Ammonium ion behaviour in feldspar: variable-temperature infrared and 2H NMR studies of synthetic buddingtonite, N(D,H)4AlSi3O8

The behaviour of the ammonium ion in synthetic buddingtonite, N(D,H)₄AlSi₃O₈, has been studied by infrared (IR) spectroscopy from 20 K to 298 K and by ²H NMR spectroscopy from 120 K to 298 K. IR spectra were collected from 500 to 3500 cm^–1. Static ²H NMR spectra collected at 298 K and 120 K are very similar, consisting of a single sharp isotropic resonance, indicating complete averaging of quadrupolar interactions and implying that at these temperatures the ammonium ion is in rapid (<1 s) randomised motion within the M-site cavity of the feldspar framework. NMR spectroscopy indicates that the splitting of the internal modes of the ammonium ion observed by IR spectroscopy is not due to freezing in of the ammonium ion. This observation rules out the formation of a preferred N–H...O hydrogen bond, with precession of the ion about it, as proposed by Kimball and Megaw (1978), because any N–H...O hydrogen bond must be very weak and transient in nature. Contraction of the cavity site upon cooling imposes a distortion upon the ammonium ion that affects vibrational modes. This distortion does not affect the motion of the ammonium ion as observed on the NMR time-scale.     

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.     

An infrared and <Superscript>1</Superscript>H MAS NMR investigation of strong hydrogen bonding in ussingite,Na<Subscript>2</Subscript>AlSi<Subscript>3</Subscript>O<Subscript>8</Subscript>(OH)

The mineral ussingite, Na₂AlSi₃O₈(OH), an interrupted tectosilicate, has strong hydrogen bonding between OH and the other nonbridging oxygen atom in the structure. Infrared spectra contain a strongly polarized, very broad OH-stretching band with an ill-defined maximum between 1500 and 1800 cm^–1, and a possible OH librational bending mode at 1295 cm^–1. The IR spectra confirm the orientation of the OH vector within the triclinic unit cell as determined from X-ray refinement (Rossi et al. 1974). There are three distinct bands in the ¹H NMR spectrum of ussingite: a predominant band at 13.5 ppm (TMS) representing 90% of the structural hydrogen, a second band at 15.9 ppm corresponding to 8% of the protons, and a third band at 11.0 ppm accounting for the remaining 2% of structural hydrogen. From the correlation between hydrogen bond length and ¹H NMR chemical shift (Sternberg and Brunner 1994), the predominant hydrogen bond length (H...O) was calculated to be 1.49 Å, in comparison to the hydrogen bond length determined from X-ray refinement (1.54 Å). The population of protons at 15.9 ppm is consistent with 5–8% Al–Si disorder. Although the ussingite crystal structure and composition are similar to those of low albite, the bonding environment of OH in low albite and other feldspars, as characterized through IR and ¹H NMR, is fundamentally different from the strong hydrogen bonding found in ussingite.     

In situ high-temperature X-ray and neutron powder diffraction study of cation partitioning in synthetic Mg(Fe<Subscript>0.5</Subscript>Al<Subscript>0.5</Subscript>)<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel

The intra-crystalline cation partitioning over T- and M-sites in a synthetic Mg(Fe,Al)₂O₄ spinel sample has been determined as a function of temperature by Rietveld structure refinements from powder diffraction data, combining in situ high-temperature neutron powder diffraction (NPD; POLARIS diffractometer, at ISIS, Rutherford Appleton Laboratory, UK), to determine the Mg and Al occupancy factors, with in situ high-temperature X-ray powder diffraction, to fix the Fe³⁺ distribution. The results obtained agree with a two-stage reaction, in which an initial exchange between Fe³⁺ and Mg, the former leaving and the latter entering tetrahedral sites, is successively followed by a rearrangement involving also Al. The measured cation distribution has then been compared and discussed with that calculated by the Maximum Configuration Entropy principle, for which only NPD patterns have been used. The cation partitioning has finally been interpreted in the light of the configuration model of O’Neill and Navrotsky.     

Calorimetric study on high-pressure transitions in KAlSi<Subscript>3</Subscript>O<Subscript>8</Subscript>

KAlSi₃O₈ sanidine dissociates into a mixture of K₂Si₄O₉ wadeite, Al₂SiO₅ kyanite and SiO₂ coesite, which further recombine into KAlSi₃O₈ hollandite with increasing pressure. Enthalpies of KAlSi₃O₈ sanidine and hollandite, K₂Si₄O₉ wadeite and Al₂SiO₅ kyanite were measured by high-temperature solution calorimetry. Using the data, enthalpies of transitions at 298 K were obtained as 65.1 ± 7.4 kJ mol^–1 for sanidine wadeite + kyanite + coesite and 99.3 ± 3.6 kJ mol^–1 for wadeite + kyanite + coesite hollandite. The isobaric heat capacity of KAlSi₃O₈ hollandite was measured at 160–700 K by differential scanning calorimetry, and was also calculated using the Kieffer model. Combination of both the results yielded a heat-capacity equation of KAlSi₃O₈ hollandite above 298 K as C_p=3.896 × 10²–1.823 × 10³T^–0.5–1.293 × 10⁷T^–2+1.631 × 10⁹T^–3 (C_p in J mol^–1 K^–1, T in K). The equilibrium transition boundaries were calculated using these new data on the transition enthalpies and heat capacity. The calculated transition boundaries are in general agreement with the phase relations experimentally determined previously. The calculated boundary for wadeite + kyanite + coesite hollandite intersects with the coesite–stishovite transition boundary, resulting in a stability field of the assemblage of wadeite + kyanite + stishovite below about 1273 K at about 8 GPa. Some phase–equilibrium experiments in the present study confirmed that sanidine transforms directly to wadeite + kyanite + coesite at 1373 K at about 6.3 GPa, without an intervening stability field of KAlSiO₄ kalsilite + coesite which was previously suggested. The transition boundaries in KAlSi₃O₈ determined in this study put some constraints on the stability range of KAlSi₃O₈ hollandite in the mantle and that of sanidine inclusions in kimberlitic diamonds.     

Characterisation of buddingtonite (NH4)[AlSi3O8] and ND4-buddingtonite (ND4)[AlSi3O8] using IR spectroscopy and Rietveld refinement of XRD spectra

Buddingtonite (NH₄)[AlSi₃O₈] and its deuterated analogue ND₄-buddingtonite (ND₄)[AlSi₃O₈] have been synthesised in 150-mg amounts at 500 and 400?°C and 500?MPa in 5-mm-wide, 4-cm-long Au capsules using René metal hydrothermal autoclaves. The resultant product consists of clumps of monoclinic crystals with diameters of 30–60?μm. The ND₄-buddingtonite contains minor amounts of NH₄-buddingtonite due to H₂ migration across the Au membrane. Using this synthesis technique resulted in >99% pure buddingtonite in 20% of the synthesis runs with the remaining synthesis runs containing very minor tobelite and quartz on the order of a few percent. IR spectra obtained from powdered samples are assigned on the basis of T _d symmetry for the ammonium molecule. They show triply degenerate vibrational bands (i.e. ν₃ and ν₄) and some overtones and combination modes from NH₄ ⁺ and ND₄ ⁺. While T _d symmetry for NH₄ ⁺ in buddingtonite is not completely correct due to distortion of the NH₄ ⁺ molecule, the non-cubic field is not large enough to cause a substantial splitting in the bands. However, this perturbation is documented in the IR spectra by a substantial increase in the FWHH as well as the occurrence of shoulders on the broadened bands. Rietveld analysis indicates that buddingtonite, like orthoclase, has a monoclinic structure with space group symmetry C2/m. Here, the NH₄ ⁺ molecule replaces the K⁺ cation on the nine fold coordinated A site which has m symmetry. Due to the larger size of the NH₄ ⁺ molecule, the N–O interatomic distances are larger than the K–O distances in pure orthoclase and range from 2.95 to 3.16?Å. This results in an increase in the volume of the polyhedron hosting the NH₄ ⁺ molecule. Also, in contrast to orthoclase, the polyhedron hosting the NH₄ ⁺ molecule becomes more regular. The rigid Al, Si tetrahedra of the framework adjust to this expansion of the A site by rotation. This results in larger unit cell parameters for buddingtonite when compared to natural and synthesised potassium feldspars. This increase is especially seen with respect to the lattice constants a and b and the monoclinic angle β which also are found to be extremely variable. In contrast, the c direction remains nearly unchanged. Investigations using IR spectroscopy indicate that it is unlikely that this variation in the a, b and β cell dimensions is caused by incorporation of H₃O⁺ or zeolitic water. Instead, it is more likely that substitution of NH₄ ⁺ for K⁺ coupled with Al, Si disorder are the chief contributors to these variations in the unit cell parameters for buddingtonite.     

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

Structure of magnetite (Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>) above the Curie temperature: a cation ordering study

A pristine magnetite (Fe₃O₄) specimen was studied by means of Neutron Powder Diffraction in the 273–1,073 K temperature range, in order to characterize its structural and magnetic behavior at high temperatures. An accurate analysis of the collected data allowed the understanding of the behavior of the main structural and magnetic features of magnetite as a function of temperature. The magnetic moments of both tetrahedral and octahedral sites were extracted by means of magnetic diffraction up to the Curie temperature (between 773 and 873 K). A change in the thermal expansion coefficient around the Curie temperature together with an increase in the oxygen coordinate value above 700 K can be observed, both features being the result of a change in the thermal expansion of the tetrahedral site. This anomaly is not related to the magnetic transition but can be explained with an intervened cation reordering, as magnetite gradually transforms from a disordered configuration into a partially ordered one. Based on a simple model which takes into account the cation-oxygen bond length, the degree of order as a function of temperature and consequently the enthalpy and entropy of the reordering process were determined. The refined values are ΔH⁰ = −23.2(1.7) kJ mol⁻¹ and ΔS⁰ = −16(2) J K⁻¹ mol⁻¹. These results are in perfect agreement with values reported in literature (Mack et al. in Solid State Ion 135(1–4):625–630, 2000; Wu and Mason in J Am Ceramic Soc 64(9):520–522, 1981).     

Structure and chemistry of (111) twin boundaries in MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel crystals from Mogok

The atomic scale structure and chemistry of (111) twins in MgAl₂O₄ spinel crystals from the Pinpyit locality near Mogok (Myanmar, formerly Burma) were analysed using complementary methods of transmission electron microscopy (TEM). To obtain a three-dimensional information on the atomic structure, the twin boundaries were investigated in crystallographic projections and Using conventional electron diffraction and high-resolution TEM (HRTEM) analysis we have shown that (111) twins in spinel can be crystallographically described by 180° rotation of the oxygen sublattice normal to the twin composition plane. This operation generates a local hcp stacking in otherwise ccp lattice and maintains a regular sequence of kagome and mixed layers. In addition to rotation, no other translations are present in (111) twins in these spinel crystals. Chemical analysis of the twin boundary was performed by energy-dispersive X-ray spectroscopy (EDS) using a variable beam diameter (VBD) technique, which is perfectly suited for analysing chemical composition of twin boundaries on a sub-nm scale. The VBD/EDS measurements indicated that (111) twin boundary in spinel is Mg-deficient. Quantitative analyses of HRTEM (phase contrast) and HAADF-STEM (Z-contrast) images of (111) twin boundary have confirmed that Mg²⁺ ions are replaced with Be²⁺ ions in boundary tetrahedral sites. The Be-rich twin boundary structure is closely related to BeAl₂O₄ (chrysoberyl) and BeMg₃Al₈O₁₆ (taaffeite) group of intermediate polysomatic minerals. Based on these results, we conclude that the formation of (111) twins in spinel is a preparatory stage of polytype/polysome formation (taaffeite) and is a result of thermodynamically favourable formation of hcp stacking due to Be incorporation on the {111} planes of the spinel structure in the nucleation stage of crystal growth. The twin structure grows as long as the surrounding geochemical conditions allow its formation. The incorporation of Be induces a 2D-anisotropy and exaggerated growth of the crystal along the (111) twin boundary.     

First-principles simulation of high-pressure polymorphs in MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript>

We have used density functional theory to investigate the stability of MgAl₂O₄ polymorphs under pressure. Our results can reasonably explain the transition sequence of MgAl₂O₄ polymorphs observed in previous experiments. The spinel phase (stable at ambient conditions) dissociates into periclase and corundum at 14 GPa. With increasing pressure, a phase change from the two oxides to a calcium-ferrite phase occurs, and finally transforms to a calcium-titanate phase at 68 GPa. The calcium-titanate phase is stable up to at least 150 GPa, and we did not observe a stability field for a hexagonal phase or periclase + Rh₂O₃(II)-type Al₂O₃. The bulk moduli of the phases calculated in this study are in good agreement with those measured in high-pressure experiments. Our results differ from those of a previous study using similar methods. We attribute this inconsistency to an incomplete optimization of a cell shape and ionic positions at high pressures in the previous calculations.     

The nature of blue luminescence from natural benitoite BaTiSi<Subscript>3</Subscript>O<Subscript>9</Subscript>

It is proved that blue luminescence from benitoite is connected with intrinsic luminescence centers, namely isolated TiO₆ octahedra. The metastable level ³T_1u is the emitting level at low temperatures with a long decay time of 1.1 ms. At higher temperatures an energy level with higher radiation probability must be involved in the emission process, and this level is situated at 0.06 eV higher than the lowest level. These two levels may be connected with ³T_1u level splitting or with closely spaced ³T_1u and ³T_2u levels. Decay time shortening and thermal quenching are connected with nonradiative decay within the TiO₆ luminescence center, while energy migration does not take place at least up to room temperature.     

Batisivite,V<Subscript>8</Subscript>Ti<Subscript>6</Subscript>[Ba(Si<Subscript>2</Subscript>O)]O<Subscript>28</Subscript>, a new mineral species from the derbylite group

Batisivite has been found as an accessory mineral in the Cr-V-bearing quartz-diopside metamorphic rocks of the Slyudyanka Complex in the southern Baikal region, Russia. A new mineral was named after the major cations in its ideal formula (Ba, Ti, Si, V). Associated minerals are quartz, Cr-V-bearing diopside and tremolite; calcite; schreyerite; berdesinskiite; ankangite; V-bearing titanite; minerals of the chromite-coulsonite, eskolaite-karelianite, dravite-vanadiumdravite, and chernykhite-roscoelite series; uraninite; Cr-bearing goldmanite; albite; barite; zircon; and unnamed U-Ti-V-Cr phases. Batisivite occurs as anhedral grains up to 0.15–0.20 mm in size, without visible cleavage and parting. The new mineral is brittle, with conchoidal fracture. Observed by the naked eye, the mineral is black and opaque, with a black streak and resinous luster. Batisivite is white in reflected light. The microhardness (VHN) is 1220–1470 kg/mm² (load is 30 g), the mean value is 1330 kg/mm². The Mohs hardness is near 7. The calculated density is 4.62 g/cm³. The new mineral is weakly anisotropic and bireflected. The measured values of reflectance are as follows (λ, nm—R _max ^′ /R _min ^′ ): 440—17.5/17.0; 460—17.3/16.7; 480—17.1/16.5; 500—17.2/16.6; 520—17.3/16.7; 540—17.4/16.8; 560—17.5/16.8; 580—17.6/16.9; 600—17.7/17.1; 620—17.7/17.1; 640—17.8/17.1; 660—17.9/17.2; 680—18.0/17.3; 700—18.1/17.4. Batisivite is triclinic, space group P \(\overline 1\); the unit-cell dimensions are: a = 7.521(1) Å, b = 7.643(1) Å, c = 9.572(1) Å, α = 110.20°(1), β = 103.34°(1), γ = 98.28°(1), V = 487.14(7) ų, Z = 1. The strongest reflections in the X-ray powder diffraction pattern [d, Å (I, %)(hkl)] are: 3.09(8)(12\(\overline 2\)); 2.84, 2.85(10)(021, 120); 2.64(8)(21\(\overline 3\)); 2.12(8)(31\(\overline 3\)); 1.785(8)(32\(\overline 4\)), 1.581(10)(24\(\overline 2\)); 1.432, 1.433(10)(322, 124). The chemical composition (electron microprobe, average of 237 point analyses, wt %) is: 0.26 Nb₂O₅, 6.16 SiO₂, 31.76 TiO₂, 1.81 Al₂O₃, 8.20 VO₂, 26.27 V₂O₃, 12.29 Cr₂O₃, 1.48 Fe₂O₃, 0.08 MgO, 11.42 BaO; the total is 99.73. The VO₂/V₂O₃ ratio has been calculated. The simplified empirical formula is (V _4.8 ³⁺ Cr_2.2V _0.7 ⁴⁺ Fe_0.3)_8.0(Ti_5.4V _0.6 ⁴⁺ )_6.0[Ba(Si_1.4Al_0.5O_0.9)]O₂₈. An alternative to the title formula could be a variety (with the diorthogroup Si₂O₇) V₈Ti₆[Ba(Si₂O₇)]O₂₂. Batisivite probably pertains to the V ₈ ³⁺ Ti ₆ ⁴⁺ [Ba(Si₂O)]O₂₈-Cr ₈ ³⁺ Ti ₆ ⁴⁺ [Ba(Si₂O)]O₂₈ solid solution series. The type material of batisivite has been deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.     

Thermoelastic properties of chromium oxide Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> (eskolaite) at high pressures and temperatures

A new synchrotron X-ray diffraction study of chromium oxide Cr₂O₃ (eskolaite) with the corundum-type structure has been carried out in a Kawai-type multi-anvil apparatus to pressure of 15 GPa and temperatures of 1873 K. Fitting the Birch–Murnaghan equation of state (EoS) with the present data up to 15 GPa yielded: bulk modulus (K _0,T0), 206 ± 4 GPa; its pressure derivative K′_0,T , 4.4 ± 0.8; (?K _0,T /?T) = ?0.037 ± 0.006 GPa K^?1; a = 2.98 ± 0.14 × 10^?5 K^?1 and b = 0.47 ± 0.28 × 10^?8 K^?2, where α _0,T  = a + bT is the volumetric thermal expansion coefficient. The thermal expansion of Cr₂O₃ was additionally measured at the high-temperature powder diffraction experiment at ambient pressure and α _0,T0 was determined to be 2.95 × 10^?5 K^?1. The results indicate that coefficient of the thermal expansion calculated from the EoS appeared to be high-precision because it is consistent with the data obtained at 1 atm. However, our results contradict α ₀ value suggested by Rigby et al. (Brit Ceram Trans J 45:137–148, 1946) widely used in many physical and geological databases. Fitting the Mie–Grüneisen–Debye EoS with the present ambient and high-pressure data yielded the following parameters: K _0,T0 = 205 ± 3 GPa, K′_0,T  = 4.0, Grüneisen parameter (γ ₀) = 1.42 ± 0.80, q = 1.82 ± 0.56. The thermoelastic parameters indicate that Cr₂O₃ undergoes near isotropic compression at room and high temperatures up to 15 GPa. Cr₂O₃ is shown to be stable in this pressure range and adopts the corundum-type structure. Using obtained thermoelastic parameters, we calculated the reaction boundary of knorringite formation from enstatite and eskolaite. The Clapeyron slope (with \({\text{d}}P/{\text{d}}T = - 0.014\) GPa/K) was found to be consistent with experimental data.     

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.     

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.     

Theoretical studies of cation adsorption on hydroxylated α-A1<Subscript>2</Subscript>O<Subscript>3</Subscript> (corundum): electronic structure calculations

 The adsorption of alkali metal cations on a hydroxylated corundum surface was investigated using high-level electronic structure calculations, with both cluster Hartree–Fock and periodic density-functional theory approaches. The work concentrates on the structural aspects of binding sites with threefold oxygen coordination at the basal (0001) surface. It was found that adsorption at different sites can give rise to a wide range of adsorption energies, which strongly depends on the freedom of surface hydrogen atoms to adjust their positions. Alkali metal adions from Li⁺ to Cs⁺ were studied with the cluster method, periodic plane-wave pseudopotential calculations being carried out for K⁺ adsorption to validate the cluster results. A site above an octahedral interstice was found to be the least preferred for cation adsorption, despite having the lowest repulsion from surface aluminium atoms. The strongest adsorption was found over an aluminium atom in the second layer, because the hydroxyl groups could reorient towards the neighbouring octahedral interstices, and hence significantly decrease repulsion with the cation. The adsorption energy and the first three interlayer spacings parallel to the basal surface change systematically with ionic size for each adsorption site. Many of these trends extend to adsorption of Ca²⁺, Co²⁺ and Pb²⁺, which were also investigated, although a redistribution of 3d electrons in Co²⁺ results in strong adsorption even at an unfavourable site. The results suggest that it may be possible not only to predict adsorption behaviour for a wide range of elements, but also to use experimental measurements of interplanar separations to gain information about contaminated surfaces. Received: 29 April 2002 / Accepted: 23 October 2002 Acknowledgements The authors thank the Natural Environment Research Council for support in carrying out this work.     

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).     

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.     

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.