Heat capacity studies of phase transitions in langbeinites II. K2Mg2(SO4)3

Heat capacity measurements have been made on a synthetic sample of langbeinite, K₂Mg₂(SO₄)₃, from 13 to 342 K in an adiabatic calorimeter. Three phase transitions, at 51.0, 54.9 and 63.8 K, have been observed in this material. Our study is the first to report the existence of such phase transitions in K₂Mg₂(S0₄)₃ and disputes predictions that none would take place below 77 K. Two models which have been proposed to explain the transition in potassium langbeinites are discussed in light of these results.     

The measurement of sulfate mineral solubilities in the Na-K-Ca-Cl-SO4-H2O system at temperatures of 100, 150 and 200°C

At T > 100°C development of thermodynamic models suffers from missing experimental data, particularly for solubilities of sulfate minerals in mixed solutions. Solubilities in Na⁺-K⁺-Ca²⁺-Cl⁻-SO₄²⁻/H₂O subsystems were investigated at 150, 200°C and at selected compositions at 100°C. The apparatus used to examine solid-liquid phase equilibria under hydrothermal conditions has been described.In the system NaCl-CaSO₄-H₂O the missing anhydrite (CaSO₄) solubilities at high NaCl concentrations up to halite saturation have been determined. In the system Na₂SO₄-CaSO₄-H₂O the observed glauberite (Na₂SO₄ · CaSO₄) solubility is higher than that predicted by the high temperature model of Greenberg and Møller (1989), especially at 200°C. At high salt concentrations, solubilities of both anhydrite and glauberite increase with increasing temperature. Stability fields of the minerals syngenite (K₂SO₄ · CaSO₄ · H₂O) and goergeyite (K₂SO₄ · 5 CaSO₄ · H₂O) were determined, and a new phase was found at 200°C in the K₂SO₄-CaSO₄-H₂O system. Chemical and single crystal structure analysis give the formula K₂SO₄ · CaSO₄. The structure is isostructural with palmierite (K₂SO₄ · PbSO₄). The glaserite (“3 K₂SO₄ · Na₂SO₄”) appears as solid solution in the system Na₂SO₄-K₂SO₄-H₂O. Its solubility and stoichiometry was determined as a function of solution composition.     

The crystal structure of alumoklyuchevskite,K<Subscript>3</Subscript>Cu<Subscript>3</Subscript>AlO<Subscript>2</Subscript>(SO<Subscript>4</Subscript>)<Subscript>4</Subscript>

The crystal structure of the unstable mineral alumoklyuchevskite K₃Cu₃AlO₂(SO₄)₄ [monoclinic, I2, a = 18.772(7), b = 4.967(2), c = 18.468(7) Å, β = 101.66(1)°, V = 1686(1) Å] was refined to R ₁ = 0.131 for 2450 unique reflections with F ≥ 4σF _hkl. The structure is based on oxocentered tetrahedrons (OAlCu ₃ ⁷⁺ ) linked into chains via edges. Each chain is surrounded by SO₄ tetrahedrons forming a structural complex. Each complex is elongated along the b axis. This type of crystal structure was also found in other fumarole minerals of the Great Tolbachik Fissure Eruption (GTFE, Kamchatka Peninsula, Russia, 1975–1976), klyuchevskite, K₃Cu₃Fe³⁺O₂(SO₄)₄; and piypite, K₂Cu₂O(SO₄)₂.     

Phase transitions in langbeinites II: Raman spectroscopic investigations of K2Cd2(SO4)3

Polarized single crystal Raman spectra of the langbeinite K₂Cd₂(SO₄)₃ were recorded for different polarisations. With a view to understanding the phase transition mechanism, the lattice vibrational spectra (0–300 cm^?1), as well as the SO₄ symmetric stretching mode v ₁ (1,022 cm^?1), were recorded at different temperatures. No soft modes were observed. From the study of the temperature variation of the integrated intensity I ₀ and band width Γ of the hard mode (1,022 cm^?1), both SO₄ libration and SO₄ order/disorder models were ruled out as possible phase transition models. On the other hand, the model of Speer and Salje (paper I), involving the distortion of the polyhedra around Cd and K ions, explains the observed temperature behaviour of the Raman spectra very well. The consequences of a possible hypothetical high-temperature phase are discussed.     

Die Kristallstruktur des Polyhalits,K2Ca2Mg[SO4]4.2H2O

Zusammenfassung Die Kristallstruktur des triklin-pinakoidalen Polyhalits, K₂Ca₂Mg[SO₄]₄. 2H₂O wurde aus photographischen Daten mit Hilfe der 3-dimensionalen Patterson-Funktion ermittelt. Die Verfeinerung mit der Methode der kleinsten Quadrate führte mit individuellen, isotropen Temperaturfaktoren für die beobachteten Reflexe auf R=0,09.In der Atomanordnung sind Sulfatgruppen durch oktaedrisch koordinierte Mg²⁺, 8-koordinierte Ca²⁺ und 11-koordinierte K⁺ miteinander verknüpft. Die H₂O-Moleküle sind jeweils an ein Mg²⁺ und ein K⁺ koordiniert. Die wahrscheinliche Lage der H-Atome wird aus den interatomaren Abständen erschlossen.

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The crystal structure of polyhalite, K₂Ca₂Mg[SO₄]₄.2H₂O

Summary The crystal structure of the triclinic-pinacoidal mineral polyhalite, K₂Ca₂Mg[SO₄].2H₂O, is derived from photographic data by means of the 3-dimensional Patterson-function. The least squares-refinement resulted for the observed reflections and individual, isotropic temperature factors in R=0.09.In the atomic arrangement the sulfate groups are linked by octahedrally coordinated Mg²⁺, 8-coordinated Ca²⁺, and 11-coordinated K⁺. Each H₂O-molecule is coordinated to one Mg²⁺ and one K⁺. The probable positions of the H-atoms are derived from the interatomic distances.

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Mit 2 Abbildungen     

Thermal phase transition of potassium sulfate,K2SO4

The thermal phase transition of K₂SO₄ has been investigated by high temperature polarized light microscopy. K₂SO₄ undergoes a first-order transition at 587° C where the orthorhombic low temperature form (Pmcn) transforms into a hexagonal high temperature modification (P6₃/mmc). Prior to the beginning of the phase transition, K₂SO₄ shows an anomalous optical behavior. The crystal apparently becomes optically uniaxial twice at 338° and 425° C, respectively, and truly optically uniaxial at 587° C. The phase transition propagates through an intermediate temperature form, which is sandwiched between the low and the high temperature forms and moves in a definite direction, 〈130〉 (orthorhombic indices), in the vicinity of the phase transition. Passing through the phase transition point on cooling, dark belts crossing each other are observed which are a result of the transformation twins parallel to {110} and {130}.     

K-H3OFe3(SO4)2(OH)6铁矾的XRD研究

根据X射线衍射(XRD)分析发现: A Fe₃(SO₄)₂(OH)₆(A=K⁺、H₃O⁺)系列铁钒的XRD数据十分相近,难以用XRD区别,需通过能谱(EDS)辅助分析,才能区分此类铁矾。另外,此类铁矾的003和107面网间距d随K⁺含量增大而增大,且呈一元三次方程的关系;而033和220面网间距d随K⁺含量增大而减小,呈一元二次方程的关系。对该现象从铁矾晶体结构方面进行解释:K⁺、H₃O⁺离子位于较大空隙中,且沿着Z轴方向排列,当K⁺、H₃O⁺离子之间相互替换时,会导致该铁矾晶体结构在Z轴方向有较明显的变化。     

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     

Dissolution of jarosite [KFe3(SO4)2(OH)6] at pH 2 and 8: Insights from batch experiments and computational modelling

Jarosite [KFe₃(SO₄)₂(OH)₆] is a mineral that is common in acidic, sulphate-rich environments, such as acid sulphate soils derived from pyrite-bearing sediments, weathering zones of sulphide ore deposits and acid mine or acid rock drainage (ARD/AMD) sites. The structure of jarosite is based on linear tetrahedral-octahedral-tetrahedral (T-O-T) sheets, made up from slightly distorted FeO₆ octahedra and SO₄ tetrahedra. Batch dissolution experiments carried out on synthetic jarosite at pH 2, to mimic environments affected by ARD/AMD, and at pH 8, to simulate ARD/AMD environments recently remediated with slaked lime (Ca(OH)₂), suggest first order dissolution kinetics. Both dissolution reactions are incongruent, as revealed by non-ideal dissolution of the parent solids and, in the case of the pH 8 dissolution, because a secondary goethite precipitate forms on the surface of the dissolving jarosite grains. The pH 2 dissolution yields only aqueous K, Fe, and SO₄. Aqueous, residual solid, and computational modelling of the jarosite structure and surfaces using the GULP and MARVIN codes, respectively, show for the first time that there is selective dissolution of the A- and T-sites, which contain K and SO₄, respectively, relative to Fe, which is located deep within the T-O-T jarosite structure. These results have implications for the chemistry of ARD/AMD waters, and for understanding reaction pathways of ARD/AMD mineral dissolution.     

The effect of sulfate on aluminum concentrations in natural waters: some stability relations in the system Al2O3-SO3-H2O at 298 K

While gibbsite and kaolinite solubilities usually regulate aluminum concentrations in natural waters, the presence of sulfate can dramatically alter these solubilities under acidic conditions, where other, less soluble minerals can control the aqueous geochemistry of aluminum. The likely candidates include alunogen, Al₂(SO₄)₃ · 17H₂O, alunite, KAl₃(SO₄)₂(OH)₆, jurbanite, Al(SO₄)(OH) · 5H₂O, and basaluminite, Al₄(SO₄)(OH)₁₀ · 5H₂O. An examination of literature values shows that the log $\text{K}{}_{\mathrm{sp}}\text{= ?85.4}$ for alunite and log $\text{K}{}_{\mathrm{sp}}\text{= ?117.7}$ for basaluminite. In this report the log $\text{K}{}_{\mathrm{sp}}\text{= ?7.0}$ is estimated for alunogen and log $\text{K}{}_{\mathrm{sp}}\text{= ?17.8}$ is estimated for jurbanite. The solubility and stability relations among these four minerals and gibbsite are plotted as a function of pH and sulfate activity at 298 K. Alunogen is stable only at pH values too low for any natural waters (<0) and probably only forms as efflorescences from capillary films. Jurbanite is stable from $\text{pH < 0}$ up to the range of 3–5 depending on sulfate activity. Alunite is stable at higher pH values than jurbanite, up to 4–7 depending on sulfate activity. Above these pH limits gibbsite is the most stable phase. Basaluminite, although kinetically favored to precipitate, is metastable for all values of pH and sulfate activity. These equilibrium calculations predict that both sulfate and aluminum can be immobilized in acid waters by the precipitation of aluminum hydroxysulfate minerals.Considerable evidence supports the conclusion that the formation of insoluble aluminum hydroxy-sulfate minerals may be the cause of sulfate retention in soils and sediments, as suggested by Adams and Rawajfih (1977), instead of adsorption.     

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

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

Isomorphic substitutions in afghanite: Identification of the mineral

Samples of afghanite, (Na,Ca,K)₈ (Al₆Si₆O₂₄)(SO₄,Cl,CO₃)₃ · H₂O, from the Malaya Bystraya and Tultui lazurite deposits in the Baikal region have been studied with X-ray diffraction and an electron microprobe. The unit-cell dimensions a and c of the examined afghanite samples range from 12.729 to 12.762 &0A and from 21.385 to 21.415 &0A. Isomorphic substitution is exhibited between Na and K; Ca content is about 2.5 apfu; the Cl/SO₄ ratio is close to 1.0 with slight variations. Parameter a was plotted versus K content. The correlation between Na and K is established. Comparative analysis has shown that afghanite from the Baikal region is characterized by a lower K content than that from volcanic complexes in Italy. In the studied samples, K₂O ranges from 0.51 to 2.28 wt %, whereas in comparable samples from volcanic complexes of Italy, it varies from 3.44 to 8.26 wt %. Sodalite-group minerals display a similar behavior of potassium. In lazurite from the Baikal region, the K₂O content ranges from 0 to 0.95 wt %; the K₂O content in hauyne from Italy varies from 0.88 to 5.75 wt %. The wider isomorphic miscibility in the feldspar minerals from volcanic-hosted assemblages in Italy as compared with the same minerals from lazurite deposits of the Baikal region reflects the difference in the geological and physicochemical conditions under which the minerals have been formed. In addition to the X-ray procedure, the Cl/SO₄ ratio determined from microprobe data may be used to identify cancrinite-group minerals found at the Baikal lazurite deposits. It is near 1.0 in afghanite; 0.8 in tounkite; and more than 2 in davyne.     

Optical absorption spectroscopy of the P213-P212121 transformation in K2Co2(SO4)3 langbeinite

Crystal field spectra have been obtained from K₂Co₂(SO₄)₃ at a number of temperatures from 20 K to room temperature. The transition to the ⁴ T _1g(P) excited state is found to be split at all temperatures owing to the trigonal distortion of the Co site. Below the P2₁3-P2₁2₁2₁ phase transition temperature an excess splitting is observed owing to the additional distortion of the site in the orthorhombic phase. This excess distortion is found to be a linear function of temperature with no first order step at T _c. Thus on a local scale the transition appears to be second order. The trigonal splitting increases with increasing temperature above T_c; this is not expected from the Speer-Salje model of the transition mechanism, which predicted decreasing distortion of the oxygen octahedra with increasing temperature. We propose that the apparent increase of asymmetry is due to the off centring of the Co atom in the high temperature phase in a more regular environment.     

Phase transitions in langbeinites I: Crystal chemistry and structures of K-double sulfates of the langbeinite type M 2 + + K2(SO4)3, M + +=Mg,Ni, Co,Zn, Ca

The crystal structures of the langbeinite type M ₂ ^{+ +} K₂(SO₄)₃ with M ^{+ +}=Mg, Ni, Co, Zn, Ca in their cubic phase (P 2 ₁ 3) and Ca₂K₂(SO₄)₃ in its orthorhombic phase (P 2 ₁ 2 ₁ 2 ₁) are determined. Whereas the SO₄-tetrahedra in these compounds are almost undistorted, the two symmetry-independent coordination polyhedra of M ^{+ +} are highly distorted octahedra with trigonal site symmetry in P 2 ₁ 3. The deformation of the oxygen octahedra and the off-centering of M ^{+ +} along the trigonal axis show systematic dependences on the ionic radii and the electronegativities of the M ^{+ +}-ions. The correlations are remarkably different for the two symmetry-independent M ^{+ +}-ions indicating different M ^{+ +} — O bonding. The octahedral deformations show also linear correlations with the phase transition temperatures (P 2 ₁ 3 — P 2 ₁ 2 ₁ 2 ₁) of the different compounds. This observation leads to a new model for the phase transition mechanism which is based on thermal instabilities of the M ^{+ +} — O and K — O polyhedral distortions. The cubic high temperature phase is characterized by high symmetric oxygen coordinations around M ^{+ +} which distort with decreasing temperature. At T _c the trigonal site symmetry is broken in such a way that the K — O coordination becomes denser at the expense of a wider and less symmetric M ^{+ +} — O coordination.     

Mineralogical, chemical, and crystallographic properties of supergene jarosite-group minerals from the Xitieshan Pb-Zn sulfide deposit, northern Tibetan Plateau, China

Supergene jarosite-group minerals are widespread in weathering profiles overlying Pb-Zn sulfide ores at Xitieshan, northern Tibetan Plateau, China. They consist predominantly of K-deficient natrojarosite, with lesser amounts of K-rich natrojarosite and plumbojarosite. Electron microprobe (EMP) analyses, scanning electron microcopy (SEM) investigation, and X-ray mapping reveal that the jarosite-group minerals are characterized by spectacular oscillatory zoning composed of alternating growth bands of K-deficient and K-bearing natrojarosite (K₂O >1 wt.%). Plumbojarosite, whenever present, occurs as an overgrowth in the outermost bands, and its composition can be best represented by K_0.29Na_0.19Pb_0.31Fe_2.66Al_0.22(SO₄)_1.65(PO₄)_0.31(AsO₄)_0.04(OH)_7.37. The substitution of monovalent for divalent cations at the A site of plumbojarosite is charge balanced by the substitution of five-valent for six-valent anions in XO₄ at the X site. Thermogravimetric analysis (TGA) of representative samples reveal mass losses of 11.46 wt.% at 446.6 °C and 21.42 wt.% at 683.4 °C due to dehydroxylation and desulfidation, respectively. TGA data also indicate that the natrojarosite structure collapses at 446.6 °C, resulting in the formation of NaFe(SO₄)₂ and minor hematite. The decomposition products of NaFe(SO₄)₂ are hematite and Na₂SO₄. Powder X-ray diffraction (XRD) analyses show that the jarosite-group minerals have mean unit-cell parameters of a?=?7.315 Å and c?=?016.598 Å. XRD and EMP data support the view that substitutions of Na for K in the A site and full Fe occupancy in the B site can considerably decrease the unit-cell parameter c, but only slightly increase a. The results from this study suggest that the observed oscillatory zoning of jarosite-group minerals at Xitieshan resulted mainly from substitutions of K for Na at the A site and P for S at the X site.     

Electron paramagnetic resonance of PO 3 2− and SO 3 − radicals in anhydrite,celestite and barite: the hyperfine structure and dynamics

Electron paramagnetic resonance (EPR) measurements of natural anhydrite CaSO₄, celestite SrSO₄ and barite BaSO₄ have revealed the presence of PO ₃ ^2? and SO ₃ ^? radicals. Hyperfine structure from³³S has been detected and measured for the first time. Low-symmetry effects, which are manifested in noncoincidence of g and hyperfine axes, were observed for SO ₃ ^? . The dynamics of one of the two SO ₃ ^? types in anhydrite has been investigated. Variations of spin Hamiltonian parameters with temperature have been attributed to thermally induced jumps between the two magnetically inequivalent sites of this center.     

Phase Chemistry Study of the Zabuye Salt Lake Brine: Isothermal Evaporation at 15°C and 25°C

Zabuye Salt Lake in Tibet, China is a carbonate-type salt lake, which has some unique characteristics that make it different from other types of salt lakes. The lake is at the latter period in its evolution and contains liquid and solid resources. Its brine is rich in Li, B, K and other useful minor elements that are of great economic value. We studied the concentration behavior of these elements and the crystallization paths of salts during isothermal evaporation of brine at 15°C and 25°C. The crystallization sequence of the primary salts from the brine at 25°C is halite (NaCl) → aphthitalite (3K2SO4·Na2SO4) → zabuyelite (Li2CO3)→ trona (Na2CO3·NaHCO3·2H2O) → thermonatrite (Na2CO3·H2O) → sylvite (KCl), while the sequence is halite (NaCl) → sylvite (KCl) → trona (Na2CO3·NaHCO3·2H2O) → zabuyelite (Li2CO3) → thermonatrite (Na2CO3·H2O) → aphthitalite (3K2SO4·Na2SO4) at 15°C. They are in accordance with the metastable phase diagram of the Na+, K+-Cl?, CO32?, SO42?-H2O quinary system at 25°C, except for Na2CO3·7H2O which is replaced by trona and thermonatrite. In the 25°C experiment, zabuyelite (Li2CO3) was precipitated in the early stage because Li2CO3 is supersaturated in the brine at 25°C, in contrast with that at 15°C, it precipitated in the later stage. Potash was precipitated in the middle and late stages in both experiments, while boron was concentrated in the early and middle stages and precipitated in the late stage.     

Application of the Pitzer ion interaction model to isopiestic data for the Fe2(SO4)3-H2SO4-H2O system at 298.15 and 323.15 K

Recent isopiestic studies of the Fe₂(SO₄)₃-H₂SO₄-H₂O system at 298.15 K are represented with an extended version of Pitzer’s ion interaction model. The model represents osmotic coefficients for aqueous {(1 − y)Fe₂(SO₄)₃ + yH₂SO₄} mixtures from 0.45 to 3.0 m at 298.15 K and 0.0435 ? y ? 0.9370. In addition, a slightly less accurate representation of a more extended molality range to 5.47 m extends over the same y values, translating to a maximum ionic strength of 45 m. Recent isopiestic data for the system at 323.15 K are represented with the extended Pitzer model over a limited range in molality and solute fraction. These datasets are also represented with the usual “3-parameter” version of Pitzer’s model so that it may be incorporated in geochemical modeling software, but is a slightly less accurate representation of thermodynamic properties for this system. Comparisons made between our ion interaction model and available solubility data display partial agreement for rhomboclase and significant discrepancy for ferricopiapite. The comparisons highlight uncertainty remaining for solubility predictions in this system as well as the need for additional solubility measurements for Fe³⁺-bearing sulfate minerals. The resulting Pitzer ion interaction models provide an important step toward an accurate and comprehensive representation of thermodynamic properties in this geochemically important system.     

Phase transition of potassium sulfate,K2SO4 (II); dielectric constant and electrical conductivity

The phase transition of K₂SO₄ has been investigated by measurements of the dielectric constant and electrical conductivity, correlated with the structural point of view. Using single crystals, the temperature dependences of the dielectric constants and electrical conductivities were measured at frequencies of 0.3, 1, 3, and 5 MHz in the temperature range from 20° to 640 °C. Within this range, the dielectric constant does not reach a maximum, but near the phase-transition temperature at 587° C, the dielectric constant along the c axis shows a larger discontinuity than those along the a and b axes. The temperature dependence of the dielectric constant is consistent with the disordered structure of the high temperature form. Based on the parabolic increase of the dielectric constant in the temperature range from 582° to 587° C, it is likely that the phase transition propagates through an intermediate state. The electrical conductivity coefficients of K₂SO₄ increase with increasing temperature, exhibiting semiconducting character above the phase-transition temperature. In the high-temperature form, the electrical conductivity along the a axis exceeds that along the c axis. Since the electrical conductivity of K₂SO₄ is mainly ionic in character, the migration of K⁺ ions makes a major contribution to the conduction process.     

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.