The kinetics of oxygen exchange between the GeO4Al12(OH)24(OH2)12(aq) molecule and aqueous solutions

We report rates of oxygen exchange with bulk solution for an aqueous complex, ^IVGeO₄Al₁₂(OH)₂₄(OH₂)₁₂⁸⁺(aq) (GeAl₁₂), that is similar in structure to both the ^IVAlO₄Al₁₂(OH)₂₄(OH₂)₁₂⁷⁺(aq) (Al₁₃) and ^IVGaO₄Al₁₂(OH)₂₄(OH₂)₁₂⁷⁺(aq) (GaAl₁₂) molecules studied previously. All of these molecules have ε-Keggin-like structures, but in the GeAl₁₂ molecule, occupancy of the central tetrahedral metal site by Ge(IV) results in a molecular charge of +8, rather than +7, as in the Al₁₃ and GaAl₁₂. Rates of exchange between oxygen sites in this molecule and bulk solution were measured over a temperature range of 274.5 to 289.5 K and 2.95 < pH < 4.58 using ¹⁷O-NMR.Apparent rate parameters for exchange of the bound water molecules (η-OH₂) are k_ex²⁹⁸ = 200 (±100) s⁻¹, ΔH^‡ = 46 (±8) kJ · mol⁻¹, and ΔS^‡ = −46 (±24) J · mol⁻¹ K⁻¹ and are similar to those we measured previously for the GaAl₁₂ and Al₁₃ complexes. In contrast to the Al₁₃ and GaAl₁₂ molecules, we observe a small but significant pH dependence on rates of solvolysis that is not yet fully constrained and that indicates a contribution from the partly deprotonated GeAl₁₂ species.The two topologically distinct μ₂-OH sites in the GeAl₁₂ molecule exchange at greatly differing rates. The more labile set of μ₂-OH sites in the GeAl₁₂ molecule exchange at a rate that is faster than can be measured by the ¹⁷O-NMR isotopic-equilibration technique. The second set of μ₂-OH sites have rate parameters of k_ex²⁹⁸ = 6.6 (±0.2) · 10⁻⁴ s⁻¹, ΔH^‡ = 82 (±2) kJ · mol⁻¹, and ΔS^‡ = −29 (±7) J · mol⁻¹ · K⁻¹, corresponding to exchanges ≈40 and ≈1550 times, respectively, more rapid than the less labile μ₂-OH sites in the Al₁₃ and GaAl₁₂ molecules. We find evidence of nearly first-order pH dependence on the rate of exchange of this μ₂-OH site with bulk solution for the GeAl₁₂ molecule, which contrasts with Al₁₃ and GaAl₁₂ molecules.     

Potentiometric and F nuclear magnetic resonance spectroscopic study of fluoride substitution in the GaAl12 polyoxocation: implications for aluminum (hydr)oxide mineral surfaces

Fluoride replacement of oxygens in the GaO₄Al₁₂(OH)₂₄(H₂O)127+(aq) molecule [GaAl₁₂] was studied via ¹⁹F nuclear magnetic resonance (NMR) at 4 < pH < 5 and 278 K in order to elucidate similar reactions at the surfaces of clays. Peaks are identified in the ¹⁹F-NMR spectra that correspond to both terminal and bridging fluorides on the GaAl₁₂ molecule, with relative peak positions similar to those previously identified in fluoridated aluminum (hydr)oxide mineral surfaces (Nordin, J. P., Sullivan, D. J., Phillips, B. L., and Casey, W. H. [1999], “Mechanisms for fluoride-promoted dissolution of bayerite [β-Al(OH)₃(s)] and boehmite [γ-AlOOH(s)]-¹⁹F-NMR spectroscopy and aqueous surface chemistry,” Geochim. Cosmochim. Acta63, 3513-3524). Fluoride substitutes for oxygen at three different sites in the GaAl₁₂ molecule, but at dramatically different rates.The kinetics of fluoride substitution follow a rate law that includes parallel and reversible transfer of fluoride from nonbridging sites to the two bridging sites. The essential features of the rate law are as follows: (1) fluoride replaces bound water molecules (η-OH₂) within minutes at 278 K at rates that are quantitatively similar to fluoride uptake by Al(H₂O)63+(aq) to form AlF²⁺(aq) at similar conditions; (2) fluoride substitutes onto the two topologically distinct μ₂-OH sites at different rates, as was previously observed for oxygen exchange, but here, the reaction is complete in hours to days at 278 K. Most importantly, rates of fluoride substitution onto μ₂-OH sites are 10² times more rapid than the corresponding rates of oxygen exchange with bulk waters, indicating that fluoride considerably labilizes the molecule, as is also observed at the surfaces of minerals. The largest cause of this labilization is the reduced molecular charge on the GaAl₁₂ upon replacement of bound waters by fluoride, which for mineral surfaces corresponds to a reduction in surface charge density.     

The flux of oxygen from the basal surface of gibbsite (α-Al(OH)3) at equilibrium

Experiments were conducted on gibbsite to determine whether oxygen-isotope exchange rates at hydroxyl bridges (μ₂-OH) on the basal sheet exhibit similar reactivity trends as in large aluminum polyoxocations, for which high-quality kinetic data exist. We followed the exchange of ¹⁸O from the mineral surface to solution by using a high-surface-area solid that had been enriched to tens of percent in ¹⁸O. To establish this high enrichment, we initially react the solid hydrothermally with highly enriched H₂¹⁸O in order to tag all oxygens near the mineral surface, and then back exchange the most reactive oxygens with isotopically normal water. This enrichment procedure isolates ¹⁸O into the least-reactive sites, which are presumably μ₂-OH on the basal surface. By analogy with aqueous aluminum complexes, including large multimers, the η-OH₂ sites exchange within fractions of a second and should be isotopically normal using this procedure.When suspended in isotopically normal electrolyte solutions, we find that the rates of release of ¹⁸O from the mineral fall close to the rates of dissolution. The lack of steady isotopic exchange of μ₂-OH on gibbsite surfaces contrasts with the aluminum polyoxocations, where the μ₂-OH exchange many hundreds of times with bulk water molecules before the molecule dissociates. Additional experiments were conducted in solutions at near-neutral pH to determine the flux of oxygens at conditions near thermodynamic equilibrium. As in more acidic solutions, rates are close to values expected from dissolution of the mineral and there is no evidence for steady exchange of hydroxyl bridges with water molecules in the bulk solution.     

Residence times for protons bound to three oxygen sites in the AlO4Al12(OH)24(H2O)12 polyoxocation

Although, the kinetic reactivity of a mineral surface is determined, in part, by the rates of exchange of surface-bound oxygens and protons with bulk solution, there are no elementary rate data for minerals. However, such kinetic measurements can be made on dissolved polynuclear clusters, and here we report lifetimes for protons bound to three oxygen sites on the AlO₄Al₁₂(OH)₂₄(H₂O)₁₂⁷⁺ (Al₁₃) molecule, which is a model for aluminum-hydroxide solids in water. Proton lifetimes were measured using ¹H NMR at pH ∼ 5 in both aqueous and mixed solvents. The ¹H NMR peak for protons on bound waters (η-H₂O) lies near 8 ppm in a 2.5:1 mixture of H₂O/acetone-d₆ and broadens over the temperature range −20 to −5 °C. Extrapolated to 298 K, the lifetime of a proton on a η-H₂O is τ²⁹⁸ ∼ 0.0002 s, which is surprisingly close to the lifetime of an oxygen in the η-H₂O (∼0.0009 s), but in the same general range as lifetimes for protons on fully protonated monomer ions of trivalent metals (e.g., Al(H₂O)₆³⁺). The lifetime is reduced somewhat by acid addition, indicating that there is a contribution from the partly deprotonated Al₁₃ molecule in addition to the fully protonated Al₁₃ at self-buffered pH conditions. Proton lifetimes on the two distinct sets of hydroxyls bridging two Al(III) (μ₂-OH) differ substantially and are much shorter than the lifetime of an oxygen at these sites. The average lifetimes for hydroxyl protons were measured in a 2:1 mixture of H₂O/dmso-d₆ over the temperature range 3.7-95.2 °C. The lifetime of a hydrogen on one of the μ₂-OH was also measured in D₂O. The τ²⁹⁸ values are ∼0.013 and ∼0.2 s in the H₂O/dmso-d₆ solution and the τ²⁹⁸ value for the μ₂-OH detectable in D₂O is τ²⁹⁸ ∼ 0.013 s. The ¹H NMR peak for the more reactive μ₂-OH broadens slightly with acid addition, indicating a contribution from an exchange pathway that involves a proton or hydronium ion. These data indicate that surface protons on minerals will equilibrate with near-surface waters on the diffusional time scale.     

Rates of oxygen exchange between the Al2O8Al28(OH)56(H2O)26(aq) (Al30) molecule and aqueous solution

Rates of steady exchange of oxygens between bulk solution and the largest known aluminum polyoxocation: Al₂O₈Al₂₈(OH)₅₆(H₂O)₂₆¹⁸⁺(aq) (Al₃₀) are reported at pH≈4.7 and 32-40°C. The Al₃₀ molecule is a useful model for geochemists because it is ≈2 nm in length, comparable to the smallest colloidal solids, and it has structural complexity greater than the surfaces of most aluminum (hydr)oxide minerals. The Al₃₀ molecule has 15 distinct hydroxyl sites and eight symmetrically distinct bound waters. Among the hydroxyl bridges are two sets of μ₃-OH, which are not present in any of the other aluminum polyoxocations that have yet been studied by NMR methods. Rates of isotopic equilibration of the μ₂-OH and μ₃-OH hydroxyls and bound water molecules fall within the same range as we have determined for other aluminum solutes, although it is impossible to determine rate laws for exchange at the large number of individual oxygen sites. After injection of ¹⁷O-enriched water, growth of the ¹⁷O-NMR peak near 37 ppm, which is assigned to μ₂-OH and μ₃-OH hydroxyl bridges, indicates that these bridges equilibrate within two weeks at temperatures near 35°C. The peak at +22 ppm in the ¹⁷O-NMR spectra, assigned to bound water molecules (η-OH₂), varies in width with temperature in a similar fashion as for other aluminum solutes, suggesting that most of the η-OH₂ sites exchange with bulk solution at rates that fall within the range observed for other aluminum complexes. Signal from one anomalous group of four η-OH₂ sites is not observed, indicating that these sites exchange at least a factor of ten more rapidly than the other η-OH₂ sites on the Al₃₀.     

Activation volumes for oxygen exchange between the GaO4Al12(OH)24(H2O)127+(aq) (GaAl12) polyoxocation and aqueous solution from variable pressure O NMR spectroscopy

Activation volumes for exchange of oxygen between bulk aqueous solution and sites in the GaO₄Al₁₂(OH)₂₄(H₂O)₁₂⁷⁺(aq) (GaAl₁₂) complex were measured by variable-pressure ¹⁷O NMR techniques. Near 322 K, rates of exchange for the less labile set of bridging hydroxyls in the GaAl₁₂ decrease by a factor of about two with increasing pressure from 0.1 to 350 MPa. These data indicate a substantially positive activation volume of ΔV^‡ = +7 ± 1 cm³/mol, which is the first activation volume measured for a bridging hydroxyl in a polynuclear complex. This result suggests significant bond-lengthening in the activation step. Electrostriction effects should be small because exchange occurs via a pH-independent path under the experimental conditions. The second, more labile set of bridging hydroxyls exchange too rapidly for the variable-pressure techniques employed here. The exchange of bound-water molecules on the GaAl₁₂ was observed at P = 350 MPa using the ¹⁷O-NMR line-broadening technique. Comparison with previous measurements at 0.1 MPa indicates decreasing line width from 0.1 to 350 MPa for temperatures at which exchange dominates, yielding an activation volume of ΔV^‡ = +3(± 1) cm³/mol. This activation volume is smaller than the value for the Al(H₂O)₆³⁺ complex, suggesting that water exchange on the larger GaAl₁₂ complex has less dissociative character although the average charge density is lower.     

Molecular dynamics simulation of the titration of polyoxocations in aqueous solution

The aqueous complex ion Al₃₀O₈(OH)₅₆(H₂O)₂₆¹⁸⁺ (Al₃₀) has a variety of bridging and terminal amphoteric surface functional groups which deprotonate over a pH range of 4-7. Their relative degree of protonation is calculated here from a series of molecular dynamics simulations in what appear to be the first molecular dynamics simulations of an acidometric titration. In these simulations, a model M₃₀O₈(OH)₅₆(H₂O)₂₆¹⁸⁺ ion is embedded in aqueous solution and titrated with hydroxide ions in the presence of a charge-compensating background of perchlorate ions. Comparison with titration of a model M₁₃O₄(OH)₂₄(H₂O)₁₂⁷⁺ reveals that the M₃₀ ion is more acidic than the M₁₃ ion due to the presence of acidic ηH₂O functional groups. The higher acidities of the functional groups on the M₃₀ ion appear to result from enhanced hydration. Metal-oxygen bond lengths are calculated for the ion in solution, an isolated ion in the gas phase, and in its crystalline hydrate sulfate salt. Gas-phase and crystalline bond lengths do not correlate well with those calculated in solution. The acidities do not relate in any simple way to the number of metals coordinating the surface functional group or the M-O bond length. Moreover, the calculated acidity in solution does not correlate with proton affinities calculated for the isolated ion in the absence of solvent. It is concluded that the search for simple indicators of structure-reactivity relationships at the level of individual reactive sites faces major limitations, unless specific information on the hydration states of the functional groups is available.     

含铝硅酸盐1)吉布斯生成自由能的预测及Y.塔达方法向高温条件下推引的可能性

近几年来,Y.塔达等人^[3-7]建立了一种预测化合物吉布斯生成自由能的经验方法。     

A computer model for the cubic sodalite structure

A computer model for cubic sodalite structures, general formula M ₈(T ₁₂O₂₄)X ₂ where M, X and T are the cavity cation and anion and framework cation respectively, has been devised. It has been used to determine the effect of changing cavity cation and anion radii on the cell edge, tilt angle of the tetrahedra and T-O-T angle for the following sodalite frameworks: (Al₆Si₆O₂₄)^6?, (Be₆Si₆O₂₄)^12?, (Al₁₂O₂₄)^12?, and (B₁₂O₂₄)^12?. After fixing the T-O distance(s), the cavity cation-framework oxygen distance and taking a value of 1.4 Å for the radius of oxygen the model was used to calculate atomic coordinates and interatomic distances and angles for selected aluminosilicate-sodalites. The structure calculated for Na₈(Al₆Si₆O₂₄)Cl₂ agrees closely with that determined for natural sodalite (Löns and Schulz, 1967). The model is also applied to the estimation of the effective radii of the tetrahedrally-coordinated cavity anions which can be accommodated in natural and synthetic sodalites: OH^? 1.48–1.51, Cl^? 1.78, Br^? 1.93, I^? 2.14–2.17, SO ₄ ^2? 2.37–2.57, MoO ₄ ^2? 2.70 and WO ₄ ^2? 2.79 Å.     

Molecular orbital studies of geometries and spectra of minerals and inorganic compounds

Extended Hückel molecular orbital theory (EHT) and simple, approximate Self-Consistent-Field MO methods are employed to explain the geometries of nontransition metal bearing minerals and inorganic compounds. The spectra of such minerals and the electronic structure of transition metal oxidic minerals are explained using the Self-Consistent-Field X α MO method. EHT provides an objective algorithm for rationalizing and correlating bond length and angle data for insular and polymerized TO ₄ ^?n tetrahedral oxyanions where T=Be, B, Al, Si, P, S, Ge, As and Se. Calculated bond overlap populations n(T-O), correlate linearly with the observed T-O bond lengths with shorter bonds tending to involve larger n(T-O) values. Such calculations show that n(T-O) is strongly dependent upon the average of the three O-T-O angles associated with a common bond, larger n(T-O) values involving wider angles. Calculations of n(T-O) as a function of the T-O-T angles in T ₂O ₇ ^?n ions, indicate that the n(T-O) values for the bonds to the bridging oxygen atoms increase nonlinearly with increasing T-O-T angle whereas those the nonbridging oxygens decrease slightly as the angle widens. In agreement with the experimental data, these results predict that shorter T-O bonds should involve wider O-T-O and T-O-T angles. The SCF-X α MO cluster model is then applied to silica and FeO. The calculations yield a satisfactory interpretation of the visible, UV and X-ray emission and X-ray photoelectron spectra of these materials. Theoretical and empirical MO diagrams are constructed and the electronic structures of the materials are discussed.     

Aluminate sodalites — A family with strained structures and ferroic phase transitions

The aluminate sodalites M ₈[Al₁₂O₂₄](XO ₄)₂, with M=Ca, Sr and X=S, Cr, Mo, W, are discussed in terms of their general structural characteristics. Special emphasis is given to the non-bonded O...O interactions between the cage anions and the sodalite framework, on one side, and to attractive cage anion — cage cation interactions, on the other side. Values for pseudocubic cell parameters, spontaneous deformation, superstructure multiplicities, transition temperatures and associated enthalpies, as well as symmetry information are given.     

The mass of the compact object in the low-mass X-ray binary 2S 0921-630

We interpret the observed radial-velocity curve of the optical star in the low-mass X-ray binary 2S 0921-630 using a Roche model, taking into account the X-ray heating of the optical star and screening of X-rays coming from the relativistic object by the accretion disk. Consequences of possible anisotropy of the X-ray radiation are considered. We obtain relations between the masses of the optical and compact (X-ray) components, m _v and m _x , for orbital inclinations i = 60°, 75°, and 90°. Including X-ray heating enabled us to reduce the compact object’s mass by ～0.5–1 M _⊙, compared to the case with no heating. Based on the K0III spectral type of the optical component (with a probable mass of m _v ? 2.9 M _⊙), we concluded that m _x ? 2.45?2.55 M _⊙ (for i = 75°?90°). If the K0III star has lost a substantial part of its mass as a result of mass exchange, as in the V404 Cyg and GRS 1905+105 systems, and its mass is m _v ? 0.65?0.75 M _⊙, the compact object’s mass is close to the standard mass of a neutron star, m _x ? 1.4 M _⊙ (for i = 75°?90°). Thus, it is probable that the X-ray source in the 2S 0921-630 binary is an accreting neutron star.     

The crystal structure of cualstibite-1M (formerly cyanophyllite), its revised chemical formula and its relation to cualstibite-1T

The crystal structure of the rare secondary mineral cualstibite-1M (formerly cyanophyllite), originally reported to have the chemical formula 10CuO·2Al₂O₃·3Sb₂O₃·25H₂O and orthorhombic symmetry, was solved from single-crystal intensity data (Mo-Kα X-radiation, CCD area detector, 293 K, 2θ_max?=?80) collected on a twinned crystal containing very minor Mg. The mineral is monoclinic, P2₁/c (no. 14), with a?=?9.938(1), b?=?8.890(1), c?=?5.493(1) Å, β?=?102.90(1)°, V?=?473.05(11) ų; R1(F)?=?0.0326. All crystals investigated turned out to be non-merohedric twins. The atomic arrangement has a distinctly layered character. Brucite-like sheets composed of two [4?+?2]-coordinated (Cu,Al,Mg) sites are linked by weak hydrogen-bonding (O···O?~?2.80 Å) to isolated regular Sb(OH)₆ octahedra (<Sb-O>?=?1.975 Å). The layered, pseudotrigonal character explains the perfect cleavage and the proneness to twinning. The Sb site is fully occupied and the two (Cu,Al,Mg) sites have occupancies of Cu_0.79Al_0.17Mg_0.04 and Cu_0.72Al_0.23Mg_0.05. The Cu-richer site shows a slightly stronger Jahn-Teller-distortion. The resulting empirical formula, which necessitates a H₂O-for-OH substitution to obtain charge balance, is (Cu_2.23Al_0.63Mg_0.14)(OH)_5.63(H₂O)_0.37[Sb⁵⁺(OH)₆]. The ideal chemical formula is (Cu,Al)₃(OH)₆[Sb⁵⁺(OH)₆], with Cu:Al = 2:1. The structure is closely related to those of trigonal cualstibite-1T [Cu₂AlSb(OH)₁₂, P-3, with ordered Cu-Al distribution in the brucite sheets], and its Zn analogue zincalstibite-1T [Zn₂AlSb(OH)₁₂]. Cualstibite-1M and cualstibite-1T are polytypes and, together with zincalstibite-1T, zincalstibite-9R and omsite, belong to the cualstibite group within the hydrotalcite supergroup, which comprises all natural members of the large family of layered double hydroxides (LDH).     

Sapphirine II

The crystal structure of aP2₁/a polymorph of sapphirine (a=11.286(3),b=14.438(2),c=9.957(2) Å, β=125.4(2) °) of composition [Mg_3.7Fe _0.1 ²⁺ Al_4.1- Fe _0.1 ³⁺ ]^IV[Si_1.8Al_4.2]^IVO₂₀ was refined using structure factors determined by both neutron and x-ray diffraction methods to conventionalR factors of 0.067 and 0.031. respectively, forF _obs>2σ. The results of the two refinements agree reasonably well, but a half-normal probability plot (Abrahams, 1974) comparing the two data sets indicates that the pooled standard deviations of the atomic coordinates have been underestimated by a factor of two. The structure of sapphirine, solved initially by Moore (1969), consists of cubic closest packed oxygens with octahedral and predominantly tetrahedral layers alternately stacked along [100]. The layer in which 70% of the octahedral sites are occupied has an Mg-Al distribution characterized by Mg-rich octahedra sharing edges mainly with Al-rich octahedra. Mean octahedral bond lengths correlate well with Al occupancy determined by neutron site refinement if the relative number of shared octahedral edges is taken into account (see Table 1). The predominantly tetrahedral layer has 10% of the octahedral sites occupied by Al and 30% of the tetrahedral sites occupied by Al-Si in the ratio 2.33∶1. There are single chains of Al-Si tetrahedra parallel toz with corner-sharing wing tetrahedra (T5 andT6) on either side in the (100) plane. The meanT-O distance is highly correlated with Al occupancy, X_Al, as determined from the neutron site refinement: $$\langle T - O\rangle = 1.656 + 0.105X_{Al} (r^2 = 0.995).$$ Details of the neutron refinement are summarized below.     

Experimental calibration of the partitioning of Fe and Mg between biotite and garnet

The cation exchange reaction Fe₃Al₂Si₃O₁₂ +KMg₃AlSi₃O₁₀(OH)₂ = Mg₃Al₂Si₃O₁₂+KFe₃-AlSi₃ O₁₀(OH)₂ has been investigated by determining the partitioning of Fe and Mg between synthetic garnet, (Fe, Mg)₃Al₂Si₃O₁₂, and synthetic biotite, K(Fe, Mg)₃AlSi₃O₁₀(OH)₂. Experimental results at 2.07 kbar and 550 °–800 ° C are consistent with In [(Mg/Fe) garnet/(Mg/Fe) biotite] = -2109/T(°K) +0.782. The preferred estimates for ¯H and ¯S of the exchange reaction are 12,454 cal and 4.662 e.u., respectively. Mixtures of garnet and biotite in which the ratio garnet/biotite=49/1 were used in the cation exchange experiments. Consequently the composition of garnet-biotite pairs could approach equilibrium values in the experiments with minimal change in garnet composition (few tenths of a mole percent). Equilibrium was demonstrated at each temperature by reversal of the exchange reaction. Numerical analysis of the experimental data yields a geothermometer for rocks containing biotite and garnet that are close to binary Fe-Mg compounds.     

Voloshinite,a new rubidium mica from granitic pegmatite of Voron’i Tundras,Kola Peninsula,Russia

Voloshinite, a new mineral of the mica group, a rubidium analogue of lepidolite, has been found from the rare-element granitic pegmatite at Mt. Vasin-Myl’k, Voron’i Tundras, Kola Peninsula, Russia. It is closely associated with pollucite and lepidolite and commonly with muscovite, albite, and quartz; K,Rb-feldspar, rubicline, spodumene, montebrasite, and elbaite are among associated minerals as well. Voloshinite, a late mineral that formed after pollucite, commonly fills polymineralic veinlets and pods within the pollucite aggregates. It occurs as rims up to 0.05 mm thick around lepidolite, as intergrowths of tabular crystals up to 0.25 mm in size, and occasionally replaces lepidolite. The new mineral is colorless, transparent, with vitreous luster. Cleavage is eminent parallel to {001}; flakes are flexible. The calculated density is 2.95 g/cm³. The new mineral is biaxial (?), with 2V = 25°, α calc = 1.511, β = 1.586, and γ = 1.590. The optical orientation is Y = b, Z = a. The chemical composition of the type material determined by electron microprobe (average of five point analyses; Li has been determined with ICP-OES) is as follows (wt %): 0.03 Na₂O, 3.70 K₂O, 12.18 Rb₂O, 2.02 Cs₂O, 4.0 Li₂O, 0.03 CaO, 0.02 MgO, 0.14 MnO, 21.33 Al₂O₃, 53.14 SiO₂, 6.41 F, -O = F₂ 2.70, total is 100.30. The empirical formula is: (Rb_0.54K_0.33Cs_0.06)_Σ0.93(Al_1.42Li_1.11Mn_0.01)_Σ2.54(Si_3.68Al_0.32)_Σ4O₁₀ (F_1.40(OH)_0.60)_Σ2. The idealized formula is as follows: Rb(LiAl_1.5□_0.5)[Al_0.5Si_3.5O₁₀]F₂. Voloshinite forms a continuous solid solution with lepidolite. According to X-ray single crystal study, voloshinite is monoclinic, space group C2/c. The unit-cell dimensions are: a = 5.191, b = 9.025, c = 20.40 Å, β = 95.37°, V= 951.5 ų, Z = 4. Polytype is 2M ₁. The strongest reflections in the X-ray powder diffraction pattern (d, Å-I[hkl]) are: 10.1-60[001]; 4.55-80[020, 110, 11\(\bar 1\)]; 3.49-50[11\(\bar 4\)]; 3.35-60[024, 006]; 3.02-45[025]; 2.575-100[11\(\bar 6\), 131, 20\(\bar 2\), 13\(\bar 4\)], 2.017-50[136, 0.0.10]. The mineral was named in honor of A.V. Voloshin (born in 1937), the famous Russian mineralogist. The type material is deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow.     

Structural changes and oxidation of ferroan phlogopite with increasing temperature: in situ neutron powder diffraction and Fourier transform infrared spectroscopy

The thermal response of the natural ferroan phlogopite-1M, K₂(Mg_4.46Fe_0.83Al_{0. 34}Ti_0.22)(Si_5.51Al_{2. 49})O₂₀[OH_3.59F_0.41] from Quebec, Canada, was studied with an in situ neutron powder diffraction. The in situ temperature conditions were set up at ?263, 25, 100°C and thereafter at a 100°C intervals up to 900°C. The crystal structure was refined by the Rietveld method (R _p=2.35–2.78%, R _wp=3.01–3.52%). The orientation of the O–H vector of the sample was determined by the refinement of the diffraction pattern. With increasing temperature, the angle of the OH bond to the (001) plane decreased from 87.3 to 72.5°. At room temperature, a = 5.13 Å, b = 9.20 Å, c = 10.21 Å, β = 100.06° and V(volume) = 491.69 ų. The expansion rate of the unit cell dimensions varied discontinuously with a break at 500°C. The shape of the M-octahedron underwent some significant changes such as flattening at 500°C. At temperatures above 500°C, the octahedral thickness and mean distance was decreased, while the octahedral flattening angle increased. Those results were attributed to the Fe oxidation and dehydroxylation processes. The dehydroxylation mechanism of the ferroan phlogopite was studied by the Fourier transform infrared spectroscopy (FTIR) after heated at temperatures ranging from 25 to 800°C with an electric furnace in a vacuum. In the OH stretching region, the intensity of the OH band associated with Fe²⁺(N _B-band) begun to decrease outstandingly at 500°C. The changes of the IR spectra confirmed that dehydroxylation was closely related to the oxidation in the vacuum of the ferrous iron in the M-octahedron. The decrease in the angle of the OH bond to the (001) plane, with increasing temperature, might be related to the imbalance of charge in the M-octahedra due to Fe oxidation.     

A new Cu-rich variety of lyonsite from fumarolic sublimates of the Tolbachik volcano (Kamchatka, Russia) and its crystal structure

A new Cu-rich variety of lyonsite has been found from fumarolic sublimates of the Tolbachik volcano (Kamchatka, Russia). The empirical formula is Cu_4.33Fe _2.37 ³⁺ Ti_0.26Al_0.26Zn_0.07(V_5.85As_0.07Mo_0.07P_0.01S_0.01)O₂₄. The crystal structure was studied on single crystal using synchrotron radiation, R = 0.0514. The mineral is orthorhombic, Pnma, a = 5.1736(7), b =10.8929(12), c = 18.220(2) Å, V = 1026.8(2) ų, and Z = 2. The structural formula is (Cu_0.6Ti_0.3Al_0.3Fe _0.2 ³⁺ □_0.6)_Σ2Cu₂(Fe _2.2 ³⁺ Cu_1.8)_Σ4(V_5.8As_0.1Mo_0.1)_Σ6O₂₄. It is proposed to recast the simplified formula of lyonsite as Cu_3+x (Fe _4?2x ³⁺ Cu_2x )(VO₄)₆, where 0 ≤ x ≤ 1.     

Discovery of low aluminium nevadaite from the Kara-Chagyr Area,Kyrgyzstan

The rare phosphate—nevadaite has been found at Kara-Chagyr (Batken region, Kyrgyzstan) in a zone of alteration of vanadium bearing “black shales”. It occurs as blue crusts of spherulitic aggregates of tiny tabular crystals (0.1–10 μm). It is associated with metahewettite, hummerite, carnotite, minyulite, fluellite, crandallite, variscite, and woodhouseite. Optical properties: n = 1.542–1.555, D _meas (for aggregates) = 2.58(1) g/cm³, D _calc = 2.582 g/cm³. The most intense X-ray powder reflections are as follows: [d/n, Å, (I _meas), (hkl)]: 9.54 (80) (020), 6.03 (100) (200), 5.61 (100) (130), 3.91 (60) (310), 3.41 (80) (041), 2.982 (100) (241), 2.804 (60) (331), 2.672 (70) (061), 1.845 (60) (352) 1.507 (70) (243). Calculated cell dimensions are: a = 12.072(10) Å, b = 18.958(15) Å, c = 4.969(5) Å, α = β = γ = 90°, V = 1137.2 ų. Electron microprobe analyses gives (wt %): (observed (average of 8 analyses); (calculated for 22H₂O)): P₂O₅ 34.69 (31.85), SiO₂ 0.25 (0.24), Al₂O₃ 25.61 (23.50), V₂O 5.58 (5.13), Fe₂O₃ 0.48 (0.46), MnO 0.03 (0.03), CuO 10.79 (9.90), ZnO 0.69 (0.65), CaO 0.18 (0.15), MgO 0.17 (0.17), K₂O 0.08 (0.08), F 7.40 (6.79), H₂O 17.16 (by diff.) (23.90), ?F₂ =O \(\bar 3\).11 (\(\bar 2\).86), total 100.00 (100.00).The crystal-chemical formula of the mineral is (Cu _2.2 ⁺² □_2.03V _1.21 ⁺³ Al_0.15Zn_0.14Fe_0.10Mg_0.07Ca_0.05K_0.03Mn_0.01)_6.00(Al_8.00(P_7.93Si_0.07O₃₂)F_6.32(OH)_2.98 · 22(H₂O) for the ideal number of water molecules. Nevadaite from Kara-Chagyr differs from that from the type locality, Gold Quarry (Nev., USA), by its lower Al content. The IR-spectrum, and microphotographs of nevadaite and associated minerals are given.     

Kasatkinite, Ba2Ca8B5Si8O32(OH)3 · 6H2O6, a new mineral from the Bazhenovskoe deposit, the Central Urals, Russia

A new mineral, kasatkinite, Ba₂Ca₈B₅Si₈O₃₂(OH)₃ · 6H₂O, has been found at the Bazhenovskoe chrysotile asbestos deposit, the Central Urals, Russia in the cavities in rhodingite as a member of two assemblages: (l) on prehnite, with pectolite, calcite, and clinochlore; and (2) on grossular, with diopside and pectolite. Kasatkinite occurs as spherulites or bunches up to 3 mm in size, occasionally combined into crusts. Its individuals are acicular to hair-like, typically split, with a polygonal cross section, up to 0.5 mm (rarely, to 6 mm) in length and to 20 μm in thickness. They consist of numerous misoriented needle-shaped subindividuals up to several dozen μm long and no more than 1 μm thick. Kasatkinite individuals are transparent and colorless; its aggregates are snow white. The luster is vitreous or silky. No cleavage was observed; the fracture is uneven or splintery for aggregates. Individuals are flexible and elastic. The Mohs’ hardness is 4–4.5. D _meas = 2.95(5), D _calc = 2.89 g/cm³. Kasatkinite is optically biaxial (+), α = 1.600(5), β = 1.603(2), γ = 1.626(2), 2V _meas = 30(20)°, 2V _calc = 40°. The IR spectrum is given. The ¹¹B MAS NMR spectrum shows the presence of BO₄ in the absence of BO₃ groups. The chemical composition of kasatkinite (wt %; electron microprobe, H₂O by gas chromatography) is as follows: 0.23 Na₂O, 0.57 K₂O, 28.94 CaO, 16.79 BaO, 11.57 B₂O₃, 0.28 Al₂O₃, 31.63 SiO₂, 0.05 F, 9.05 H₂O, ?0.02 ?O=F₂; the total is 99.09. The empirical formula (calculated on the basis of O + F = 41 apfu, taking into account the TGA data) is: Na_0.11K_0.18Ba_1.66Ca_7.84B_5.05Al_0.08Si_8.00O_31.80(OH)_3.06F_0.04 · 6.10H₂O. Kasatkinite is monoclinic, space group P2₁/c, P2/c, or Pc; the unit-cell dimensions are a = 5.745(3), b = 7.238(2), c = 20.79 (1) Å, β = 90.82(5)°, V = 864(1) ų, Z = 1. The strongest reflections (d Å–I[hkl]) in the X-ray powder diffractions pattern are: 5.89–24[012], 3.48–2.1[006], 3.36–24[114]; 3.009–100[ $12\bar 1$ , 121, $10\bar 6$ ], 2.925–65[106, $12\bar 2$ , 122], 2.633–33[211, 124], 2.116–29[ $13\bar 3$ , 133, 028]. Kasatkinite is named in honor of A.V. Kasatkin (b. 1970), a Russian amateur mineralogist and mineral collector who has found this mineral. Type specimen is deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.