Equation of state of adamite up to 11 GPa: a synchrotron X-ray diffraction study

The compression behavior of natural adamite [Zn₂AsO₄OH] has been investigated up to 11.07 GPa at room temperature utilizing in situ angle-dispersive X-ray diffraction and a diamond anvil cell. No phase transition has been observed within the pressure range investigated. A third-order Birch–Murnaghan equation of state fitted to all of the data points yielded V ₀ = 430.1(4) Å³, K ₀ = 80(3) GPa, K′ ₀ = 1.9(5). The K ₀ was obtained as 69(1) GPa when K′ ₀ was fixed at 4. Analysis of axial compressible moduli shows the intense compression anisotropy of adamite: K _a0 = 37(3) GPa, K _b0 = 153(6) GPa, K _c0 = 168(8) GPa; hence, a axis is the most compressible and the compressibility of b and c axis is comparable. Furthermore, the comparisons among the compressional properties of adamite, libethenite (Cu₂PO₄OH, also belongs to olivenite group), and andalusite (Al₂SiO₄O has the similar structure with adamite) at high pressure were made.     

Behavior of epidote at high pressure and high temperature: a powder diffraction study up to 10 GPa and 1,200 K

The thermo-elastic behavior of a natural epidote [Ca_1.925 Fe_0.745Al_2.265Ti_0.004Si_3.037O₁₂(OH)] has been investigated up to 1,200 K (at 0.0001 GPa) and 10 GPa (at 298 K) by means of in situ synchrotron powder diffraction. No phase transition has been observed within the temperature and pressure range investigated. P–V data fitted with a third-order Birch–Murnaghan equation of state (BM-EoS) give V ₀ = 458.8(1)ų, K _T0 = 111(3) GPa, and K′ = 7.6(7). The confidence ellipse from the variance–covariance matrix of K _T0 and K′ from the least-square procedure is strongly elongated with negative slope. The evolution of the “Eulerian finite strain” vs “normalized stress” yields Fe(0) = 114(1) GPa as intercept values, and the slope of the regression line gives K′ = 7.0(4). The evolution of the lattice parameters with pressure is slightly anisotropic. The elastic parameters calculated with a linearized BM-EoS are: a ₀ = 8.8877(7) Å, K _T0(a) = 117(2) GPa, and K′(a) = 3.7(4) for the a-axis; b ₀ = 5.6271(7) Å, K _T0(b) = 126(3) GPa, and K′(b) = 12(1) for the b-axis; and c ₀ = 10.1527(7) Å, K _T0(c) = 90(1) GPa, and K’(c) = 8.1(4) for the c-axis [K _T0(a):K _T0(b):K _T0(c) = 1.30:1.40:1]. The β angle decreases with pressure, β_P(°) = β_P0 −0.0286(9)P +0.00134(9)P ² (P in GPa). The evolution of axial and volume thermal expansion coefficient, α, with T was described by the polynomial function: α(T) = α₀ + α₁ T ^−1/2. The refined parameters for epidote are: α₀ = 5.1(2) × 10⁻⁵ K⁻¹ and α₁ = −5.1(6) × 10⁻⁴ K^1/2 for the unit-cell volume, α₀(a) = 1.21(7) × 10⁻⁵ K⁻¹ and α₁(a) = −1.2(2) × 10⁻⁴ K^1/2 for the a-axis, α₀(b) = 1.88(7) × 10⁻⁵ K⁻¹ and α₁(b) = −1.7(2) × 10⁻⁴ K^1/2 for the b-axis, and α₀(c) = 2.14(9) × 10⁻⁵ K⁻¹ and α₁(c) = −2.0(2) × 10⁻⁴ K^1/2 for the c-axis. The thermo-elastic anisotropy can be described, at a first approximation, by α₀(a): α₀(b): α₀(c) = 1 : 1.55 : 1.77. The β angle increases continuously with T, with β_T(°) = β_T0 + 2.5(1) × 10⁻⁴ T + 1.3(7) × 10⁻⁸ T ². A comparison between the thermo-elastic parameters of epidote and clinozoisite is carried out.     

Structural evolution of a 2M 1 phengite mica up to 11 GPa: an in situ single-crystal X-ray diffraction study

The structural evolution at high pressure of a natural 2M ₁-phengite [(K_0.98Na_0.02)_Σ=1.00(Al_1.55Mg_0.24Fe_0.21Ti_0.02)_Σ=2.01(Si_3.38Al_0.62)O₁₀(OH)₂; a = 5.228(2), b = 9.057(3), c = 19.971(6)Å, β = 95.76(2)°; space group: C2/c] from the metamorphic complex of Cima Pal (Sesia Zone, Western Alps, Italy) was studied by single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions up to ~11 GPa. A series of 12 structure refinements were performed at selected pressures within the P range investigated. The compressional behaviour of the same phengite sample was previously studied up to ~25 GPa by synchrotron X-ray powder diffraction, showing an irreversible transformation with a drastic decrease of the crystallinity at P > 15–17 GPa. The elastic behaviour between 0.0001 and 17 GPa was modelled by a third-order Birch–Murnaghan Equation of State (BM-EoS), yielding to K _T0 = 57.3(10) GPa and K′ = ?K _T0/?P = 6.97(24). The single-crystal structure refinements showed that the significant elastic anisotropy of the 2M ₁-phengite (with β(a):β(b):β(c) = 1:1.17:4.60) is mainly controlled by the anisotropic compression of the K-polyhedra. The evolution of the volume of the inter-layer K-polyhedron as a function of P shows a negative slope, Fitting the P–V(K-polyhedron) data with a truncated second-order BM-EoS we obtain a bulk modulus value of K _T0(K-polyhedron) = 26(1) GPa. Tetrahedra and octahedra are significantly stiffer than the K-polyhedron. Tetrahedra behave as quasi-rigid units within the P range investigated. In contrast, a monotonic decrease is observed for the octahedron volume, with K _T0 = 120(10) GPa derived by a BM-EoS. The anisotropic response to pressure of the K-polyhedron affects the P-induced deformation mechanism on the tetrahedral sheet, consisting in a cooperative rotation of the tetrahedra and producing a significant ditrigonalization of the six-membered rings. The volume of the K-polyhedron and the value of the ditrigonal rotation parameter (α) show a high negative correlation (about 93%), though a slight discontinuity is observed at P >8 GPa. α increases linearly with P up to 7–8 GPa (with ?α/?P ≈ 0.7°/GPa), whereas at higher Ps a “saturation plateau” is visible. A comparison between the main deformation mechanisms as a function of pressure observed in 2M ₁- and 3T-phengite is discussed.     

High-pressure investigations on Piplia Kalan eucrite meteorite using in-situ X-ray diffraction and ~(57)Fe Mssbauer spectroscopic technique up to 16 GPa

We report here high-pressure investigations on Piplia Kalan eucrite—a member of HED(Howardite-Eucrite-Diogenite) family from asteroid 4-Vesta based on synchrotron X-ray diffraction(up to 16 GPa)and ~(57)Fe Mossbauer spectroscopy(up to 8 GPa).Dominant with anorthite-rich plagioclase,pigeonite-rich pyroxene and clino-ferrosilite,the sample displayed various phase transitions attaining amorphous character at 16 GPa.These phase transitions of individual components could be explained simultaneously through variations in high-pressure XRD patterns and the Mossbauer parameters.Most prominent P2_1/c to C2/c transition of pigeonite and ferrosilite was exhibited both as sudden variation in Mossbauer parameters and population inversion of Fe~(2+) in Ml and M2 sites between 2.9 and 3.8 GPa and variation in intensity profile in XRD patterns at 3.56 GPa.Anorthite seemed to respond more to such impact than other components in the sample.Complete amorphization in anorthite which occurred at lower pressure of- 12 GPa implied residual stress experienced due to shock impact.The presence of high pressure(monoclinic) phase of pigeonite and ferrosilite at ambient condition in this eucrite sample confirmed earlier suggestions of an early shock event.This report is an attempt to emphasize the role of anorthite in the determination of the residual stress due to impact process in the parent body thus to understand the behavioral differences amongst HED members.     

Thermo-elastic behaviour of Be2BO3OH (hambergite) up to 7 GPa and 1,100 K

The thermo-elastic behaviour of Be₂BO₃(OH)_0.96F_0.04 (i.e. natural hambergite, Z = 8, a = 9.7564(1), b = 12.1980(2), c = 4.4300(1) Å, V = 527.21(1) ų, space group Pbca) has been investigated up to 7 GPa (at 298 K) and up to 1,100 K (at 0.0001 GPa) by means of in situ single-crystal X-ray diffraction and synchrotron powder diffraction, respectively. No phase transition or anomalous elastic behaviour has been observed within the pressure range investigated. P?V data fitted to a third-order Birch–Murnaghan equation of state give: V ₀ = 528.89(4) ų, K _T0 = 67.0(4) GPa and K′ = 5.4(1). The evolution of the lattice parameters with pressure is significantly anisotropic, being: K _T0(a):K _T0(b):K _T0(c) = 1:1.13:3.67. The high-temperature experiment shows evidence of structure breakdown at T > 973 K, with a significant increase in the full-width-at-half-maximum of all the Bragg peaks and an anomalous increase in the background of the diffraction pattern. The diffraction pattern was indexable up to 1,098 K. No new crystalline phase was observed up to 1,270 K. The diffraction data collected at room-T after the high-temperature experiment showed that the crystallinity was irreversibly compromised. The evolution of axial and volume thermal expansion coefficient, α, with T was described by the polynomial function: α(T) = α ₀ + α ₁ T ^?1/2. The refined parameters for Be₂BO₃(OH)_0.96F_0.04 are: α ₀ = 7.1(1) × 10^?5 K^?1 and α ₁ = ?8.9(2) × 10^?4 K ^?1/2 for the unit-cell volume, α ₀(a) = 1.52(9) × 10^?5 K^?1 and α ₁(a) = ?1.4(2) × 10^?4 K ^?1/2 for the a-axis, α ₀(b) = 4.4(1) × 10^?5 K^?1 and α ₁(b) = ?5.9(3) × 10^?4 K ^?1/2 for the b-axis, α ₀(c) = 1.07(8) × 10^?5 K^?1 and α ₁(c) = ?1.5(2) × 10^?4 K ^?1/2 for the c-axis. The thermo-elastic anisotropy can be described, at a first approximation, by α ₀(a):α ₀(b):α ₀(c) = 1.42:4.11:1. The main deformation mechanisms in response to the applied temperature, based on Rietveld structure refinement, are discussed.     

Cation substitution in synthetic meridianiite (MgSO4·11H2O) II: variation in unit-cell parameters determined from X-ray powder diffraction data

We have prepared aqueous MgSO₄ solutions doped with various divalent metal cations (Ni²⁺, Zn²⁺, Mn²⁺, Cu²⁺, Fe²⁺, and Co²⁺) in proportions up to and including the pure end-members. These liquids have been solidified into fine-grained polycrystalline blocks of metal sulfate hydrate + ice by rapid quenching in liquid nitrogen. In a companion paper (Fortes et al., in Phys Chem Min 39) we reported the identification of various phases using X-ray powder diffraction, including meridianiite-structured undecahydrates, melanterite- and epsomite-structured heptahydrates, novel enneahydrates and a new octahydrate. In this work we report the changes in unit-cell parameters of these crystalline products where they exist over sufficient dopant concentrations. We find that there is a linear relationship between the rate of change in unit-cell volume as a function of dopant concentration and the ionic radius of the dopant cation; large ions such as Mn²⁺ produce a substantial inflation of the hydrates’ unit-cell volume, whereas smaller ions such as Ni²⁺ produce a modest reduction in unit-cell volume. Indeed, when the data for all hydrates are normalised (i.e., divided by the number of formula units per unit-cell, Z, and the hydration number, n), we find a quantitatively similar relationship for different values of n. Conversely, there is no relationship between the degree of unit-cell inflation or deflation and the limit to which a given cation will substitute into a certain hydrate structure; for example, Co²⁺ and Zn²⁺ affect the unit-cell volume of MgSO₄·11H₂O to a very similar degree, yet the solubility limits inferred in our companion paper are >60 mol. % Co²⁺ and <30 mol. % Zn²⁺.     

Octahedral cation ordering in olivine at high temperature. II: an in situ neutron powder diffraction study on synthetic MgFeSiO4 (Fa50)

 The partitioning of Fe and Mg between the M1 and M2 octahedral sites of olivine has been investigated by in situ time-of-flight neutron powder diffraction. The degree of M-cation order was determined from direct measurements of site occupancies in a synthetic sample of Fo50Fa50 heated to 1250 °C at the Fe-FeO oxygen buffer. Fe shows slight preference for M1 at temperatures below about 600 °C, progressively disordering on heating to this temperature. Above 630 °C, the temperature at which site preferences cross over (T _cr), Fe preferentially occupies M2, becoming progressively more ordered into M2 on increasing temperature. The cation-ordering behaviour is discussed in relation to the temperature dependence of the M1 and M2 site geometries, and it is suggested that vibrational entropy, crystal field effects and changes in bond characteristics play a part in the cross-over of partitioning behaviour. The temperature dependence of site ordering is modelled using a Landau expansion of the free energy of ordering of the type ΔG = −hQ + gTQ +  (T − T _c)Q ² +  Q ⁴, with a/h = 0.00406 K⁻¹, b/h = 2.3, T _c = 572 K and g/h = 0.00106 K⁻¹. These results suggest that the high-temperature ordering behaviour across the forsterite-fayalite join will have a bearing on the activity-composition relations of this important rock-forming mineral, and indicate that Fe-Mg olivine solid solutions become less ideal as temperature increases. Received: 12 August 1999 / Accepted: 25 April 2000     

Electronic state of Fe2+ in (Mg,Fe)(Si,Al)O3 perovskite and (Mg,Fe)SiO3 majorite at pressures up to 81 GPa and temperatures up to 800 K

Despite a large number of studies of iron spin state in silicate perovskite at high pressure and high temperature, there is still disagreement regarding the type and P–T conditions of the transition, and whether Fe²⁺ or Fe³⁺ or both iron cations are involved. Recently, our group published results of a Mössbauer spectroscopy study of the iron behaviour in (Mg,Fe)(Si,Al)O₃ perovskite at pressures up to 110 GPa (McCammon et al. 2008), where we suggested stabilization of the intermediate spin state for 8- to 12-fold coordinated ferrous iron (^[8–12]Fe²⁺) in silicate perovskite above 30 GPa. In order to explore the behaviour in related systems, we performed a comparative Mössbauer spectroscopic study of silicate perovskite (Fe_0.12Mg_0.88SiO₃) and majorite (with two compositions—Fe_0.18Mg_0.82SiO₃ and Fe_0.11Mg_0.88SiO₃) at pressures up to 81 GPa in the temperature range 296–800 K, which was mainly motivated by the fact that the oxygen environment of ferrous iron in majorite is quite similar to that in silicate perovskite. The ^[8–12]Fe²⁺ component, dominating the Mössbauer spectra of majorites, shows high quadrupole splitting (QS) values, about 3.6 mm s^?1, in the entire studied P–T region (pressures to 58 GPa and 296–800 K). Decrease of the QS of this component with temperature at constant pressure can be described by the Huggins model with the energy splitting between low-energy e _g levels of ^[8–12]Fe²⁺ equal to 1,500 (50) cm^?1 for Fe_0.18Mg_0.82SiO₃ and to 1,680 (70) cm^?1 for Fe_0.11Mg_0.88SiO₃. In contrast, for the silicate perovskite dominating Mössbauer component associated with ^[8–12]Fe²⁺ suggests the gradual change of the electronic properties. Namely, an additional spectral component with central shift close to that for high-spin ^[8–12]Fe²⁺ and QS about 3.7 mm s^?1 appeared at ~35 (2) GPa, and the amount of the component increases with both pressure and temperature. The temperature dependence of QS of the component cannot be described in the framework of the Huggins model. Observed differences in the high-pressure high-temperature behaviour of ^[8–12]Fe²⁺ in the silicate perovskite and majorite phases provide additional arguments in favour of the gradual high-spin—intermediate-spin crossover in lower mantle perovskite, previously reported by McCammon et al. (2008) and Lin et al. (2008).     

A powder neutron diffraction study of the crystal structure of the fluoroperovskite NaMgF<Subscript>3</Subscript> (neighborite) from 300 to 3.6 K

The cell dimensions and crystal structures of the fluoroperovskite NaMgF₃ (neighborite), synthesized by solid state methods, have been determined by powder neutron diffraction and Rietveld refinement over the temperature range 300–3.6 K using Pt metal as an internal standard for calibration of the neutron wavelength. These data show that Pbnm NaMgF₃ does not undergo any phase transitions to structures of lower symmetry with decreasing temperature. The cell dimensions and atomic coordinates together with polyhedron volumes and distortion indices are given for Pbnm NaMgF₃ at 25 K intervals from 300 to 3.6 K. Decreases in the a and c cell dimensions reach a saturation point at 50 K, whereas the b dimension becomes saturated at 150 K. The distortion of the structure of Pbnm NaMgF₃ from the aristotype cubic structure is described in terms of the tilting of the MgF₆ octahedra according to the tilt scheme a ⁻ a ⁻ c ⁺ . With decreasing temperature the antiphase tilt (a ⁻) increases from 14.24° to 15.39°, whereas the in-phase tilt (c ⁺ ) remains effectively constant at ∼10.7°. Changes in the tilt angles are insufficient to cause changes in the coordination sphere of Na that might induce a low temperature phase transition. The structure of Pbnm NaMgF₃ is also described in terms of normal mode analysis and displacements of the condensed normal modes are compared with those of Pbnm KCaF₃.     

Taipingite-(Ce), (Ce73+, Ca2)Σ9Mg(SiO4)3[SiO3(OH)]4F3, a new mineral from the Taipingzhen REE deposit,North Qinling Orogen,central China

A new cerite group mineral species, taipingite-(Ce), ideally (Ce₇³⁺, Ca₂)_Σ9Mg(SiO₄)₃[SiO₃(OH)]₄F₃, has been found in the Taipingzhen rare earth element (REE) deposit in the North Qinling Orogen (NQO), Central China. It forms subhedral grains (up to approximately 100 ​μm ​× ​200 ​μm) commonly intergrown with the REE mineral assemblages and is closely associated with allanite-(Ce), gatelite-(Ce), törnebohmite-(Ce), fluocerite-(Ce), fluocerite-(La), fluorite, bastnäsite-(Ce), parisite-(Ce) and calcite. Taipingite-(Ce) is light red to pinkish brown under a binocular microscope and pale brown to colorless in thin section, and it is translucent to transparent with a grayish-white streak and vitreous luster. This mineral is brittle with conchoidal fracture; has a Mohs hardness value of approximately 5½ and exhibits no cleavage twinning or parting. The calculated density is 4.900(5) g/cm³. Optically, taipingite-(Ce) is uniaxial (+), with ω ​= ​1.808(5), ε ​= ​1.812(7), c ​= ​ε, and a ​= ​b ​= ​ω. Furthermore, this mineral is insoluble in HCl, HNO₃ and H₂SO₄. Electron microprobe analysis demonstrated that the sample was relatively pure, yielding the empirical formula (with calculated H₂O): (Ce_4.02La_1.64Nd_1.49Pr_0.41Sm_0.10Gd_0.02Ho_0.02Tm_0.01Lu_0.02Y_0.03Ca_0.66Mg_0.05Th_0.01–0.51)_Σ9(Mg_0.75Fe_0.25³⁺)_Σ1(SiO₄)₃{[SiO₃(OH)]_3.98[PO₃(OH)]_0.02}_Σ4(F_1.81OH_1.17Cl_0.02)_Σ3. Taipingite-(Ce) is trigonal and exhibits space group symmetry R3c with unit cell parameters a ​= ​10.7246(3) Å, c ​= ​37.9528(14) Å, V ​= ​3780.39(20) ų and Z ​= ​6. The strongest eight lines in the X-ray diffraction pattern are [d in Å(I)(hkl)]: 4.518(50)(202), 3.455(95)(122), 3.297(85)(214), 3.098(35)(300), 2.941(100)(02,10), 2.683(65)(220), 1.945(40)(238) and 1.754(40)(30,18). The crystal structure has been refined to a R₁ factor of 0.025, calculated for the 2312 unique observed reflections (Fo ​≥ ​4σ). The mineral is named after its discovery locality and is characterized as the F-dominant analogue of cerite-(Ce).     

Cation substitution in synthetic meridianiite (MgSO<Subscript>4</Subscript>·11H<Subscript>2</Subscript>O) I: X-ray powder diffraction analysis of quenched polycrystalline aggregates

Meridianiite, MgSO₄·11H₂O, is the most highly hydrated phase in the binary MgSO₄–H₂O system. Lower hydrates in the MgSO₄–H₂O system have end-member analogues containing alternative divalent metal cations (Ni²⁺, Zn²⁺, Mn²⁺, Cu²⁺, Fe²⁺, and Co²⁺) and exhibit extensive solid solution with MgSO₄ and with one another, but no other undecahydrate is known. We have prepared aqueous MgSO₄ solutions doped with these other cations in proportions up to and including the pure end-members. These liquids have been solidified into fine-grained polycrystalline blocks of metal sulfate hydrate + ice by rapid quenching in liquid nitrogen. The solid products have been characterised by X-ray powder diffraction, and the onset of partial melting has been quantified using a thermal probe. We have established that of the seven end-member metal sulfates studied, only MgSO₄ forms an undecahydrate; ZnSO₄ forms an orthorhombic heptahydrate (synthetic goslarite), MnSO₄, FeSO₄, and CoSO₄ form monoclinic heptahydrates (syn. mallardite, melanterite, bieberite, respectively), and CuSO₄ crystallises as the well-known triclinic pentahydrate (syn. chalcanthite). NiSO₄ forms a new hydrate which has been indexed with a triclinic unit cell of dimensions a = 6.1275(1) Å, b = 6.8628(1) Å, c = 12.6318(2) Å, α = 92.904(2)°, β = 97.678(2)°, and γ = 96.618(2)°. The unit-cell volume of this crystal, V = 521.74(1) ų, is consistent with it being an octahydrate, NiSO₄·8H₂O. Further analysis of doped specimens has shown that synthetic meridianiite is able to accommodate significant quantities of foreign cations in its structure; of the order 50 mol. % Co²⁺ or Mn²⁺, 20–30 mol. % Ni²⁺ or Zn²⁺, but less than 10 mol. % of Cu²⁺ or Fe²⁺. In three of the systems we examined, an ‘intermediate’ phase occurred that differed in hydration state both from the Mg-bearing meridianiite end-member and the pure dopant end-member hydrate. In the case of CuSO₄, we observed a melanterite-structured heptahydrate at Cu/(Cu + Mg) = 0.5, which we identify as synthetic alpersite [(Mg_0.5Cu_0.5)SO₄·7H₂O)]. In the NiSO₄- and ZnSO₄-doped systems we characterised an entirely new hydrate which could also be identified to a lesser degree in the CuSO₄- and the FeSO₄-doped systems. The Ni-doped substance has been indexed with a monoclinic unit-cell of dimensions a = 6.7488(2) Å, b = 11.9613(4) Å, c = 14.6321(5) Å, and β = 95.047(3)°, systematic absences being indicative of space-group P2₁/c with Z = 4. The unit-cell volume, V = 1,176.59(5) ų, is consistent with it being an enneahydrate [i.e. (Mg_0.5Ni_0.5)SO₄·9H₂O)]. Similarly, the new Zn-bearing enneahydrate has refined unit cell dimensions of a = 6.7555(3) Å, b = 11.9834(5) Å, c = 14.6666(8) Å, β = 95.020(4)°, V = 1,182.77(7) ų, and the new Fe-bearing enneahydrate has refined unit cell dimensions of a = 6.7726(3) Å, b = 12.0077(3) Å, c = 14.6920(5) Å, β = 95.037(3)°, and V = 1,190.20(6) ų. The observation that synthetic meridianiite can form in the presence of, and accommodate significant quantities of other ions increases the likelihood that this mineral will occur naturally on Mars—and elsewhere in the outer solar system—in metalliferous brines.     

In situ high-temperature powder X-ray diffraction study on the spinel solid solutions (Mg1?xMnx)Cr2O4

Using a conventional high-T furnace, the solid solutions between magnesiochromite and manganochromite, (Mg_1−x Mn _x )Cr₂O₄ with x = 0.00, 0.19, 0.44, 0.61, 0.77 and 1.00, were synthesized at 1,473 K for 48 h in open air. The ambient powder X-ray diffraction data suggest that the V–x relationship of the spinels does not show significant deviation from the Vegard’s law. In situ high-T powder X-ray diffraction measurements were taken up to 1,273 K at ambient pressure. For the investigated temperature range, the unit-cell parameters of the spinels increase smoothly with temperature increment, indicating no sign of cation redistribution between the tetrahedral and octahedral sites. The V–T data were fitted with a polynomial expression for the volumetric thermal expansion coefficient (a_T = a₀ + a₁ T + a₂ T ^{- 2} \alpha_{T} = a_{0} + a_{1} T + a_{2} T^{ - 2} ), which yielded insignificant a ₂ values. The effect of the composition on a ₀ is adequately described by the equation a ₀ = [17.7(8) − 2.4(1) × x] 10⁻⁶ K⁻¹, whereas that on a ₁ by the equation a ₁ = [8.6(9) + 2.1(11) × x] 10⁻⁹ K⁻².     

Synchrotron X-ray analysis of norbergite, Mg2.98Fe0.01Ti0.02Si0.99O4(OH0.31F1.69) structure at high pressure up to 8.2?GPa

The natural norbergite, Mg_2.98Fe_0.01Ti_0.02Si_0.99O₄(OH_0.31F_1.69) is examined by synchrotron X-ray diffraction analysis at pressures up to 8.2 GPa. The measured linear compressibilities of the crystallographic axes are β _a  = 2.18(4) × 10⁻³, β _b  = 2.93(7) × 10⁻³, and β _c  = 2.77(7) × 10⁻³ (GPa⁻¹), respectively and the calculated isothermal bulk modulus of the norbergite is K _T = 113(2) GPa based on the Birch–Murnaghan equation of state assuming a pressure derivative of K′ = 4. The crystal structures of norbergite are refined at room temperature and pressures of 4.7, 6.3, and 8.2 GPa, yielding R values for the structure refinements of 4.6, 5.3, and 5.3%, respectively. The bulk moduli of the polyhedral sites are 293(15) GPa for the tetrahedron, 106(5) GPa for the M2 octahedron, 113(2) GPa for the M3 octahedron, and 113(3) GPa for the total void space. The bulk modulus exhibits a good linear correlation with the filling factor for polyhedral sites in structures of the humite minerals and forsterite, reflecting the Si⁴⁺ + 4O²⁻ ⇔ □ + 4(OH, F)⁻ substitution in the humite minerals. Moreover, two simply linear trends were observed in the relationship between bulk modulus and packing index for natural minerals and dense hydrous magnesium silicate minerals. This relationship would reflect that the differences in compression mechanism were involved with hydrogen bonding in these minerals. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Biachellaite, (Na,Ca,K)8(Si6Al6O24)(SO4)2(OH)0.5 · H2O, a new mineral species of the cancrinite group

Biachellaite, a new mineral species of the cancrinite group, has been found in a volcanic ejecta in the Biachella Valley, Sacrofano Caldera, Latium region, Italy, as colorless isometric hexagonal bipyramidal-pinacoidal crystals up to 1 cm in size overgrowing the walls of cavities in a rock sample composed of sanidine, diopside, andradite, leucite and hauyne. The mineral is brittle, with perfect cleavage parallel to {10$ \bar 1 $ \bar 1 0} and imperfect cleavage or parting (?) parallel to {0001}. The Mohs hardness is 5. D_meas = 2.51(1) g/cm³ (by equilibration with heavy liquids). The densities calculated from single-crystal X-ray data and from X-ray powder data are 2.515 g/cm³ and 2.520 g/cm³, respectively. The IR spectrum demonstrates the presence of SO₄²⁻, H₂O, and absence of CO₃²⁻. Biachellaite is uniaxial, positive, ω = 1.512(1), ɛ = 1.514(1). The weight loss on ignition (vacuum, 800°C, 1 h) is 1.6(1)%. The chemical composition determined by electron microprobe is as follows, wt %: 10.06 Na₂O, 5.85 K₂O, 12.13 CaO, 26.17 Al₂O₃, 31.46 SiO₂, 12.71 SO₃, 0.45 Cl, 1.6 H₂O (by TG data), −0.10 −O=Cl₂, total is 100.33. The empirical formula (Z = 15) is (Na_3.76Ca_2.50K_1.44)_Σ7.70(Si_6.06Al_5.94O₂₄)(SO₄)_1.84Cl_0.15(OH)_0.43 · 0.81H₂O. The simplified formula is as follows: (Na,Ca,K)₈(Si₆Al₆O₂₄)(SO₄)₂(OH)_0.5 · H₂O. Biachellaite is trigonal, space group P3, a =12.913(1), c = 79.605(5) ?; V = 11495(1) ?³. The crystal structure of biachellaite is characterized by the 30-layer stacking sequence (ABCABCACACBACBACBCACBACBACBABC)_∞. The tetrahedral framework contains three types of channels composed of cages of four varieties: cancrinite, sodalite, bystrite (losod) and liottite. The strongest lines of the X-ray powder diffraction pattern [d, ? (I, %) (hkl)] are as follows: 11.07 (19) (100, 101), 6.45 (18) (110, 111), 3.720 (100) (2.1.10, 300, 301, 2.0.16, 302), 3.576 (18) (1.0.21, 2.0.17, 306), 3.300 (47) (1.0.23, 2.1.15), 3.220 (16) (2.1.16, 222). The type material of biachellaite has been deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, registration number 3642/1.     

The thermal expansion and crystal structure of mirabilite (Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>·10D<Subscript>2</Subscript>O) from 4.2 to 300 K,determined by time-of-flight neutron powder diffraction

We have collected high resolution neutron powder diffraction patterns from Na₂SO₄·10D₂O over the temperature range 4.2–300 K following rapid quenching in liquid nitrogen, and over a series of slow warming and cooling cycles. The crystal is monoclinic, space-group P2₁/c (Z = 4) with a = 11.44214(4) Å, b = 10.34276(4) Å, c = 12.75486(6) Å, β = 107.847(1)°, and V = 1436.794(8) Å³ at 4.2 K (slowly cooled), and a = 11.51472(6) Å, b = 10.36495(6) Å, c = 12.84651(7) Å, β = 107.7543(1)°, V = 1460.20(1) Å³ at 300 K. Structures were refined to R _P (Rietveld powder residual, \( R_{P} = {{\sum {\left| {I_{\text{obs}} - I_{\text{calc}} } \right|} } \mathord{\left/ {\vphantom {{\sum {\left| {I_{\text{obs}} - I_{\text{calc}} } \right|} } {\sum {I_{\text{obs}} } }}} \right. \kern-\nulldelimiterspace} {\sum {I_{\text{obs}} } }} \)) better than 2.5% at 4.2 K (quenched and slow cooled), 150 and 300 K. The sulfate disorder observed previously by Levy and Lisensky (Acta Cryst B34:3502–3510, 1978) was not present in our specimen, but we did observe changes with temperature in deuteron occupancies of the orientationally disordered water molecules coordinated to Na. The temperature dependence of the unit-cell volume from 4.2 to 300 K is well represented by a simple polynomial of the form V = ? 4.143(1) × 10^?7 T ³ + 0.00047(2) T² ? 0.027(2) T + 1437.0(1) Å³ (R ² = 99.98%). The coefficient of volume thermal expansion, α _V , is positive above 40 K, and displays a similar magnitude and temperature dependence to α _V in deuterated epsomite and meridianiite. The relationship between the magnitude and orientation of the principal axes of the thermal expansion tensor and the main structural elements are discussed; freezing in of deuteron disorder in the quenched specimen affects the thermal expansion, manifested most obviously as a change in the behaviour of the unit-cell parameter β.     

X-ray single-crystal and Raman study of (Na<Subscript>0.86</Subscript>Mg<Subscript>0.14</Subscript>)(Mg<Subscript>0.57</Subscript>Ti<Subscript>0.43</Subscript>)Si<Subscript>2</Subscript>O<Subscript>6</Subscript>, a new pyroxene synthesized at 7 GPa and 1700 °C

A new pyroxene with formula (Na_0.86Mg_0.14)(Mg_0.57Ti_0.43)Si₂O₆, synthesized in a high-pressure toroidal ‘anvil-with-hole’ apparatus at P = 7 GPa and T = 1700 °C, was characterized by X-ray single-crystal diffraction and Raman spectroscopy. The compound was found to be monoclinic (R1 = 2.56 %), space group C2/c, with lattice parameters a = 9.687(2), b = 8.814(1), c = 5.290(1) Å, β = 107.853(2)°, V = 430.08(1) ų. The coexistence of Mg and Ti⁴⁺ at the M1 site does not induce strong modifications either to the M1 site or to the adjacent M2 site. The Raman spectrum of synthetic Na–Ti-pyroxene was obtained for the first time and compared with that of Mg₂Si₂O₆ (with very low concentrations of Na and Ti). The structural characterization of the Na–Ti–Mg-pyroxene is important, because the study of its thermodynamic constants provides new constraints on thermobarometry of the upper mantle assemblages.