Effects of temperature on the crystal structure of epidote: a neutron single-crystal diffraction study at 293 and 1,070 K

The effects of temperature on the crystal structure of a natural epidote [Ca_1.925 Fe_0.745Al_2.265Ti_0.004Si_3.037O₁₂(OH), a = 8.890(6), b = 5.630(4), c = 10.150(6) Å and β = 115.36(5)°, Sp. Gr. P2₁ /m] have been investigated by means of neutron single-crystal diffraction at 293 and 1,070 K. At room conditions, the structural refinement confirms the presence of Fe³⁺ at the M₃ site [%Fe(M3) = 73.1(8)%] and all attempts to refine the amount of Fe at the M(1) site were unsuccessful. Only one independent proton site was located. Two possible hydrogen bonds, with O(2) and O(4) as acceptors [i.e. O(10)–H(1)···O(2) and O(10)–H(1)···O(4)], occur. However, the topological configuration of the bonds suggests that the O(10)–H(1)···O(4) is energetically more favourable, as H(1)···O(4) = 1.9731(28) Å, O(10)···O(4) = 2.9318(22) Å and O(10)–H(1)···O4 = 166.7(2)°, whereas H(1)···O(2) = 2.5921(23) Å, O(10)···O(2) = 2.8221(17) Å and O(10)–H(1)···O2 = 93.3(1)°. The O(10)–H(1) bond distance corrected for “riding motion” is 0.9943 Å. The diffraction data at 1,070 K show that epidote is stable within the T-range investigated, and that its crystallinity is maintained. A positive thermal expansion is observed along all the three crystallographic axes. At 1,070 K the structural refinement again shows that Fe³⁺ share the M(3) site along with Al³⁺ [%Fe(M3)_1,070K = 74(2)%]. The refined amount of Fe³⁺ at the M(1) is not significant [%Fe(M1)_1,070K = 1(2)%]. The tetrahedral and octahedral bond distances and angles show a slight distortion of the polyhedra at high-T, but a significant increase of the bond distances compared to those at room temperature is observed, especially for bond distances corrected for “rigid body motions”. The high-T conditions also affect the inter-polyhedral configurations: the bridging angle Si(2)–O(9)–Si(1) of the Si₂O₇ group increases significantly with T. The high-T structure refinement shows that no dehydration effect occurs at least within the T-range investigated. The configuration of the H-bonding is basically maintained with temperature. However, the hydrogen bond strength changes at 1,070 K, as the O(10)···O(4) and H(1)···O(4) distances are slightly longer than those at 293 K. The anisotropic displacement parameters of the proton site are significantly larger than those at room condition. Reasons for the thermal stability of epidote up to 1,070 K observed in this study, the absence of dehydration and/or non-convergent ordering of Al and Fe³⁺ between different octahedral sites and/or convergent ordering on M(3) are discussed.     

A variable-temperature neutron diffraction study of ussingite; a strong asymmetric hydrogen bond in an aluminosilicate framework

A powder neutron diffraction study of ussingite, Na₂AlSi₃O₈(OH), over the temperature range 4–850 K has been undertaken. The strong hydrogen bond that exists in this mineral has been accurately determined with the O–H distance at 1.070(8) Å and an O(donor)–O(acceptor) separation of 2.481(5) Å at 4 K, The distribution of hydrogen along the O–O direction remains asymmetric between 4 and 850 K with the H atom being fully ordered at a single site, rather than partially disordered over two sites of a double-potential well, as in serandite. A gradual increase in the bonded O–H distance at higher temperatures is observed, indicative of a broadening of the potential well in which the hydrogen atom is sited. Below 50 K, the material shows negative thermal expansion, likely to be associated with reduced bending motion of the O–H bond.     

On the crystal structure and compressional behavior of talc: a mineral of interest in petrology and material science

The crystal structure of a natural triclinic talc (1Tc polytype) [with composition: (Mg_2.93Fe_0.06)_Σ2.99(Al_0.02Si_3.97)_Σ3.99O₁₀(OH)_2.10] has been investigated by single-crystal X-ray diffraction at 223 and 170 K and by single-crystal neutron diffraction at 20 K. Both the anisotropic X-ray refinements (i.e. at 223 and 170 K) show that the two independent tetrahedra are only slightly distorted. For the two independent Mg-octahedra, the bond distances between cation-hydroxyl groups are significantly shorter than the others. The ditrigonal rotation angle of the six-membered ring of tetrahedra is modest (α ~ 4°). The neutron structure refinement shows that the hydrogen-bonding scheme in talc consists of one donor site and three acceptors (i.e. trifurcated configuration), all the bonds having O···O ≤ 3.38 Å, H···O ~ 2.8 Å, and O–H···O ~ 111–116°. The three acceptors belong to the six-membered ring of tetrahedra juxtaposed to the octahedral sheet. The vibrational regime of the proton site appears being only slightly anisotropic. The elastic behavior of talc was investigated by means of in situ synchrotron single-crystal diffraction up to 16 GPa (at room temperature) using a diamond anvil cell. No evidence of phase transition has been observed within the pressure range investigated. P–V data fit, with an isothermal third-order Birch-Murnaghan equation of state, results as follows: V ₀ = 454.7(10) Å³, K _T0 = 56(3) GPa, and K′ = 5.4(7). The “Eulerian finite strain” versus “normalized stress” plot yields: Fe(0) = 56(2) GPa and K′ = 5.3(5). The compressional behavior of talc is strongly anisotropic, as reflected by the axial compressibilities (i.e. β(a):β(b):β(c) = 1.03:1:3.15) as well as by the magnitude and orientation of the unit-strain ellipsoid (with ε ₁:ε ₂:ε ₃ = 1:1.37:3.21). A comparison between the elastic parameters of talc obtained in this study with those previously reported is carried out.     

High-temperature neutron diffraction study of deuterated brucite

To study the structural behavior of brucite at high temperature, we conducted in situ neutron diffraction experiments of a deuterated brucite powder sample, Mg(OD)₂, in the temperature range 313–583 K. The sample was stable up to 553 K, above which it started to decompose into periclase (MgO) and D₂O vapor. Rietveld analyses of the obtained data were performed using both single-site and three-site split-atom hydrogen models. Our results show that with increasing temperature, unit-cell parameter c increases at a rate ~7.7 times more rapidly than a. This large anisotropy of thermal expansion is primarily due to rapid increase in the interlayer thickness along the c-axis on heating. The amplitudes of thermal vibration for Mg, O, and D increase linearly with increasing temperature; however, the rate of the increase for the lighter D is much larger. In addition, D vibrates anisotropically with a higher magnitude within the (001) plane, as confirmed by our first-principles phonon calculations. On heating, the interatomic distances between a given D and its associated O and D from the adjacent [MgO₆] layer increase, whereas the O–D bond length decreases. This behavior suggests weakened D···O and D···D interlayer interactions but strengthened O–D bonding with increasing temperature.     

Hydrogen bonding in coquimbite, nominally Fe2(SO4)3·9H2O, and the relationship between coquimbite and paracoquimbite

Using single-crystal X-ray diffraction at 293, 200 and 100 K, and neutron diffraction at 50 K, we have refined the positions of all atoms, including hydrogen atoms (previously undetermined), in the structure of coquimbite ( $ P {\bar 3}1c $ , a?=?10.924(2)/10.882(2) Å, c?=?17.086(3) / 17.154(3) Å, V?=?1765.8(3)/1759.2(5) ų, at 293 / 50 K, respectively). The use of neutron diffraction allowed us to determine precise and accurate hydrogen positions. The O–H distances in coquimbite at 50 K vary between 0.98 and 1.01 Å. In addition to H₂O molecules coordinated to the Al³⁺ and Fe³⁺ ions, there are rings of six “free” H₂O molecules in the coquimbite structure. These rings can be visualized as flattened octahedra with the distance between oxygen and the geometric center of the polyhedron of 2.46 Å. The hydrogen-bonding scheme undergoes no changes with decreasing temperature and the unit cell shrinks linearly from 293 to 100 K. A review of the available data on coquimbite and its “dimorph” paracoquimbite indicates that paracoquimbite may form in phases closer to the nominal composition of Fe₂(SO₄)₃·9H₂O. Coquimbite, on the other hand, has a composition approximating Fe_1.5Al_0.5(SO₄)₃·9H₂O. Hence, even a “simple” sulfate Fe_2-x Al _x (SO₄)₃·9H₂O may be structurally rather complex.     

Spectroscopic and bond-topological investigation of interstitial volatiles in beryl from Slovakia

Nine beryl samples from Western Carpathians, Slovakia, were investigated by infrared and Raman spectroscopy and differential thermal analysis. Two types of water H₂O I and H₂O II were detected. Infrared spectroscopy proved the presence of water type I and II in the presence of alkali cations with several bands: (1) symmetric stretching vibration—ν₁; (2) antisymmetric stretching mode—ν₃; (3) bending vibration—ν₂. The presence of singly and doubly coordinated type II water (IIs and IId) was confirmed by single-crystal IR spectroscopy. From Raman spectra a band at 3606 cm^?1 was assigned to ν₁ of water type I and the range of 3597–3600 cm^?1 to water type II. The presence of doubly coordinating water indicates a relatively highly hydrated environment with the presence of alkali ions including Na as the dominant cation coordinated by H₂O II. CO₂ bands were detected only by single-crystal IR spectroscopy. Thermal analysis proved total water loss in the range of 1.4–2.0 wt% and three main dehydration events. Based on the study of bond-topological arrangements two molecules of H₂O IId are each bound with two H···O1 bonds and one Na–O_W bond with an angular distortion, and by releasing one H₂O molecule more stable H₂O IIs is produced. The H₂O I molecule is bound only by two equivalent hydrogen bonds. The H₂O IIs molecule with a Na–O_W bond strength of 0.28 vu and two H···O1 bonds of 0.14 vu without any forced angular distortion is the most stable of all.     

Crystal structure, Raman and FTIR spectroscopy, and equations of state of OH-bearing MgSiO3 akimotoite

MgSiO₃ akimotoite is stable relative to majorite-garnet under low-temperature geotherms within steeply or rapidly subducting slabs. Two compositions of Mg–akimotoite were synthesized under similar conditions: Z674 (containing about 550 ppm wt H₂O) was synthesized at 22 GPa and 1,500 °C and SH1101 (nominally anhydrous) was synthesized at 22 GPa and 1,250 °C. Crystal structures of both samples differ significantly from previous studies to give slightly smaller Si sites and larger Mg sites. The bulk thermal expansion coefficients of Z674 are (153–839 K) of a ₁ = 20(3) × 10^?9 K^?2 and a ₀ = 17(2) × 10^?6 K^?1, with an average of α ₀ = 27.1(6) × 10^?6 K^?1. Compressibility at ambient temperature of Z674 was measured up to 34.6 GPa at Sector 13 (GSECARS) at Advanced Photon Source Argonne National Laboratory. The second-order Birch–Murnaghan equation of state (BM2 EoS) fitting yields: V ₀ = 263.7(2) Å³, K _T0 = 217(3) GPa (K′ fixed at 4). The anisotropies of axial thermal expansivities and compressibilities are similar: α _a  = 8.2(3) and α _c  = 10.68(9) (10^?6 K^?1); β _a  = 11.4(3) and β _c  = 15.9(3) (10^?4 GPa). Hydration increases both the bulk thermal expansivity and compressibility, but decreases the anisotropy of structural expansion and compression. Complementary Raman and Fourier transform infrared (FTIR) spectroscopy shows multiple structural hydration sites. Low-temperature and high-pressure FTIR spectroscopy (15–300 K and 0–28 GPa) confirms that the multiple sites are structurally unique, with zero-pressure intrinsic anharmonic mode parameters between ?1.02 × 10^?5 and +1.7 × 10^?5 K^?1, indicating both weak hydrogen bonds (O–H···O) and strong OH bonding due to long O···O distances.     

Boron K-edge XANES of boron oxides: tetrahedral B–O distances and near-surface alteration

Synchrotron radiation boron K-edge XANES spectra collected in fluorescence yield mode are reported for monoclinic metaboric acid [HBO₂(II)], sinhalite (MgAlBO₄), and a selection of boron oxides in which B is exclusively in trigonal coordination (^[3]B). The anomalously high divergence of tetrahedral (^[4]B–O) bond lengths in HBO₂(II) and sinhalite is used to resolve fine structure at the ^[4]B K edge due to splitting of σ*(t₂) antibonding orbitals. For HBO₂(II), XANES peaks at 196.9 and 199.3?eV are assigned to ^[4]B–O distances of 1.564 and ～1.440 (×3) Å, respectively, and, for sinhalite, peaks at 196.8, 197.9, and 199.6?eV are assigned to distances of 1.586, 1.483 (×2), and 1.442?Å, respectively. A correlation between peak splitting at the ^[4]B K edge and divergence of tetrahedral bond length is established for borates and borosilicates using data for sinhalite, HBO₂(II), ferroaxinite, danburite, datolite, and BPO₄. B K-edge XANES spectra collected in total electron yield mode, which probes to <60?Å, show that almost all ^[4]B in HBO₂(II) and about one-third of the ^[4]B in sinhalite are converted to ^[3]B in the near-surface structure. Moreover, HBO₂(II), HBO₂(III), sassolite (boric acid; H₃BO₃), and v-B₂O₃, which have markedly different bulk structures, have a similar near-surface layer composed of a relaxed anhydrous network of BO₃ groups.     

Natrolite,part I: Refinement of high-order data,separation of internal and external vibrational amplitudes from displacement parameters

A single crystal of natrolite, Na₂Al₂Si₃O₁₀·2H₂O, was studied by X-ray diffraction methods at room temperature. The intensities were measured with MoKα radiation (λ = 0.7107 Å) in a complete sphere of reflection up to sin θ/λ = 0.903 Å^?1. The structure was refined in the orthorhombic space group Fdd2 with a = 18.2929 (7) Å, b = 18.6407(9) Å, c = 6.5871(6) Å, V = 2246 ų, Z = 8. A refinement of high-order diffraction data yielded reliability factors of R(F) = 0.9%, R _w(F) = 0.8%, GoF = 1.40 for 1856 high-angle reflections (0.7 ?in θ/λ <0.903 Å^?1) and R(F) = 1.0%, R _W(F) = 1.2%, GoF = 3.07 for all 3471 independent reflections in the complete sphere of reflection. The T-O distances as well as the T-O-T angles were found to be strongly influenced by the different bond strengths received by the individual oxygen atoms. The T O distances calculated using Baur's extended valence rule agree on average within 0.003 Å with the observed values. An analysis of the mean square displacement amplitudes allowed a separation of the external and internal vibrational amplitudes along the T-O bonds as well as along the Na O and H₂O-O bond directions and the calculation of force constants. The internal vibrational amplitudes (ΔU) of the T-O vibrations are in the range of 5 to 11 × 10^-4 Ų, that is about one order of magnitude smaller than the mean square displacement amplitudes of the external vibrations. The corresponding force constants are F = 354 to 824 Nm^?1. The values of the force constants of the motion of the Na-ion and the water molecule against the framework oxygen atoms lie in the range between F = 57 and 293 Nm^?1. This is the first instance where displacement amplitudes from a zeolite structure refinement could be apportioned between contributions from internal and external vibrations for individual bonds.     

The crystal structure and thermal expansion tensor of MgSO<Subscript>4</Subscript>–11D<Subscript>2</Subscript>O(meridianiite) determined by neutron powder diffraction

We have collected high-resolution neutron powder diffraction patterns from MgSO₄·11D₂O over the temperature range 4.2–250 K. The crystal is triclinic, space-group \( \text{P} \bar{1} \) (Z = 2) with a = 6.72746(6) Å, b = 6.78141(6) Å, c = 17.31803(13) Å, α = 88.2062(6)°, β = 89.4473(8)°, γ = 62.6075(5)°, and V = 701.140(6) ų at 4.2 K, and a = 6.75081(3) Å, b = 6.81463(3) Å, c = 17.29241(6) Å, α = 88.1183(3)°, β = 89.4808(3)°, γ = 62.6891(3)°, and V = 706.450(3) ų at 250 K. Structures were refined to wRp = 3.99 and 2.84% at 4.2 and 250 K, respectively. The temperature dependence of the lattice parameters over the intervening range have been fitted with a modified Einstein oscillator model which was used to obtain the coefficients of the thermal expansion tensor. The volume thermal expansion, α_V, is considerably smaller than ice Ih at all temperatures, and smaller even than MgSO₄·7D₂O (although ?α_V/?T is very similar for both sulfates); MgSO₄·11D₂O exhibits negative α_V below 55 K (compared to 70 K in D₂O ice Ih and 20 K in MgSO₄·7D₂O) The relationship between the magnitude and orientation of the principal axes of the expansion tensor and the main structural elements are discussed.     

Natrolite,part II: Determination of deformation electron densities by the X-X method

A single crystal of natrolite, Na₂Al₂Si₃O₁₀ ·2H₂O (space group Fdd2), was studied by X-ray diffraction methods at room temperature. The intensities were measured in a complete sphere of reflection up to sinΘ/ λ=0.903 Å^?1. A refinement of high-order diffraction data yielded residuals of R/(F)=0.9%, R_w(F)=0.8%, GoF=1.40 for 1856 high-angle reflections (0.7≤sinΘ/ λ≤0.903 Å^?1) and R(F)=1.0%, R_w(F)=1.2%, GoF=3.07 for all 3471 independent reflections in the complete sphere of reflection. The X-X method was used to calculate deformation electron densities (DED) in natrolite. Within all tetrahedra, residual electron density-was found in the T-O bond directions indicating a considerable covalent contribution to the chemical bond. The range of the interatomic peak heights was from 0.19 to 0.34 e/ų in the SiO₄ tetrahedra and from 0.11 to 0.23 e/ų in the AlO₄ tetrahedron. The ionic contribution to the chemical bond manifests itself in the displacement of the peaks towards the oxygen atoms. Charge displacement due to interaction of nonframework cations with framework oxygen atoms as well as electron densities attributable to the lone pair orbitals in the water molecule have been observed.     

Thermodynamic properties of synthetic calcium-free carbonate cancrinite

Calcium-free carbonate cancrinite with formula unit Na_8.28[Al_5.93Si_6.07O₂₄](CO₃)_0.93(OH)_0.49·3.64H₂O (CAN) has been synthesized under hydrothermal conditions. The product has been characterized by the methods of scanning electronic microscopy and energy dispersive X-ray analysis, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis with FTIR of evolved gases (TGA–FTIR), and X-ray powder diffraction. The heat capacity of CAN has been measured from 6 to 259 K via low-temperature adiabatic calorimetry. A linear combination of Einstein functions has been used to approximate the obtained data on the heat capacity. The thermal contributions to the entropy and enthalpy of CAN in the temperature range 0–300 K have been calculated from these data. The heat capacity and third-law absolute entropy of CAN at 298.15 K are 1,047 ± 30 and 1,057 ± 35 J mol^?1 K^?1, respectively. High-temperature oxide-melt solution calorimetry has been used to determine the enthalpy of formation from elements of CAN at 298.15 K; the value equals ?14,684 ± 50 kJ mol^?1. The Gibbs energy of formation from elements at 298.15 K has been calculated and totaled ?13,690 ± 51 kJ mol^?1.     

Behavior of 10-Å phase at low temperatures

The thermal evolution of 10-Å phase Mg₃Si₄O₁₀(OH)₂·H₂O, a phyllosilicate which may have an important role in the storage/release of water in subducting slabs, was studied by X-ray single-crystal diffraction in the temperature range 116–293 K. The lattice parameters were measured at several intervals both on cooling and heating. The structural model was refined with intensity data collected at 116 K and compared to the model refined at room temperature. As expected for a layer silicate on cooling in this temperature range, the a and b lattice parameters undergo a small linear decrease, α _a  = 1.7(4) 10^?6 K^?1 and α _b  = 1.9(4) 10^?6 K^?1, where α is the linear thermal expansion coefficient. The greater variation is along the c axis and can be modeled with the second order polynomial c _T  = c ₂₉₃(1 + 6.7(4)10^?5 K^?1ΔT + 9.5(2.5)10^?8 K^?2(ΔT)²) where ΔT = T ? 293 K; the monoclinic angle β slightly increased. The cell volume thermal expansion can be modeled with the polynomial V _T  = V ₂₉₃ (1 + 8.0 10^?5 K^?1 ΔT + 1.4 10^?7 K^?2 (ΔT)²) where ΔT = T ? 293 is in K and V in ų. These variations were similar to those expected for a pressure increase, indicating that T and P effects are approximately inverse. The least-squares refinement with intensity data measured at 116 K shows that the volume of the SiO₄ tetrahedra does not change significantly, whereas the volume of the Mg octahedra slightly decreases. To adjust for the increased misfit between the tetrahedral and octahedral sheets, the tetrahedral rotation angle α changes from 0.58° to 1.38°, increasing the ditrigonalization of the silicate sheet. This deformation has implications on the H-bonds between the water molecule and the basal oxygen atoms. Furthermore, the highly anisotropic thermal ellipsoid of the H₂O oxygen indicates positional disorder, similar to the disorder observed at room temperature. The low-temperature results support the hypothesis that the disorder is static. It can be modeled with a splitting of the interlayer oxygen site with a statistical distribution of the H₂O molecules into two positions, 0.6 Å apart. The resulting shortest O_bas–O_W distances are 2.97 Å, with a significant shortening with respect to the value at room temperature. The low-temperature behavior of the H-bond system is consistent with that hypothesized at high pressure on the basis of the Raman spectra evolution with P.     

Titanium- and water-rich metamorphic olivine in high-pressure serpentinites from the Voltri Massif (Ligurian Alps,Italy): evidence for deep subduction of high-field strength and fluid-mobile elements

Titanium- and water-rich metamorphic olivine (Fo 86–88) is reported from partially dehydrated serpentinites from the Voltri complex, Ligurian Alps. The rocks are composed of mostly antigorite and olivine in addition to magnetite, chlorite, clinopyroxene and Ti-clinohumite. In situ secondary ion mass spectrometry (SIMS) data show that metamorphic olivine has very high and strongly correlated H₂O (up to 0.7 wt%) and TiO₂ contents (up to 0.85 wt%). Ti-rich olivine shows colourless to yellow pleochroism. Olivine associated with Ti-clinohumite contains low Ti, suggesting that Ti-rich olivine is not the breakdown product of Ti-clinohumite. Fourier transform infrared spectroscopy (FTIR) absorption spectra show peaks of serpentine, Ti-clinohumite and OH-related Si vacancies. Combining FTIR and SIMS data, we suggest the presence of clustered planar defects or nanoscale exsolutions of Ti-clinohumite in olivine. These defects or exsolutions contain more H₂O (x ~ 0.1 in the formula 4Mg₂SiO₄·(1?x)Mg(OH,F)₂·xTiO₂) than Ti-clinohumite in the sample matrix (x = 0.34–0.46). In addition to TiO₂ and H₂O, secondary olivine contains significant Li (2–60 ppm), B (10–20 ppm), F (10–130 ppm) and Zr (0.9–2.1 ppm). It is enriched in ¹¹B (δ¹¹B = +17 to +23 ‰). Our data indicate that secondary olivine may play a significant role in transporting water, high-field strength and fluid-mobile elements into the deeper mantle as well as introduce significant B isotope anomalies. Release of hydrogen from H₂O-rich olivine subducted into the deep mantle may result in strongly reduced mantle domains.     

Synthesis and magnetic properties of centennialite: a new <Emphasis Type="Italic">S</Emphasis> = ½ Kagomé antiferromagnet and comparison with herbertsmithite and kapellasite

Minerals of the atacamite group such as herbertsmithite and kapellasite have recently attracted enormous attention as the S = ½ Kagomé antiferromagnets for achieving the quantum spin liquid (QSL) state with diverse technological applications. Herein we report on the synthesis of the newly discovered mineral centennialite by using an unconventional “solid-state” reaction method at 463 K. Synthetic centennialite, Ca_1.06Cu_2.94Cl_2.01(OH)_5.99·0.73H₂O, has been characterized by scanning electron microscopy, electron microprobe analyses, Fourier-transform infrared spectroscopy, thermogravimetric and differential scanning calorimetric analyses, single-crystal X-ray diffraction structure refinements, and magnetic susceptibility measurements. The crystal structure of centennialite is characterized by a perfect (threefold symmetry) Kagomé layer with <5 % substitution between Ca and Cu and therefore differs from those of herbertsmithite and kapellasite, in which 15–25 % mixing between similar Zn and Cu atoms dramatically affects the QSL state. Centennialite remains antiferromagnetic down to ~7 K with a moderate spin frustration (i.e., a Weiss temperature θ = ?56 K and a spin frustration parameter f = 8), but exhibits a canted antiferromagnetic ordering with a ferromagnetic component at lower temperatures.     

Five-coordinate Cd in the crystal structure of triploidite-type Cd2(AsO4)(OH)

This paper reports on hydrothermal synthesis and crystal structure refinement of dicadmium arsenate hydroxide, Cd₂(AsO₄)(OH), obtained at 220 °C and autogenous pressure. Its crystal structure is monoclinic, space group P2₁/a, with a = 13.097(3), b = 14.089(3), c = 10.566(2) Å, β = 108.38(3)°, V = 1850.2(6) ų (Z = 16). It is isotypic with the members of the triploidite group of minerals and synthetic compounds, and thus shows a close topological relationship with the triplite group. The complex framework contains edge- and corner-sharing CdO₄(OH) and CdO₄(OH)₂ polyhedra, linked via corner-sharing to AsO₄ tetrahedra (average As—O distances range between 1.682 and 1.688 Å). Four five-coordinated Cd sites are at the centers of distorted trigonal bipyramids (average Cd—O distances are between 2.225 and 2.251 Å), whereas the remaining four Cd sites have a distorted octahedral coordination environment (average Cd—O distances are between 2.297 and 2.320 Å). The positions of all the hydrogen atoms were located in a difference-Fourier map and refined with an isotropic displacement parameter. The hydrogen-bonds are weak to very weak. The unusual five-coordination of Cd is briefly discussed in relation to comparable minerals and compounds. Among triploidite-type compounds, Cd₂(AsO₄)(OH) is the member with the largest unit cell reported so far, and the second known arsenate member.     

Thermal expansion of hydrated six-coordinate silicon in thaumasite,Ca<Subscript>3</Subscript>Si(OH)<Subscript>6</Subscript>(CO<Subscript>3</Subscript>)(SO<Subscript>4</Subscript>)·12H<Subscript>2</Subscript>O

Thaumasite, Ca₃Si(OH)₆(CO₃)(SO₄)12H₂O, occurs as a low-temperature secondary alteration phase in mafic igneous and metamorphic rocks, and is recognized as a product and indicator of sulfate attack in Portland cement. It is also the only mineral known to contain silicon in six-coordination with hydroxyl (OH)^? that is stable at ambient P–T conditions. Thermal expansion of the various components of this unusual structure has been determined from single-crystal X-ray structure refinements of natural thaumasite at 130 and 298 K. No phase transitions were observed over this temperature range. Cell parameters at room temperature are: a= 11.0538(6) Å, c=10.4111(8) Å and V=1101.67(10) ų, and were measured at intervals of about 50 K between 130 and 298 K, resulting in mean axial and volumetric coefficients of thermal expansion (×10^?5K^?1); α _a =1.7(1), α _c =2.1(2), and α _V =5.6(2). Although the unit cell and ^VIIICaO₈ polyhedra show significant positive thermal expansion over this temperature range, the silicate octahedron, sulfate tetrahedron, and carbonate group show zero or negative thermal expansion, with α _V (^VISiO₆) = ?0.6 ± 1.1, α _V (^IVSO₄)=?5.8 ± 1.4, and α _R (C–O)= 0.0 ± 1.8 (×10^?5 K^?1). Most of the thermal expansion is accommodated by lengthening of the R(O...O) hydrogen bond distances by on average 5σ, although the hydrogen bonds involving hydroxyl sites on ^VISi expand twice as much as those on molecular water, causing the [Ca₃Si(OH)₆(H₂O)₁₂]⁴⁺ columns to expand in diameter more than they move apart over this temperature range. The average Si–OH bond length of the six-coordinated Si atom 〈R(^VISi–OH)〉 in thaumasite is 1.783(1) Å, being about 0.02 Å (?20σ) shorter than ^VISi–OH in the dense hydrous magnesium silicate, phase D, MgSi₂H₂O₆.     

A high-pressure neutron diffraction study of the ferroelastic phase transition in RbCaF3

The fluoroperovskite phase RbCaF₃ has been investigated using high-pressure neutron powder diffraction in the pressure range ~0–7.9 GPa at room temperature. It has been found to undergo a first-order high-pressure structural phase transition at ~2.8 GPa from the cubic aristotype phase to a hettotype phase in the tetragonal space group I4/mcm. This transition, which also occurs at ~200 K at ambient pressure, is characterised by a linear phase boundary and a Clapeyron slope of 2.96 × 10^?5 GPa K^?1, which is in excellent agreement with earlier, low-pressure EPR investigations. The bulk modulus of the high-pressure phase (49.1 GPa) is very close to that determined for the low-pressure phase (50.0 GPa), and both are comparable with those determined for the aristotype phases of CsCdF₃, TlCdF₃, RbCdF₃, and KCaF₃. The evolution of the order parameter with pressure is consistent with recent modifications to Landau theory and, in conjunction with polynomial approximations to the pressure dependence of the lattice parameters, permits the pressure variation of the bond lengths and angles to be predicted. On entering the high-pressure phase, the Rb–F bond lengths decrease from their extrapolated values based on a third-order Birch–Murnaghan fit to the aristotype equation of state. By contrast, the Ca–F bond lengths behave atypically by exhibiting an increase from their extrapolated magnitudes, resulting in the volume and the effective bulk modulus of the CaF₆ octahedron being larger than the cubic phase. The bulk moduli for the two component polyhedra in the tetragonal phase are comparable with those determined for the constituent binary fluorides, RbF and CaF₂.     

Kobyashevite, Cu5(SO4)2(OH)6·4H2O, a new devilline-group mineral from the Vishnevye Mountains, South Urals, Russia

A new mineral kobyashevite, Cu₅(SO₄)₂(OH)₆·4H₂O (IMA 2011–066), was found at the Kapital’naya mine, Vishnevye Mountains, South Urals, Russia. It is a supergene mineral that occurs in cavities of a calcite-quartz vein with pyrite and chalcopyrite. Kobyashevite forms elongated crystals up to 0.2 mm typically curved or split and combined into thin crusts up to 1?×?2 mm. Kobyashevite is bluish-green to turquoise-coloured. Lustre is vitreous. Mohs hardness is 2½. Cleavage is {010} distinct. D(calc.) is 3.16 g/cm³. Kobyashevite is optically biaxial (?), α 1.602(4), β 1.666(5), γ 1.679(5), 2 V(meas.) 50(10)°. The chemical composition (wt%, electron-microprobe data) is: CuO 57.72, ZnO 0.09, FeO 0.28, SO₃ 23.52, H₂O(calc.) 18.39, total 100.00. The empirical formula, calculated based on 18 O, is: Cu_4.96Fe_0.03Zn_0.01S_2.01O_8.04(OH)_5.96·4H₂O. Kobyashevite is triclinic, $ P\overline{\,1 } $ , a 6.0731(6), b 11.0597(13), c 5.5094(6)?Å, α 102.883(9)°, β 92.348(8)°, γ 92.597(9)°, V 359.87(7)?ų, Z?=?1. Strong reflections of the X-ray powder pattern [d,Å-I(hkl)] are: 10.84–100(010); 5.399–40(020); 5.178–12(110); 3.590–16(030); 2.691–16(20–1, 040, 002), 2.653–12(04–1, 02–2), 2.583–12(2–11, 201, 2–1–1), 2.425–12(03–2, 211, 131). The crystal structure (single-crystal X-ray data, R?=?0.0399) сontains [Cu₄(SO₄)₂(OH)₆] corrugated layers linked via isolated [CuO₂(H₂O)₄] octahedra; the structural formula is CuCu₄(SO₄)₂(OH)₆·4H₂O. Kobyashevite is a devilline-group member. It is named in memory of the Russian mineralogist Yuriy Stepanovich Kobyashev (1935–2009), a specialist on mineralogy of the Urals.