (determined by the Penfield method) 4.94, total 98.96. The empirical formula calculated on the basis of (PO₄,AsO₄)₂ is (Zn_0.76Ca_0.02)_Σ0.78(Fe _1.72 ³⁺ Al_0.36)_Σ2.08[(PO₄)_1.86(AsO₄)_0.14]_Σ2.00(OH)_{1. 80} · 0.17H₂O. The simplified formula is ZnFe ₂ ³⁺ (PO₄)₂(OH)₂. Zinclipscombite is tetragonal, space group P4₃2₁2 or P4₁2₁2; a = 7.242(2) Å, c = 13.125(5) Å, V = 688.4(5) ų, Z = 4. The strongest reflections in the X-ray powder diffraction pattern (d, (I, %) ((hkl)) are 4.79(80)(111), 3.32(100)(113), 3.21(60)(210), 2.602(45)(213), 2.299(40)(214), 2.049(40)(106), 1.663(45)(226), 1.605(50)(421, 108). Zinclipscombite is an analogue of lipscombite, Fe²⁺Fe ₂ ³⁺ (PO₄)₂(OH)₂ (tetragonal), with Zn instead of Fe²⁺. The mineral is named for its chemical composition, the Zn-dominant analogue of lipscombite. The type material of zinclipscombite is deposited in the Mineralogical Collection of the Technische Universität Bergakademie Freiberg, Germany.     

Jonassonite (AuBi<Subscript>5</Subscript>S<Subscript>4</Subscript>) from the Dakripen gold deposit in Vietnam

For the first time in ore deposits of Vietnam, a mineral phase containing Au, Bi, and S as major elements was found in the gold ore of the Dakripen deposit. Pb is also present as a minor isomorphic impurity. Rare irregular or ellipsoid grains of that mineral up to 35 μm in size were identified in polished sections together with pyrite; galena; sphalerite; bismuthinite; ikunolite; native bismuth; and, occasionally, gold. All these species are related to the third stage of the ore formation. In reflected light, the mineral is bluish white, the reflectance is comparable with ikunolite and slightly higher than that of bismuthinite, and weak pleochroism and visible anisotropy are established. The mineral is opaque and brittle without internal reflections. According to 18 microprobe analyses, its average chemical composition and quantitative variations for the major elements are as follows (wt %): 14.02 (13.11–14.58) Au, 76.37 (74.93–76.91) Bi, 0.49 (0.10–1.00) Pb, 9.80 (8.87–10.07) S, and 100.68 in total. The empirical formula calculated for the average element contents—Au_0.96(Bi_4.91Pb_0.03)_4.94S_4.10—is similar to the idealized formula of jonassonite from the Nagybörzsöny deposit (Hungary)—Au(Bi,Pb)₅S₄—approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association. The typical mineral assemblage, optical properties, and chemical composition of this mineral allow us to regard it as a low-Pb variety of jonassonite. It may be assumed that the real formula of jonassonite without sporadic impurities of Pb and other elements must be AuBi₅S₄, as was stated before in the first communication of the Commission on New Minerals and Mineral Names of the International Mineralogical Association and is typical of minerals from Kazakhstan, Russia, Japan, Germany, the Czech Republic, the United States, and Australia.     

Elastic behavior of vanadinite,Pb<Subscript>10</Subscript>(VO<Subscript>4</Subscript>)<Subscript>6</Subscript>Cl<Subscript>2</Subscript>, a microporous non-zeolitic mineral

The high-pressure behavior of a vanadinite (Pb₁₀(VO₄)₆Cl₂, a = b = 10.3254(5), c = 7.3450(4) Å, space group P6₃/m), a natural microporous mineral, has been investigated using in-situ HP-synchrotron X-ray powder diffraction up to 7.67 GPa with a diamond anvil cell under hydrostatic conditions. No phase transition has been observed within the pressure range investigated. Axial and volume isothermal Equations of State (EoS) of vanadinite were determined. Fitting the P–V data with a third-order Birch-Murnaghan (BM) EoS, using the data weighted by the uncertainties in P and V, we obtained: V ₀ = 681(1) Å³, K ₀ = 41(5) GPa, and K′ = 12.5(2.5). The evolution of the lattice constants with P shows a strong anisotropic compression pattern. The axial bulk moduli were calculated with a third-order “linearized” BM-EoS. The EoS parameters are: a ₀ = 10.3302(2) Å, K ₀(a) = 35(2) GPa and K′(a) = 10(1) for the a-axis; c ₀ = 7.3520(3) Å, K ₀(c) = 98(4) GPa, and K′(c) = 9(2) for the c-axis (K ₀(a):K ₀(c) = 1:2.80). Axial and volume Eulerian-finite strain (fe) at different normalized stress (Fe) were calculated. The weighted linear regression through the data points yields the following intercept values: Fe _a (0) = 35(2) GPa for the a-axis, Fe _c (0) = 98(4) GPa for the c-axis and Fe _V (0) = 45(2) GPa for the unit-cell volume. The slope of the regression lines gives rise to K′ values of 10(1) for the a-axis, 9(2) for the c-axis and 11(1) for the unit cell-volume. A comparison between the HP-elastic response of vanadinite and the iso-structural apatite is carried out. The possible reasons of the elastic anisotropy are discussed.     

Equation of state and crystal structure of Sb<Subscript>2</Subscript>S<Subscript>3</Subscript> between 0 and 10 GPa

High-precision unit-cell volume data of stibnite, collected in the pressure range of 0–10 GPa, was used for fitting a third-order Birch–Murnaghan equation of state. The zero-pressure volume, bulk modulus and its pressure derivative were found to be 487.73(6) ų, 26.91(14) GPa and 7.9(1), respectively. A series of X-ray intensity data was collected in the same pressure range using a CCD-equipped Bruker diffractometer. The high-pressure structures were all refined to R1(|F₀|>4) values of approximately 0.03. Crystal-chemical parameters as polyhedron volume, centroid and eccentricity were calculated for the seven coordinated cation positions using the software IVTON. The cation eccentricity appears to be a very useful tool for quantification of the lone electron pair activity. U₂S₃, Dy₂S₃ and Nd₂Te₃ are all isostructural with stibnite, but the cations in these materials have no lone electron pair. Their eccentricity is much smaller than that of Sb, and close to zero. This confirms that the stibnite structure type alone does not force eccentricity upon the cations involved and it is the lone electron pairs of Sb that generate the eccentricity of cation positions in the structures of stibnite. At increasing pressure the eccentricity of Sb is decreasing. It is therefore reasonable to conclude that the lone electron pair activity is decreasing with increasing pressure.     

Ab-initio calculations of the proton location in topaz-OH,Al<Subscript>2</Subscript>SiO<Subscript>4</Subscript>(OH)<Subscript>2</Subscript>

The position of hydrogen in the structure of topaz-OH was determined by means of ab-initio quantum-mechanic calculations. Static lattice energy calculations predict the existence of four non-equivalent positions of protons, which are characterized by O4–H1... O1, O4–H2... O2, O4–H3... O3 and O4–H4... O4 hydrogen bonds. The distribution of the protons between positions of local equilibrium is controlled by the proton–proton avoidance rule and the strength of the hydrogen bonds. The most favourable configuration of hydrogen atoms is achieved for adjacent protons, which form O4–H3... O3 and O4–H4... O4 hydrogen bonds, respectively. The thermal excitation of atoms at a temperature of 55 K is large enough for the hydrogen atoms occasionally to change their positions to form O4–H1... O1 and O4–H2... O2 bonds. At ambient pressures and higher temperatures the protons are in a dynamic exchange between the allowed positions of local minima. As a consequence, for nearly room-temperature conditions, the dynamic change between different structural configurations leads to the violation of all possible symmetry elements and with that to space group #E5/E5#1. The flipping of the protons between different sites is achieved by simple rotation of the OH-dipole and does not produce any significant distortion of the framework of topaz, whose symmetry remains that of the space group Pbnm. Therefore, no reduction of symmetry has been observed in former X-ray studies on topaz-OH. Calculated IR absorption spectra of topaz-OH were found to be in good agreement with measured spectra. According to the calculations, the two favourable configurations of protons might correspond to the measured peak splitting within the OH-stretching range. An experimentally observed low-frequency band at 3520 cm^–1 was assigned to the OH-stretching of the O4–H3... O3 bond, while the band at 3600 cm^–1 was attributed to OH-stretching of the O4–H4... O4 hydrogen bond. The broad peak in FAR-IR frequency range at 100–150 cm^–1 is attributed to the stretching of H3... O3 and H4... O4 contacts. The rate of proton exchange at 670 K among different sites was estimated by ab-inito molecular dynamic simulations. The calculations predict that flipping of adjacent protons between O4–H3... O3 and O4–H4... O4 bonds at 670 K occur at a rate of about 1.96 THz.     

Stability of (Cs,K)Al<Subscript>4</Subscript>Be<Subscript>5</Subscript>B<Subscript>11</Subscript>O<Subscript>28</Subscript> (londonite) at high pressure and high temperature: a potential neutron absorber material

The stability and the thermo-elastic behaviour of a natural londonite

Magnetic phase diagrams of Mg-, Fe-, (Cu + Ti)- and Cu-substituted braunite Mn<Superscript>2+</Superscript>Mn<Subscript>6</Subscript><Superscript>3+</Superscript>SiO<Subscript>12</Subscript>

 The magnetic behavior of the Jahn-Teller structure braunite, (Mn²⁺ _1−yM _y )(Mn³⁺ _{6− x} M_x)SiO₁₂, is strongly influenced by the incorporation of elements substituting manganese. Magnetic properties of well-defined synthetic samples were investigated in dependence on the composition. The final results are presented in magnetic phase diagrams. To derive the necessary data, ac susceptibility and magnetization of braunites with the substitutional elements M = Mg, Fe, (Cu+Ti) and Cu were measured. Whereas the antiferromagnetic ordering temperature, T _N , of pure braunite is hardly affected by the substitution of nonmagnetic Mg, it is rapidly suppressed by the substitution of magnetic atoms at the Mn positions. Typically for a concentration (x, y) ≥ 0.7 of the substituted elements, a spin glass phase occurs in the magnetic phase diagrams. Additionally, for the braunite system with Fe³⁺ substitutions, we observe in the concentration range 0.2 < x< 0.7 a double transition from the paramagnetic state, first to the antiferromagnetic state, followed by a transition to a spin glass state at lower temperatures. The unusual change of the magnetic properties with magnetic substitution at the Mn positions is attributed to the peculiar antiferromagnetic structure of braunite, which has been resolved recently. Received: 19 April 2001 / Accepted: 6 September 2001     

High-pressure thermo-elastic properties of beryl (Al<Subscript>4</Subscript>Be<Subscript>6</Subscript>Si<Subscript>12</Subscript>O<Subscript>36</Subscript>) from ab initio calculations,and observations about the source of thermal expansion

Ab initio calculations of thermo-elastic properties of beryl (Al₄Be₆Si₁₂O₃₆) have been carried out at the hybrid HF/DFT level by using the B3LYP and WC1LYP Hamiltonians. Static geometries and vibrational frequencies were calculated at different values of the unit cell volume to get static pressure and mode-γ Grüneisen’s parameters. Zero point and thermal pressures were calculated by following a standard statistical-thermodynamics approach, within the limit of the quasi-harmonic approximation, and added to the static pressure at each volume, to get the total pressure (P) as a function of both temperature (T) and cell volume (V). The resulting P(V, T) curves were fitted by appropriate EoS’, to get bulk modulus (K ₀) and its derivative (K′), at different temperatures. The calculation successfully reproduced the available experimental data concerning compressibility at room temperature (the WC1LYP Hamiltonian provided K ₀ and K′ values of 180.2 Gpa and 4.0, respectively) and the low values observed for the thermal expansion coefficient. A zone-centre soft mode \( P6/mcc \to P\bar{1} \) phase transition was predicted to occur at a pressure of about 14 GPa; the reduction of the frequency of the soft vibrational mode, as the pressure is increased, and the similar behaviour of the majority of the low-frequency modes, provided an explanation of the thermal behaviour of the crystal, which is consistent with the RUM model (Rigid Unit Model; Dove et al. in Miner Mag 59:629–639, 1995), where the negative contribution to thermal expansion is ascribed to a geometric effect connected to the tilting of rigid polyhedra in framework silicates.     

Atomistic model of diopside-K-jadeite (CaMgSi<Subscript>2</Subscript>O<Subscript>6</Subscript>-KAlSi<Subscript>2</Subscript>O<Subscript>6</Subscript>) solid solution

Atomistic model was proposed to describe the thermodynamics of mixing in the diopside-K-jadeite solid solution (CaMgSi₂O6-KAlSi₂O₆). The simulations were based on minimization of the lattice energies of 800 structures within a 2 × 2 × 4 supercell of C2/c diopside with the compositions between CaMgSi₂O₆ and KAlSi₂O₆ and with variable degrees of order/disorder in the arrangement of Ca/K cations in M2 site and Mg/Al in Ml site. The energy minimization was performed with the help of a force-field model. The results of the calculations were used to define a generalized Ising model, which included 37 pair interaction parameters. Isotherms of the enthalpy of mixing within the range of 273–2023 K were calculated with a Monte Carlo algorithm, while the Gibbs free energies of mixing were obtained by thermodynamic integration of the enthalpies of mixing. The calculated T-X diagram for the system CaMgSi₂O₆-KAlSi₂O₆ at temperatures below 1000 K shows several miscibility gaps, which are separated by intervals of stability of intermediate ordered compounds. At temperatures above 1000 K a homogeneous solid solution is formed. The standard thermodynamic properties of K-jadeite (KAlSi₂O₆) evaluated from quantum mechanical calculations were used to determine location of several mineral reactions with the participation of the diopside-K-jadeite solid solution. The results of the simulations suggest that the low content of KalSi₂O₆ in natural clinopyroxenes is not related to crystal chemical factors preventing isomorphism, but is determined by relatively high standard enthalpy of this end member.     

Aktashite Cu6Hg3As4S12 from the Aktash deposit, Altai, Russia: Refinement and crystal chemical analysis of the structure

The composition and structure of aktashite from the Aktash deposit, Gorny Altai, Russia, have been studied by electron microprobe and X-ray structural analysis. On the basis of close compositions and crystal structures, the identity of aktashite from the Gal-Khaya and Aktash deposits has been demonstrated. Crystals of aktashite are of trigonal symmetry; the unit-cell dimensions are: a = 13.7500(4), c = 9.3600(3) Å, V = 532.54(8) ų, space group R3, Z = 3 for the composition of Cu₆Hg₃As₄S₁₂, R = 0.043. The structure of aktashite as a framework of vertex-shared HgS_4? and CuS_4? tetrahedrons of the same orientation is intimately related to the sphalerite-type structure. The earlier identified uncommon cluster group [As₄] has been verified and its parameters have been refined. It is shown that the structure may be represented as construction blocks (As₄S₁₂)_12? packed according to the law of the distorted cubic I-cell.     

Hydroxylborite,Mg<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)(OH)<Subscript>3</Subscript>, a new mineral species and isomorphous series fluoborite-hydroxylborite

Hydroxylborite, a new mineral species, an analogue of fluoborite with OH > F, has been found at the Titovsky deposit (57°41′N, 125°22′E), the Chersky Range, Dogdo Basin, Sakha-Yakutia Republic, Russia. Prismatic crystals of the new mineral are dominated by the {10\(\overline 1 \)0} faces without distinct end forms and reach (1?1.5) × (0.1?0.2) mm in size. Radial aggregates of such crystals occur in the mineralized marble adjacent to the boron ore (suanite-kotoite-ludwigite). Calcite, dolomite, Mg-rich ludwigite, kotoite, szaibelyite, clinohumite, magnetite, serpentine, and chlorite are associated minerals. Hydroxylborite is transparent colorless, with a white streak and vitreous luster. The new mineral is brittle. The Mohs’ hardness is 3.5. The cleavage is imperfect on {0001}. The density measured with equilibration in heavy liquids is 2.89(1) g/cm³; the calculated density is 2.872 g/cm³. The wave numbers of the absorption bands in the IR spectrum of hydroxylborite are (cm^?1; sh is shoulder): 3668, 1233, 824, 742, 630sh, 555sh, 450sh, and 407. The new mineral is optically uniaxial, negative, ω = 1.566(1), and ε = 1.531(1). The chemical composition (electron microprobe, H₂O measured with the Penfield method, wt %) is 18.43 B₂O₃, 65.71 MgO, 10.23 F, 9.73 H₂O, 4.31-O = F₂, where the total is 99.79. The empirical formula calculated on the basis of 6 anions pfu is as follows: Mg_3.03B_0.98[(OH)_2.00F_1.00]O_3.00. Hydroxylborite is hexagonal, and the space group is P6₃/m. The unit-cell dimensions are: a = 8.912(8) Å, c = 3.112(4) Å, V = 214.05(26) ų, and Z = 2. The strongest reflections in the X-ray powder pattern [d, Å (I, %)(hkil)] are: 7.69(52)(01\(\overline 1 \)0), 4.45(82)(11\(\overline 2 \)0), 2.573(65)(03\(\overline 3 \)0), 2.422(100)(02\(\overline 2 \)1), and 2.128(60)(12\(\overline 3 \)1). The compatibility index 1 ? (K _p/K _c) is 0.038 (excellent) for the calculated density and 0.044 (good) for the measured density. The type material of hydroxylborite is deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow (inventory number 91968) and the Geological Museum of the All-Russia Institute of Mineral Resources, Moscow (inventory number M-1663).     

Minerals at high pressure. Mechanics of compression from quantum mechanical calculations in a case study: the beryl (Al<Subscript>4</Subscript>Be<Subscript>6</Subscript>Si<Subscript>12</Subscript>O<Subscript>36</Subscript>)

The Bader topological analysis has been applied to ab initio computed electron densities of beryl, in order to clarify its mechanism of compression. Full structural optimization and total energy (E) calculations were performed at different cell volumes (V _c). The pressure at each volume and the equation of state were estimated from the first and second derivatives of the resultant E(V _c) curve. The total (negative) potential energy of the crystal, sum of both attractive and repulsive electrostatic terms, was found to systematically decrease (i.e., it moved to more negative values) up to the highest pressure considered (28.4 GPa), indicating that interelectronic and internuclear repulsions are not the only terms controlling the compressibility, at least in the pressure range investigated. Electronic kinetic energy increases as the cell volume is reduced, leading to a parallel increase of the total energy. Both structure at equilibrium and compressibility are therefore due to the balance between the opposing kinetic and potential energy terms. The Bader theory has been used to identify the topological atoms within the structure and to calculate their properties, with particular attention to the forces driving the structural relaxation at high pressure. On a qualitative basis, the obtained results are expected to be transferable to the discussion of compressibility of other mineral phases.