Zur Synthese und Charakterisierung von Ca(HSO4)2 · 2 H2SO4 bzw. H2[Ca(HSO4)4]

Synthesis and Characterization of Ca(HSO₄)₂ · 2 H₂SO₄ or H₂[Ca(HSO₄)₄], respectively ?Ca(HSO₄)₂ · 2 H₂SO₄”? crystallizes from CaSO₄ saturated hot H₂SO_4c below 310 K. With SOCl₂ containing ether it is possible, to remove two moles H₂SO₄ and to prepare Ca(HSO₄)₂. ?Ca(HSO₄)₂ · 2 H₂SO₄”? shows two endothermal effects at T_p1 = 336 K and T_p2 = 477 K during the thermal analysis. Whereas T_p2 corresponds to the segregation of H₂SO₄ from Ca(HSO₄)₂, T_p1 is attributed to the loss of two moles H₂SO₄. These results are supported by x-ray heating measurements on single crystals. From oscillation and Weissenberg photographs with CuKα the unit cell was determined. In agreement with these parameters, the compound is to formulate as the complex acid H₂[Ca(HSO₄)₄].     

Zum Hydratisierungs- und Dehydratisierungsverhalten von Magnesiumhydrogensulfat

On the Hydration and Dehydration Behaviour of Magnesium Hydrogensulfate Mg(HSO₄)₂ · H₂O has been prepared by crystallization from diluted sulfuric acid. The thermal destruction leads to the formation of siccative Mg(HSO₄)₂. This process is reversible, i. e., Mg(HSO₄)₂ rehydrates during cooling under formation of Mg(HSO₄)₂. H₂O. Due to further water reception Mg(HSO₄)₂ · H₂O decomposes at a temperature of 50°C forming Kieserite (MgSO₄ · H₂), MgSO₄ · 1,25H₂O, sulfuric acid, and water. During gentle warming of this heterogenous phase mixture it reacts under reconstruction of Mg(HSO₄)₂ · H₂O. The thermal measurements have been carried out in dynamic as well as in static gas atmosphere. The strong dependence of the decomposition temperatures from the partial pressure of the gaseous products could be demonstrated by in situ X-ray measurements.     

Synthese und Kristallstruktur von Hydrogensulfaten einwertiger Metalle – Ag(H3O)(HSO4)2, Ag2(HSO4)2(H2SO4), AgHSO4 und Hg2(HSO4)2

Synthesis and Crystal Structure of Metal(I) Hydrogen Sulfates – Ag(H₃O)(HSO₄)₂, Ag₂(HSO₄)₂(H₂SO₄), AgHSO₄, and Hg₂(HSO₄)₂ Hydrogen sulfates Ag(H₃O)(HSO₄)₂, Ag₂(HSO₄)₂ · (H₂SO₄), and AgHSO₄ have been synthesized from Ag₂SO₄ and sulfuric acid. Hg₂(HSO₄)₂ was obtained from metallic mercury and 96% sulfuric acid as starting materials. The compounds were characterized by X‐ray single crystal structure determination. Ag(H₃O)(HSO₄)₂ belongs to the structure type of Na(H₃O)(HSO₄). The silver atom is coordinated by 6 + 2 oxygen atoms. In the structure, there are dimers and chains of hydrogen bonded HSO₄^– tetrahedra. Dimers and chains are connected by the H₃O⁺ ion to form a three dimensional hydrogen network. Ag₂(HSO₄)₂(H₂SO₄) crystallizes isotypic to Na₂(HSO₄)₂(H₂SO₄). The coordination number of silver is 6 + 1. The structure of Ag₂(HSO₄)₂(H₂SO₄) is characterized by hydrogen bonded trimers of HSO₄^– tetrahedra, which are further connected to chains. For the recently published structure of AgHSO₄ the hydrogen bonding system was discussed. There are tetrameres and chains, connected by bifurcated hydrogen bonds. The structure of Hg₂(HSO₄)₂ contains Hg₂²⁺ cations with Hg–Hg distance of 2.509 Å. Every mercury atom is coordinated by one oxygen atom at shorter distance (2.18 Å) and three ones at longer distances (2.57 to 3.08 Å). The HSO₄^– tetrahedra form zigzag chains by hydrogen bonds.     

Etude de la deshydratation de Ca(H2PO4)2·H2O et de sa reaction avec CaSO4 par ATD-ATG couplees

The dehydration of Ca(H₂PO₄)₂·H₂O was examined with simultaneous DTA and TG. This dehydration permitted clearly the apparation of the following phases: Ca(H₂PO₄)₂·0.5H₂O, Ca(H₂PO₄)₂, Ca₃(HP₂O₇)₂, Ca₂HP₃O₁₀ et Ca(PO₃)₂. The reaction of Ca(H₂PO₄)₂·H₂O and CaSO₄ was also examined with the same technics. It was found that the decomposition of CaSO₄ takes place for relatively low temperature (between 600°C and 800°C).     

Ein neues Kaliumhydrogensulfat,K(H3O)(HSO4)2 — Synthese und Struktur

A New Potassium Hydrogensulfate, K(H₃O)(HSO₄)₂ — Synthesis and Structure Single crystals of the new compound K(H₃O)(SO₄)₂ are synthesized from the system potassium sulfate/sulfuric acid. The up to day not described compound crystallizes in the monoclinic space group P2₁/c with the unit cell parameters a = 7.203(1) b = 13.585(2) and c = 8.434(1) Å, β = 105.54(1)°, V = 795.1 ų, Z = 4 and D_x = 2.107 g · cm^?3. There are two crystallographically different tetrahedral SO₃(OH) anions. The first kind S1 tetrahedra forms dimers, whereas the second kind S2 forms infinite chains bonded via hydrogen bridges. The S1 dimers are linked to the S2 chains via oxonium ions (hydrogen bonds). Potassium is coordinated by 8 oxygen atoms which belong to four different SO₃(OH) tetrahedra. These potassium oxygen polyhedra are connected by common edges forming chains running parallel z.     

Phase equilibria in the Na,K,Mg,Ca∥SO<Subscript>4</Subscript>,Cl–H<Subscript>2</Subscript>O system at 50°С in the polyhalite crystallization region

Phase equilibria of the Na,K,Mg,Ca||SO₄,Cl–H₂O system at 50°С in the polyhalite (K₂SO₄ · MgSO₄ · 2CaSO₄ · 2H₂O) crystallization region were studied using the translation method. Polyhalite was found to be involved, as an equilibrium phase of the title system at 50°С, in 17 invariant points, 36 monovariant curves, and 24 divariant fields. A fragment of equilibrium phase diagram of the title system in the polyhalite crystallization region was constructed.     

Hydrogensulfate mit fehlgeordneten Wasserstoffatomen – Synthese und Struktur von Li[H(HSO4)2](H2SO4)2 sowie Verfeinerung der Struktur von α-NaHSO4

Hydrogen Sulfates with Disordered Hydrogen Atoms – Synthesis and Structure of Li[H(HSO₄)₂](H₂SO₄)₂ and Refinement of the Structure of α-NaHSO₄ The structure of Li[H(HSO₄)₂](H₂SO₄)₂ has been determined for the first time whereas the structure of α-NaHSO₄ has been refined, so that direct determination of the hydrogen positions was possible. Both compounds crystallize triclinic in the space group P1 with the lattice constants a = 6.708(2), b = 6.995(1), c = 7.114(1) Å, α = 75.53(1), β = 84.09(2) and γ = 87.57(2)° (Z = 4) for α-NaHSO₄ and a = 4.915(1), b = 7.313(1), c = 8.346(2) Å, α = 82.42(3), β = 86.10(3) and γ = 80.93(3)° (Z = 1) for Li[H(HSO₄)₂](H₂SO₄)₂. In both compounds there are disordered hydrogen positions. In the structure of α-NaHSO₄ there are two crystallographically different HSO₄^? tetrahedra and two different coordinated Na atoms. The system of hydrogen bonds can be described by chains in [0–11] direction. The disordering of the H atoms reduces the differences between the S? O and S? OH distances (1.45 and 1.50 Å) while in the ordered HSO₄ unit “regular” bond lengths are observed (1.45 und 1,57 Å). In the structure of Li[H(HSO₄)₂](H₂SO₄)₂ there are two crystallographically different SO₄-tetrahedra. The first one belongs to the [H(HSO₄)₂]^? unit while the second one represents H₂SO₄ molecules. The H atom which is located nearby the symmetry centre and connects two HSO₄ units by a short O…?O distance of 2.44 Å. Li is located on a symmetry centre and is slightly distorted octahedrally coordinated by oxygen atoms of six different SO₄ tetrahedra. The system of hydrogen bonds can be regarded as consisting of double layers parallel to the xy-plane.     

A whole zoo of hydrogen bonds in one crystal structure: tris(isonicotinium) hydrogensulfate sulfate monohydrate

Depending on the reaction partner, the organic ditopic molecule isonicotinic acid (Hina) can act either as a Brønsted acid or base. With sulfuric acid, the pyridine ring is protonated to become a pyridinium cation. Crystallization from ethanol affords the title compound tris(4‐carboxypyridinium) hydrogensulfate sulfate monohydrate, 3C₆H₆NO₂⁺·HSO₄⁻·SO₄²⁻·H₂O or [(H₂ina)₃(HSO₄)(SO₄)(H₂O)]. This solid contains 11 classical hydrogen bonds of very different flavour and nonclassical C—H…O contacts. All N—H and O—H donors find at least one acceptor within a suitable distance range, with one of the three pyridinium H atoms engaged in bifurcated N—H…O hydrogen bonds. The shortest hydrogen‐bonding O…O distance is subtended by hydrogensulfate and sulfate anions, viz. 2.4752 (19) Å, and represents one of the shortest hydrogen bonds ever reported between these residues.     

In situ synchrotron PXRD investigation of fresh and aged Equisetum ramosissimum Desf. (horsetail grass)

As one of the oldest species, horsetail grass (Equisetum ramosissimum Desf.) is known as a living fossil plant, dating back to the Mesozoic era. Horsetail grass is also considered one of the most important sources of bio-silica due to its ability to accumulate high amounts of silica in all parts of the plant; various minerals can also be isolated by heat treatment. Fresh and aged horsetail grass stored for 2 years under ambient conditions was investigated by synchrotron powder X-ray diffraction (PXRD). Clear crystallites were not observed in a fresh sample stored at room temperature; surprisingly, high amounts of gypsum (CaSO₄·2H₂O) and syngenite (K₂Ca[SO₄]₂·H₂O) were observed in the 2-day dried and 2-year aged samples, respectively. However, crystalline silica materials were not observed. In addition, in situ thermal treatment of up to 700°C was applied to investigate the crystals and phase transitions by focusing the X-ray beam onto a single stem. In situ synchrotron PXRD revealed that dehydration occurred in gypsum in the 2-day dried sample with an increase in temperature to hemihydrate (CaSO₄·xH₂O, 0.5 ≤ x ≤ 0.8) and anhydrite (CaSO₄). On the other hand, syngenite was transformed to calciolangbeinite (K₂Ca₂[SO₄]₃) at high temperatures in 2-year aged horsetail grass.     

Synthese,Struktur und Reaktivität der Erdalkalihydrogensulfate des Mg,Ca, Sr und Ba

Synthesis, Structure, and Reactivity of the Alkaline Earth Hydrogen Sulfates of Mg, Ca, Sr, and Ba The pure compounds of Ba(HSO₄)₂, Sr(HSO₄)₂ and Mg(HSO₄)₂ were prepared and identified as real hydrogensulfates. In contact with moist air Mg(HSO₄)₂ forms the monohydrate Mg(HSO₄)₂ · H₂O which can be dehydrated into Mg(HSO₄)₂ at about 120°C. The hydration of Mg(HSO₄)₂ proceeds crystallographically oriented. The unit cell parameters and the d-values of the new compounds were determined. Ba(HSO₄)₂ and Sr(HSO₄)₂ cristallize orthorhombic, Mg(HSO₄)₂ monoclinic and Mg(HSO₄)₂ · H₂O triclinic. Sr(HSO₄)₂ is isotypic in regard to Ca(HSO₄)₂. The relationship between the crystal structures and the chemical properties of the earth alkaline sulfates are discussed.     

Formation of calcium sulfate whiskers from CaCO3-bearing desulfurization gypsum

Calcium sulfate whiskers can be used as the reinforcing agents in many composites, such as polymers, ceramics, cements, and papers, etc. This paper investigated the feasibility of preparing calcium sulfate whiskers using desulfurization gypsum as the raw material. The desulfurization gypsum composed mainly of CaSO₄·2H₂O (93.45 wt%) and CaCO₃ (1.76 wt%) were treated with dilute H₂SO₄ at room temperature to convert CaCO₃ to CaSO₄; the latter was then treated at 110?C150 °C to form CaSO₄·0.5H₂O whiskers. The removal of the CaCO₃ impurity from the desulfurization gypsum favored the formation of CaSO₄·0.5H₂O whiskers with high aspect ratios.     

Tribochemistry and kinetics of Al2(SO4)3·xH2O decomposition

The thermal decomposition of tribochemically activated Al₂(SO₄)₃·xH₂O was studied by TG, DTA and EMF methods. For some of the intermediate solids, X-ray diffraction and IR-spectroscopy were applied to learn more about the reaction mechanism. Thermal and EMF studies confirmed that, even after mechanical activation of Al₂(SO₄)₃·xH₂O, Al₂O(SO₄)₂ is formed as an intermediate. Isothermal kinetic experiments demonstrated that the thermochemical sulphurization of inactivated Al₂(SO₄)₃·xH₂O has an activation energy of 102.2 kJ·mol^?1 in the temperature range 850–890 K. The activation energy for activated Al₂(SO₄)₃·xH₂O in the range 850–890 K is 55.0 kJ·mol^?1. The time of thermal decomposition is almost halved when Al₂(SO₄)₃·xH₂O is activated mechanically. The results permit conclusions concerning the efficiency of the tribochemical activation of Al₂(SO₄)₃·xH₂O and the chemical and kinetic mechanisms of the desulphurization process.     

Proton mobility in complex [RhL4Cl2]HSO4 · nH2SO4 · mH2O salts (L = Py, γ-picoline)

The method of deposition from solutions was used to synthesize [RhL ₄Cl₂]HSO₄ · nH₂SO₄ · mH₂O complex salts (L = Py, γ-picoline), n ≈ 0.5−0.6, m ≈ 5−6. According to the data of X-ray phase analysis, the crystal structure of these salts is formed by layers of cations separated by layers consisting of anions molecules of sulfuric acid and water connected through a system of hydrogen bonds. Calorimetric methods were used to study phase transitions and the range of thermal stability of salts. The method of ¹H NMR spectroscopy discovered that protons within the {HSO₄⁻ · nH₂SO₄ · mH₂O} subsystem featured enhanced conductivity. Conductivity studies showed that trans-[RhL ₄Cl₂]HSO₄ · nH₂SO₄ · mH₂O samples had high proton conductivity.     

Synthese und Struktur neuer Natriumhydrogensulfate Na(H3O)(HSO4)2; Na2(HSO4)2(H2SO4) und Na(HSO4)(H2SO4)2

Synthesis and Structure of New Sodium Hydrogen Sulfates Na(H₃O)(HSO₄)₂, Na₂(HSO₄)₂(H₂SO₄), and Na(HSO₄)(H₂SO₄)₂ Three acidic sodium sulfates have been synthesized from the system sodium sulfate/sulfuric acid and have been crystallographically characterized. Na(H₃O)(HSO₄)₂ ( A ) crystallizes in the space group P2₁/c with the unit cell parameters a = 6.974(2), b = 13.086(2), c = 8.080(3) Å, α = 105.90(4)°, V = 709.1 Å3, Z = 4. Na₂(HSO₄)₂(H₂SO₄) ( B ) is orthorhombic (space group Pna2₁) with the unit cell parameters a = 9.970(2), b = 6.951(1), c = 13.949(3) Å, V = 966.7 ų and Z = 4. Na(HSO₄)(H₂SO₄)₂ ( C ) crystallizes in the triclinic space group P1 with the unit cell parameters a = 5.084(1), b = 8.746(1), c = 11.765(3) Å, α = 68.86(2)°, β = 88.44(2)°, γ = 88.97(2)°, V = 487.8 Å3 and Z = 2. All three compounds contain SO₄ tetrahedra as HSO₄^? anions and additionally in B and C in form of H₂SO₄ molecules. The ratio H:SO₄ determines the connectivity degree in the hydrogen bond system. In A , there are zigzag chains and dimers additionally connected via oxonium ions. Complex chains consisting of cyclic trimers (two HSO₄^? and one H₂SO₄) are present in B . In structure C , several parallel chains are connected to columns due to the greater content of H₂SO₄. Sodium cations show a distorted octahedral coordination by oxygen in all three structures, the NaO₆ octahedra being “isolated” (connected via SO₄ tetrahedra only) in A . Pairs of octahedra with common edge form Na₂O₁₀ dimeric units in C . Such double octahedra are connected via common corners forming zigzag chains in B .     

DSC investigation of the thermal behaviour of (NH4)2SO4, NH4HSO4 and NH4NH2SO3

Gaseous products evolved from (NH₄)₂SO₄, NH₄HSO₄ and NH₄NH₂SO₃ during successive heating and cooling cycles were flushed with inert gas into analyzer Dräger tubes hooked tightly to the terminal port of the DSC cell base. This simple procedure allowed the starting temperature of the decomposition to be determined and the amount of the individual gases in the mixture to be identified and even estimated. NH₄NH₂SO₃ at 523 K in humid air produced HNH₂SO₃ initially and, on further cycling, (NH₄)₂SO₄ and NH₄HSO₄ also appeared. The ΔH_f values for NH₄HSO₄ were (kJ mole^?1): in an airtight sample holder 12.67, in a dry argon atmosphere 11.93, and in a static air atmosphere 10.92. Endothermic peaks for (NH₄)₂SO₄ and 498 and 411 K represented the incongruent melting point and the polymorphic transition of (NH₄)₂SO₄·NH₄HSO₄. After the first heating in air to 530 K, (NH₄)₂SO₄ and NH₄HSO₄ exhibited closely similar cyclic DSC curves. The endothermic peaks at about 393–420 K may be assigned to different combinations of (NH₄)₂SO₄ and NH₄HSO₄.     

Structure of the first nitrosoammine complexes of ruthenium with a coordinated sulfate ion: [Ru(NO)(NH3)4(SO4)](HSO4)·H2O and [Ru(NO)(NH3)3Cl(SO4)]·2H2O

We present a thermogravimetric study of the thermolysis of trans-[Ru(NO)(NH₃)₄(H₂O)](HSO₄)SO₄ in a helium atmosphere. The intermediate product of thermolysis (at 186°C) is treated with a 2 M H₂SO₄ solution to obtain the first example of a sulfate ammine complex of nitrosoruthenium [Ru(NO)(NH₃)₄(SO₄)](HSO₄)·H₂O (I) with a ∼70% yield. The product of higher temperature thermolysis (220°C) is treated with acids (H₂SO₄ and HCl) to obtain a triammine complex [Ru(NO)(NH₃)₃Cl(SO₄)]·2H₂O (II). The structure of the compounds is found by single crystal XRD: Pna2₁ space group, a = 10.8005(2) ?, b = 14.9032(3) ?, c = 7.7603(1) ?) (I) and P2_1/n space group, a = 8.9397(1) ?, b = 8.3276(1) ?, c = 13.8993(2) ?; β = 97.358(1)° (II).     

Kaliumhydrogensulfat-Dihydrogensulfat,K(HSO4)(H2SO4) – Synthese und Kristallstruktur

Potassium Hydrogensulfate Dihydrogensulfate, K(HSO₄)(H₂SO₄) – Synthesis and Crystal Structure Single crystals with the composition KH₃(SO₄)₂ have been synthesized from the system Potassium sulfate/sulfuric acid. The hitherto crystallographically not investigated compound crystallizes in the monoclinic space group P2₁/c (14) with the unit cell parameters a = 7.654(3), b = 11.473(5) and c = 8.643(3) Å, β = 112.43(3)°, V = 701.6 ų, Z = 4 and D_x = 2.22 g · cm^?3. The structure contains two types of tetrahedra, SO₃(OH) and SO₂(OH)₂. These tetrahedra form tetramers via hydrogen bonds consisting of both, two SO₃(OH) and two SO₂(OH)₂ tetrahedra. The tetramers are linked to each other via hydrogen bonds. Potassium is coordinated by 9 oxygen atoms which belong to both kinds of tetrahedra. These potassium oxygen polyhedra are connected by common faces forming chains running parallel z.     

Zur Kenntnis der Sulfito-kobalt(III)-Ammine. III. Hydrogensulfitokobalt(III)-Ammine

Sulphito Cobalt(III) Ammines. III. Hydrogensulphito Cobalt(III) Ammines Concentrated acids react with [CoSO₃(NH₃)₅]⁺ salts hydrogen- sulphitopentaamminecobalt(III) complexes. [Co(HSO₃)(NH₃)₅]Cl₂, [Co(HSO₃)(NH₃)₅]Br₂ and [Co(HSO₃)(NH₃)₅](HSO₄)₂·H₂O have been isolated. These substances are yellow coloured in contrast to an earlier work which reported red colour. Furthermore, the hydrogensulphitoacidotetreaammine complexes [Co(HSO₃)Cl(NH₃)₄]Cl, [Co(HSO₃)Cl(NH₃)₄]ClO₄·H₂O, [Co(HSO₃)Br(NH₃)₄]Br and [Co(HSO₃) CN(NH₃)₄]Cl habe been prepared. [Co(HSO₃)Br(NH₃)₄]Br is losing spontaneously HBr forming [CoSO₃Br(NH₃)₄]. The neutral complex [Co(HSO₃)SO₃(NH₃)₄]·1/2H₂O has been obtained from cis- NH₄[Co(SO₃)₂(NH₃)₄] and HCl. The absorption spectra in the IR, visible and UV region are reported and discussed. The HSO₃ group is coordinated to Co through the S atom. The Co? S bond is weaker than in the sulphito complexes as concluded from the RAMAN spectrum. In the new complexes, the hydrogensulphito ligand causes a minor trans effect than the sulphito ligand.     

Boron–π interactions in two 3-(dihydroxyboryl)anilinium salts analyzed by crystallographic studies and supported by the noncovalent interactions (NCI) index theoretical approach

In the title compounds, 3-(dihydroxyboryl)anilinium bisulfate monohydrate, C₆H₉BNO₂⁺·HSO₄⁻·H₂O ( I ), and 3-(dihydroxyboryl)anilinium methyl sulfate, C₆H₉BNO₂⁺·CH₃SO₄⁻ ( II ), the almost planar boronic acid molecules are linked by pairs of O—H…O hydrogen bonds, forming centrosymmetric motifs that can be described by the graph-set R₂²(8) motif. In both crystals, the B(OH)₂ group acquires a syn–anti conformation (with respect to the H atoms). The presence of the hydrogen-bonding functional groups B(OH)₂, NH₃⁺, HSO₄⁻, CH₃SO₄⁻ and H₂O generates three-dimensional hydrogen-bonded networks, in which the bisulfate (HSO₄⁻) and methyl sulfate (CH₃SO₄⁻) counter-ions act as the central building blocks within the crystal structures. Furthermore, in both structures, the packing is stabilized by weak boron–π interactions, as shown by noncovalent interactions (NCI) index calculations.     

Crystallization and Phase Stability of CaSO<Subscript>4</Subscript> and CaSO<Subscript>4</Subscript> – Based Salts

Summary.  Calcium sulfate occurs in nature in form of three different minerals distinguished by the degree of hydration: gypsum (CaSO₄·2H₂O), hemihydrate (CaSO₄·0.5H₂O) and anhydrite (CaSO₄). On the one hand the conversion of these phases into each other takes place in nature and on the other hand it represents the basis of gypsum-based building materials. The present paper reviews available phase diagram and crystallization kinetics information on the formation of calcium sulfate phases, including CaSO₄-based double salts and solid solutions. Uncertainties in the solubility diagram CaSO₄–H₂O due to slow crystallization kinetics particularly of anhydrite cause uncertainties in the stable branch of crystallization. Despite several attempts to fix the transition temperatures of gypsum–anhydrite and gypsum–hemihydrate by especially designed experiments or thermodynamic data analysis, they still vary within a range from 42–60°C and 80–110°C. Electrolyte solutions decrease the transition temperatures in dependence on water activity. Dry or wet dehydration of gypsum yields hemihydrates (α-, β-) with different thermal and re-hydration behaviour, the reason of which is still unclear. However, crystal morphology has a strong influence. Gypsum forms solid solutions by incorporating the ions HPO₄ ²⁻, HAsO₄ ²⁻, SeO₄ ²⁻, CrO₄ ²⁻, as well as ion combinations Na⁺(H₂PO₄)⁻ and Ln³⁺(PO₄)³⁻. The channel structure of calcium sulfate hemihydrate allows for more flexible ion substitutions. Its ion substituted phases and certain double salts of calcium sulfate seem to play an important role as intermediates in the conversion kinetics of gypsum into anhydrite or other anhydrous double salts in aqueous solutions. The same is true for the opposite process of anhydrite hydration to gypsum. Knowledge about stability ranges (temperature, composition) of double salts with alkaline and alkaline earth sulfates (esp. Na₂SO₄, K₂SO₄, MgSO₄, SrSO₄) under anhydrous and aqueous conditions is still very incomplete, despite some progress made for the systems Na₂SO₄–CaSO₄ and K₂SO₄–CaSO₄–H₂O. Corresponding author. E-mail: daniela.freyer@chemie.tu-freiberg.de Received December 17, 2002; accepted January 10, 2003 Published online April 3, 2003