Extent of immiscibility in the ettringite-thaumasite system

Abstract Solid solutions between thaumasite and the related phase ettringite were prepared and characterised by X-ray diffraction and infrared spectroscopy. A miscibility gap in the solid solution is identified and defined. Crystallographically, the miscibility gap is identified as a gap in the unit cell dimensions of the solid phases formed between c ≅20.95 (10.475) and c ≅21.3 A. A combination of quantitative X-ray powder diffraction and infrared spectroscopy enabled us to define the miscibility gap in terms of Al:Si ratio. Ettringite can accept the replacement of 1/2 its Al by Si, while thaumasite tolerates little or no Al in its structure.