Study of the system Ca₃(AsO₄)₂–CaSO₄by solid-state reaction

Abstract

Study of the system Ca,(AsO,),-CaSO, has led to the preparation of a series of solid solutions whose general formula is [(Ca,, --x 0,,,)(As0,),,~,,(S0,),, O,] with O < x < 1. These solid solutions are isostructural with tricalcium orthoarsenate Ca,(AsO,),. The limit of solubility of sulfate ions in tricalcium orthoarsenate corresponds to the ionic ratio R=SOZ-/AsOZ-=0.167 (corresponding to x = 1 in the general formula). The limit compound, calcium arsenosulfate Ca,,(AsO,),,(SO,),, is characterized by its infrared absorption spectra, crystallographic unit-cell parameter in t h e rhombohedral system a,= 10.78(2) 8, and density [p=3.469(5) g ~m -~] .A mechanism of SO:-ion integration in t h e lattice of tricalcium orthoarsenate is proposed.

Solid-solid reactions in the Ca3(AsO4)2-Ca2SiO4-CaSO4 ternary system at elevated temperatures

“…In recent years, considerable progress has been made in the development of fixed-grid enthalpy-based macroscopic models, [1] in order to simulate transport phenomena during alloy solidification processes. In such models, enthalpy is related to the liquid fraction of a control volume in the phase changing domain, in an implicit fashion, without the requirement of any explicit imposition of interfacial boundary conditions, which is in sharp contrast with the so-called moving grid front-tracking algorithms.…”

“…Recent experimental studies [1,2] have demonstrated that solid-solid reactions exist between CaSO 4 and Ca 3 (AsO 4 ) 2 at elevated temperatures. These two compounds appear to form an intermediate phase with the following composition: Ca 10 (AsO 4 ) 6 (SO 4 ).…”

“…These two compounds appear to form an intermediate phase with the following composition: Ca 10 (AsO 4 ) 6 (SO 4 ). [1,2] Coursol et al, were able to estimate the Gibbs energy of formation for the Ca 10 (AsO 4 ) 6 (SO 4 ) stoichiometric compound. The mechanism by which the sulfate ions dissolve in Ca 3 (AsO 4 ) 2 was related to its crystal structure.…”

The Ca(SO4)-Na2(SO4)-Ca3(AsO4)2-Na3(AsO4) phase diagram

“…The binary solid solution Ca 3 (AsO 4 ) 2 -CaSO 4 was studied by Marraha et al [24] In a previous publication, [5] we proposed a model for this binary solid solution (Figure 2). The crystal structure of Ca 3 (AsO 4 ) 2 has been studied in detail.…”

“…Consider When CaSO 4 dissolves in Ca 3 (AsO 4 ) 2 , it was demonstrated by Marraha et al [24] that the ion substitutes for an ion, which is of similar size. Furthermore, substitution for an {AsO 4 } on the A chain, with a vacancy occupying the [Ca,Va] site, should be a preferred configuration because this will result in local charge neutrality.…”