Calcium and Magnesium‐bearing Sabugalite from the Tono Uranium Deposit,Central Japan

Calcium and magnesium‐bearing sabugalite occurs as aggregations of yellowish platy crystals in veinlets or druses in conglomerate from the oxidized parts of the Tono uranium deposit, Central Japan. X‐ray powder diffractometry of this mineral has reflections consistent with previous powder diffraction data of sabugalite. It is included in the monoclinic system with space group C2/m and calculated cell parameters of a = 19.68Å, b = 9.89Å, c = 9.82Å α = γ = 90°, β‐96.93° and V = 1897.83ų. Chemical analysis yields a formula of (Ca_0.10 Mg_0.09)_Σ0.19Al_0.53(UO₂)_2.04((PO₄)_1.99(AsO₄)_0.01)_Σ2.00·11.22H₂O. EMPA mapping shows that the mineral is compositionally uniform with no micron‐scale layering. Charge of cations including Ca and Mg in the cation‐H₂O layer is 1.98 being identical to that of autunite group minerals. This suggests that the charge balance in the cation‐H₂O layer of the mineral could be made by the alkaline earth or alkaline elements rather than by hydrogen ions.     

The crystal structure of Ca5(PO4)2SiO4 (Silico-Carnotite)

Summary The crystal structure of Ca₅(PO₄)₂SiO₄ (silico-carnotite) has been determined from 3358 x-ray diffraction data collected by a counter method and has been refined toR _w =0.038,R=0.045, in space group Pnma. The unit cell parameters area=6.737 (1) Å,b=15.508 (2) Å andc=10.132 (1) Å at 24°C;Z=4. The observed density is 3.06 and the calculated density is 3.03 g · cm^–3. The crystal contains about 2.5% V₂O₅ as an impurity. The bond lengths within the tetrahedral anions suggest that substitution or disorder of PO₄ ^3–, SiO₄ ^4– and possibly VO₄ ^3– occurs among the anion sites. The structure has some relationship to that of Ca₅(PO₄)₃OH, the predominant inorganic phase in the human body, but suggests that the Ca₅(PO₄)₃OH type structure may not be stable without some of the OH positions being filled. Ca₅(PO₄)₂SiO₄ is more closely related to K₃Na(SO₄)₂ (glaserite) if it is considered that there are systematic cation vacancies in Ca₅(PO₄)₂SiO₄.This type of structure is consistent with the view that cation vacancies in the glaserite-type structure account for solid solutions between Ca₂SiO₄ and Ca₃(PO₄)₂ and between Ca₃(PO₄)₂ and CaNaPO₄.

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Die Kristallstruktur vonCa ₅(PO ₄)₂ SiO ₄ (Silicocarnotit)

Zusammenfassung Die Kristallstruktur von Ca₅(PO₄)₂SiO₄ (Silicocarnotit) wurde aus 3358 Röntgendiffraktometer-Daten bestimmt und in Raumgruppe Pnma aufR _w =0,038,R=0,045 verfeinert. Die Gitterkonstanten (bei 24° C) sind:a=6,737 (1) Å,b=15,508 (2) Å undc=10,132 (1) Å,Z=4; D_obs.=3,06 g · cm^–3, D_exp.=3,03 g · cm^–3. Der Kristall enthält etwa 2,5% V₂O₅ als Verunreinigung. Die Bindungslängen in den tetraedrischen Anionen legen nahe, daß unter den Anionenplätzen gegenseitige Vertretung oder Unordnung von PO₄ ^3–, SiO₄ ^4– und möglicherweise VO₄ ^3– auftritt. Die Struktur zeigt einige Verwandtschaft zu der von Ca₅(PO₄)₃OH, der wichtigsten anorganischen Substanz im menschlichen Körper, weist aber darauf hin, daß eine Struktur vom Ca₅(PO₄)₃OH-Typ ohne Besetzung eines Teiles der OH-Position nicht stabil ist. Ca₅(PO₄)₂SiO₄ zeigt engere Beziehungen zu K₃Na(SO₄)₂ (Glaserit), wenn man berücksichtigt, daß in Ca₅(PO₄)₃SiO₄ systematische Kationen-Leerstellen sind. Dieser Strukturtyp ist mit derAuffassung in Übereinstimmung, daß Kationenleerstellen für die festen Lösungen zwischen Ca₂SiO₄ und Ca₃(PO₄)₂ und zwischen Ca₃(PO₄)₂ und CaNaPO₄ verantwortlich sind.

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With 9 Figures     

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 crystal structure of Ca7Mg9(Ca,Mg)2(PO4)12

Summary The unit cell of Ca₇Mg₉(Ca,Mg)₂(PO₄)₁₂ isa=22.841(3) Å,b=9.994(1) Å,c=17.088(5) Å and =99.63(3)° at 24° C. The space-group is C2/c with four formula weights per cell. The crystal structure has been determined from 6330 X-ray reflections measured from a single crystal by a counter method and has been refined toR _w =0.044,R=0.046 (based on 4227 observed reflections and 322 of the unobserved reflections). One cation site may be occupied by Ca or Mg and gives rise to variability in composition as is reflected in the formula give above. In the sample studied, Ca and Mg occupy the site approximately equally. The direction in the unit cell is pseudo-hexagonal. The structure of Ca₇Mg₉(Ca,Mg)₂(PO₄)₁₂ is related to that of K₃Na(SO₄)₂ in that along it has columns of cations and columns of cations and anions. These columns are arranged in a K₃Na(SO₄)₂-type pseudo-cell. In the cation-anion columns, every other cation site in K₃Na(SO₄)₂ is vacant in Ca₇Mg₉(Ca,Mg)₂(PO₄)₁₂.

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Die Kristallstruktur von Ca₇Mg₉(Ca,Mg)₂(PO₄)₁₂

Zusammenfassung Die Gitterkonstanten von Ca₇Mg₉(Ca,Mg)₂(PO₄)₁₂ sind (bei 24° C)a=22,841(3) Å,b=9,994(1) Å,c=17,088(5) Å und =99,63(3)°; Raumgruppe: C2/c;Z=4. Die Kristallstruktur wurde aus 6330 Röntgendiffraktometer-Einkristalldaten bestimmt und (auf der Basis von 4227 beobachteten und 322 nicht-beobachteten Reflexen) aufR _w =0,044 undR=0,046 verfeinert. Eine Kationenlage kann von Ca oder Mg besetzt werden, was eine Variabilität der Zusammensetzung ergibt, wie sie obige Formel ausdrückt. In der untersuchten Probe besetzen Ca und Mg diese Punktlage etwa zu gleichen Teilen. Die -Richtung der Elementarzelle ist pseudo-hexagonal. Die Struktur von Ca₇Mg₉(Ca,Mg)₂(PO₄)₁₂ ist zu der von K₃Na(SO₄)₂ darin verwandt, daß sie längs Säulen von Kationen und Säulen von Anionen hat. Diese Säulen sind in einer Pseudozelle vom K₃Na(SO₄)₂-Typ angeordnet. In den Kation-Anion-Säulen ist jede zweite Kationen-Lage des K₃Na(SO₄)₂ in Ca₇Mg₉(Ca,Mg)₂(PO₄)₁₂ unbesetzt.

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With 6 Figures     

The stability of primary alluaudites in granitic pegmatites: an experimental investigation of the Na2(Mn2−2x Fe1+2x )(PO4)3 system

In order to assess the geothermometric potential of the Na₂(Mn_2−2x Fe_1+2x )(PO₄)₃ system (x = 0–1), which represents the compositions of natural weakly oxidized alluaudites, we performed hydrothermal experiments between 400 and 800°C, at 1 kbar, under an oxygen fugacity (f(O₂)) controlled by the Ni–NiO (NNO), Fe₂O₃–Fe₃O₄ (HM), Cu₂O–CuO (CT), and Fe–Fe₃O₄ (MI) buffers. When f(O₂) is controlled by NNO, single-phase alluaudites crystallize at 400 and 500°C, whereas the association alluaudite + marićite appears between 500 and 700°C. The limit between these two fields corresponds to the maximum temperature that can be reached by alluaudites in granitic pegmatites, because marićite has never been observed in these geological environments. Because alluaudites are very sensitive to variations of oxygen fugacity, the field of hagendorfite, Na₂MnFe²⁺Fe³⁺(PO₄)₃, has been positioned in the f(O₂)–T diagram, and provides a tool that can be used to estimate the oxygen fugacity conditions that prevailed in granitic pegmatites during the crystallization of this phosphate.     

Structural investigation of the synthetic CaAn(PO4)2 (An = Th and Np) cheralite-like phosphates

The crystallographic structures of the synthetic cheralite, CaTh(PO₄)₂, and its homolog CaNp(PO₄)₂ have been investigated by X-ray diffraction at room temperature. Rietveld analyses showed that both compounds crystallize in the monoclinic system and are isostructural to monazite LnPO₄ (Ln = La to Gd). The space group is P2₁/n (I.T. = 14) with Z = 2. The refined lattice parameters of CaTh(PO₄)₂ are a = 6.7085(8) Å, b = 6.9160(6) Å, c = 6.4152(6) Å, and β = 103.71(1)° with best fit parameters R _wp = 4.87%, R _p = 3.69% and R _B = 3.99%. For CaNp(PO₄)₂, we obtained a = 6.6509(5) Å, b = 6.8390(3) Å, c = 6.3537(8) Å, and β = 104.12(6)° and R _wp = 6.74%, R _p = 5.23%, and R _B = 6.05%. The results indicate significant distortions of bond length and angles of the PO₄ tetrahedra in CaTh(PO₄)₂ and to a lesser extent in CaNp(PO₄)₂. The structural distortions were confirmed by Raman spectroscopy of CaTh(PO₄)₂. A comparison with the isostructural compounds LnPO₄ (Ln = Ce and Sm) confirmed that the substitution of the large rare earth trivalent cations with Ca²⁺ and Th⁴⁺ introduces a distortion of the PO₄ tetrahedra.     

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

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

Description and crystal structure of albrechtschraufite, MgCa4F2[UO2(CO3)3]2⋅17-18H2O

Albrechtschraufite, MgCa₄F₂[UO₂(CO₃)₃]₂?17-18H₂O, triclinic, space group Pī, a?=?13.569(2), b?=?13.419(2), c?=?11.622(2) Å, α?=?115.82(1), β?=?107.61(1), γ?=?92.84(1)° (structural unit cell, not reduced), V?=?1774.6(5) ų, Z?=?2, D _c?=?2.69 g/cm³ (for 17.5 H₂O), is a mineral that was found in small amounts with schröckingerite, NaCa₃F[UO₂(CO₃)₃](SO₄)?10H₂O, on a museum specimen of uranium ore from Joachimsthal (Jáchymov), Czech Republic. The mineral forms small grain-like subhedral crystals (≤ 0.2 mm) that resemble in appearance liebigite, Ca₂[UO₂(CO₃)₃]??~?11H₂O. Colour pale yellow-green, luster vitreous, transparent, pale bluish green fluorescence under ultraviolet light. Optical data: Biaxial negative, nX?=?1.511(2), nY?=?1.550(2), nZ?=?1.566(2), 2?V?=?65(1)° (λ?=?589 nm), r < v weak. After qualitative tests had shown the presence of Ca, U, Mg, CO₂ and H₂O, the chemical formula was determined by a crystal structure analysis based on X-ray four-circle diffractometer data. The structure was later on refined with data from a CCD diffractometer to R1?=?0.0206 and wR2?=?0.0429 for 9,236 independent observed reflections. The crystal structure contains two independent [UO₂(CO₃)₃]^4? anions of which one is bonded to two Mg and six Ca while the second is bonded to only one Mg and three Ca. Magnesium forms a MgF₂(O_carbonate)₃(H₂O) octahedron that is linked via the F atoms with three Ca atoms so as to provide each F atom with a flat pyramidal coordination by one Mg and two Ca. Calcium is 7- and 8-coordinate forming CaFO₆, CaF₂O₂(H₂O)₄, CaFO₃(H₂O)₄ and CaO₂(H₂O)₆ coordination polyhedra. The crystal structure is built up from MgCa₃F₂[UO₂(CO₃)₃]?8H₂O layers parallel to (001) which are linked by Ca[UO₂(CO₃)₃]?5H₂O moieties into a framework of the composition MgCa₄F₂[UO₂(CO₃)₃]?13H₂O. Five additional water molecules are located in voids of the framework and show large displacement parameters. One of the water positions is partly vacant, leading to a total water content of 17-18H₂O per formula unit. The MgCa₃F₂[UO₂(CO₃)₃]?8H₂O layers are pseudosymmetric according to plane group symmetry cmm. The remaining constituents do not sustain this pseudosymmetry and make the entire structure truly triclinic. A characteristic paddle-wheel motif Ca[UO₂(CO₃)₃]₄Ca relates the structure of albrechtschraufite partly to that of andersonite and two synthetic alkali calcium uranyl tricarbonates.     

Crystal chemical characteristics of α-CaSi2O5, a new high pressure calcium silicate with five-coordinated silicon synthesized at 1500°C and 10 GPa

The crystal structure of α-CaSi₂O₅ synthesized at conditions of 1500°C and 10 GPa, has been solved and refined in centrosymmetric space group P , using single crystal X-ray diffraction data. The composition (Z=4) and unit cell are Ca_1.02Si_1.99O₅ by EPMA analysis and a=7.243(2) Å, b=7.546(4) Å, c=6.501(4) Å, α=81.43(5)°, β=84.82(4)°, γ=69.60(3)°, V=329.5(3) ų, yielding the density value, 3.55 g/cm³. The structure is closely related to that of titanite, CaTiSiO₅ and features the square-pyramid five-fold coordination of silicon by oxygen. The ionic radius for five-coordinated Si calculated from the bond distances is 0.33 Å. The substantial deviation of valence sum for Ca indicates the existence of local strain and the instability of α-CaSi₂O₅ at room pressure.     

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.     

Structural and compositional characterization of synthetic (Ca,Sr)-tremolite and (Ca,Sr)-diopside solid solutions

Tremolite (Ca_xSr_1–x)₂Mg₅[Si₈O₂₂/(OH)₂] and diopside (Ca_xSr_1–x)Mg[Si₂O₆] solid solutions have been synthesized hydrothermally in equilibrium with a 1 molar (Ca,Sr)Cl₂ aqueous solution at 750°C and 200 MPa. The solid run products have been investigated by optical, electron scanning and high resolution transmission electron microscopy, electron microprobe, X-ray-powder diffraction and Fourier-transform infrared spectroscopy. The synthesized (Ca,Sr)-tremolites are up to 2000 µm long and 30 µm wide, the (Ca,Sr)-diopsides are up to 150 µm long and 20 µm wide. In most runs the tremolites and diopsides are well ordered and chain multiplicity faults are rare. Nearly pure Sr-tremolite (tr_0.02Sr-tr_0.98) and Sr-diopside (di_0.01Sr-di_0.99) have been synthesized. A continuous solid solution series, i.e. complete substitution of Sr²⁺ for Ca²⁺ on M₄-sites exists for (Ca,Sr)-tremolite. Total substitution of Sr²⁺ for Ca²⁺ on M₂-sites can be assumed for (Ca,Sr)-diopsides. For (Ca,Sr)-tremolites the lattice parameters a, b and β are linear functions of composition and increase with Sr-content whereas c is constant. For the diopside series all 4 lattice parameters are a linear function of composition; a, b, c increase and β decreases with rising Sr-content. The unit cell volume for tremolite increases 3.47% from 906.68 ų for tremolite to 938.21 ų for Sr-tremolite. For diopside the unit cell volume increases 4.87 % from 439.91 ų for diopside to 461.30 ų for Sr-diopside. The observed splitting of the OH stretching band in tremolite is caused by different configurations of the next nearest neighbors (multi mode behavior). Resolved single bands can be attributed to the following configurations on the M₄-sites: SrSr, SrCa, CaCa and CaMg. The peak positions of these 4 absorption bands are a linear function of composition. They are shifted to lower wavenumbers with increasing Sr-content. No absorption band due to the SrMg configuration on the M₄-site is observed. This indicates a very low or negligible cummingtonite component in Sr-rich tremolites, which is also supported by electron microprobe analysis.     

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

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

Transformation of curite into metaautunite paragenesis and electrokinetic properties

Genesis of metaautinute [Ca(UO₂/PO₄)₂ · 7H₂O] starting from curite hints at the existence of an intermediate hydrogen autunite stage [HUO₂PO₄ · 4H₂O]. The substitution of protons in hydrogen autunite by Ca²⁺ ions is proved by electrokinetic measurements. As a consequence of the similarity between X-ray powder patterns of hydrogen autunite and meta-autunite a glycolation method has been applied in order to distinguish the two species. The cell dimensions have been determined from Guinier X-ray diffraction patterns. Both minerals are tetragonal with a=6.981±0.005 Å and c=8.448±0.005 Å for metaautunite and a=7.084±0.005 Å and c=8.777±0.005 Å for hydrogen autunite. For both minerals, the zeta-potential is mostly negative and is strongly influenced by temperature, pH and concentration of cations in the suspension. The surface conductivity has been calculated from the value of the zetapotential. The electrokinetic properties of metaautunite are very similar to those of metatorbernite.     

Lead orthophosphates—III. Stabilities of fluoropyromorphite and bromopyromorphite at 25°C

The reactions of secondary lead orthophosphate with approximately 10^?1 M sodium fluoride and sodium bromide solutions have been investigated at 25°C. Interpretation of the solubility data resulted in solubility product constants for fluoropyromorphite and bromopyromorphite of 10^?71.6 and 10^?78.1, respectively. According to these constants, the stability sequence for lead pyromorphites is Pb₅(PO₄)₃Cl > Pb₅(PO₄)₃Br > Pb₅(PO₄)₃OH > Pb₅(PO₄)₃F. The derived free energy data have been used to evaluate the respective stabilities of fluoro-pyromorphite and bromopyromorphite within the systems PbF₂-PbO-P₂O₅-H₂O and PbBr₂-PbO-P₂O₅-H₂O and to predict the equilibrium behavior of the Pb₅(PO₄)₃F-Pb₅(PO₄)₃OH solid solution under aqueous conditions.     

Crystal chemistry of natural and synthetic lead oxohalides: II. Crystal structure of Pb7O4(OH)4Br2

Crystals of lead oxobromide Pb₇O₄(OH)₄Br₂ have been synthesized by hydrothermal method. The structure of the new compound has been studied with X-ray single-crystal diffraction analysis. The compound is monoclinic, space group C112₁; unit-cell dimensions are a = 5.852(4), b = 13.452(7), c = 19.673(9) Å, γ = 90.04°, V = 1548.7(15) ų. The structure has been solved by direct methods and refined to R ₁ = 0.1138 for 1847 observed Pb₇O₄(OH)₄Br₂ unique reflections. The structure contains seven symmetrically independent bivalent Pb atoms. The coordination polyhedrons of Pb are strongly distorted due to stereochemical activity of unshared electron pair 6s ². Oxygen atoms are tetrahedrally coordinated by four Pb²⁺ cations with the formation of oxocentered tetrahedrons OPb₄. The compound is based on [O₂Pb₃]²⁺ double chains formed by OPb₄ tetrahedrons. (OH)Pb₂ dimers combine the [O₂Pb₃]²⁺ chains into 3D framework. Channels in the framework are parallel to [100] and are occupied by Br anions.     

Thermodynamic modeling of REE behavior in oxidized hydrothermal fluids of high sulfate sulfur concentrations

Thermodynamic calculations using the HCh software were made for mineral equilibriums including REEs in the fluoride–sulfide–chloride–carbonate–sulfate–system in the presence of Na, Ca, and P with fluids of various acidities–alkalinities [11]. The obtained thermodynamic characteristics of thenardite allowed us to carry out the calculations for this phase under complicated hydrothermal conditions simulating the presence of oxidized fluids at 500–100°C and 2000–125 bar. Among other solid phases, REEs–fluorite, monazite, and REE–F–apatite were formed as CaF₂–(Ln,Y)F₃, LnPO₄, and Ca₅(PO₄)₃F–(Ln,Y)₃(PO₄)₃ ideal solid solutions, respectively, where Ln is La, Ce, Pr, Nd, Sm, Eu, and Gd. Xenotime, anhydrite, elemental sulfur, and calcite were found as well.     

Structural investigations,high temperature behavior and phase transition of Na6Ca4(SO4)6F2

Polycrystalline material of a sulfate apatite with chemical composition Na₆Ca₄(SO₄)₆F₂ or (Na₂Ca₄)Na₄(SO₄)₆F₂ has been synthesized by solid state reactions. Basic crystallographic data are as follows: hexagonal symmetry, a?=?9.3976(1) Å, c?=?6.8956(1) Å, V?=?527.39(1) ų, Z?=?1, space group P6₃/m. For structural investigations the Rietveld method was employed. Thermal expansion has been studied between 25 and 600 °C. High temperature (HT) powder diffraction data as well as thermal analysis indicate that the apatite-type compound undergoes a reconstructive phase transition in the range between 610 and 630 °C. Single-crystals of the HT-polymorph were directly grown from the melt. Structural investigations based on single-crystal diffraction data of the quenched crystals performed at ?100 °C showed orthorhombic symmetry (space group Pna2₁) with a?=?12.7560(8) Å, b?=?8.6930(4) Å, c?=?9.8980(5) Å, V?=?1097.57(10) ų and Z?=?2. Unit cell parameters for a quenched polycrystalline sample of the HT-form obtained at ambient conditions from a LeBail-fit are as follows: a?=?12.7875(1) Å, b?=?8.7255(1) Å, c?=?9.9261(1) Å, V?=?1107.53(2) ų. The lattice parameters of both modifications are related by the following approximate relationships: a _HT?≈?2c _RT, b _HT?≈?-(½a _RT?+?b _RT), c _HT?≈?a _RT. The HT-modification is isotypic with the corresponding potassium compound K₆Ca₄(SO₄)₆F₂. The pronounced disorder of the sulphate group even at low temperatures has been studied by maximum entropy calculations. Despite the first-order character of the transformation clusters of sulfate groups surrounding the fluorine anions can be identified in both polymorphs. Each of the three next neighbor SO₄-tetrahedra within a cluster is in turn surrounded by 8–9 M-cations (M: Na,Ca) defining cage-like units. However, in the apatite structure the corresponding three tricapped trigonal prisms are symmetry equivalent. Furthermore, the central fluorine atom of each cluster is coordinated by three next M-neighbors (FM₃-triangles), whereas in the HT-polymorph a four-fold coordination is observed (FM₄-tetrahedra).     

Ca2Mg(NO3)6×12H2O–structural investigations on a new compound retrieved from chimney deposits of a combined heat and power plant

Phase analysis of incrustations retrieved from chimney deposits of a combined heat and power plant in Malchow/Germany by X-ray powder diffraction gave evidence for the existence of a previously unknown hydrous calcium magnesium nitrate. Optical investigations of the sample showed the presence of colorless platy crystals with a maximum diameter of about 250 μm embedded in a partly polycrystalline and partly glassy matrix. Aided by EDX-analysis and Raman spectroscopy, a single-crystal diffraction study performed at ambient conditions demonstrated that the material represents a phase with composition Ca₂Mg(NO₃)₆×12H₂O. Basic crystallographic data are as follows: trigonal symmetry, space group type R \( \overline{3} \) , a?=?10.5583(5) Å, c?=?19.5351(10) Å, V?=?1885.97(16) ų, Z?=?3, (R(|F|) = 0.0248). The magnesium ions are coordinated by water molecules to form distorted Mg(H₂O)₆-octahedra. The calcium atoms are surrounded by nine ligands. The resulting CaO₉ tricapped trigonal prisms involve oxygen atoms from additional water moieties as well as from three different bidentate nitrate groups, respectively. Hydrogen bonds link one octahedron with two adjacent prisms into trimers. The trimers in turn are stacked in columns running parallel to [001]. Further hydrogen bonding between neighboring columns results in the formation of a three-dimensional network. To our best knowledge, Ca₂Mg(NO₃)₆×12H₂O represents a new structure type. However, column-like topologies with rods consisting of different types of polyhedra have been also observed in other trigonal hydrous nitrates. The structural relationships between these compounds are discussed. It is interesting to note that in previous phase equilibrium studies on the ternary system Ca(NO₃)₂-Mg(NO₃)₂-H₂O no other hydrous double salt has been described. Finally, the results of the structure analysis allowed a qualitative and quantitative phase analysis of the crystalline part of the chimney deposit by the Rietveld method.     

Hydroxycalciopyrochlore,A New Mineral Species from Sichuan,China

Hydroxycalciopyrochlore, ideally(Ca,Na,U,□)2(Nb,Ti)2O6(OH), cubic, is a new mineral species(IMA2011-026) within the pyrochlore supergroup that was found occurring at the Maoniuping mine, Mianning County, Xichang prefecture, Sichuan Province, southwest China. The mineral is found in an alkali feldspar granite rare-earth ore deposit(26–27 Ma). Associated minerals include calcite, barite, celestine, albite, aegirine, aegirine-augite, fluorite, parasite-(Ce), thorite, thorianite, zircon, galena, sphalerite, magnetite, and pyrite. Crystals occur mostly as octahedra, and less often as dodecahedra and tetrahexahedra or combinations thereof. Some occur with an allotriomorphic habit with a thick triangular tabular form. Crystals generally range from 0.1 to 1 mm in size. The mineral is brownishblack, greenish-black and black on fresh sections with a brown streak. The crystal is translucent, and has a greasy lustre on fresh sections. It is metamict without any observed parting or cleavage and with a conchoidal fracture. The Vickers microhardness is 572 kg/mm2(5–6 on the Mohs hardness scale). The density measured by hydrostatic weighing is 5.10(3) g/cm3. The strongest four reflections in the X-ray powder-diffraction pattern [d in(I) hkl] are: 2.9657(100) 2 2 2, 1.8142(34) 0 4 4, 1.5463(21) 2 2 6, 2.5688(18) 0 0 4. The unit-cell parameters are a = 10.381(4), V = 1118.7(7)3, Z = 8. The structure was solved and refined in the space group Fd3m with R = 0.09. The empirical formula is(Ca0.74Na0.58U0.40Ce0.05Fe0.02□0.21)2.00(Nb1.15Ti0.80Ta0.03Al0.01Mg0.01)2.00O6.02 [(OH)1.01F0.09]1.10, on the basis of 2 atoms of B pfu; the simplified formula is(Ca,Na,U,□)2(Nb,Ti)2O6(OH). Type material is deposited in the Geological Museum of China, Beijing, People's Republic of China, catalogue number M11800.     

Origin of birefringence in andradite from Arizona, Madagascar, and Iran

The crystal structure of four birefringent andradite samples (two from Arizona, one from Madagascar, and one from Iran) was refined with the Rietveld method, space group $Ia\overline{3} d$ , and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Each sample contains an assemblage of three different cubic phases. From the electron-microprobe (EMPA) results, fine-scale intergrowths in the Arizona-2 and Madagascar samples appear homogeneous with nearly identical compositions of {Ca_2.99Mg_0.01}_Σ3[ ${\text{Fe}}_{1.99}^{3 + }$ ${\text{Mn}}_{0.01}^{3 + }$ ]_Σ2(Si_2.95Al_0.03 ${\text{Fe}}_{0.02}^{3 + }$ )_Σ3O₁₂, Adr₉₈ (Arizona-2), and Adr₉₇ (Madagascar). Both samples are near-end-member andradite, ideally {Ca₃}[ ${\text{Fe}}_{2}^{3 + }$ ](Si₃)O₁₂, so cation ordering in the X, Y, or Z sites is not possible. Because of the large-scale intergrowths, the Arizona-1 and Iran samples contain three different compositions. Arizona-1 has compositions Adr₉₇ (phase-1), Adr₉₃Grs₄ (phase-2), and Adr₈₇Grs₁₁ (phase-3). Iran sample has compositions Adr₈₆Uv₁₂ (phase-1), Adr₆₉Uv₃₀ (phase-2), and Adr₇₆Uv₂₂ (phase-3). The crystal structure of the three phases within each sample was modeled quite well as indicated by the Rietveld refinement statistics of reduced χ² and overall R (F ²) values of, respectively, 1.980 and 0.0291 (Arizona-1); 1.091 and 0.0305 (Arizona-2); 1.362 and 0.0231 (Madagascar); and 1.681 and 0.0304 (Iran). The dominant phase for each sample has the following unit-cell parameters (Å) and weight fractions (%): a = 12.06314(1), 51.93(9) (Arizona-1); 12.04889(1), 52.47(1) (Arizona-2); 12.06276(1), 52.21(8) (Madagascar); and 12.05962(2), 63.3(1) (Iran). For these dominant phases, the distances and site occupancy factors (sofs) in terms of neutral atoms at the Ca(X), Fe(Y), and Si(Z) sites are as follows: <Ca–O> = 2.4348, Fe–O = 2.0121(6), Si–O = 1.6508(6) Å; Ca(sof) = 0.955(2), Fe(sof) = 0.930(2), and Si(sof) = 0.917(2) (Arizona-1); <Ca–O> = 2.4288, Fe–O = 2.0148(7), Si–O = 1.6476(7) Å; Ca(sof) = 0.953(2), Fe(sof) = 0.891(2), and Si(sof) = 0.927(2) (Arizona-2); <Ca–O> = 2.4319, Fe–O = 2.0220(6), Si–O = 1.6460(6) Å; Ca(sof) = 0.955(2), Fe(sof) = 0.941(2), and Si(sof) = 0.939(2) (Madagascar); and <Ca–O> = 2.4344, Fe–O = 2.0156(8), Si–O = 1.6468(8) Å; Ca(sof) = 0.928(2), Fe(sof) = 0.908(2), and Si(sof) = 0.932(3) (Iran). The sofs based on the EMPA results are similar to those obtained from the Rietveld refinement. Each phase in the HRPXRD results can be correlated with a specific chemical composition. For example, the Iran sample composition Adr₆₃Uv₃₀ corresponds to phase-3 that has the smallest unit-cell parameter; Adr₇₆Uv₂₂ corresponds to phase-1 that has the intermediate cell value; and Adr₈₆Uv₁₃ corresponds to phase-2 that has the largest unit-cell parameter. The bond distances compare well with those obtained from radii sum. The three different cubic phases in each sample cause strain that arises from the mismatch of the cubic unit-cell parameters and give rise to birefringence.