Co2+ in trigonal fields of oxygen based structures: Electronic absorption spectra of buetschliite-type K2Co(SeO3)2, K2Co2(SeO3)3 and zemannite-type K2Co2(SeO3)3 · 2H2O

Polarized single crystal absorption spectra, in the spectral range 40 000–5 000 cm^-1, were obtained on Co²⁺ in trigonally distorted octahedral oxygen fields of buetschliite-type K₂Co(SeO₃)₂ (I), K₂Co₂(SeO₃)₃ (II) and zemannite-type K₂Co₂(SeO₃)₃ · 2H₂O (III). Site symmetries of Co²⁺ are m (D_3d) in I, 3m (C_3v) in II, and 3 (C₃) in III. The spectra can be interpreted on the basis of an electric dipole mechanism, wherein transitions of Co²⁺ in the centrosymmetric site in I gain intensity from dynamic removal of the inversion centre by vibronic coupling. In accordance with the elongation of the CoO₆ octahedra along the trigonal axis, the split component E_(g) of the ground state ⁴T_1g in octahedral fields is the ground state in all three compounds. Trigonal field parameters Dq_(trig), D, D and the Racah parameters B have been fitted to the energies of spin allowed transitions (293 K) as follows: I: 744, 94, -16, and 838 cm^-1, resp.; II: 647, 227, 42, and 798 cm^-1, resp.; III: 667, 181, 21, and 809 cm^-1, respectively. Racah parameters C were estimated from the energy of some observed spin-forbidden transitions to be 3770 (I), 3280 (II), and 3465 cm^-1 (III). Values of Dq and of the Racah parameters B and C indicate slight differences of Co²⁺-O bonding in I as compared to II and III, with somewhat higher covalency in compounds II and III which contain face-sharing CoO₆ octahedra with short Co-Co contacts. Also, in II and III the observed D values do not agree with theoretical D values, predicted from the magnitude of the mean octahedral distortions.     

Crystal structure of yegorovite Na4[Si4O8(OH)4] · 7H2O

Doklady Earth Sciences - Using X-ray analysis, the crystal structure of yegorovite Na4[Si4O8(OH)4] · 7H2O, a newly-discovered mineral from the Lovozero alkaline complex (Kola Peninsula,...     

A study on the effects of drying conditions on the stability of NaNd(CO3)2·6H2O and NaEu(CO3)2·6H2O

The effect of different drying conditions on the stability of NaNd(CO₃)·6H₂O and NaEu(CO₃)·6H₂O and the identity of the decomposition product have been investigated. The rate of decomposition and the nature of the altered phases are dependant on the drying conditions used. When the phases are oven dried at 120 °C, the decomposition is immediate and the phase completely alters to Nd₂(CO₃)₃ or Eu₂(CO₃)₃ respectively. Under less severe drying conditions, the Na rare earth carbonate phases alter to Nd₂(CO₃)₃·8H₂O and Eu₂(CO₃)₃·8H₂O over a period of 24–48 h, but they can be kept indefinitely in a water saturated environment. The implications for using Nd and Eu as actinide analogues are discussed.     

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 structure of KMn3+ [SeO4]2 — a triclinic distorted member of the Yavapaiite family

Summary The crystal structure of synthetic KMn[SeO₄]₂ was determined by single crystal X-ray diffraction methods in space group , a = 4.827(2) Å, b = 4.988(2) Å, c = 7.981(3) Å, = 83.18(1)°, = 85.32(2)°, = 67.92(1)°, V = 176.66 ų, Z = 1; 1564 unique data, measured up to 2 = 70° (MoK-radiation); R, R(I)_w = 0.034, 0.074.KMn[SeO₄]₂ is closely related to monoclinic yavapaiite, KFe[SO₄]₂, and isotypic compounds. Jahn-Teller distorted MnO₆ octahedra are alternately linked with KO₁₀ polyhedra along [001]. The mean values of the Mn-O and Se-O distances are 2.007 Å and 1.637 Å, respectively.

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Die Kristallstruktur vonKMn ³⁺[SeO₄]-einem triklin verzerrten Vertreter der Yavapaiite-Familie

Zusammenfassung Die Kristallstruktur von synthetisch dargestelltem KMn[SeO₄]₂ wurde mittels Einkristallröntgenmethoden in der Raumgruppe bestimmt: a = 4.827(2) Å, b = 4.988(2) Å, c = 7.981(3) Å, = 83.18(1)°, = 85.32(2)°, = 67.92(1)°, V = 176.66 ų, Z = 1; 1564 unabhängige Daten bis 2 = 70° (MoK-Strahlung); R, R(I)_w = 0.034, 0.074.KMn[SeO₄]₂ ist eng mit dem monoklinen Mineral Yavapaiit, KFe[SO₄]₂ und einer Reihe damit isotyper Verbindungen verwandt. Jahn-Teller verzerrte MnO₆ Oktaeder sind alternierend mit KO₁₀ Polyedern parallel [001] verbunden. Die Mittelwerte der Mn-O und Se-O Abstände sind 2.007 Å bzw. 1.637 Å.

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With 1 Figure     

Solubility of ettringite (Ca6[Al(OH)6]2(SO4)3 · 26H2O) at 5–75°C

The solubility of ettringite (Ca₆[Al(OH)₆]₂(SO₄)₃ · 26H₂O) was measured in a series of dissolution and precipitation experiments at 5–75°C and at pH between 10.5 and 13.0 using synthesized material. Equilibrium was established within 4 to 6 days, with samples collected between 10 and 36 days. The log K_SP for the reaction Ca₆[Al(OH)₆]₂(SO₄)₃ · 26H₂O ⇌ 6Ca²⁺ + 2Al(OH)₄⁻ + 3SO₄²⁻ + 4OH⁻ + 26H₂O at 25°C calculated for dissolution experiments (−45.0 ± 0.2) is not significantly different from the log K_SP calculated for precipitation experiments (−44.8 ± 0.4) at the 95% confidence level. There is no apparent trend in log K_SP with pH and the mean log K_SP,298 is −44.9 ± 0.3. The solubility product decreased linearly with the inverse of temperature indicating a constant enthalpy of reaction from 5 to 75°C. The enthalpy and entropy of reaction ΔH^°_r and ΔS^°_r, were determined from the linear regression to be 204.6 ± 0.6 kJ mol⁻¹ and 170 ± 38 J mol⁻¹ K⁻¹. Using our values for log K_SP, ΔH^°_r, and ΔS^°_r and published partial molal quantities for the constituent ions, we calculated the free energy of formation ΔG^°_f,298, the enthalpy of formation ΔH^°_f,298, and the entropy of formation ΔS^°_f,298 to be −15211 ± 20, −17550 ± 16 kJ mol⁻¹, and 1867 ± 59 J mol⁻¹ K⁻¹. Assuming ΔC_P,r is zero, the heat capacity of ettringite is 590 ± 140 J mol⁻¹ K⁻¹.     

Redefinition of Lemanskiite: New Mineralogical Data,Crystal Structure,and Revised Formula NaCaCu5(AsO4)4Cl · 3H2O

Geology of Ore Deposits - The crystal structure of lemanskiite is determined for the first time (R = 0.019) and the mineral is redefined. Its chemical formula, crystal system, space group and...     

The crystal structure of galgenbergite-(Ce), CaCe2(CO3)4∙H2O

Galgenbergite-(Ce) from the type locality, the railroad tunnel Galgenberg between Leoben and St. Michael, Styria, Austria, was investigated. There it occurs in small fissures of an albite-chlorite schist as very thin tabular crystals building rosette-shaped aggregates associated with siderite, ancylite-(Ce), pyrite and calcite. Electron microprobe analyses gave CaO 9.49, Ce₂O₃ 28.95, La₂O₃ 11.70, Nd₂O₃ 11.86, Pr₂O₃ 3.48, CO₂ 30.00, H₂O 3.07, total 98.55 wt.%. CO₂ and H₂O calculated by stoichiometry. The empirical formula (based on Ca + REE ∑3.0) is $ \mathrm{C}{{\mathrm{a}}_{1.00 }}{{\left( {\mathrm{C}{{\mathrm{e}}_{1.04 }}\mathrm{L}{{\mathrm{a}}_{0.42 }}\mathrm{N}{{\mathrm{d}}_{0.42 }}\mathrm{P}{{\mathrm{r}}_{0.12 }}} \right)}_{2.00 }}{{\left( {\mathrm{C}{{\mathrm{O}}_3}} \right)}_4}\cdot {{\mathrm{H}}_2}\mathrm{O} $ , and the simplified formula is $ \mathrm{CaC}{{\mathrm{e}}_2}{{\left( {\mathrm{C}{{\mathrm{O}}_3}} \right)}_4}\cdot {{\mathrm{H}}_2}\mathrm{O} $ . According to X-ray single crystal diffraction galgenbergite-(Ce) is triclinic, space group $ P\overline{1},a=6.3916(5) $ , b?=?6.4005(4), c?=?12.3898(9) Å, α?=?100.884(4), β?=?96.525(4), γ?=?100.492(4)°, V?=?483.64(6) ų, Z?=?2. The eight strongest lines in the powder X-ray diffraction pattern are [d _calc in Å/(I)/hkl]: 5.052/(100)/011; 3.011/(70)/0-22; 3.006/(66)/004; 5.899/(59)/-101; 3.900/(51)/1-12; 3.125/(46)/-201; 2.526/(42)/022; 4.694/(38)/-102. The infrared absorption spectrum reveals H₂O (OH-stretching mode at 3,489 cm^?1, HOH bending mode at 1,607 cm^?1) and indicates the presence of distinctly non-equivalent CO₃-groups by double and quadruple peaks of their ν1, ν2, ν3 and ν4 modes. The crystal structure of galgenbergite-(Ce) was refined with X-ray single crystal data to R1?=?0.019 for 2,448 unique reflections (I?>?2σ(I)) and 193 parameters. The three cation sites of the structure Ca(1), Ce(2) and Ce(3) have a modest mixed site occupation by Ca and small amount of REE (Ce, La, Pr, Nd) and vice versa. The structure is based on double layers parallel to (001), which are composed of Ca(1)Ce(2)(CO₃)₂ single layers with an ordered chessboard like arrangement of Ca and Ce, and with a roof tile-like stacking of the CO₃ groups. Perpendicular to (001) the double layers are connected to a triclinic framework structure with good cleavage parallel to (001) by a differently organized and more open part of the structure formed by Ce(3)(CO₃)₂(H₂O). Based on the topology of the CaCe(CO₃)₂ single layer in galgenbergite-(Ce), structural relationships to rutherfordine, to aragonite and ancylite type minerals, and to lanthanite are outlined.     

Experimental study of the K2ZrSi3O9 (wadeite)–K2TiSi3O9 and K2(Zr,Ti)Si3O9–phlogopite systems at 2–3 GPa

Experiments ranging from 2 to 3 GPa and 800 to 1300 °C and at 0.15 GPa and 770 °C were performed to investigate the stability and mutual solubility of the K₂ZrSi₃O₉ (wadeite) and K₂TiSi₃O₉ cyclosilicates under upper mantle conditions. The K₂ZrSi₃O₉–K₂TiSi₃O₉ join exhibits complete miscibility in the P–T interval investigated. With increasing degree of melting the solid solution becomes progressively enriched in Zr, indicating that K₂ZrSi₃O₉ is the more refractory end member. At 2 GPa, in the more complex K₂ZrSi₃O₉–K₂TiSi₃O₉–K₂Mg₆Al₂Si₆O₂₀(OH)₄ system, the presence of phlogopite clearly limits the extent of solid solution of the cyclosilicate to more Zr-rich compositions [Zr/(Zr + Ti) > 0.85], comparable to wadeite found in nature, with TiO₂ partitioning strongly into the coexisting mica and/or liquid. However, at 1200 °C, with increasing pressure from 2 to 3 GPa, the partitioning behaviour of TiO₂ changes in favour of the cyclosilicate, with Zr/(Zr + Ti) of the K₂(Zr,Ti)Si₃O₉ phase decreasing from ∼0.9 to ∼0.6. The variation in the Ti content of the coexisting phlogopite is related to its degree of melting to forsterite and liquid, following the major substitution ^VITi+^VI□=2^VIMg. Received: 26 January 1999 / Accepted: 10 January 2000     

Thermodynamic properties of copper carbonates — malachite Cu2(OH)2CO3 and azurite Cu3(OH)2(CO3)2

The thermodynamic properties of the copper carbonates malachite and azurite have been studied by adiabatic calorimetry, by heat-flux Calvet Calorimetry, by differential thermal analysis (DTA) and by thermogravimetrie (TGA) analysis. The heat capacities, C _p ⁰ of natural malachite and azurite have been measured between 3.8 and 300 K by low-temperature adiabatic calorimetry. The heat capacity of azurite exhibits anomalous behavior at low temperatures. At 298.15 K the molar heat capacities C _p ⁰ and the third law entropies S _298.15 ⁰ are 228.5±1.4 and 254.4±3.8 J mol^?1 K^?1 for azurite and 154.3±0.93 and 166.3±2.5 J mol^?1 K^?1 for malachite. Enthalpies of solution at 973 K in lead borate 2PbO·B₂O₃ have been measured for heat treated malachite and azurite. The enthalpies of decomposition are 105.1±5.8 for azurite and 66.1±5.0 kJ mol^? for malachite. The enthalpies of formation from oxides of azurite and malachite determined by oxide melt solution calorimetry, are ?84.7±7.4 and ?52.5±5.9 kJ mol^?1, respectively. On the basis of the thermodynamic data obtained, phase relations of azurite and malachite in the system Cu²⁺-H₂O-CO₂ at 25 and 75 °C have been studied.     

Solubility of Ca6[Al(OH)6]2(CrO4)3·26H2O,the chromate analog of ettringite; 5–75°C

Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O, the chromate analog of the sulfate mineral ettringite, was synthesized and characterized by X-ray diffraction, Fourier transform infra-red spectroscopy, thermogravimetric analyses, energy dispersive X-ray spectrometry, and bulk chemical analyses. The solubility of the synthesized solid was measured in a series of dissolution and precipitation experiments conducted at 5–75°C and at initial pH values between 10.5 and 12.5. The ion activity product (IAP) for the reaction Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O⇌6Ca²⁺+2Al(OH)⁻₄+3CrO²⁻₄+4OH⁻+26H₂O varies with pH unless a CaCrO_4(aq) complex is included in the speciation model. The log K for the formation of this complex by the reaction Ca²⁺+CrO²⁻₄=CaCrO_4(aq) was obtained by minimizing the variance in the IAP for Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O. There is no significant trend in the formation constant with temperature and the average log K is 2.77±0.16 over the temperature range 5–75°C. The log solubility product (log K_SP) of Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O at 25°C is −41.46±0.30. The temperature dependence of the log K_SP is log K_SP=A−B/T+D log(T) where A=498.94±48.99, B=27,499±2257, and D=−181.11±16.74. The values of ΔG⁰_r,298 and ΔH⁰_r,298 for the dissolution reaction are 236.6±3.9 and 77.5±2.4 kJ mol⁻¹. the values of ΔC⁰_P,r,298 and ΔS⁰_r,298 are −1506±140 and −534±83 J mol⁻¹ K⁻¹. Using these values and published standard state partial molal quantities for constituent ions, ΔG⁰_f,298=−15,131±19 kJ mol⁻¹, ΔH⁰_f,298=−17,330±8.6 kJ mol⁻¹, ΔS⁰₂₉₈=2.19±0.10 kJ mol⁻¹ K⁻¹, and ΔC⁰_Pf,298=2.12±0.53 kJ mol⁻¹ K⁻¹, were calculated.     

Equilibrium experiments in the system MgO–SiO2–H2O (MSH): stability fields of clinohumite-OH [Mg9Si4O16(OH)2], chondrodite-OH [Mg5Si2O8(OH)2] and phase A (Mg7Si2O8(OH)6)

The water-pressure and temperature stability fields of clinohumite-OH, chondrodite-OH and phase A were determined in reversed equilibrium experiments up to 100 kbar within the system MgO–SiO₂–H₂O. Their PT-fields differ from results from former synthesis experiments. Bracketing experiments on the reaction phase A + low P-clinoenstatite ⇆ forsterite + water resulted in a slightly steeper dP/dT-slope compared to earlier experiments for this equilibrium. Clinohumite-OH and chondrodite-OH both have large stability fields which extend over pressure ranges of more than 80 kbar. However, they are hardly relevant as hydrous minerals within the subducted oceanic lithosphere. Both are too Mg-rich for a typical mantle bulk composition. In addition, the dehydration of subducted oceanic lithosphere – due to (forsterite + water)-forming reactions – will occur before the two humite-group phases even become stable. Restricted to the cool region of cold subducting slabs, phase A, however, might be formed via the reactions phase A + low P-/high P-clinoenstatite ⇆ forsterite + water or antigorite + brucite ⇆ phase A + water, before dehydration of the oceanic lithosphere occurs. Received: 22 July 1997 / Accepted: 12 March 1998     

Hydrogen incorporation into forsterite in Mg2SiO4–K2Mg(CO3)2–H2O and Mg2SiO4–H2O–C at 7.5–14.0 GPa

Experiments on water solubility in forsterite in the systems Mg₂SiO₄–K₂Mg(CO₃)₂–H₂O and Mg₂SiO₄–H₂O–C were conducted at 7.5–14.0 GPa and 1200–1600 °C. The resulting crystals contain 448 to 1480 ppm water, which is 40–70% less than in the forsterite–water system under the same conditions. This can be attributed to lower water activity in the carbonate-bearing melt. The water content of forsterite was found to vary systematically with temperature and pressure. For instance, at 14 GPa in the system forsterite–carbonate–H₂O the H₂O content of forsterite drops from 1140 ppm at 1200 °C to 450 ppm at 1600 °C, and at 8 GPa it remains constant or increases from 550 to 870 ppm at 1300–1600 °C. Preliminary data for D-H-bearing forsterite are reported. Considerable differences were found between IR spectra of D-H- and H-bearing forsterite. The results suggest that CO₂ can significantly affect the width of the olivine-wadsleyite transition, i.e., the 410-km seismic discontinuity, which is a function of the water content of olivine and wadsleyite.     

Crystal structures of Zn2SiO4 III and IV synthesized at 6.5–8 GPa and 1,273 K

We report the crystal structures determined under ambient condition for two Zn₂SiO₄ polymorphs synthesized at 6.5 GPa and 1,273 K (phase III) and 8 GPa and 1,273 K (phase IV) and also compare their ²⁹Si MAS NMR spectroscopic characteristics with those of other Zn₂SiO₄ polymorphs (phases I, II and V). Electron microprobe analysis revealed that both of phases III and IV are stoichiometric like the lower-pressure polymorphs (phases I and II), contrary to previous report. The crystal structures were solved using an ab initio structure determination technique from synchrotron powder X-ray diffraction data utilizing local structural information from ²⁹Si MAS NMR as constraints and were further refined with the Rietveld technique. Phase III is orthorhombic (Pnma) with a = 10.2897(5), b = 6.6711(3), c = 5.0691(2) Å. It is isostructural with the high-temperature (Zn_1.1Li_0.6Si_0.3)SiO₄ phase and may be regarded as a ‘tetrahedral olivine’ type that resembles the ‘octahedral olivine’ structure in the (approximately hexagonally close packed) oxygen arrangement and tetrahedral Si positions, but has Zn in tetrahedral, rather than octahedral coordination. Phase IV is orthorhombic (Pbca) with a = 10.9179(4), b = 9.6728(4), c = 6.1184(2) Å. It also consists of tetrahedrally coordinated Zn and Si and features unique edge-shared Zn₂O₆ dimers. The volumes per formula under ambient condition for phases III and IV are both somewhat larger than that of the lower-pressure polymorph, phase II, suggesting that the two phases may have undergone structural changes during temperature quench and/or pressure release.     

Vibrational analysis of dioptase Cu6Si6O18 · 6(H2O) and its puckered six-membered ring

The normal modes of vibration and their frequencies are calculated for dioptase, a mineral whose crystal structure (space group R or C _3i ² ) consists of puckered six-membered silicate rings (Si₆O₁₈) linked by Cu²⁺ ions and H₂O groups. The calculation employs a valence force potential consisting of central interactions between nearest neighbors and bond-bending interactions centered at the Si⁴⁺ and Cu²⁺ ions. The force constants are determined by fitting the calculated frequencies to values obtained by measuring the single-crystal Raman spectra. The calculated frequencies are in reasonable agreement with experiment, permitting assignment of normal modes to the observed spectral frequencies. Considerable mixing of Cu and H₂O motions with those of the ring is found for the Raman-active modes below 430 cm^-1. The normal modes and frequencies of the hypothetical isolated ring with C _3i symmetry are determined by neglecting all interactions between the rings and the surrounding Cu and H₂O. The identification of normal modes characteristic of the puckered six-membered silicate rings and the effect of the environment on these modes may prove useful in the interpretation of the Raman spectra of amorphous silicates.     

Orschallite,Ca3(SO3)2 · SO4 · 12H2O,a new calcium-sulfite-sulfate-hydrate mineral

Summary The new mineral orschallite, Ca₃(SO₃)₂SO₄ · 12H₂O, was found at the Hannebacher Ley near Hannebach, Eifel, Germany. Crystal structure analysis of the mineral, chemical analysis and water determination on synthetic material gave the composition Ca₃(SO₃)₂SO₄ · 12H₂O. The mineral crystallizes in space group with a = 11.350(1), c = 28.321(2) Å, V = 3159.7 ų, Z = 6, D_c = 1.87 Mg/m³, D_m = 1.90(3) Mg/m³. It is uniaxial positive with the optical constants = 1.4941, = 1.4960(4). The strongest lines in the powder pattern are (d-value (Å), I, hkl) 5.73, 100, 1 0 4/8.11, 80, 0 1 2/2.69, 80, 3 0 6/3.63, 60, 1 1 6/3.28, 40, 3 0 0. Refinement of the crystal structure led to a weighted residual of R_w = 0.043 for 600 observed reflections with I > 2(I) and 52 variable parameters.

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Orschallit, Ca₃(SO₃)₂SO₄ · 12H₂O, ein neues Kalzium-Sulfat-Sulfat-Hydrat-Mineral

Zusammenfassung Das neue Mineral Orschallit, Ca₃(SO₃)₂SO₄ · 12H₂O, wurde in der Hannebacher Ley bei Hannebach, Eifel, Deutschland gefunden. Eine Analyse der Kristallstruktur an einem Einkristall des natürlichen Materials, chemische Analyse und Wasserbestimmung an synthetischem Material ergaben die Zusammensetzung Ca₃(SO₃)₂SO₄ · 12H₂O. Das Mineral kristallisiert in der Raumgruppe mit a = 11.350(1), c = 28.321(2) Å, V = 3159.7 ų, Z = 6, D_c = 1.87 Mg/m³, D_m = 1.90(3) Mg/m³. Es ist optisch einachsig mit den optischen Konstanten = 1.4941, = 1.4960(4). Die stärksten Linien des Pulver-diagramms liegen bei (d-Wert (Å), I, hkl) 5.73, 100, 1 0 4/8.11, 80, 0 1 2/2.69, 80, 3 0 6/3.63, 60; 1 1 6/3.28, 40, 3 0 0. Die Verfeinerung der Kristallstruktur ergab einen gewichteten Residualwert R_w = 0.043 für 600 beobachtete Reflexe mit I > 2(I) und 52 variable Parameter.

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Thermodynamic properties of tooeleite,Fe63+(As3+O3)4(SO4)(OH)4·4H2O

Tooeleite, nominally Fe₆³⁺(As³⁺O₃)₄(SO₄)(OH)₄·4H₂O, is a relatively uncommon mineral of some acid-mine drainage systems. Yet, if it does occur, it does so in large quantities, indicating that some specific conditions favor the formation of this mineral in the system Fe-As-S-O-H. In this contribution, we report the thermodynamic properties of synthetic tooeleite. The sample was characterized by powder X-ray diffraction, scanning electron microscopy, extended X-ray absorption fine-structure spectroscopy, and Mössbauer spectroscopy. These methods confirmed that the sample is pure, devoid of amorphous impurities of iron oxides, and that the oxidation state of arsenic is 3+. Using acid-solution calorimetry, the enthalpy of formation of this mineral from the elements at the standard conditions was determined as −6196.6 ± 8.6 kJ mol⁻¹. The entropy of tooeleite, calculated from low-temperature heat capacity data measured by relaxation calorimetry, is 899.0 ± 10.8 J mol⁻¹ K⁻¹. The calculated standard Gibbs free energy of formation is −5396.3 ± 9.3 kJ mol⁻¹. The log K_sp value, calculated for the reaction Fe₆(AsO₃)₄(SO₄)(OH)₄·4H₂O + 16H⁺ = 6Fe³⁺ + 4H₃AsO₃ + SO₄²⁻ + 8H₂O, is −17.25 ± 1.80. Tooeleite has stability field only at very high activities of aqueous sulfate and arsenate. As such, it does not appear to be a good candidate for arsenic immobilization at polluted sites. An inspection of speciation diagrams shows that the predominance field of Fe³⁺ and As³⁺ overlap only at strongly basic conditions. The formation of tooeleite, therefore, requires strictly selective oxidation of Fe²⁺ to Fe³⁺ and, at the same time, firm conservation of the trivalent oxidation state of arsenic. Such conditions can be realized only by biological systems (microorganisms) which can selectively oxidize one redox-active element but leave the other ones untouched. Hence, tooeleite is the first example of an “obligatory” biomineral under the conditions prevailing at or near the Earth's surface because its formation under these conditions necessitates the action of microorganisms.