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.     

Crystal chemistry of selenates with mineral-like structures: V. Crystal structures of (H3O)2[(UO2)(SeO4)2(H2O)](H2O)2 and (H3O)2[(UO2)(SeO4)2(H2O)](H2O), new compounds with rhomboclase and goldichite topology

The crystal structures of two new compounds (H₃O)₂[(UO₂)(SeO₄)₂(H₂O)](H₂O)₂ (1, orthorhombic, Pnma, a = 14.0328(18), b = 11.6412(13), c = 8.2146(13) Å, V = 134.9(3) ų) and (H₃O)₂[(UO₂)(SeO₄)₂(H₂O)](H₂O) (2, monoclinic, P2₁/c, a = 7.8670(12), b = 7.5357(7), c = 21.386(3) Å, β = 101.484(12)°, V = 1242.5(3) ų) have been solved by direct methods and refined to R ₁ = 0.076 and 0.080, respectively. The structures of both compounds contain sheet complexes [(UO₂)(SeO₄)₂]^2? formed by cornershared [(UO₂)O₄(H₂O)] bipyramids and SeO₄ tetrahedrons. The sheets are parallel to the (100) plane in structure 1 and to (?102) in structure 2. The [(UO₂)(SeO₄)₂(H₂O)]^2? layers are linked by hydrogen bonds via interlayer groups H₂O and H₃O⁺. The sheet topologies in structures 1 and 2 are different and correspond to the topologies of octahedral and tetrahedral complexes in rhomboclase (H₂O₂)⁺[Fe(SO₄)₂(H₂O)₂] and goldichite K[Fe(SO₄)₂(H₂O)₂](H₂O)₂, respectively.     

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.     

Crystal structure of daqingshanite

Daqinshanite, a new mineral recently discovered in China, is a kind of strontium-rare earth-bearing phosphate-carbonate. The ideal formula is: (Sr, Ca, Ba), RF(PO₄)(CO₃)₃. The mineral belongs to the trigonal system. The intensity data were collected with a single-crystal four-circle diffractometer. The lattice parameters are given as:a=10.058(±0.002) Å;c=9.225(±0.003) Å. Space group=R3m,Z=3. The crystal structure was solved by Patterson and electron density synthesis methods. The least-squares refinement led to a final conventional factorR=0.079. Crystal structural analysis shows that daqingshanite is of calcite-type structure and the ordered model of cations is similar to that of huntite (Mg₃Ca(CO₃)₄).     

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

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

The crystal structure of ilinskite, NaCu5O2(SeO3)2Cl3, and review of mixed-ligand CuOmCln coordination geometries in minerals and inorganic compounds

The crystal structure of ilinskite, NaCu₅O₂(SeO₃)₂Cl₃, a rare copper selenite chloride from volcanic fumaroles of the Great fissure Tolbachik eruption (Kamchatka peninsula, Russia), has been solved by direct methods and refined to R ₁?=?0.044 on the basis of 2720 unique observed reflections. The mineral is orthorhombic, Pnma, a?=?17.769(7), b?=?6.448(3), c?=?10.522(4) Å, V?=?1205.6(8) ų, Z?=?4. The The CuO_mCl_n coordination polyhedra share edges to form tetramers that have 'additional' O1 and O2 atoms as centers. The O1Cu₄ and O2Cu₄ tetrahedra share common Cu atoms to form [O₂Cu₅]⁶⁺ sheets. The SeO₃ groups and Cl atoms are adjacent to the [O₂Cu₅]⁶⁺ sheets to form complex layers parallel to (100). The Na⁺ cations are located in between the layers. A review of mixed-ligand CuO_mCl_n coordination polyhedra in minerals and inorganic compounds is given. There are in total 26 stereochemically different mixed-ligand Cu-O-Cl coordinations.     

Depmeierite Na8[Al6Si6O24](PO4,CO3)1−x · 3H2O (x < 0.5): A new cancrinite-group mineral species from the Lovozero alkaline pluton of the Kola Peninsula

A new mineral depmeierite, the first cancrinite-group member with the species-forming extraframework anion PO ₄ ^3? , has been found at Mt. Karnasurt in the Lovozero alkaline pluton on the Kola Peninsula in Russia. Natrolite and depmeierite are the major components of a hydrothermal peralkaline veinlet 1.5 cm thick, which cross cuts the foyaite-urtite-lujavrite complex. The associated minerals are steenstrupine-(Ce), vuonnemite, epistolite, sodalite, aegirine, serandite, natisite, and vitusite-(Ce). Depmeierite occurs as colorless transparent isometric grains up to 1 cm in size. Its luster is vitreous. The mineral is brittle, and its cleavage (100) is perfect. Its Mohs hardness is 5, and D(meas) = 2.321(1) and D(calc) = 2.313 g/cm³. Depmeierite is optically biaxial positive, ω = 1.493(2), and ? = 1.497(2). The IR spectrum is given. The chemical composition is as follows (wt %, the average of 10 microprobe analyses with the H₂O and CO₂ determined by selective sorption): 23.04 Na₂O, 0.54 K₂O, 0.03 Fe₂O₃, 29.07 Al₂O₃, 36.48 SiO₂, 3.30 P₂O₅, 0.08 SO₃, 0.97 CO₂, and 5.93 H₂O; the total is 99.44. The empirical formula based on (Si,Al)₁₂O₂₄ is (Na₇₅₈K_0.12)_Σ7.70(Si_6.19Al_5.81O₂₄)[(PO₄)_0.47(CO₃)_0.22(OH)_0.02(SO₄)_0.01]_Σ0.72 · 3.345H₂O. The simplified formula is Na₈[Al₆Si₆O₂₄](CO₃)_{1 ? x} · 3H₂O (x < 0.05). Depmeierite is hexagonal with space group P6₃, and the unit-cell dimensions are a = 12.7345(2), c = 5.1798(1), V = 727.46(2) ų, and Z = 1. The strongest reflections of the X-ray powder pattern (d, Å (I, %) [hkl]) are as follows: 6.380(30) [110], 4.695(91) [101], 3.681(37) [300], 3.250(100) [211], 2.758 (33) [400], 2.596(31) [002], and 2.121(24) [330, 302]. The crystal structure was studied using a single crystal, and R _hkl = 0.0362. Depmeierite differs from cancrinite in the development of wide channels containing Na cations, H₂O molecules, prevailing PO ₄ ^3? -anionic groups, and CO ₃ ^2? . The mineral is named in honor of the German crystallographer Wulf Depmeier (born in 1944). The type specimen is deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences in Moscow. The cancrinite sensu stricto subgroup separated within the cancrinite group comprises six minerals with AB frameworks, the smallest unit cell is (a ≈ 12.55–12.75, c ≈ 5.1–5.4 Å), and the chain […Na…H₂O…]^∞ exists in narrow channels: cancrinite, vishnevite, cancrisilite, hydroxycancrinite, kyanoxalite, and depmeierite. The P-bearing varieties of the cancrinite-group minerals are discussed, as well as the formation conditions of the noncarbonate members of the group related to intrusive alkaline complexes.     

Minerals with Brackebuschite-Like Structures: A Novel Solid Solution System Involving Cr<Superscript>6+</Superscript> and V<Superscript>5+</Superscript>

A novel complex continuous system of solid solutions involving vauquelinite Pb₂Cu(CrO₄)(PO₄)(OH), bushmakinite Pb₂Al(VO₄)(PO₄)(OH), ferribushmakinite Pb2Fe³⁺(VO₄)(PO₄)(OH), and a phase with the endmember formula Pb₂Cu(VO₄)(PO₄)(H₂O) or Pb₂Cu(VO₄)(РО₃ОН)(ОН) is studied based on samples from the oxidation zone of the Berezovskoe, Trebiat, and Pervomaisko-Zverevsky deposits in the Urals, Russia. This is the first natural system in which chromate and vanadate anions show a wide range of substitutions and the most extensive solid solution system involving (CrO₄)^2– found in nature. The major couple substitution is Cr⁶⁺ + Cu²⁺ ? V⁵⁺ + M³⁺, where M = Fe, Al. The correlation coefficients calculated from 125 point analyses are: 0.96 between V and (Fe + Al), 0.96 between Cr and (Cu + Zn),–0.96 between V and (Cu + Zn),–0.97 between Cr and (Fe + Al), and–0.97 between (Fe + Al) and (Cu + Zn). The substitutions V⁵⁺ ? Cr⁶⁺ (correlation coefficient–0.98) and to a lesser extent P⁵⁺ ? As⁵⁺ (correlation coefficient–0.86) occur at two types of tetrahedral sites, whereas the metal–nonmetal/metalloid substitutions, i.e., V or Cr for P or As, are minor. The substitution Fe³⁺ ? Al³⁺ is also negligible in this solid solution system.     

Crystal chemistry of selenates with mineral-like structures. VI. Hydrogen bonds in the crystal structure of [(H5O2)(H3O)(H2O)][(UO2)(SeO4)2]

The crystal structure of a new compound, [(H₅O₂)(H₃O)(H₂O)][(UO₂)(SeO₄)₂] (monoclinic, P2₁/n a = 8.3105(15), b = 11.0799(14), c = 13.227(2) Å, β = 103.880(13)°, V = 1182.4(3) ų), has been solved by direct methods and refined to R ₁ = 0.036. The structure is based on [(UO₂)(SeO₄)₂]^2? sheet complexes formed by corner-shared UO₇ pentagonal bipyramids and SeO₄ tetrahedrons. The sheets are parallel to the ( $ \bar 1 The crystal structure of a new compound, [(H₅O₂)(H₃O)(H₂O)][(UO₂)(SeO₄)₂] (monoclinic, P2₁/n a = 8.3105(15), b = 11.0799(14), c = 13.227(2) ?, β = 103.880(13)°, V = 1182.4(3) ?³), has been solved by direct methods and refined to R ₁ = 0.036. The structure is based on [(UO₂)(SeO₄)₂]²⁻ sheet complexes formed by corner-shared UO₇ pentagonal bipyramids and SeO₄ tetrahedrons. The sheets are parallel to the (01) plane. Oxonium ions and water molecules forming [(H₃O)·(H₂O)·(H₅O₂)]²⁺ complexes are interlayer. Among minerals, the existence of (H₅O₂)⁺ has been unambiguously confirmed only in rhomboclase, (H₅O₂)⁺[Fe₂(SO₄)₂(H₂O)₂]. Original Russian Text ? S.V. Krivovichev, 2008, published in Zapiski Rossiiskogo Mineralogicheskogo Obshchestva, 2008, No. 2, pp. 123–130.     

Refinement of the crystal structure of fornacite using the rietveld method

The crystal structure of fornacite Pb₂(Cu,Fe)[CrO₄(As,P)O₄OH] from the Berezovskii deposit (Central Urals, Russia) was refined by X-ray powder diffraction data using the Rietveld method. Fornacite is monoclinic, space group P2₁/c, the unit cell dimensions are a = 8.09015(12), b = 5.90913(9), c = 17.4839(2) Å, β = 109.99(2), V = 785.5(3) ų, and Z = 4. The structure was refined in the isotropic approximation of the atomic displacement parameters up to R _p = 0.0516, R _wp = 0.0692, R _B = 0.0229, and R _F = 0.0200. The fornacite structure is similar to that of minerals of the brackebuschite-group and consists of heteropolyhedral chains, built by the columns of edge-sharing Cu²⁺O₆ octahedra connected with isolated Cr⁶⁺O₄ and As⁵⁺O₄ tetrahedra. The chains are linked by ninefold Pb²⁺ polyhedra.     

Crystal chemistry of selenates with mineral-like structures: VIII. Butlerite chains in the structure of K(UO<Subscript>2</Subscript>)(SeO<Subscript>4</Subscript>)(OH)(H<Subscript>2</Subscript>O)

A new potassium uranyl selenate compound K(UO₂)(SeO₄)(OH)(H₂O) has been synthesized for the first time using the technique of evaporation from water solution. Its crystal structure has been solved by direct methods (monoclinic, P2₁/c,a = 8.0413(9) Å, b = 8.0362(9) Å, c = 11.6032(14) Å, β = 106.925(2)°, V = 717.34(14) ų) and refined to R ₁ = 0.0319 (wR ₂ = 0.0824) for 1285 reflections with |F ₀| > 4σ _F . The structure consists of [(UO₂(SeO₄)(OH)(H₂O)]^? chains extending along axis b. In the chains, the uranyl pentagonal bipyramids are linked via bridged hydroxyl anions and tetrahedral oxoanions [SeO₄]^2?. Potassium ions are situated between these chains. No chains of that type have been observed in uranyl compounds earlier, but they had been detected in the structures of butlerite, parabutlerite, uklonskovite, fibroferrite, and a number of synthetic compounds.     

Crystal chemistry of selenates with mineral-like structures: VII. The structure of (H<Subscript>3</Subscript>O)[(UO<Subscript>2</Subscript>)(SeO<Subscript>4</Subscript>)(SeO<Subscript>2</Subscript>OH)] and some structural features of selenite-selenates

The crystal structure of a new compound, (H₃O)[(UO₂)(SeO₄)(SeO₂OH)] (monoclinic, P2₁/n, a = 8.6682(19), b = 10.6545(16), c = 9.846(2) Å, β = 97.881(17)°, V = 900.7(3) ų), was solved by direct methods and refined to R ₁ = 0.050. The structure contains two symmetrically different Se atoms. The Se1 site is coordinated by three O atoms as is characteristic of Se⁴⁺ cations. The Se2 site is coordinated by four O atoms and forms selenate anion SeO ₄ ^2? . The structure is based on selenite-selenate sheets [(UO₂)(SeO₄)(SeO₂OH)]^? linked by the interlayer H₃O^? ions. The sheets are parallel to (101). The structure is compared to that of schmiederite, Pb₂Cu₂(SeO₃)(SeO₄)(OH)₄.     

The crystal and magnetic structure of Li-aegirine LiFe3+Si2O6: a temperature-dependent study

Single crystals of Li-aegirine LiFe³⁺Si₂O₆ were synthesized at 1573?K and 3?GPa, and a polycrystalline sample suitable for neutron diffraction was produced by ceramic sintering at 1223?K. LiFe³⁺Si₂O₆ is monoclinic, space group C2/c, a=9.6641(2)?Å, b= 8.6612(3)?Å, c=5.2924(2)?Å, β=110.12(1)^° at 300?K as refined from powder neutron data. At 229?K Li-aegirine undergoes a phase transition from C2/c to P2₁ /c. This is indicated by strong discontinuities in the temperature variation of the lattice parameters, especially for the monoclinic angle β and by the appearance of Bragg reflections (hkl) with h+k≠2n. In the low-temperature form two non-equivalent Si-sites with 〈SiA–O〉=1.622?Å and 〈SiB–O〉=1.624?Å at 100?K are present. The bridging angles of the SiO₄ tetrahedra O3–O3–O3 are 192.55(8)° and 160.02(9)° at 100?K in the two independent tetrahedral chains in space group P2₁ /c, whereas it is 180.83(9)° at 300?K in the high-temperature C2/c phase, i.e. the chains are nearly fully expanded. Upon the phase transition the Li-coordination changes from six to five. At 100?K four Li–O bond lengths lie within 2.072(4)–2.172(3)?Å, the fifth Li–O bond length is 2.356(4)?Å, whereas the Li–O3?A bond lengths amount to 2.796(4)?Å. From ⁵⁷Fe Mössbauer spectroscopic measurements between 80 and 500?K the structural phase transition is characterized by a small discontinuity of the quadrupole splitting. Temperature-dependent neutron powder diffraction experiments show first occurrence of magnetic reflections at 16.5?K in good agreement with the point of inflection in the temperature-dependent magnetization of LiFe³⁺Si₂O₆. Distinct preordering phenomena can be observed up to 35?K. At the magnetic phase transition the unit cell parameters exhibit a pronounced magneto-striction of the lattice. Below T _N Li-aegirine shows a collinear antiferromagnetic structure. From our neutron powder diffraction experiments we extract a collinear antiferromagnetic spin arrangement within the a–c plane.     

Arcanite from Fumarole Exhalations of the Tolbachik Volcano (Kamchatka,Russia) and Its Crystal Structure

The crystal structure (R = 0.0194) of arcanite β-K₂SO₄ was studied on a single crystal from exhalations of the Arsenatnaya fumarole, Tolbachik Volcano (Kamchatka, Russia). The mineral crystallizes at a temperature of ≥350–430°C and associates with langbeinite, aphthitalite, hematite, tenorite, johillerite, and others. Arcanite is orthorhombic, Pnma, a = 7.4763(2) Å, b = 5.77262(16) Å, c = 10.0630(3) Å, V = 434.30(2) ų, Z = 4. Its structure contains isolated SO₄ tetrahedra, whereas K cations center ten- and nine-fold polyhedra.     

Sodium trisilicate: A new high-pressure silicate structure (Na2Si[Si2O7])

Crystals of sodium trisilicate (Na₂Si₃O₇) have been grown in the presence of melt at 9 GPa, 1200 °C using the MA6/8 superpress at Edmonton, and the X-ray structure determined at room pressure (R=2.0%). Na₂Si₃O₇ is monoclinic with a=8.922(2) Å, b= 4.8490(5) Å, c=11.567(1) Å, β=102.64(1)° (C2/c), D _x = 3.295 g·cm^-3. Silicon occurs in both tetrahedral and octahedral coordination (^[6]Si∶^[4]Si = l∶2). The SiO₄ tetrahedra form a diorthosilicate [Si₂O₇] group and are linked by the isolated SiO₆ octahedra via shared corners into a framework of 6-membered (^[4]Si-^[4]Si-^[6]Si^[4]Si-^[4] Si-^[6]Si) and 4-membered (^[4]Si-^[6]Si-^[4]Sr-^[6]Si) rings: 〈^[6]Si-O〉=1.789 Å, 〈^[4]Si-O〉= 1.625 Å, ^[4]Si-O-^[4]Si=132.9° and the bridging oxygen is overbonded (s = 2.22). Channels parallel to b-axis and [110] accommodate Na in irregular 6-fold coordination: 〈Na-O〉 = 2.511 Å.     

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.     

Paleoenvironmental significance of aluminum phosphate-sulfate minerals in the upper Cretaceous ooidal ironstones,E-NE Aswan area,southern Egypt

Aluminum phosphate-sulfate (APS) minerals are present as small, disseminated crystals in the upper Cretaceous shallow marine ooidal ironstones, E-NE Aswan area, southern Egypt. Their association with the ironstones is considered as a proxy of subaerial weathering and post-diagenetic meteoric water alteration. The mineralogical composition of the ooidal ironstones was investigated by optical and scanning electron microscopes, X-ray diffraction, Fourier transform infrared and Raman spectroscopy. The ooidal ironstones are composed mainly of ooids and groundmass, both of which consist of a mixture of detrital (quartz) and diagenetic (fluorapatite, chamosite and pyrite) mineral assemblages. These mineral assemblages are destabilized under acidic and oxidizing, continental conditions. These conditions resulted from the oxidation of pyrite and probably organic matter under warm and humid, tropical climate followed the Santonian Sea regression and subaerial exposure. These pedogenic conditions promoted corrosion of quartz, dissolution of chamosite and apatite and hydrolysis of feldspars of the nearby exposed granitoids. The released Si, Al and Sr from quartz, chamosite and feldspars; Fe and S from pyrite and P, Ca and light rare earth elements (LREE) from apatite are reprecipitated as hematite, kaolinite, apatite and APS minerals from the pore fluids or along fractures. The paragenetic sequence and textural relationships of this post-diagenetic mineral assemblage indicate that hematite was formed by replacement of chamosite followed by formation of a secondary generation of pore filling chlorapatite and APS minerals and finally the precipitation of kaolinite in the remaining pore spaces. The formation of APS minerals and chlorapatite is simultaneous, but APS minerals are stable at shallow depths under acidic to neutral pH conditions, whereas chlorapatite is stable under alkaline pH conditions. Alkaline conditions were maintained at greater depths when the infiltrated acidic fluids reacted with chamosite. The APS minerals display a homogeneous chemical composition in all ironstone locations in Aswan area, corresponding to a solid solution between crandallite (CaAl₃(PO₄)₂(OH)₅·H₂O), goyazite (SrAl₃(PO₄)₂(OH)₅·H₂O), svanbergite (SrAl₃(PO₄)(SO₄)(OH)₆) and woodhouseite (CaAl₃(PO₄)(SO₄)(OH)₆) end-members. The variations in the APS mineral chemistry (AB₃(XO₄)₂(OH)₆) are essentially due to variable substitutions of Sr and LREE for Ca at the A site and limited S for P at the X site. The spatial distribution of APS minerals and their composition in the ooidal ironstones of Aswan area permitted to consider them as good tracers of physicochemical and paleoenvironmental changes, in particular those associated with subaerial exposure and pedogenesis. The post-diagenetic phosphatization and kaolinization of the Aswan ironstones decrease their economic potentiality; thus, understanding paragenetic sequence and textural relationships is essential for the iron ore beneficiation.     

Phase E: A high pressure hydrous silicate with unique crystal chemistry

The unique cation-disordered crystal structures of two samples of phase E, a non-stoichiometric, hydrous silicate synthesized in a uniaxial, split-sphere, multi-anvil apparatus at conditions above 13 GPa and 1000° C, have been solved and refined in space group $\bar 3$ . The compositions and unit cells for the two materials, assuming six oxygens per cell, are Mg_2.08Si_1.16H_3.20O₆, a=2.9701(1) Å, c=13.882(1) Å V = 106.05(4) ų for sample 1, and Mg_2.17Si_1.01H_3.62O₆, a=2.9853(6) Å, c=13.9482(7) Å, V= 107.65(4) ų for sample 2. The structure contains layers with many features of brucite-type units, with the layers stacked in a rhombohedral arrangement. The layers are cross linked by silicon in tetrahedral coordination and magnesium in octahedral coordination, as well as hydrogen bonds. Interlay er octahedra share edges with intralayer octahedra. Interlayer tetrahedra would share faces with intralayer octahedra. To avoid this situation, there are vacancies within the layers. There is, however, no long-range order in the occupation of these sites, as indicated by the lack of a superstructure. Selected-area electron diffraction patterns show walls of diffuse intensity similar in geometry and magnitude to those observed in short-range-ordered alloys and Hågg phases. Phase E thus appears to represent a new class of disordered silicates, which may be thermodynamically metastable.     

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.