Isomorphic substitutions in afghanite: Identification of the mineral

Samples of afghanite, (Na,Ca,K)₈ (Al₆Si₆O₂₄)(SO₄,Cl,CO₃)₃ · H₂O, from the Malaya Bystraya and Tultui lazurite deposits in the Baikal region have been studied with X-ray diffraction and an electron microprobe. The unit-cell dimensions a and c of the examined afghanite samples range from 12.729 to 12.762 &0A and from 21.385 to 21.415 &0A. Isomorphic substitution is exhibited between Na and K; Ca content is about 2.5 apfu; the Cl/SO₄ ratio is close to 1.0 with slight variations. Parameter a was plotted versus K content. The correlation between Na and K is established. Comparative analysis has shown that afghanite from the Baikal region is characterized by a lower K content than that from volcanic complexes in Italy. In the studied samples, K₂O ranges from 0.51 to 2.28 wt %, whereas in comparable samples from volcanic complexes of Italy, it varies from 3.44 to 8.26 wt %. Sodalite-group minerals display a similar behavior of potassium. In lazurite from the Baikal region, the K₂O content ranges from 0 to 0.95 wt %; the K₂O content in hauyne from Italy varies from 0.88 to 5.75 wt %. The wider isomorphic miscibility in the feldspar minerals from volcanic-hosted assemblages in Italy as compared with the same minerals from lazurite deposits of the Baikal region reflects the difference in the geological and physicochemical conditions under which the minerals have been formed. In addition to the X-ray procedure, the Cl/SO₄ ratio determined from microprobe data may be used to identify cancrinite-group minerals found at the Baikal lazurite deposits. It is near 1.0 in afghanite; 0.8 in tounkite; and more than 2 in davyne.     

青金石中的结构有序化和调制及其与光性异常间的关系

青金石结构有无序和有序之别。无序青金石为光性均质体,但它的有序化及由此引起的结构调制,则有可能使青金石产生光性异常。使结构仍保持立方对称的有序化并不改变它光性上的均质性,而其他形式的有序化以及结构调制,如果它们是多方向的 且在空间取向上具有立方对称的特征,则晶体仍是光性均质体,否则便转变为光性非均质体 。此外,还提出了根据选区电子衍射花样对结构调制方向进行三维判识的可能性;报道了在所研究青金石中发现的沿［110］方向n=4的有公度调制的有序结构,以及所观察到的其他有序结构和无公度调制结构。     

Vladimirivanovite, Na6Ca2[Al6Si6O24](SO4,S3,S2,Cl)2 · H2O, a new mineral of sodalite group

The results of an examination of vladimirivanovite, a new mineral of the sodalite group, found at the Tultui deposit in the Baikal region are discussed. The mineral occurs in the form of outer rims (0.01–3 mm thick) of lazurite, elongated segregations without faced crystals (0.2 to 3–4 mm in size; less frequently, 4 × 12–15 × 20 mm), and rare veinlets (up to 5 mm) hosted in calciphyre and marble. Vladimirivanovite is irregular and patchy dark blue. The mineral is brittle; on average, the microhardness VHN is 522–604, 575 kg/mm²; and the Mohs hardness is 5.0–5.5. The measured and calculated densities are 2.48(3) and 2.436 g/cm³, respectively. Vladimirivanovite is optically biaxial; 2V _meas = 63(±1)°, 2V _calc = 66.2°; the refractive indices are α = 1.502–1.507 (±0.002), N _m = 1.509–1.514 (±0.002), and N _g = 1.512–1.517 (±0.002). The chemical composition is as follows, wt %: 32.59 SiO₂, 27.39 Al₂O₃, 7.66 CaO, 17.74 Na₂O, 11.37 SO₃, 1.94 S, 0.12 Cl, and 1.0 H₂O; total is 99.62. The empirical formula calculated based on (Si + Al) = 12 with sulfide sulfur determined from the charge balance is Na_6.36Ca_1.52(Si_6.03Al_5.97)_Σ12O_23.99(SO₄)_1.58(S₃)_0.17(S₂)_0.08 · Cl_0.04 · 0.62H₂O; the idealized formula is Na₆Ca₂[Al₆Si₆O₂₄](SO₄,S₃,S₂,Cl)₂ · H₂O. The new mineral is orthorhombic, space group Pnaa; the unit-cell dimensions are a = 9.066, b = 12.851, c = 38.558 Å, V = 4492 ų, and Z = 6. The strongest reflections in the X-ray powder diffraction pattern (dÅ—I[hkl]) are: 6.61–5[015], 6.43–11[020, 006], 3.71–100[119, 133], 2.623–30[20.12, 240], 2.273–6[04.12], 2.141–14[159, 13.15], 1.783–9[06.12, 04.18], and 1.606–6[080, 00.24]. The crystal structure has been solved with a single crystal. The mineral was named in memoriam of Vladimir Georgievich Ivanov (1947–2002), Russian mineralogist and geochemist. The type material of the mineral is deposited at the Mineralogical Museum of St. Petersburg State University, St. Petersburg, Russia.     

The nature of the stability of an incommensurate 3D structural modulation in Baikal lazurite: Experimental data at 550°C

The nature of the stability of an incommensurate 3D modulation (ITM) in the structure of Baikal lazurite was evaluated using the methods of experimental geochemistry and X-ray photoelectron spectroscopy. It was shown that ITM with a period of 4.6a is preserved in the lazurite structure at 550°C almost without changes within the time interval from t = 100 h to at least 2000 h, although its initial (t = 0) development was not restored. In contrast to higher temperatures (≥ 600°C), the activities of gas species have no significant influence on the process of modulation release, except for the region of low O₂, S₂, and SO₂ fugacities, where the type of modulation changes, and the monosulfide ion appears in the lazurite composition. At T = 550°C and probably at lower temperatures, SO₂ fugacity ceases to be the critical parameter of ITM existence. The ordered state of polysulfide and sulfate clusters corresponding to the ITM period of Baikal cubic lazurite is stable at T = 550°C and is an example of forced equilibrium. It develops in response to a crystal chemical event occurring at a temperature of T _x within 600–550°C and is related to the thermal compression of the structure resulting in the isolation of structural cages containing clusters with different states of sulfur. Their mutual interaction, which leads to the rapid release of the modulation at higher temperatures owing to the equalizing of cluster sizes in the cages, ceases. As a result, the proportions of reduced (S₂²⁻, and S _x ²⁻) and oxidized (SO₄²⁻, So₃²⁻, and S₂O₃²⁻ sulfur species show negligible variations, and there is only partial reduction of sulfate to sulfite and thiosulfate. Lazurite samples with disulfide and polysulfide ions behave similarly, which suggests that an important condition for the preservation of ITM is the presence of sulfur-bearing anions with different sizes rather than particular sulfur species in structural cages. The degree of ordering in the distribution of clusters attained at T _x remains unchanged owing to the development of forced equilibrium maintained by the energy balance between framework deformation and cluster ordering. Natural lazurite with an ITM structure could not form at temperatures higher than T _x , i.e., above 550–600°C     

Mechanism and numerical simulation of multicomponent solute transport in sodic soils reclaimed by calcium sulfate

The mechanism for reclaiming sodic soils using calcium sulfate (CaSO₄) could provide a theoretical basis for the field application of CaSO₄ substitutes, including the by-products of flue gas desulfurization (BFGD), fly ash, and phosphorus gypsum. In this study, Ca²⁺ application experiment was conducted to analyze the dynamic changes of the cations in the reclamation of sodic soils with CaSO₄. A multicomponent solute transport model (UNSATCHEM) that considers ion adsorption exchange and dynamic changes in the soil’s hydraulic conductivity was subsequently used to simulate and predict the movement of ions. The Ca²⁺ application experiment consisted of four treatments with four CaSO₄ concentrations (0.5, 1, 1.5, and 2 g L^?1). When the Ca²⁺ concentrations in the supplied water were 14.71, 22.06, and 29.41 mmol L^?1, Ca²⁺ achieved penetration, and this process was faster when the Ca²⁺ concentration in the supplied water was higher. Ca²⁺ did not achieve penetration when the Ca²⁺ concentration was 7.35 mmol L^?1. UNSATCHEM was able to simulate the transportation mechanism of Ca²⁺ and Na⁺ in the soil solution in the Ca²⁺ application experiment, the adsorption and exchange between the Na⁺ in the soil colloid and Ca²⁺ in the soil solution, and the precipitation and dissolution of CaSO₄ with a high degree of accuracy. Sodic soil reclamation with CaSO₄ was not a short-term process. Compared with applying CaSO₄ only once, applying CaSO₄ in batches decreased the accumulation of soil salts and promoted its dissolution.     

Water soluble components of PM10 Chongqing, China

The concentrations of water soluble ions (Na⁺, NH₄ ⁺, K⁺, Mg²⁺, Ca²⁺, NO₃ ^-, Cl^-, and SO ₄ ^2- ) in PM₁₀ samples collected on cellulose filters by a medium-volume cascade impactor were determined, which were obtained from three kinds of areas in Chongqing: industrial area (Jiulongpo district), commercial and residential area (Jiangbei district) and background area (Jinyun Mountain in the Beibei district). The results showed that except for the background site, the annual average values of PM₁₀ are 23% – 61% higher than the national air quality standard (GradeII) (0.1 mg/m³), even that the value of the control site is still 20% higher than American standard (0.05 mg/m³). This implied that serious pollution of fine particles occurred in Chongqing. Nine kinds of soluble ions in water of PM₁₀ were analyzed by ion chromatography (IC) and the annual average concentrations follow the order of [SO ₄ ^2- ] > [NO³ _-] > [Cl^-] > [F^-], and [Ca²⁺] > [NH₄ ⁺] > [K⁺] > [Na⁺] > [Mg²⁺]. Their values were different in these areas: the industrial area > the commercial and living area > the control area. As for NH_{4 +}, K⁺, Ca²⁺, NO₃ ^- and SO ₄ ^2- , their seasonal average concentrations show a similar variation trend: the values in spring and fall were higher than those in summer and winter. The seasonal average concentrations of [Cl^-], [F^-], [Na⁺] and [Mg²⁺] are much lower than those of other ions. However, the concentrations of [Na⁺] changed more greatly in different seasons than those of the other three ions. Correlation coefficients showed that the three areas have been polluted by coal smoke and dust to different extents, while some local resources of pollution should be taken into consideration as well.     

Equation of state of hexagonal aluminous phase in basaltic composition to 63 GPa at 300 K

A laser-heated diamond-anvil cell that is capable of operating up to a pressure of 63 GPa, with X-ray diffraction facilities using a synchrotron radiation source at the SPring-8, has been developed to observe the compressibility of a hexagonal aluminous phase, [K_0.15Na_1.66Ca_0.11Mg_1.29Fe²⁺ _0.86Al_3.13Ti_0.09Si_1.98] _Σ9.27O₁₂. The hexagonal aluminous phase is a potassium host mineral from the subducted oceanic crust in the Earth's lower mantle. A sample was heated using a YAG laser at each pressure increment to relax the deviatoric stress in the sample. X-ray diffraction measurements were carried out at 300 K using an angle-dispersive technique. Pressure was measured using an internal platinum pressure calibrant. The observed unit-cell volumes were used to obtain a third-order Birch–Murnaghan equation of state: unit-cell volume V _o=185.94(±16) ų, density ρ _o=4.145 g/cm³, and bulk modulus K _o=198(±3) GPa when the first pressure is derivative of the bulk modulus K ^′ _o is fixed to 4. The density of hexagonal aluminous phase is lower than that of coexisting Mg-perovskite in the subducted oceanic crust.     

The crystal structure of ilinskite, NaCu5O2(SeO3)2Cl3, and review of mixed-ligand CuOmCln coordination geometries in minerals and inorganic compounds

The crystal structure of ilinskite, NaCu₅O₂(SeO₃)₂Cl₃, a rare copper selenite chloride from volcanic fumaroles of the Great fissure Tolbachik eruption (Kamchatka peninsula, Russia), has been solved by direct methods and refined to R ₁?=?0.044 on the basis of 2720 unique observed reflections. The mineral is orthorhombic, Pnma, a?=?17.769(7), b?=?6.448(3), c?=?10.522(4) Å, V?=?1205.6(8) ų, Z?=?4. The The CuO_mCl_n coordination polyhedra share edges to form tetramers that have 'additional' O1 and O2 atoms as centers. The O1Cu₄ and O2Cu₄ tetrahedra share common Cu atoms to form [O₂Cu₅]⁶⁺ sheets. The SeO₃ groups and Cl atoms are adjacent to the [O₂Cu₅]⁶⁺ sheets to form complex layers parallel to (100). The Na⁺ cations are located in between the layers. A review of mixed-ligand CuO_mCl_n coordination polyhedra in minerals and inorganic compounds is given. There are in total 26 stereochemically different mixed-ligand Cu-O-Cl coordinations.     

Crystal chemistry of natural and synthetic lead oxohalides: II. Crystal structure of Pb7O4(OH)4Br2

Crystals of lead oxobromide Pb₇O₄(OH)₄Br₂ have been synthesized by hydrothermal method. The structure of the new compound has been studied with X-ray single-crystal diffraction analysis. The compound is monoclinic, space group C112₁; unit-cell dimensions are a = 5.852(4), b = 13.452(7), c = 19.673(9) Å, γ = 90.04°, V = 1548.7(15) ų. The structure has been solved by direct methods and refined to R ₁ = 0.1138 for 1847 observed Pb₇O₄(OH)₄Br₂ unique reflections. The structure contains seven symmetrically independent bivalent Pb atoms. The coordination polyhedrons of Pb are strongly distorted due to stereochemical activity of unshared electron pair 6s ². Oxygen atoms are tetrahedrally coordinated by four Pb²⁺ cations with the formation of oxocentered tetrahedrons OPb₄. The compound is based on [O₂Pb₃]²⁺ double chains formed by OPb₄ tetrahedrons. (OH)Pb₂ dimers combine the [O₂Pb₃]²⁺ chains into 3D framework. Channels in the framework are parallel to [100] and are occupied by Br anions.     

Kyzylkumite: a finding in the southern Baikal region, Russia and refinement of its crystal chemical formula

Kyzylkumite has been found in Cr-V-bearing metamorphic rocks of the Sludyanka Complex, Southern Baikal region; it has been identified by X-ray powder diffraction method. This is a late secondary mineral developed after Ti-V-oxides (schreyerite, berdesinskiite) and V-bearing rutile and titanite. Kyzylkumite represents a new structural type with composition Ti₄V ₂ ³⁺ O₁₀(OH)₂ corresponding to octahedral coordination of Ti⁴⁺ and V³⁺. Its unit-cell dimensions are: a = 8.4787(1), b = 4.5624(1), c = 10.0330(1) Å, β = 93.174(1)°. The ideal formula of kyzylkumite Ti₄V ₂ ³⁺ O₁₀(OH)₂ corresponds to composition, wt %: 65.56 TiO₂, 30.75 V₂O₃, 3.69 H₂O. Indeed, the contents (wt %) of these constituents range from 62 to 70 TiO₂ and from 23 to 33 V₂O₃. Variations in contents and the Ti/V value are caused by partial substitution V³⁺ for V⁴⁺, isovalent substitutions Ti⁴⁺ and V³⁺ for V⁴⁺ and Cr³⁺, respectively, and coupled substitution V³⁺ + OH^? ? Ti⁴⁺ + O^2?. Smyslova et al. (1981)—the discovereres of kyzylkumite—assumed its composition to be the same as for schreyerite V ₂ ³⁺ Ti₃O₉ that principally different from kyzylkumite from the Sludyanka Complex. Therefore, re-examination of the kyzylkumite holotype or cotype from its type locality is needed.     

Theoretical modeling of the Al paramagnetic center and its precursors in stishovite

Previous electron paramagnetic resonance (EPR) spectroscopic study of gamma-ray-irradiated stishovite at 77 K detected an Al hole center, which was proposed to be an [O₂ ³⁻–Al³⁺] defect. First-principles quantum-mechanical calculations show that the unpaired spin is 85% localized on one of the six oxygen atoms at an AlO₆ octahedron, while the calculated ²⁷Al hyperfine constants are similar to those determined by EPR experiments. Theoretical results allow us to propose the Al center to represent an [AlO₆]⁰ defect, and hole hoping among equivalent oxygen atoms is responsible for its detection only at cryogenic temperatures. Theoretical calculations also show that the diamagnetic precursors [AlO₆/H⁺]⁰, [AlO₆/Li⁺]⁰, and [AlO₆/Na⁺]⁰ are stable in stishovite. The calculated OH bond distance and orientation are in excellent agreement with those inferred from FTIR spectra and previous theoretical calculations. The calculated [AlO₆/Li⁺]⁰ and [AlO₆/Na⁺]⁰ defects suggest that the monovalent cations such as Li⁺ and Na⁺ are potentially important in accommodating Al in stishovite in the lower mantle.     

Molar volume and thermal expansion of glaucophane

The molar volume of glaucophane [Na₂Mg₃Al₂Si₈O₂₂(OH)₂] has been determined in this study by correcting synthetic glaucophane-rich amphiboles made in the system Na₂O–MgO–Al₂O₃–SiO₂–H₂O for very small deviations from ideal glaucophane composition using recent volume data on key amphibole components. The derived unit-cell volume for end-member glaucophane is 862.7±1.6 Å³, which gives a molar volume of 259.8±0.5 cm³/mol and a calculated density of 3.016±0.006 g/cm³. This value has been corroborated through an essentially independent method by correcting the volumes of natural sodic amphiboles reported in the literature for non-glaucophane components, particularly including calcium-rich components, to yield a value of 861.2±1.9 Å³. The unit-cell volume derived from the synthetic amphiboles, which is considered here to be more reliable, is somewhat smaller than that reported previously in the literature. A thermal expansion (α_V) at 298 K of 1.88±0.06×10^?5/K was derived from unit-cell volumes measured in the range of 25–500°C for a synthetic glaucophane sample, which is noticeably smaller than previously reported.     

The measurement of sulfate mineral solubilities in the Na-K-Ca-Cl-SO4-H2O system at temperatures of 100, 150 and 200°C

At T > 100°C development of thermodynamic models suffers from missing experimental data, particularly for solubilities of sulfate minerals in mixed solutions. Solubilities in Na⁺-K⁺-Ca²⁺-Cl⁻-SO₄²⁻/H₂O subsystems were investigated at 150, 200°C and at selected compositions at 100°C. The apparatus used to examine solid-liquid phase equilibria under hydrothermal conditions has been described.In the system NaCl-CaSO₄-H₂O the missing anhydrite (CaSO₄) solubilities at high NaCl concentrations up to halite saturation have been determined. In the system Na₂SO₄-CaSO₄-H₂O the observed glauberite (Na₂SO₄ · CaSO₄) solubility is higher than that predicted by the high temperature model of Greenberg and Møller (1989), especially at 200°C. At high salt concentrations, solubilities of both anhydrite and glauberite increase with increasing temperature. Stability fields of the minerals syngenite (K₂SO₄ · CaSO₄ · H₂O) and goergeyite (K₂SO₄ · 5 CaSO₄ · H₂O) were determined, and a new phase was found at 200°C in the K₂SO₄-CaSO₄-H₂O system. Chemical and single crystal structure analysis give the formula K₂SO₄ · CaSO₄. The structure is isostructural with palmierite (K₂SO₄ · PbSO₄). The glaserite (“3 K₂SO₄ · Na₂SO₄”) appears as solid solution in the system Na₂SO₄-K₂SO₄-H₂O. Its solubility and stoichiometry was determined as a function of solution composition.     

Heterovalent isomorphism in the magnesium-iron borates

Orthorhombic magnesium-iron ludwigite-vonsenite forms a continuous isomorphic series Mg₂Fe³⁺[BO₃]O₂-Fe ₂ ²⁺ Fe[BO₃]O₂; its composition at the magnesioskarn and other deposits varies from magnesian to ferriferous members. In addition, they demonstrate isovalent substitution of Mn for Mg (in pinakiolite, blatterite, and others) and practically complete substitution of Ni for Mg (in bonaccordite). Ferric iron in the borates is substituted by isovalent Al and Cr. The incorporation of Ti, Sn, Sb, and V via heterovalent substitution has been studied in less detail. Our research revealed new manifestations of Ti-and Sn-bearing borates. They are magnesioludwigite and azoproite with variable Ti content, as well as by Sn-bearing aluminian borates formed via the 2Fe³⁺ → (Ti⁴⁺ + Mg)⁶⁺ and/or (Sn⁴⁺ + Mg)⁶⁺ substitution. The incorporation of pentavalent elements according to the scheme 3Fe³⁺ → (Sb⁵⁺ + 2Mg)⁹⁺ or (V⁵⁺ + 2Mg)⁹⁺ is not excluded. The highest Ti borates were found in the marbles and calciphyres of the Tazheran deposit in the Baikal region and Nalednoe, Dokuchan, and Titovskoe deposits in Yakutia, where azoproites contain more than 50 and even higher 75 mol % of the Mg₂(TiMg)_0.5[BO₃]O₂ end member. Aluminum magnesioludwigites from Yakutia and Chukotka simultaneously contain tin and titanium. Mount Brooks, Alaska, contains tin-bearing azoproite or its tin-bearing varieties. New data are reported on Sb-and V-bearing orthoborates. Calciphyres of Alaska contain monoclinic magnesiohulsite (Mg,Fe)₂(SnMg) _0.5 ⁶⁺ [BO₃]O₂, which is replaced by schoenfliesite MgSn(OH)₆. The studied borate occurrences belong to hypabyssal magnesian skarns of the periclase and monticellite metasomatic PT facies at contacts of dolomites with granitoid intrusions of increasing alkalinity or leucocratic granites. Their formation was related to interaction between disequilibrium kotoite and early oxides and spinellides of various compositions, on the one hand, and, on the other hand, to the influx of Ti-and Sn-bearing hydrothermal solutions.     

Günterblassite, (K,Ca)3 − x Fe[(Si,Al)13O25(OH,O)4] · 7H2O, a new mineral: the first phyllosilicate with triple tetrahedral layer

A new mineral, günterblassite, has been found in the basaltic quarry at Mount Rother Kopf near Gerolstein, Rheinland-Pfalz, Germany as a constituent of the late assemblage of nepheline, leucite, augite, phlogopite, åkermanite, magnetite, perovskite, a lamprophyllite-group mineral, götzenite, chabazite-K, chabazite-Ca, phillipsite-K, and calcite. Günterblassite occurs as colorless lamellar crystals up to 0.2 × 1 × 1.5 mm in size and their clusters. The mineral is brittle, with perfect cleavage parallel to (001) and less perfect cleavage parallel to (100) and (010). The Mohs hardness is 4. The calculated and measured density is 2.17 and 2.18(1) g/cm³, respectively. The IR spectrum is given. The new mineral is optically biaxial and positive as follows: α = 1.488(2), β = 1.490(2), γ = 1.493(2), 2V _meas = 80(5)°. The chemical composition (electron microprobe, average of seven point analyses, H₂O is determined by gas chromatography, wt %) is as follows: 0.40 Na₂O, 5.18 K₂O, 0.58 MgO, 3.58 CaO, 4.08 BaO, 3.06 FeO, 13.98 Al₂O₃, 52.94 SiO₂, 15.2 H₂O, and the total is 98.99. The empirical formula is Na_0.15K_1.24Ba_0.30Ca_0.72Mg_0.16F _0.48 ²⁺ [Si_9.91Al_3.09O_25.25(OH)_3.75] · 7.29H₂O. The crystal structure has been determined from a single crystal, R = 0.049. Günterblassite is orthorhombic, space group Pnm2₁; the unit-cell dimensions are a = 6.528(1), b = 6.970(1), c = 37.216(5) Å, V = 1693.3(4) ų, Z = 2. Günterblassite is a member of a new structural type; its structure is based on three-layer block [Si₁₃O₂₅(OH,O)₄]. The strong reflections in the X-ray powder diffraction pattern [d Å (I, %) are as follows: 6.532 (100), 6.263 (67), 3.244 (49), 3.062 (91), 2.996 (66), 2.955 (63), and 2.763 (60). The mineral was named in honor of Günter Blass (born in 1943), a well-known amateur mineralogist and specialist in electron microprobe and X-ray diffraction. The type specimen of günterblassite is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, with the registration number 4107/1.     

Rate Equations and an Ion-pair Mechanism for Batch Dissolution of Gypsum: Repositioning the Shrinking Object Model at the Core of Hydrodynamic Modelling

Recent improvements to experiments and modelling of batch dissolution in a turbulent reactor, based upon the shrinking object model, are extended to middle loadings of gypsum, that is, in the region between low and high loadings, which lead, respectively, to high under-saturation or saturation with a great excess of solid left undissolved. Dissolved calcium sulphate concentration was monitored by change in electrical conductivity. This investigation uses an improved, ion-pair model for CaSO ₄ ⁰ to allow for the presence of calcium or sulphate added as common ions. The study demonstrates that the full dissolution curve for 5.82 mM loadings of 106-μm particles of gypsum (~1.00 g L^?1) in de-ionised water barely changed in the presence of either 4.64 or 8.09 mM calcium chloride, or 4.39 mM sodium sulphate. However, this masked a doubling of dissolution rate imposed by comparable increases in ionic strength from sodium chloride. The results are consistent with the ion pair, CaSO ₄ ⁰ , being the key species in the rate-determining step of the back-reaction, and perhaps all salt dissolutions, including calcium carbonate. In this case, the rate equation is as follows: \( {\frac{{{\text{d}}c}}{{{\text{d}}t}}} = \frac{S}{V} \cdot (k_{1} - k_{2}^{\prime } \cdot [{\text{CaSO}}_{ 4}^{0} ]) \), where k ₁ and k ₂′ are rate constants. The reported observations are interpreted as effects of ionic strength and common ion concentrations upon the formation equilibrium for the ion pair. This rate equation readily transforms mathematically to one involving the product of [Ca²⁺] and [SO₄ ^2?] in the back-reaction. The parallel of this with the well-known PWP equation used in calcium carbonate dissolution is discussed, with the CaHCO₃ ⁺ ion pair of the equation being replaced by that of CaCO ₃ ⁰ . Meanwhile, the earlier use of the product, [Ca²⁺]^½ × [CO₃ ^2?]^½, in the back-reaction term of another dissolution rate equation for calcite is shown to be incorrect. Finally, it is argued that the shrinking object model should be repositioned as a logical derivative of the hydrodynamical approach to dissolution.     

Ferroelastic phase transition in cryolite,Na3AlF6, a mixed fluoride perovskite: High temperature single crystal X-ray diffraction study and symmetry analysis of the transition mechanism

Cryolite, Na₃AlF₆[ = 2Na⁺(Na_0.5 ⁺Al_0.5 ³⁺)F₃] is a mixed fluoride perovskite, in which the corner-sharing octahedral framework is formed by alternating [NaF₆] and [AlF₆] octahedra and the cavities are occupied by Na⁺ ions. At 295 K, it is monoclinic (α phase), space group P2 ₁/n with a = 5.4139 (7), b = 5.6012 (5) and c = 7.7769 (8) Å and β = 90.183 (3)°, Z = 2. A high temperature single crystal X-ray diffraction study in the range 295–900 K indicates a fluctuation-induced first-order phase transition from monoclinic to orthorhombic symmetry at T ₀ ～ 885 K, in contrast to a previous report that it becomes cubic at ～823 K. The space group of the high temperature β phase is Immm with a = 5.632 (4), b = 5.627 (3) and c = 7.958 (4) Å, Z = 2 at 890 K. Above T ₀, the coordination number of the Na⁺ ion in the cavity increases from eight to twelve and the zigzag Na1 — Al octahedral chains parallel to c become straight with the Na1-F-Al angle = 180 °. The phase transition is driven by two coupled primary order parameters. The first corresponds to the rotation of the nearly rigid [AlF₆] group and transforms according to the Γ ₄ ⁺ irreducible representation of Immm. Coupled to the [AlF₆] rotation is a second primary order parameter corresponding to the displacement of the Na2⁺ ion in the cavity from its equilibrium position. This order parameter transforms according to the X ₃ ⁺ irreducible representation of Immm. Following Immm → P2 ₁ /n phase transition, four equivalent domains of P2 ₁/n are determined relative to Immm, which are in an antiphase and/or twin relationship. The abrupt shortening of the octahedral Al-F and Na-F bonds and a sudden change in orientations of the atomic thermal vibration ellipsoids above T ₀ indicate a crossover from displacive to an order-disorder mechanism near the transition temperature. The β phase is interpreted as a dynamic average of four micro-twin and -antiphase domains of the a phase. This view is consistent with the entropy of phase transition, ΔS_trans (11.43 JK^?1 mol^?1) calculated from heat capacity measurements (Anovitz et al. 1987), which corresponds closely to R ln4 (11.53 JK^?1 mol^?1), where 4 is the number of domains formed during the phase transition. The dynamic nature of the β phase is independently confirmed from a considerable narrowing of the ²⁷Al nuclear magnetic resonance (NMR) line-shape above T ₀ (Stebbins et al. 1992).     

High pressure single-crystal synthesis, structure and compressibility of the garnet Mn2+ 3Mn3+ 2[SiO4]3

Single crystals of the garnet Mn²⁺ ₃Mn³⁺ ₂[SiO₄]₃ and coesite were synthesised from MnO₂-SiO₂ oxide mixtures at 1000°C and 9 GPa in a multianvil press. The crystal structure of the garnet [space group Ia3¯d, a=11.801(2) Å] was refined at room temperature and 100 K from single-crystal X-ray data to R1=2.36% and R1=2.71%, respectively. In contrast to tetragonal Ca₃Mn³⁺ ₂[GeO₄]₃ (space group I4₁/a), the high-pressure garnet is cubic and does not display an ordered Jahn-Teller distortion of octahedral Mn³⁺. A disordered Jahn-Teller distortion either dynamic or static is evidenced by unusual high anisotropic displacement parameters. The room temperature structure is characterised by following bond lengths: Si-O=1.636(4) Å (tetrahedron), Mn³⁺-O=1.995 (4) Å (octahedron), Mn²⁺-O=2.280(5) and 2.409(4) Å (dodecahedron). The cubic structure was preserved upon cooling to 100 K [a=11.788(2) Å] and upon compressing up to 11.8 GPa in a diamond-anvil cell. Pressure variation of the unit cell parameter expressed by a third-order Birch-Murnaghan equation of state led to a bulk modulus K ₀=151.6(8) GPa and its pressure derivatives K′=6.38(19). The peak positions of the Raman spectrum recorded for Mn²⁺ ₃Mn³⁺ ₂[SiO₄]₃ were assigned based on a calderite Mn²⁺ ₃Fe³⁺ ₂[SiO₄]₃ model extrapolated from andradite and grossular literature data.     

The thermal stability of sideronatrite and its decomposition products in the system Na2O–Fe2O3–SO2–H2O

The thermal stability of sideronatrite, ideally Na₂Fe³⁺(SO₄)₂(OH)·3(H₂O), and its decomposition products were investigated by combining thermogravimetric and differential thermal analysis, in situ high-temperature X-ray powder diffraction (HT-XRPD) and Fourier transform infrared spectroscopy (HT-FTIR). The data show that for increasing temperature there are four main dehydration/transformation steps in sideronatrite: (a) between 30 and 40 °C sideronatrite transforms into metasideronatrite after the loss of two water molecules; both XRD and FTIR suggest that this transformation occurs via minor adjustments in the building block. (b) between 120 and 300 °C metasideronatrite transforms into metasideronatrite II, a still poorly characterized phase with possible orthorhombic symmetry, consequently to the loss of an additional water molecule; X-ray diffraction data suggest that metasideronatrite disappears from the assemblage above 175 °C. (c) between 315 and 415 °C metasideronatrite II transforms into the anhydrous Na₃Fe(SO₄)₃ compound. This step occurs via the loss of hydroxyl groups that involves the breakdown of the [Fe³⁺(SO₄)₂(OH)] _∞ ^2? chains and the formation of an intermediate transient amorphous phase precursor of Na₃Fe(SO₄)₃. (d) for T > 500 °C, the Na₃Fe(SO₄)₃ compound is replaced by the Na-sulfate thenardite, Na₂SO₄, plus Fe-oxides, according to the Na₃Fe³⁺(SO₄)₃ → 3/2 Na₂(SO₄) + 1/2 Fe₂O₃ + SO_x reaction products. The Na–Fe sulfate disappears around 540 °C. For higher temperatures, the Na-sulfates decomposes and only hematite survives in the final product. The understanding of the thermal behavior of minerals such as sideronatrite and related sulfates is important both from an environmental point of view, due to the presence of these phases in evaporitic deposits, soils and sediments including extraterrestrial occurrences, and from the technological point of view, due to the use of these materials in many industrial applications.