Phase relation of CaSO<Subscript>4</Subscript> at high pressure and temperature up to 90 GPa and 2300 K

Calcium sulfate (CaSO₄), one of the major sulfate minerals in the Earth’s crust, is expected to play a major role in sulfur recycling into the deep mantle. Here, we investigated the crystal structure and phase relation of CaSO₄ up to ~90 GPa and 2300 K through a series of high-pressure experiments combined with in situ X-ray diffraction. CaSO₄ forms three thermodynamically stable polymorphs: anhydrite (stable below 3 GPa), monazite-type phase (stable between 3 and ~13 GPa) and barite-type phase (stable up to at least 93 GPa). Anhydrite to monazite-type phase transition is induced by pressure even at room temperature, while monazite- to barite-type transition requires heating at least to 1500 K at ~20 GPa. The barite-type phase cannot always be quenched from high temperature and is distorted to metastable AgMnO₄-type structure or another modified barite structure depending on pressure. We obtained the pressure–volume data and density of anhydrite, monazite- and barite-type phases and found that their densities are lower than those calculated from the PREM model in the studied P–T conditions. This suggests that CaSO₄ is gravitationally unstable in the mantle and fluid/melt phase into which sulfur dissolves and/or sulfate–sulfide speciation may play a major role in the sulfur recycling into the deep Earth.     

Effect of KCl on melting in the Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript>–MgSiO<Subscript>3</Subscript>–H<Subscript>2</Subscript>O system at 5 GPa

To examine the effect of KCl-bearing fluids on the melting behavior of the Earth’s mantle, we conducted experiments in the Mg₂SiO₄–MgSiO₃–H₂O and Mg₂SiO₄–MgSiO₃–KCl–H₂O systems at 5 GPa. In the Mg₂SiO₄–MgSiO₃–H₂O system, the temperature of the fluid-saturated solidus is bracketed between 1,200–1,250°C, and both forsterite and enstatite coexist with the liquid under supersolidus conditions. In the Mg₂SiO₄–MgSiO₃–KCl–H₂O systems with molar Cl/(Cl + H₂O) ratios of 0.2, 0.4, and 0.6, the temperatures of the fluid-saturated solidus are bracketed between 1,400–1,450°C, 1,550–1,600°C, and 1,600–1,650°C, respectively, and only forsterite coexists with liquid under supersolidus conditions. This increase in the temperature of the solidus demonstrates the significant effect of KCl on reducing the activity of H₂O in the fluid in the Mg₂SiO₄–MgSiO₃–H₂O system. The change in the melting residues indicates that the incongruent melting of enstatite (enstatite = forsterite + silica-rich melt) could extend to pressures above 5 GPa in KCl-bearing systems, in contrast to the behavior in the KCl-free system.     

Phase relations of potassium-bearing clinopyroxene in the system CaMgSi<Subscript>2</Subscript>O<Subscript>6</Subscript>-KAlSi<Subscript>2</Subscript>O<Subscript>6</Subscript> at 7 GPa

The pseudo-binary system CaMgSi₂O₆-KAlSi₂O₆, modeling the potassium-bearing clinopyroxene (KCpx) solid solution, has been studied at 7 GPa and 1,100–1,650 °C. The KCpx is a liquidus phase of the system up to 60 mol% of KAlSi₂O₆. At higher content of KAlSi₂O₆ in the system, grossular-rich garnet becomes a liquidus phase. Above 75 mol% of KAlSi₂O₆ in the system, KCpx is unstable at the solidus as well, and garnet coexists with kalsilite, Si-wadeite and kyanite. No coexistence of KCpx with kyanite was observed. Above the solidus, KAlSi₂O₆ content of the KCpx coexisting with melt increases with decreasing temperature. Near the solidus of the system (about 1,250 °C) KCpx contains up to 5.6 wt% of K₂O, i.e. about 22–26 mol% of KAlSi₂O₆. Such high concentration of potassium in KCpx is presumably the maximal content of KAlSi₂O₆ in the Fe-free clinopyroxene at 7 GPa. In addition to the major substitution Mg^M1C²Al¹K², the KCpx solid solution contains Ca-Eskola and only minor Ca-Tschermack components. Our experimental results indicate that the natural assemblage KCpx+grossular-rich garnet might be a product of crystallization of the ultra-potassic SiO₂-rich alumino-silicate mantle melts (>200 km).Editorial responsibility: J. Hoefs     

Attikaite,Ca<Subscript>3</Subscript>Cu<Subscript>2</Subscript>Al<Subscript>2</Subscript>(AsO<Subscript>4</Subscript>)<Subscript>4</Subscript>(OH)<Subscript>4</Subscript> · 2H<Subscript>2</Subscript>O,a new mineral species

Attikaite, a new mineral species, has been found together with arsenocrandalite, arsenogoyazite, conichalcite, olivenite, philipsbornite, azurite, malachite, carminite, beudantite, goethite, quartz, and allophane at the Christina Mine No. 132, Kamareza, Lavrion District, Attiki Prefecture (Attika), Greece. The mineral is named after the type locality. It forms spheroidal segregations (up to 0.3 mm in diameter) consisting of thin flexible crystals up to 3 × 20 × 80 μm in size. Its color is light blue to greenish blue, with a pale blue streak. The Mohs’ hardness is 2 to 2.5. The cleavage is eminent mica-like parallel to {001}. The density is 3.2(2) g/cm³ (measured in heavy liquids) and 3.356 g/cm³ (calculated). The wave numbers of the absorption bands in the infrared spectrum of attikaite are (cm^?1; sh is shoulder; w is a weak band): 3525sh, 3425, 3180, 1642, 1120w, 1070w, 1035w, 900sh, 874, 833, 820, 690w, 645w, 600sh, 555, 486, 458, and 397. Attikaite is optically biaxial, negative, α = 1.642(2), β = γ = 1.644(2) (X = c) 2V _means = 10(8)°, and 2V _calc = 0°. The new mineral is microscopically colorless and nonpleochroic. The chemical composition (electron microprobe, average over 4 point analyses, wt %) is: 0.17 MgO, 17.48 CaO, 0.12 FeO, 16.28 CuO, 10.61 Al₂O₃, 0.89 P₂O₅, 45.45 As₂O₅, 1.39 SO₃, and H₂O (by difference) 7.61, where the total is 100.00. The empirical formula calculated on the basis of (O,OH,H₂O)₂₂ is: Ca_2.94Cu _1.93 ²⁺ Al_1.97Mg_0.04Fe _0.02 ²⁺ [(As_3.74S_0.16P_0.12)_Σ4.02O_16.08](OH)_3.87 · 2.05H₂ O. The simplified formula is Ca₃Cu₂Al₂(AsO₄)₄(OH)₄ · 2H₂O. Attikaite is orthorhombic, space group Pban, Pbam or Pba2; the unit-cell dimensions are a = 10.01(1), b = 8.199(5), c = 22.78(1) Å, V = 1870(3) ų, and Z = 4. In the result of the ignition of attikaite for 30 to 35 min at 128–140°, the H₂O bands in the IR spectrum disappear, while the OH-group band is not modified; the weight loss is 4.3%, which approximately corresponds to two H₂O molecules per formula; and parameter c decreases from 22.78 to 18.77 Å. The strongest reflections in the X-ray powder diffraction pattern [d, Å (I, %)((hkl)] are: 22.8(100)(001), 11.36(60)(002), 5.01(90)(200), 3.38(5)(123, 205), 2.780(70)(026), 2.682(30)(126), 2.503(50)(400), 2.292(20)(404). The type material of attikaite is deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow. The registration number is 3435/1.     

The stability and equation of state for the cotunnite phase of TiO2 up to 70 GPa

The stability and equation of state for the cotunnite phase in TiO₂ were investigated up to a pressure of about 70 GPa by high-pressure in situ X-ray diffraction measurements using a laser-heated diamond anvil cell. The transition sequence under high pressure was rutile → α-PbO₂ phase → baddeleyite phase → OI phase → cotunnite phase with increasing pressure. The cotunnite phase was the most stable phase at pressures from 40 GPa to at least 70 GPa. The equation of state parameters for the cotunnite phase were established on the platinum scale using the volume data at pressures of 37–68 GPa after laser annealing, in which the St value, an indicator of the magnitude of the uniaxial stress component in the samples, indicates that these measurements were performed under quasi-hydrostatic conditions. The third-order Birch-Murnaghan equation of state at K ₀′ = 4.25 yields V ₀ = 15.14(5) cm³/mol and K ₀ = 294(9), and the second-order Birch-Murnaghan equation of state yields V ₀ = 15.11(5) cm³/mol and K ₀ = 306(9). Therefore, we conclude that the bulk modulus for the cotunnite phase is not comparable to that of diamond.     

High-pressure X-ray diffraction and Raman spectroscopy of CaFe<Subscript>2</Subscript>O<Subscript>4</Subscript>-type β-CaCr<Subscript>2</Subscript>O<Subscript>4</Subscript>

In situ high-pressure synchrotron X-ray diffraction and Raman spectroscopic studies of orthorhombic CaFe₂O₄-type β-CaCr₂O₄ chromite were carried out up to 16.2 and 32.0 GPa at room temperature using multi-anvil apparatus and diamond anvil cell, respectively. No phase transition was observed in this study. Fitting a third-order Birch–Murnaghan equation of state to the P–V data yields a zero-pressure volume of V ₀ = 286.8(1) ų, an isothermal bulk modulus of K ₀ = 183(5) GPa and the first pressure derivative of isothermal bulk modulus K ₀′ = 4.1(8). Analyses of axial compressibilities show anisotropic elasticity for β-CaCr₂O₄ since the a-axis is more compressible than the b- and c-axis. Based on the obtained and previous results, the compressibility of several CaFe₂O₄-type phases was compared. The high-pressure Raman spectra of β-CaCr₂O₄ were analyzed to determine the pressure dependences and mode Grüneisen parameters of Raman-active bands. The thermal Grüneisen parameter of β-CaCr₂O₄ is determined to be 0.93(2), which is smaller than those of CaFe₂O₄-type CaAl₂O₄ and MgAl₂O₄.     

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.     

Single-crystal X-ray diffraction study of cation distribution in MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript>–MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel solid solution

Synthesis experiments in the system MgAl₂O₄–MgFe₂O₄ [MgAl_2–xFe_xO₄ (0 x 2)] were carried out using a PbF₂ flux. The crystalline products synthesized in the compositional range of 0.6 <x 1.2 consisted of two spinel phases, whereas those synthesized in the compositional ranges of 0.0 x 0.6 and 1.2 < x 2.0 crystallized as single spinel phases. Structure refinements of the spinel single crystals, which grew in the ranges of 0.0 x 0.6 and 1.2 < x 2.0, show that the degree of randomness of cation distribution between A and B sites increases as x approaches the two-phase region. This means that the degree of the size mismatch among Mg²⁺, Fe³⁺ and Al³⁺occupying each equivalent mixing site increases as x approaches the two-phase region. Consequently, if the coexistence of two spinels observed in the intermediate compositions reveals the existence of a miscibility gap at low temperatures, this increase in the degree of the size mismatch among the three cations is suggested as a factor of energetic destabilization to form the miscibility gap.     

Behaviour of Fe<Subscript>4</Subscript>O<Subscript>5</Subscript>–Mg<Subscript>2</Subscript>Fe<Subscript>2</Subscript>O<Subscript>5</Subscript> solid solutions and their relation to coexisting Mg–Fe silicates and oxide phases

Experiments at high pressures and temperatures were carried out (1) to investigate the crystal-chemical behaviour of Fe₄O₅–Mg₂Fe₂O₅ solid solutions and (2) to explore the phase relations involving (Mg,Fe)₂Fe₂O₅ (denoted as O₅-phase) and Mg–Fe silicates. Multi-anvil experiments were performed at 11–20 GPa and 1100–1600 °C using different starting compositions including two that were Si-bearing. In Si-free experiments the O₅-phase coexists with Fe₂O₃, hp-(Mg,Fe)Fe₂O₄, (Mg,Fe)₃Fe₄O₉ or an unquenchable phase of different stoichiometry. Si-bearing experiments yielded phase assemblages consisting of the O₅-phase together with olivine, wadsleyite or ringwoodite, majoritic garnet or Fe³⁺-bearing phase B. However, (Mg,Fe)₂Fe₂O₅ does not incorporate Si. Electron microprobe analyses revealed that phase B incorporates significant amounts of Fe²⁺ and Fe³⁺ (at least ~?1.0 cations Fe per formula unit). Fe-L_2,3-edge energy-loss near-edge structure spectra confirm the presence of ferric iron [Fe³⁺/Fe_tot?=?~?0.41(4)] and indicate substitution according to the following charge-balanced exchange: [4]Si⁴⁺?+?[6]Mg²⁺?=?2Fe³⁺. The ability to accommodate Fe²⁺ and Fe³⁺ makes this potential “water-storing” mineral interesting since such substitutions should enlarge its stability field. The thermodynamic properties of Mg₂Fe₂O₅ have been refined, yielding H°_1bar,298?=???1981.5 kJ mol^??1. Solid solution is complete across the Fe₄O₅–Mg₂Fe₂O₅ binary. Molar volume decreases essentially linearly with increasing Mg content, consistent with ideal mixing behaviour. The partitioning of Mg and Fe²⁺ with silicates indicates that (Mg,Fe)₂Fe₂O₅ has a strong preference for Fe²⁺. Modelling of partitioning with olivine is consistent with the O₅-phase exhibiting ideal mixing behaviour. Mg–Fe²⁺ partitioning between (Mg,Fe)₂Fe₂O₅ and ringwoodite or wadsleyite is influenced by the presence of Fe³⁺ and OH incorporation in the silicate phases.     

Crystal structure and refined formula of garyansellite Mg<Subscript>2</Subscript>Fe<Superscript>3+</Superscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>(OH) · 2H<Subscript>2</Subscript>O

Single-crystal study of the structure (R = 0.0268) was performed for garyansellite from Rapid Creek, Yukon, Canada. The mineral is orthorhombic, Pbna, a = 9.44738(18), b = 9.85976(19), c = 8.14154(18) Å, V = 758.38(3) ų, Z = 4. An idealized formula of garyansellite is Mg₂Fe³⁺(PO₄)₂(OH) · 2H₂O. Structurally the mineral is close to other members of the phosphoferrite–reddingite group. The structure contains layers of chains of M(2)O₄(OH)(H₂O) octahedra which share edges to form dimers and connected by common edges with isolated from each other M(1)O₄(H₂O)₂ octahedra. The neighboring chains are connected to the layer through the common vertices of M(2) octahedra and octaahedral layers are linked through PO₄ tetrahedra.     

Near-solidus melts of MORB + 4 wt% H<Subscript>2</Subscript>O at 0.8–2.8 GPa applied to issues of subduction magmatism and continent formation

Experiments on MORB?+?4 wt% H₂O at 0.8–2.8 GPa and 700–950 °C (Liu in High pressure phase equilibria involving the amphibolite–eclogite transformation. PhD dissertation, Stanford University, Stanford, California, 1997; Liu et al. in Earth Planet Sci Lett 143:161–171, 1996) were reexamined for their major and trace element melt compositions and melting relations. Degree of melting diminishes at greater pressures, with corresponding evolution of melt from andesitic at the lowest pressures and hottest temperatures to high-silica rhyolitic at the greatest pressure and coolest temperature. Quartz contributes greatly to the production of near-solidus melts of basaltic eclogite, with the result that melt productivity falls markedly following quartz exhaustion. This limits the extent of melting attainable in the basaltic eclogite portions of sub-arc subducting plates to no more than ~?2?×?the modal wt% quartz in the mafic eclogite protolith. Synthesized residual mineral assemblages lack an epidote-series mineral at temperatures?>?750 °C, and as a result, melts from the rutile eclogite and rutile-amphibole eclogite facies have elevated concentrations of light rare earth elements, U, Th, have elevated Ba, K, and Sr, high Sr/Y, and are strongly depleted in Nb, Y, and the heavy rare earth elements. Models of eclogite partial melt reacting with peridotite of the mantle wedge reproduce major and trace element characteristics of parental arc magmas so long as the proportions of infiltrating melt to peridotite are relatively high, consistent with channelized ascent. Melt mass is estimated to increase roughly three- to ten-fold, consistent with H₂O concentrations of 3–7 wt% in the magmas produced by reaction. Partial melts of subducting basaltic eclogite are predicted to have positive Sr concentration anomalies, relative to Ce and Nd, that persist through melt-peridotite reactions. Primitive arc magmas commonly have positive Sr anomalies, whereas such anomalies are smaller in estimates of the bulk continental crust. Overall, Sr anomalies diminish passing from primitive to more evolved arc volcanic rocks, consistent with extensive mineral-melt differentiation (crystallization, partial remelting) involving plagioclase. On the order of 50 wt% differentiation would be necessary to eliminate Sr positive anomalies, based on geochemical variations in the Cascade and western Aleutian magmatic arcs. Loss to the mantle of cumulates and restites with high Sr anomalies, in abundances broadly equal to the mass of the preserved crust, would be required to form the continents via processes similar to present-day subduction magmatism.     

Elastic and thermoelastic properties of synthetic Ca<Subscript>2</Subscript>MgSi<Subscript>2</Subscript>O<Subscript>7</Subscript> (åkermanite) and Ca<Subscript>2</Subscript>ZnSi<Subscript>2</Subscript>O<Subscript>7</Subscript> (hardystonite)

Elastic and thermoelastic constants of large single crystals of Ca₂MgSi₂O₇ and Ca₂ZnSi₂O₇ have been derived from ultrasonic resonance frequencies of plane-parallel plates and their shift upon variation of temperature, respectively. In addition, coefficients of thermal expansion and dielectric constants were determined. Both species possess quite similar properties. As observed in other isotypic magnesium and zinc compounds, the mean elastic stiffness and the deviation from the Cauchy relations are significantly larger in the zinc compound, due to a covalent contribution of the Zn–O bond. Positive thermoelastic constants T₄₄ and T₆₆ in Ca₂MgSi₂O₇ allow temperature-independent ultrasonic generators and oscillators to be manufactured.     

Structural changes and valence states in the MgCr<Subscript>2</Subscript>O<Subscript>4</Subscript>–FeCr<Subscript>2</Subscript>O<Subscript>4</Subscript> solid solution series

The influence on the structure of Fe²⁺ Mg substitution was studied in synthetic single crystals belonging to the MgCr₂O₄–FeCr₂O₄ series produced by flux growth at 900–1200 °C in controlled atmosphere. Samples were analyzed by single-crystal X-ray diffraction, electron microprobe analyses, optical absorption-, infrared- and Mössbauer spectroscopy. The Mössbauer data show that iron occurs almost exclusively as ^IVFe²⁺. Only minor Fe³⁺ (<0.005 apfu) was observed in samples with very low total Fe. Optical absorption spectra show that chromium with few exceptions is present as a trivalent cation at the octahedral site. Additional absorption bands attributable to Cr²⁺ and Cr³⁺ at the tetrahedral site are evident in spectra of end-member magnesiochromite and solid-solution crystals with low ferrous contents. Structural parameters a₀, u and T–O increase with chromite content, while the M–O bond distance remains nearly constant, with an average value equal to 1.995(1) Å corresponding to the Cr³⁺ octahedral bond distance. The ideal trend between cell parameter, T–O bond length and Fe²⁺ content (apfu) is described by the following linear relations: a₀=8.3325(5) + 0.0443(8)Fe²⁺ (Å) and T–O=1.9645(6) + 0.033(1)Fe²⁺ (Å) Consequently, Fe²⁺ and Mg tetrahedral bond lengths are equal to 1.998(1) Å and 1.965(1) Å, respectively.     

Do carbonate liquids become denser than silicate liquids at pressure? Constraints from the fusion curve of K<Subscript>2</Subscript>CO<Subscript>3</Subscript> to 3.2 GPa

Brackets on the melting temperature of K₂CO₃ were experimentally determined at 1.86 ± 0.02 GPa (1,163–1,167°C), 2.79 ± 0.03 GPa (1,187–1,195°C), and 3.16 ± 0.04 GPa (1,183–1,189°C) in a piston-cylinder apparatus. These new data, in combination with published experiments at low pressure (<0.5 GPa), establish the K₂CO₃ fusion curve to 3.2 GPa. On the basis of these experiments and published thermodynamic data for crystalline and liquid K₂CO₃, the high-pressure density and compressibility of K₂CO₃ liquid were derived from the fusion curve. The pressure dependence of the liquid compressibility (K′₀ = dK ₀/dP, where K ₀ = 1/β₀) is between 16.2 and 11.6, with a best estimate of 13.7, in a third-order Birch–Murnaghan equation of state (EOS). This liquid K′₀ leads to a density of 2,175 ± 36 kg/m³ at 4 GPa and 1,500°C, which is ∼30% lower than that reported in the literature on the basis of the falling-sphere method at the same conditions. The uncertainty in the liquid K′₀ leads to an error in melt density of ± 2% at 4 GPa; the error decreases with decreasing pressure. With a K′₀ of 13.7, the compressibility of K₂CO₃ at 1,500°C and 1 bar (K ₀ = 3.8 GPa) drops rapidly with increasing pressure ( ), which prevents a density crossover with silicate melts, such as CaAlSi₂O₈ and CaMgSi₂O₆, at upper mantle depths.     

Phase relations in the CaMgSi<Subscript>2</Subscript>O<Subscript>6</Subscript>–KAlSi<Subscript>3</Subscript>O<Subscript>8</Subscript> join at 6 and 3.5 GPa as a model for formation of some potassium-bearing deep-seated mineral assemblages

The join CaMgSi₂O₆–KAlSi₃O₈ has been studied at 6 GPa (890–1,500°C) and 3.5 GPa (1,000–1,100°C). K-rich melts in the join produce assemblages Cpx + Grt, Cpx + Opx, Cpx + San, and Cpx + Grt + San at 1,100–1,300°C. At N_San^system<~70 mol%, sanidine is unstable on the solidus and appears at the liquidus, if N_San^system>90 mol%. This explains a scarcity of San in mantle Cpx-rich assemblages and its association with high-K aluminosilicate melt inclusions in diamonds. In absence of San, KCpx is the only host for potassium. The K-jadeite content in KCpx systematically increases with decreasing temperature and reaches 10–12 mol% near the solidus. However, KCpx coexists with San at N_San^system>70 mol% and <1,300°C, being formed via reaction San + L=KCpx. The KJd content in KCpx is controlled by the equilibrium San=KJd + SiO₂^L that displaces to the right with increasing pressure and decreasing both the temperature and This equilibrium is considered to be responsible for the formation of San lamellae in natural UHP Cpx. In our experiments at 3.5 GPa, garnet is absent whereas the KJd and Ca-Eskola contents in Cpx are low, and the join CaMgSi₂O₆–KAlSi₃O₈ is close to binary (with the eutectic Cpx + San + L). Different topologies of the join at 6 and 3.5 GPa define a sequence of mineral crystallization from K-rich aluminosilicate melts during cooling and decompression: from KCpx + Grt without San at P>4 GPa to Cpx + San at P<4 GPa. Similar sequence of assemblages is observed in some eclogitic xenoliths from kimberlites and Grt–Cpx rocks of the Kokchetav Complex (Northern Kazakhstan).     

Mangazeite,Al<Subscript>2</Subscript>(SO<Subscript>4</Subscript>)(OH)<Subscript>4</Subscript> · 3H<Subscript>2</Subscript>O,a new mineral species

Mangazeite, a new mineral species, has been found at the Mangazeya silver deposit (300 km east of the Lena River, 65°43′40″ N and 130°20′ E) in eastern Yakutia (Sakha Republic, Siberia, Russia). The new mineral was described from fractured, sericitized, and pyritized granodiorite adjacent to a quartz-arsenopyrite vein. Associated minerals are gypsum and chlorite. The new mineral occurs as radial fibrous segregations of thin lamellar crystals. The size of the fibers does not exceed 40 μm in length and 1 μm across. The mineral is white, with a white streak and a vitreous luster. Mangazeite is transparent in isolated grains. No fluorescence is observed. The Mohs hardness is 1–2. The calculated density is 2.15 g/cm³. The new mineral is biaxial; its optical character was not determined; α = 1.525(9), β was not measured, and γ = 1.545(9). The average chemical composition is as follows (wt %): Al₂O₃ 36.28, SO₃ 28.81, H₂O⁺ 34.35, total 99.44, H₂O^? 9.27. The H₂O^? content was neither included in the total nor used in formula calculation. The empirical formula is Al_1.99(SO₄)_1.01(OH)_3.94 · 3.37H₂O. The simplified formula is Al₂(SO₄)(OH)₄ · 3H₂O. The theoretical chemical composition calculated from this formula is (wt %) Al₂O₃ 37.47, SO₃ 29.42, H₂O 33.11, total 100.00. The new mineral is triclinic; the unit cell parameters refined from X-ray powder diffraction data are a = 8.286(5), b = 9.385(5), c = 11.35(1) Å, α = 96.1(1), β = 98.9(1), γ = 96.6(1)°, and Z = 4. The strongest lines in the X-ray powder diffraction pattern (d(I, %)) are 8.14(19), 7.59(49), 7.16(46), 4.258(100), 4.060(48), and 3.912(43). Mangazeite is supergene in origin and crystallized in a favorable aluminosilicate environment in the presence of sulfate ion due to pyrite oxidation.     

<Emphasis Type="Italic">P</Emphasis>–<Emphasis Type="Italic">V</Emphasis>–<Emphasis Type="Italic">T</Emphasis> equation of state of CaAl<Subscript>4</Subscript>Si<Subscript>2</Subscript>O<Subscript>11</Subscript> CAS phase

The thermoelastic parameters of the CAS phase (CaAl₄Si₂O₁₁) were examined by in situ high-pressure (up to 23.7 GPa) and high-temperature (up to 2,100 K) synchrotron X-ray diffraction, using a Kawai-type multi-anvil press. P–V data at room temperature fitted to a third-order Birch–Murnaghan equation of state (BM EOS) yielded: V _0,300 = 324.2 ± 0.2 Å³ and K _0,300 = 164 ± 6 GPa for K′ _0,300 = 6.2 ± 0.8. With K′ _0,300 fixed to 4.0, we obtained: V _0,300 = 324.0 ± 0.1 Å³ and K _0,300 = 180 ± 1 GPa. Fitting our P–V–T data with a modified high-temperature BM EOS, we obtained: V _0,300 = 324.2 ± 0.1 Å³, K _0,300 = 171 ± 5 GPa, K′ _0,300 = 5.1 ± 0.6 (?K _0,T /?T) _P  = ?0.023 ± 0.006 GPa K^?1, and α_0,T  = 3.09 ± 0.25 × 10^?5 K^?1. Using the equation of state parameters of the CAS phase determined in the present study, we calculated a density profile of a hypothetical continental crust that would contain ~10 vol% of CaAl₄Si₂O₁₁. Because of the higher density compared with the coexisting minerals, the CAS phase is expected to be a plunging agent for continental crust subducted in the transition zone. On the other hand, because of the lower density compared with lower mantle minerals, the CAS phase is expected to remain buoyant in the lowermost part of the transition zone.     

Measurements and calculations of solid-liquid equilibria in the ternary systems NaBr–Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>–H<Subscript>2</Subscript>O and KBr–K<Subscript>2</Subscript>SO<Subscript>4</Subscript>–H<Subscript>2</Subscript>O at 348 K

According to the compositions of the underground brine resources in the west of Sichuan Basin, solubilities of the ternary systems NaBr–Na₂SO₄–H₂O and KBr–K₂SO₄–H₂O were investigated by isothermal method at 348 K. The equilibrium solid phases, solubilities of salts, and densities of the solutions were determined. On the basis of the experimental data, the phase diagrams and the density-composition diagrams were plotted. In the two ternary systems, the phase diagrams consist of two univariant curves, one invariant point and two crystallization fields. Neither solid solution nor double salts were found. The equilibrium solid phases in the ternary system NaBr–Na₂SO₄–H₂O are NaBr and Na₂SO₄, and those in the ternary system KBr–K₂SO₄–H₂O are KBr and K₂SO₄. Using the solubilities data of the two ternary subsystems at 348 K, mixing ion-interaction parameters of Pitzer’s equation θxxx, Ψxxx and Ψxxx were fitted by multiple linear regression method. Based on the chemical model of Pitzer’s electrolyte solution theory, the solubilities of phase equilibria in the two ternary systems NaBr–Na₂SO₄–H₂O and KBr–K₂SO₄–H₂O were calculated with corresponding parameters. The calculation diagrams were plotted. The results showed that the calculated values have a good agreement with experimental data.     

Solubility of niobium in the system CaCO<Subscript>3</Subscript>–CaF<Subscript>2</Subscript>–NaNbO<Subscript>3</Subscript> at 0.1 GPa pressure: implications for the crystallization of pyrochlore from carbonatite magma

Experimental data are presented for the solubility of NaNbO₃ in the ternary system CaCO₃–CaF₂–NaNbO₃ (or calcite–fluorite–lueshite) over the temperature range 500–1,000°C at 0.1 GPa pressure. Liquidus to solidus phase relationships are given for the pseudo-binary join ([CaCO₃]₆₀[CaF₂]₄₀)_100-x–(NaNbO₃)_x (0<x<60 wt%). These data show that the maximum solubility of NaNbO₃ in these liquids is about 17 wt% (or 13.8 wt% Nb₂O₅) at approximately 930°C, and is represented by the appearance of pyrochlore as the primary liquidus phase. The sub-liquidus assemblages with decreasing temperature for NaNbO₃ contents of 20–50 wt% are: pyrochlore + liquid; pyrochlore + CaF₂ + liquid; pyrochlore + CaF₂ + CaCO₃ + liquid. The solidus assemblage is pyrochlore + CaF₂ + CaCO₃ at temperatures of approximately 700°C (20 wt% NaNbO₃) and 600°C (40 wt% NaNbO₃). NaNbO₃ is present only in sub-solidus assemblages. These data show that in this fluorine-bearing anhydrous system pyrochlore is the principal Nb-hosting supra-solidus phase, in contrast to fluorine-free hydrous melts from which perovskite-structured compounds crystallize. The crystallization of pyrochlore and/or perovskite-structured compounds from haplocarbonatite liquids is thus considered to be dependent upon the F/OH ratio of the melt.     

Electrical conductivity of polycrystalline Mg(OH)<Subscript>2</Subscript> at 2 GPa: effect of grain boundary hydration–dehydration

The effect of intergranular water on the conductivity of polycrystalline brucite, Mg(OH)₂, was investigated using impedance spectroscopy at 2 GPa, during consecutive heating–cooling cycles in the 298–980 K range. The grain boundary hydration levels tested here span water activities from around unity (wet conditions) down to 10⁻⁴ (dry conditions) depending on temperature. Four orders of magnitude in water activity result in electrical conductivity variations for about 6–7 orders of magnitude at 2 GPa and room temperature. Wet brucite samples containing, initially, about 18 wt% of evaporable water (i.e. totally removed at temperatures below 393 K in air), display electrical conductivity values above 10⁻²–10⁻³ S/m. A.C. electrical conductivity as a function of temperature follows an Arrhenius behaviour with an activation energy of 0.11 eV. The electrical conductivity of the same polycrystalline brucite material dried beforehand at 393 K (dry conditions) is lower by about 5–6 orders of magnitude at room temperature and possesses an activation energy of 0.8–0.9 eV which is close to that of protonic diffusion in (001) brucitic planes. Above ca. 873 K, a non-reversible conductivity jump is observed which is interpreted as a water transfer from mineral bulk to grain boundaries (i.e. partial dehydration). Cooling of such partially dehydrated sample shows electrical conductivities much higher than those of the initially dry sample by 4 orders of magnitude at 500 K. Furthermore, the corresponding activation energy is decreased by a factor of about four (i.e. 0.21 eV). Buffering of the sample at low water activity has been achieved by adding CaO or MgO, two hygroscopic compounds, to the starting material. Then, sample conductivities reached the lowest values encountered in this study with the activation energy of 1.1 eV. The strong dependency of the electrical conductivity with water activity highlights the importance of the latter parameter as a controlling factor of diffusion rates in natural processes where water availability and activity may vary grandly. Water exchange between mineral bulk and mineral boundary suggests that grain boundary can be treated as an independent phase in dehydroxylation reactions.