Structural characterization of high-pressure C–Na<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>5</Subscript> by single-crystal diffraction and <Superscript>29</Superscript>Si MAS NMR

Single crystals of C–Na₂Si₂O₅ have been synthesized from the hydrothermal recrystallization of a glass. The title compound is monoclinic, space group P2₁/c with Z= 8 and unit-cell parameters a= 4.8521 (4)Å, b=23.9793(16)Å, c=8.1410(6)Å, β=90.15(1)° and V=947.2(2)ų. The structure has been determined by direct methods and belongs to the group of phyllosilicates. It is based on layers of tetrahedra with elliptically six-membered rings in chair conformation. The sequence of directedness within a single ring is UDUDUD. The sheets are parallel to (010) with linking sodium cations in five- and sixfold coordination. Concerning the shape and the conformation of the rings, C–Na₂Si₂O₅ is closely related to β-Na₂Si₂O₅. However, both structures differ in the stacking sequences of the layers. A possible explanation for the frequently observed polysynthetic twinning of phase C is presented. In the ²⁹Si MAS-NMR spectrum of C–Na₂Si₂O₅ four well-resolved lines of equal intensity are observed at ?86.0, ?86.3, ?87.4, and ?88.2?ppm. The narrow range of isotropic chemical shifts reflects the great similarity of the environments of the different Si sites. This lack of pronounced differences in geometry renders a reliable assignment of the resonance lines to the individual sites on the basis of known empiric correlations and geometrical features impossible.     

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.     

A new oxygen-deficient perovskite phase Ca(Fe<Subscript>0.4</Subscript>Si<Subscript>0.6</Subscript>)O<Subscript>2.8</Subscript> and phase relations along the join CaSiO<Subscript>3</Subscript>–CaFeO<Subscript>2.5</Subscript> at transition zone conditions

A new oxygen-deficient perovskite with the composition Ca(Fe_0.4Si_0.6)O_2.8 has been synthesised at high-pressure and -temperature conditions relevant to the Earths transition zone using a multianvil apparatus. In contrast to pure CaSiO₃ perovskite, this new phase is quenchable under ambient conditions. The diffraction pattern revealed strong intensities for pseudocubic reflections, but the true lattice is C-centred monoclinic with a=9.2486 Å, b=5.2596 Å, c=21.890 Å and =97.94°. This lattice is only slightly distorted from rhombohedral symmetry. Electron-diffraction and high-resolution TEM images show that a well-ordered ten-layer superstructure is developed along the monoclinic c* direction, which corresponds to the pseudocubic [111] direction. This unique type of superstructure likely consists of an oxygen-deficient double layer with tetrahedrally coordinated silicon, alternating with eight octahedral layers of perovskite structure, which are one half each occupied by silicon and iron as indicated by Mössbauer and Si K electron energy loss spectroscopy. The maximum iron solubility in CaSiO₃ perovskite is determined at 16 GPa to be 4 at% on the silicon site and it increases significantly above 20 GPa. The phase relations have been analysed along the join CaSiO₃–CaFeO_2.5, which revealed that no further defect perovskites are stable. An analogous phase exists in the aluminous system, with Ca(Al_0.4Si_0.6)O_2.8 stoichiometry and diffraction patterns similar to that of Ca(Fe_0.4Si_0.6)O_2.8. In addition, we discovered another defect perovskite with Ca(Al_0.5Si_0.5)O_2.75 stoichiometry and an eight-layer superstructure most likely consisting of a tetrahedral double layer alternating with six octahedral layers. The potential occurrence of all three defect perovskites in the Earths interior is discussed.     

Characterization of synthetic hedenbergite (CaFeSi<Subscript>2</Subscript>O<Subscript>6</Subscript>)–petedunnite (CaZnSi<Subscript>2</Subscript>O<Subscript>6</Subscript>) solid solution series by X-ray powder diffraction and <Superscript>57</Superscript>Fe Mössbauer spectroscopy

Clinopyroxenes along the solid solution series hedenbergite (CaFeSi₂O₆)–petedunnite (CaZnSi₂O₆) were synthesized under hydrothermal conditions and different oxygen fugacities at temperatures of 700 to 1200 °C and pressures of 0.2 to 2.5 GPa. Properties were determined by means of X-ray diffraction, electron microprobe analysis and ⁵⁷Fe Mössbauer spectroscopy at 298 K. Unit-cell parameters display a linear dependency with changing composition. Parameters a₀ and b₀ exhibit a linear decrease with increasing Zn content while the monoclinic angle increases linearly. Parameter c₀ is not affected by composition and remains constant at a value of 5.248 Å. The molar volume can be described according to the equation V_mol (ccm mol^–1)=33.963(16)–0.544(31)*Zn pfu. The isomer shifts of ferrous iron on the octahedral M1 site in hedenbergite are not affected by composition along the hedenbergite–petedunnite solid solution series and remain constant at an average value of 1.18 mm s^–1. Quadrupole splittings of Fe²⁺ on the M1 are, however, strongly affected by composition, and they decrease linearly with increasing petedunnite component in hedenbergite, ranging from 2.25 mm s^–1 for pure hedenbergite end member to 1.99 mm s^–1 for a solid solution containing 84 mole% petedunnite. The half-widths of intermediate solid solutions vary between 0.26 and 0.33 mm s^–1, indicating, in accordance with the microprobe analyses and X-ray diffraction, that samples are homogeneous and well-crystallized. The data from this study demonstrate that the crystallinity of hedenbergitic clinopyroxenes can be improved by using oxide mixtures as starting materials. Crystal sizes for intermediate compositions range up to 70 m, suitable for standard single-crystal X-ray analysis.This paper is dedicated to Prof. Dr. Georg Amthauer, Salzburg, on occasion of his 60th birthday     

Dachiardite-K, (K<Subscript>2</Subscript>Ca)(Al<Subscript>4</Subscript>Si<Subscript>20</Subscript>O<Subscript>48</Subscript>) · 13H<Subscript>2</Subscript>O,a new zeolite from Eastern Rhodopes,Bulgaria

Dachiardite-K (IMA No. 2015-041), a new zeolite, is a K-dominant member of the dachiardite series with the idealized formula (К₂Са)(Al₄Si₂₀O₄₈) · 13H₂О. It occurs in the walls of opal–chalcedony veinlets cutting hydrothermally altered effusive rocks of the Zvezdel paleovolcanic complex near the village of Austa, Momchilgrad Municipality, Eastern Rhodopes, Bulgaria. Chalcedony, opal, dachiardite-Ca, dachiardite-Na, ferrierite-Mg, ferrierite-K, clinoptilolite-Ca, clinoptilolite-K, mordenite, smectite, celadonite, calcite, and barite are associated minerals. The mineral forms radiated aggregates up to 8 mm in diameter consisting of split acicular individuals. Dachiardite-K is white to colorless. Perfect cleavage is observed on (100). D _meas = 2.18(2), D _calc = 2.169 g/cm³. The IR spectrum is given. Dachiardite-K is biaxial (+), α = 1.477 (calc), β = 1.478(2), γ = 1.481(2), 2V _meas = 65(10)°. The chemical composition (electron microprobe, mean of six point analyses, H₂O determined by gravimetric method) is as follows, wt %: 4.51 K₂O, 3.27 CaO, 0.41 BaO, 10.36 A1₂O₃, 67.90 SiO₂, 13.2 H₂O, total is 99.65. The empirical formula is H_26.23K_1.71Ca_1.04Ba_0.05Al_3.64Si_20.24O₆₁. The strongest reflections in the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 9.76 (24) (001), 8.85 (58) (200), 4.870 (59) (002), 3.807 (16) (202), 3.768 (20) (112, 020), 3.457 (100) (220), 2.966 (17) (602). Dachiardite-K is monoclinic, space gr. C2/m, Cm or C2; the unit cell parameters refined from the powder X-ray diffraction data are: a = 18.670(8), b = 7.511(3), c = 10.231(4) Å, β = 107.79(3)°, V= 1366(1) ų, Z = 1. The type specimen has been deposited in the Earth and Man National Museum, Sofia, Bulgaria, with the registration number 23927.     

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.     

Yegorovite,Na<Subscript>4</Subscript>[Si<Subscript>4</Subscript>O<Subscript>8</Subscript>(OH)<Subscript>4</Subscript>]·7H<Subscript>2</Subscript>O,a new mineral from the Lovozero alkaline pluton,Kola Peninsula

A new mineral, yegorovite, has been identified in the late hydrothermal, low-temperature assemblage of the Palitra hyperalkaline pegmatite at Mt. Kedykverpakhk, Lovozero alkaline pluton, Kola Peninsula, Russia. The mineral is intimately associated with revdite and megacyclite, earlier natrosilite, microcline, and villiaumite. Yegorovite occurs as coarse, usually split prismatic (up to 0.05 × 0.15 × 1 mm) or lamellar (up to 0.05 × 0.7 × 0.8 mm) crystals. Polysynthetic twins and parallel intergrowths are typical. Mineral individuals are combined in bunches or chaotic groups (up to 2 mm); radial-lamellar clusters are less frequent. Yegorovite is colorless, transparent with vitreous luster. Cleavage is perfect parallel to (010) and (001). Fracture is splintery; crystals are readily split into acicular fragments. The Mohs hardness is ～2. Density is 1.90(2) g/cm³ (meas) and 1.92 g/cm³ (calc). Yegorovite is biaxial (?), with α = 1.474(2), β = 1.479(2), and γ = 1.482(2), 2V _meas > 70°, 2V _calc = 75°. The optical orientation is X ∧ a ～ 15°, Y = c, Z = b. The IR spectrum is given. The chemical composition determined using an electron microprobe (H₂O determined from total deficiency) is (wt %): 23.28 Na₂O, 45.45 SiO₂, 31.27 H₂O_calc; the total is 100.00. The empirical formula is Na_3.98Si_4.01O_8.02(OH)_3.98 · 7.205H₂O. The idealized formula is Na₄[Si₄O₈(OH)₄] · 7H₂O. Yegorovite is monoclinic, space group P2₁/c. The unit-cell dimensions are a = 9.874, b= 12.398, c = 14.897 Å, β = 104.68°, V = 1764.3 ų, Z = 4. The strongest reflections in the X-ray powder pattern (d, Å (I, %)([hkl]) are 7.21(70)[002], 6.21(72)[012, 020], 4.696(44)[022], 4.003(49)[211], 3.734(46)[\(\bar 2\) 13], 3.116(100)[024, 040], 2.463(38)[\(\bar 4\)02, \(\bar 2\)43]. The crystal structure was studied by single-crystal method, R _hkl = 0.0745. Yegorovite is a representative of a new structural type. Its structure consists of single chains of Si tetrahedrons [Si₄O₈(OH)₄]∞ and sixfold polyhedrons of two types: [NaO(OH)₂(H₂O)₃] and [NaO(OH)(H₂O)₄] centered by Na. The mineral was named in memory of Yu. K. Yegorov-Tismenko (1938–2007), outstanding Russian crystallographer and crystallochemist. The type material of yegorovite has been deposited at the Fersman Mineralogical Museum of Russian Academy of Sciences, Moscow.     

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.     

Middendorfite,K<Subscript>3</Subscript>Na<Subscript>2</Subscript>Mn<Subscript>5</Subscript>Si<Subscript>12</Subscript>(O,OH)<Subscript>36</Subscript> · 2H<Subscript>2</Subscript>O,a new mineral species from the Khibiny pluton,Kola Peninsula

Middendorfite, a new mineral species, has been found in a hydrothermal assemblage in Hilairite hyperperalkaline pegmatite at the Kirovsky Mine, Mount Kukisvumchorr apatite deposit, Khibiny alkaline pluton, Kola Peninsula, Russia. Microcline, sodalite, cancrisilite, aegirine, calcite, natrolite, fluorite, narsarsukite, labuntsovite-Mn, mangan-neptunite, and donnayite are associated minerals. Middendorfite occurs as rhombshaped lamellar and tabular crystals up to 0.1 × 0.2 × 0.4 mm in size, which are combined in worm-and fanlike segregations up to 1 mm in size. The color is dark to bright orange, with a yellowish streak and vitreous luster. The mineral is transparent. The cleavage (001) is perfect, micalike; the fracture is scaly; flakes are flexible but not elastic. The Mohs hardness is 3 to 3.5. Density is 2.60 g/cm³ (meas.) and 2.65 g/cm³ (calc.). Middendorfite is biaxial (?), α = 1.534, β = 1.562, and γ = 1.563; 2V (meas.) = 10°. The mineral is pleochroic strongly from yellowish to colorless on X through brown on Y and to deep brown on Z. Optical orientation: X = c. The chemical composition (electron microprobe, H₂O determined with Penfield method) is as follows (wt %): 4.55 Na₂O, 10.16 K₂O, 0.11 CaO, 0.18 MgO, 24.88 MnO, 0.68 FeO, 0.15 ZnO, 0.20 Al₂O₃, 50.87 SiO₂, 0.17 TiO₂, 0.23 F, 7.73 H₂O; ?O=F₂?0.10, total is 99.81. The empirical formula calculated on the basis of (Si,Al)₁₂(O,OH,F)₃₆ is K_3.04(Na_2.07Ca_0.03)_Σ2.10(Mn_4.95Fe_0.13Mg_0.06Ti_0.03Zn_0.03)_Σ5.20(Si_11.94Al_0.06)_Σ12O_27.57(OH)_8.26F_0.17 · 1.92H₂O. The simplified formula is K₃Na₂Mn₅Si₁₂(O,OH)₃₆ · 2H₂O. Middenforite is monoclinic, space group: P2₁/m or P2₁. The unit cell dimensions are a = 12.55, b = 5.721, c = 26.86 Å; β = 114.04°, V = 1761 ų, Z = 2. The strongest lines in the X-ray powder pattern [d, Å, (I)(hkl)] are: 12.28(100)(002), 4.31(81)(11\(\overline 4 \)), 3.555(62)(301, 212), 3.063(52)(008, 31\(\overline 6 \)), 2.840(90)(312, 021, 30\(\overline 9 \)), 2.634(88)(21\(\overline 9 \), 1.0.\(\overline 1 \)0, 12\(\overline 4 \)), 2.366(76)(22\(\overline 6 \), 3.1.\(\overline 1 \)0, 32\(\overline 3 \)), 2.109(54)(42–33, 42–44, 51\(\overline 9 \), 414), 1.669(64)(2.2.\(\overline 1 \)3, 3.2.\(\overline 1 \)3, 62\(\overline 3 \), 6.1.\(\overline 1 \)3), 1.614(56)(5.0.\(\overline 1 \)6, 137, 333, 71\(\overline 1 \)). The infrared spectrum is given. Middendorfite is a phyllosilicate related to bannisterite, parsenttensite, and the minerals of the ganophyllite and stilpnomelane groups. The new mineral is named in memory of A.F. von Middendorff (1815–1894), an outstanding scientist, who carried out the first mineralogical investigations in the Khibiny pluton. The type material of middenforite has been deposited at the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.     

Parageorgbokiite, β-Cu<Subscript>5</Subscript>O<Subscript>2</Subscript>(SeO<Subscript>3</Subscript>)<Subscript>2</Subscript>Cl<Subscript>2</Subscript>, a new mineral species from volcanic exhalations,Kamchatka Peninsula,Russia

Parageorgbokiite, β-Cu₅O₂(SeO₃)₂Cl₂, has been found at the second cinder cone of the Great Fissure Tolbachik Eruption, Kamchatka Peninsula, Russia. Ralstonite, tolbachite, melanothallite, chalcocyanite, euchlorine, Fe oxides, tenorite, native gold, sophiite, Na, Ca, and Mg sulfates, cotunnite, and some copper oxoselenites are associated minerals. The estimated temperature of the mineral formation is 400–625°C. The color is green, with a vitreous luster; the streak is light green. The mineral is brittle, with the Mohs hardness ranging from 3 to 4. Cleavage is not observed. The calculated density is 4.70 g/cm³. Parageorgbokiite is biaxial (+); α = 2.05(1), β = 2.05(1), and γ = 2.08(1); 2V _(meas.) is ～0³, and 2V _(calc.) = 0(5)°. The optical orientation is X = a; other details remain unclear. The mineral is pleochroic, from grass green on X and Y to yellowish green on Z. The empirical formula calculated on the basis of O + Cl = 10 is Cu_4.91Pb_0.02O_1.86(ScO₃)₂Cl_2.14. The simplified formula is Cu₅O₂(ScO₃)₂Cl₂. Parageorgbokiite pertains to a new structural type of inorganic compounds. Its name points out its dimorphism with georgbokiite, which was named in honor of G.B. Bokii, the prominent Russian crystal chemist (1909–2000).     

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).     

Characterization,dissolution and solubility of the hydroxypyromorphite–hydroxyapatite solid solution [(Pb<Subscript>x</Subscript>Ca<Subscript>1−x</Subscript>)<Subscript>5</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>OH] at 25 °C and pH 2–9

Background

The interaction between Ca-HAP and Pb²⁺ solution can result in the formation of a hydroxyapatite–hydroxypyromorphite solid solution [(Pb_xCa_1?x)₅(PO₄)₃(OH)], which can greatly affect the transport and distribution of toxic Pb in water, rock and soil. Therefore, it’s necessary to know the physicochemical properties of (Pb_xCa_1?x)₅(PO₄)₃(OH), predominantly its thermodynamic solubility and stability in aqueous solution. Nevertheless, no experiment on the dissolution and related thermodynamic data has been reported.

Results

Dissolution of the hydroxypyromorphite–hydroxyapatite solid solution [(Pb_xCa_1?x)₅(PO₄)₃(OH)] in aqueous solution at 25 °C was experimentally studied. The aqueous concentrations were greatly affected by the Pb/(Pb + Ca) molar ratios (X_Pb) of the solids. For the solids with high X_Pb [(Pb_0.89Ca_0.11)₅(PO₄)₃OH], the aqueous Pb²⁺ concentrations increased rapidly with time and reached a peak value after 240–720 h dissolution, and then decreased gradually and reached a stable state after 5040 h dissolution. For the solids with low X_Pb (0.00–0.80), the aqueous Pb²⁺ concentrations increased quickly with time and reached a peak value after 1–12 h dissolution, and then decreased gradually and attained a stable state after 720–2160 h dissolution.

Conclusions

The dissolution process of the solids with high X_Pb (0.89–1.00) was different from that of the solids with low X_Pb (0.00–0.80). The average K _sp values were estimated to be 10^?80.77±0.20 (10^?80.57–10^?80.96) for hydroxypyromorphite [Pb₅(PO₄)₃OH] and 10^?58.38±0.07 (10^?58.31–10^?58.46) for calcium hydroxyapatite [Ca₅(PO₄)₃OH]. The Gibbs free energies of formation (ΔG _f ^o ) were determined to be ?3796.71 and ?6314.63 kJ/mol, respectively. The solubility decreased with the increasing Pb/(Pb + Ca) molar ratios (X_Pb) of (Pb_xCa_1?x)₅(PO₄)₃(OH). For the dissolution at 25 °C with an initial pH of 2.00, the experimental data plotted on the Lippmann diagram showed that the solid solution (Pb_xCa_1?x)₅(PO₄)₃(OH) dissolved stoichiometrically at the early stage of dissolution and moved gradually up to the Lippmann solutus curve and the saturation curve for Pb₅(PO₄)₃OH, and then the data points moved along the Lippmann solutus curve from right to left. The Pb-rich (Pb_xCa_1?x)₅(PO₄)₃(OH) was in equilibrium with the Ca-rich aqueous solution.

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Graphical abstractLippmann diagrams for dissolution of the hydroxypyromorphite–hydroxyapatite solid solution [(Pb_xCa_1?x)₅(PO₄)₃OH] at 25??C and an initial pH of 2.00.

     

The solubility of Gd<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript> and the hydrolysis of Gd<Superscript>3+</Superscript> in acidic solutions at 250°C and saturation vapor pressure

The solubility of Gd₂Ti₂O₇ ceramic in acidic solutions (HCl and HClO₄) was studied at 250°C and saturation vapor pressure within pH 2.5–5.2. The dissolution process occurs mainly via two reactions: 0.5 Gd₂Ti₂O_7(cr) + 3H⁺ = Gd³⁺ + TiO_2(cr) + 1.5 H₂O at pH < 3 and 0.5Gd₂Ti₂O₇(cr) + H⁺ + 0.5H₂O = Gd(OH) ₂ ⁺ TiO₂(cr) at pH 3–5. The thermodynamic equilibrium constants were calculated at the 0.95 confidence level as log K ₍₁₎ ^o = 4.12 ± 0.47; = ?0.97 ± 0.16 at 250°C. It was shown that Gd³⁺ undergoes hydrolysis in solutions with pH > 3, and the species Gd(OH) ₂ ⁺ dominates up to at least pH 5. At pH < 3, Gd occurs in solutions as Gd³⁺. The second constant of Gd³⁺ hydrolysis was determined at 250°C as K ^o = ?5.09 ± 0.5, and the thermodynamic characteristics of the initial Gd₂Ti₂O₇ solid phase were determined: S _298.15 ^o = 251.4 J/(mol K) and ΔfG _298.15 ^o = ?3630 ± 10 kJ/mol.     

Influence of oxygen fugacity on the solubility of carbon and hydrogen in FeO-Na<Subscript>2</Subscript>O-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> melts in equilibrium with liquid iron at 1.5 GPa and 1400°C

Equilibria in the model melt (NaAlSi₃O₈(80) + FeO(20))-C-H₂ system were experimentally studied at ΔlogfO₂(IW) from −2.2 to −5.6, a pressure of 1.5 GPa, and a temperature of 1400°C. The experiments were conducted in a piston-cylinder apparatus using Pt capsules. The low fO₂ values were imposed during the experiments by adding 2, 5, and 7 wt % of finely dispersed SiC to NaAlSi₃O₈(80) + FeO(20) powder. The experimental products were investigated by electron microprobe analysis and Raman spectroscopy. The investigations showed that melting at 1.5 GPa and 1400°C in the stability field of a metallic iron phase produces silicate liquids containing both oxidized and reduced H and C species. Carbon and hydrogen are dissolved in the melt as C-H (CH₄) complexes. In addition, OH⁻ groups, molecular hydrogen H₂, and molecular water H₂O were observed in the melts. The proportions of dissolved C and H species strongly depend on oxygen fugacity. With decreasing fO₂, the content of O-H species decreases and that of H-C species increases. The obtained data and previous results (Kadik et al., 2004, 2006) allow us to suppose a fundamental change in the character of magmatic transfer of C-O-H components during the evolution of the redox state of the Earth’s mantle in geologic time toward higher fO₂ in its interiors.     

In situ high-temperature X-ray diffraction and spectroscopic study of fibroferrite,FeOH(SO<Subscript>4</Subscript>)·5H<Subscript>2</Subscript>O

The thermal dehydration process of fibroferrite, FeOH(SO₄)·5H₂O, a secondary iron-bearing hydrous sulfate, was investigated by in situ high-temperature synchrotron X-ray powder diffraction (HT-XRPD), in situ high-temperature Fourier transform infrared spectroscopy (HT-FTIR) and thermal analysis (TGA-DTA) combined with evolved gas mass spectrometry. The data analysis allowed the determination of the stability fields and the reaction paths for this mineral as well as characterization of its high-temperature products. Five main endothermic peaks are observed in the DTA curve collected from room T up to 800 °C. Mass spectrometry of gases evolved during thermogravimetric analysis confirms that the first four mass loss steps are due to water emission, while the fifth is due to a dehydroxylation process; the final step is due to the decomposition of the remaining sulfate ion. The temperature behavior of the different phases occurring during the heating process was analyzed, and the induced structural changes are discussed. In particular, the crystal structure of a new phase, FeOH(SO₄)·4H₂O, appearing at about 80 °C due to release of one interstitial H₂O molecule, was solved by ab initio real-space and reciprocal-space methods. This study contributes to further understanding of the dehydration mechanism and thermal stability of secondary sulfate minerals.     

Trace element partitioning between orthopyroxene and anhydrous silicate melt on the lherzolite solidus from 1.1 to 3.2 GPa and 1,230 to 1,535°C in the model system Na<Subscript>2</Subscript>O–CaO–MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>

We determined experimentally the Nernst distribution coefficient between orthopyroxene and anhydrous silicate melt for trace elements i in the system Na₂O–CaO–MgO–Al₂O₃–SiO₂ (NCMAS) along the dry model lherzolite solidus from 1.1 GPa/1,230°C up to 3.2 GPa/1,535°C in a piston cylinder apparatus. Major and trace element composition of melt and orthopyroxene were determined with a combination of electron microprobe and ion probe analyses. We provide partitioning data for trace elements Li, Be, B, K, Sc, Ti, V, Cr, Co, Ni, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Sm, Nd, Yb, Lu, Hf, Ta, Pb, U, and Th. The melts were chosen to be boninitic at 1.1 and 2.0 GPa, picritic at 2.3 GPa and komatiitic at 2.7 and 3.2 GPa. Orthopyroxene is Tschermakitic with 8 mol% Mg-Tschermaks MgAl[AlSiO₆] at 1.1 GPa while at higher pressure it has 18–20 mol%. The rare earth elements show a continuous, significant increase in compatibility with decreasing ionic radius from D _La^opx−melt ∼ 0.0008 to D _Lu^opx−melt ∼ 0.15. For the high-field-strength elements compatibility increases from D _Th^opx−melt ∼ 0.001 through D _Nb^opx−melt ∼ 0.0015, D _U^opx−melt ∼ 0.002, D _Ta^opx−melt ∼ 0.005, D _Zr^opx−melt ∼ 0.02 and D _Hf^opx−melt ∼ 0.04 to D _Ti^opx−melt ∼ 0.14. From mathematical and graphical fits we determined best-fit values for D ₀^M1, D ₀^M2, r ₀^M1, r ₀^M2, E ₀^M1, and E ₀^M2 for the two different M sites in orthopyroxene according to the lattice strain model and calculated the intracrystalline distribution between M1 and M2. Our data indicate extreme intracrystalline fractionation for most elements in orthopyroxene; for the divalent cations D _i ^M2−M1 varies by three orders of magnitude between D _Co^M2−M1 = 0.00098–0.00919 and D _Ba^M2−M1 = 2.3–28. Trivalent cations Al and Cr almost exclusively substitute on M1 while the other trivalent cations substitute on M2; D _La^M2−M1 reaches extreme values between 6.5 × 10⁷ and 1.4 × 10¹⁶. Tetravalent cations Ti, Hf, and Zr almost exclusively substitute on M1 while U and Th exclusively substitute on M2. Our new comprehensive data set can be used for polybaric-polythermal melting models along the Earth’s mantle solidus. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Chesnokovite,Na<Subscript>2</Subscript>[SiO<Subscript>2</Subscript>(OH)<Subscript>2</Subscript>] · 8H<Subscript>2</Subscript>O,the first natural sodium orthosilicate from the Lovozero alkaline pluton,Kola Peninsula: Description and crystal structure of a new mineral species

Chesnokovite, a new mineral species, is the first natural sodium orthosilicate. It has been found in an ussingite vein uncovered by underground mining at Mt. Kedykverpakhk, Lovozero alkaline pluton, Kola Peninsula, Russia. Natrolite, sodalite, vuonnemite, steenstrupine-(Ce), phosinaite-(Ce), natisite, gobbinsite, villiaumite, and natrosilite are associated minerals. Chesnokovite occurs as intergrowths with natrophospate in pockets up to 4 × 6 × 10 cm in size consisting of chaotic segregations of coarse lamellar crystals (up to 0.05 × 1 × 2 cm in size) flattened along [010]. The crystals are colorless and transparent. The aggregates are white to pale brownish yellowish, with a white streak and a vitreous luster. The cleavage is perfect parallel to (010) and distinct to (100) and (001). The fracture is stepped. The Mohs’ hardness is 2.5. The measured density is 1.68 g/cm³; the density calculated on the basis of an empirical formula is 1.60 g/cm³ and 1.64 g/cm³ on the basis of an idealized formula. The new mineral is optically biaxial, positive, α = 1.449, β = 1.453, γ = 1.458, 2V _meas = 80°, and Z = b. The infrared spectrum is given. The chemical composition (Si determined with electron microprobe; Na, K, and Li, with atomic emission analysis; and H₂O, with the Alimarin method) is as follows, wt %: 21.49 Na₂O, 0.38 K₂O, 0.003 Li₂O, 21.42 SiO₂, 54.86 H₂O, total is 98.153. The empirical formula calculated on the basis of O₂(OH)₂ is as follows: (Na_1.96K_0.02)_Σ1.98Si_1.005O₂(OH)₂ · 7.58H₂O. The simplified formula (Z = 8) is Na₂[SiO₂(OH)₂] · 8H₂O. The new mineral is orthorhombic, and the space group is Ibca. The unit-cell dimensions are: a = 11.7119, b = 19.973, c = 11.5652 Å, and V = 2299.0 ų. The strongest reflections in the X-ray powder pattern [d, Å (I, %)(hkl)] are: 5.001(30)(211), 4.788(42)(022), 3.847(89)(231), 2.932(42)(400), 2.832(35)(060), 2.800(97)(332, 233), and 2.774(100)(341, 143, 114). The crystal structure was studied using the Rietveld method, R _p = 5.77, R _wp = 7.77, R _B = 2.07, and R _F = 1.74. The structure is composed of isolated [SiO₂(OH)₂] octahedrons and the chains of edge-shared [Na[H₂O)₆] octahedrons. The Si and Na polyhedrons are linked only by H-bonds, and this is the cause of the low stability of chesnokovite under atmospheric conditions. The new mineral is named in memory of B.V. Chesnokov (1928–2005), an outstanding mineralogist. The type material of chesnokovite is deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.     

X-ray single-crystal and Raman study of knorringite,Mg3(Cr1.58Mg0.21Si0.21)Si3O12, synthesized at 16 GPa and 1,600 °C

The crystal structure of a knorringite-type compound, Mg₃(Cr_1.58Mg_0.21Si_0.21)Si₃O₁₂, synthesized in a multi-anvil press at P = 16 GPa and T = 1,600 °C, was refined from single-crystal X-ray diffraction data up to R = 2.36 % for 314 independent reflections. Garnet was found to be cubic and have space group Ia $\overline{3}$ d, with the unit cell parameters a = 11.5718 (1) Å, V = 1,549.54 (2) Å³. The knorringite crystal studied contains 21 mol% of majorite end-member. The structural characterization of knorringitic garnet is important because the study of its thermodynamic constants provides new constraints on thermobarometry of peridotitic garnet assemblages of the lowermost upper mantle. The Raman spectra of synthetic knorringite have been obtained for the first time.     

Interaction of Fe and Fe<Subscript>3</Subscript>C with hydrogen and nitrogen at 6–20 GPa: a study by in situ X-ray diffraction

A method of in situ X-ray diffraction at Spring-8 (Japan) was used to analyze simultaneously the hydrogen incorporation into Fe and Fe₃C, as well as to measure the relative stability of carbides, nitrides, sulfides, and hydrides of iron at pressures of 6–20 GPa and temperatures up to 1600 K. The following stability sequence of individual iron compounds was established in the studied pressure and temperature interval: FeS > FeN > FeC > FeH > Fe. A change in the unit-cell volume as compared to the known equations of state was used to estimate the hydrogen contents in carbide Fe₃C and hydride FeH_x. Data on hydride correspond to stoichiometry with x ≈ 1. Unlike iron sulfides and silicides, the solubility of hydrogen in Fe₃C seemed to be negligibly low—within measurement error. Extrapolating obtained data to pressures of the Earth’s core indicates that carbon and hydrogen are mutually incpompatible in the iron–nickel core, while nitrogen easily substitutes carbon and may be an important component of the inner core in the light of the recent models assuming the predominance of iron carbide in its composition.