Daniela Freyer

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

Abstract 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 Moller (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.

9Mg(OH)2 · MgCl2 · 4H2O, a High Temperature Phase of the Magnesia Binder System

The metastable phase 9Mg(OH)(2)·MgCl(2)·4H(2)O (9-1-4 phase) was found at the extended metastable isotherm of Mg(OH)(2) in the system MgO-MgCl(2)-H(2)O at 120 °C and occurs as intermediate binder phase during setting of magnesia cement due to temperature development of the setting reaction. The crystal structure of the 9-1-4 phase was solved from high resolution laboratory X-ray powder diffraction data in space group I2/m (C2/m) (a = 22.2832(3) Å, b = 3.13501(4) Å, c = 8.1316(2) Å, β = 97.753(1)°, V = 562.86(2) Å(3), and Z = 1). Structural and characteristical relations of the phases in the system MgO-MgCl(2)-H(2)O can be derived, with which the development of the cement or concrete qualities becomes explainable.

Water channel structure of bassanite at high air humidity: crystal structure of CaSO4·0.625H2O.

Structure analysis using single-crystal diffraction was carried out as a contribution to the dispute about the nature of the water channel structure of bassanite (CaSO(4)·0.5H(2)O). A recent result of Weiss & Bräu (2009) for the crystal structure of bassanite (monoclinic, space group C2) at ambient conditions of air humidity was confirmed. In the presence of high relative air humidity the crystal structure of bassanite transformed due to the incorporation of additional water of hydration. The crystal structure of CaSO(4)·0.625H(2)O was solved by single-crystal diffraction at 298 K and 75% relative air humidity. The experimental results provided an insight into both crystal structures. A model explaining the phase transition from CaSO(4)·0.625H(2)O to CaSO(4)·0.5H(2)O was derived. The monoclinic cell setting of CaSO(4)·0.5H(2)O and the trigonal cell setting of CaSO(4)·0.625H(2)O were confirmed by powder diffraction.

CaSeO4-0.625H2O - Water Channel Occupation in a bassanite Related Structure

Calcium selenate subhydrate, CaSeO(4)·0.625H(2)O, was prepared by hydrothermal conversion of CaSeO(4)·2H(2)O at 463 K. From the single crystals obtained in the shape of hexagonal needles, 50-300 µm in length, the crystal structure could be solved in a trigonal unit cell with space group P3(2)21. The cell was confirmed and refined by high-resolution synchrotron powder diffraction. The subhydrate was characterized by thermal analysis and Raman spectroscopy.

Structure solution and refinement of stacking-faulted NiCl(OH)

Two samples of pure NiCl(OH) were produced by hydrothermal synthesis and characterized by chemical analysis, IR spectroscopy, high-resolution laboratory X-ray powder diffraction and scanning electron microscopy. Layers composed of edge-sharing distorted NiCl6x(OH)6−6x octahedra were identified as the main building blocks of the crystal structure. NiCl(OH) is isostructural to CoOOH and crystallizes in space group R \overline{{3}}m [a = 3.2606 (1), c = 17.0062 (9) A]. Each sample exhibits faults in the stacking pattern of the layers. Crystal intergrowth of (AγB)(BαC)(CβA) and (AγB)(AγB) [C6 like, β-Ni(OH)2 related] stacked layers was identified as the main feature of the microstructure of NiCl(OH) by DIFFaX simulations. A recursion routine for creating distinct stacking patterns of rigid-body-like layers in real space with distinct faults (global optimization) and a Rietveld-compatible approach (local optimization) was realized and implemented in a macro for the program TOPAS for the first time. This routine enables a recursive creation of supercells containing (AγB)(BαC)(CβA), (AγB)(AγB) and (CβA)(BαC)(AγB) stacking patterns, according to user-defined transition probabilities. Hence it is an enhancement of the few previously published Rietveld-compatible approaches. This routine was applied successfully to create and adapt a detailed microstructure model to the measured data of two stacking-faulted NiCl(OH) samples. The obtained microstructure models were supported by high-resolution scanning electron microscopy images.

Thermal Behaviour and Crystal Structure of Sodium-Containing Hemihydrates of Calcium Sulfate

Summary. A metastable sodium-containing hemihydrate ((6-x)CaSO4ċxNa2SO4ċ3H2O, 0 ≤ x ≤ 1) limited by the pentasalt composition Na2SO4ċ5CaSO4ċ3H2O occurs as an intermediate solid phase in the systems Na2SO4-CaSO4-H2O, NaCl-Na2SO4-CaSO4-H2O, and NaCl-CaSO4-H2O. X-Ray structure determination of a crystal with pentasalt composition results in a super-structure of the pure hemihydrate (CaSO4ċ0.5H2O), in which one Ca2+ion is statistically replaced by two Na+ ions. One Na+ cation is situated in a Ca2+ position in only one of the three chains of CaSO4 forming an axis of nearly three fold symmetry along the c-axis. The second Na+ is located in the water channel neighbouring to the first Na+. The hydrate crystallized in a monoclinic space group C121 (No. 5) with a=24.1781(11), b=13.805(2), c=12.7074(12) Å, β=90.089(12)°. The dehydration temperature of the hydrates depends on their Na+ ion content. A high Na+ content (in water channels) blocks the water escape strongly, and the dehydration temperature increases. Thermal behaviour is also effected by the crystal sizes. The thermograms of small crystals as opposed to large ones show a exothermic effect adjoining the endothermic dehydration. This may be indicative for a change in the dehydration mechanism upon crystal size.Zusammenfassung. Ein metastabiles natriumhaltiges Calciumsulfathemihydrat ((6−x) CaSO4ċxNa2SO4ċ3H2O, 0 ≤ x ≤ 1) tritt als intermediäre Phase in den Systemen Na2SO4-CaSO4-H2O, NaCl-Na2SO4-CaSO4-H2O und NaCl-CaSO4-H2O auf. Die Phase mit dem höchsten Natriumionengehalt entspricht dem sogenannten Natriumpentasalz (Na2SO4ċ5CaSO4ċ3H2O). Die Röntgenstrukturanalyse, welche an einem Einkristall mit Pentasalz-Zusammensetzung durchgeführt wurde, ergab eine Überstruktur des reinen Hemihydrats (CaSO4ċ0.5H2O), wobei ein Ca2+-Ion statistisch durch zwei Na+-Ionen ersetzt ist. Die Ca2+-Substitution erfolgt nur in einer der drei CaSO4-Ketten, welche eine nahezu dreizählige Achse entlang der c-Achse bilden. Ein Ca2+-Ion wird durch ein Na+-Ion ersetzt, das zweite Na+-Ion ist in unmittelbarer Nähe zum ersten in den Wasserkanälen eingelagert. Das Hydrat kristallisiert in der monoklinen Raumgruppe C121 (Nr.5) mit den Gitterparametern a=24.178(11), b=13.805(2), c=12.7074(12) Å, β=90.089(12)°. Die Dehydratationstemperatur variiert mit dem Na+-Gehalt. Je größer dieser ist, um so stärker wird der Austritt der H2O-Moleküle blockiert und zu höheren Temperaturen verschoben. Ein weiterer Unterschied im thermischen Verhalten wurde bei gleichem Na+-Gehalt für unterschiedliche Kristallitgrößen gefunden. Bei kleinen Kristallgrößen (Nadellänge < 20 μm) beginnt die Entwässerung früher im Vergleich zu größeren Kristallen (Nadellänge > 50 μm). Ein exothermer Effekt im Anschluß an die Dehydratation wird nur für kleine Kristalle beobachtet und deutet auf eine abweichende Entwässerungskinetik in Abhänigkeit von der Kristallitgröße hin.

The phase diagram of the system Na2SO4 - CaSO4

Abstract Quenching of Na 2 SO 4 - CaSO 4 melts down to room temperature results in different forms of metastable solid solutions dependent on the CaSO 4 content. The transformation of these solid solutions into stable phases were investigated after various times and temperatures by X-ray powder diffraction and thermal analyses. An equilibrium phase diagram is derived, which is in accordance with all experimental facts.

Ni3Cl2.1(OH)3.9·4H2O, the Ni analogue to Mg3Cl2(OH)4·4H2O.

For the first time a basic transition-metal hydrate, Ni3Cl2.1(OH)3.9·4H2O, is found to be isostructural to a main-group metal phase, Mg3Cl2.0(OH)4.0·4H2O. The Ni phase was found as crystalline solid in the course of investigations into the formation of basic nickel(II) chloride phases at 25 and 40 °C in alkaline, concentrated nickel(II) chloride solutions. Ni3Cl2.1(OH)3.9·4H2O was characterized by thermal analysis, IR spectroscopy, scanning electron microscopy, and X-ray powder diffraction. The crystal structure was determined from high-resolution laboratory X-ray powder diffraction data. Ni3Cl2.1(OH)3.9·4H2O crystallizes in space group C2/m (12) with Z = 2, a = 14.9575(4) Å, b = 3.1413(1) Å, c = 10.4818(5) Å, β = 101.482(1)°, and V = 482.50(3) Å(3). The main building unit of the structure is an infinite triple chain of edge-linked distorted NiO6 octahedra. These chains are separated by interstitial one-dimensional zigzag chains of disordered Cl(-) ions and H2O molecules. The crystal structures of Ni3Cl2.1(OH)3.9·4H2O and the isostructural magnesium salt hydrate Mg3Cl2(OH)4·4H2O (2-1-4 phase) are compared in detail.

Water channel structure of bassanite at high air humidity: crystal structure of CaSO4·0.625H2O. Corrigendum

The correspondence author in the paper by Schmidt et al. [(2011), Acta Cryst. B67, 467–475] is corrected.