Monazite structure from dehydrated CaSeO_4·2H_2O

摘要

The structure of the high-temperature form of CaSeO_4, formed by dehydration of the gyspum-type structure dihydrate is presented. The material is equivalent to that described previously as a P2_12_12_1 form, but is however, a monazite with unit cell a = 6.85661(16) A, b = 7.04962(15) A, c = 6.68817(15) A and β = 104.2675(21)° in space group P12_11. Also presented is evidence for two intermediate trigonal and pseudo-trigonal phases related to the structurally similar minerals rhabdophane, bassanite, and γ-CaSO_4. This result permits closer comparisons between intermediate-temperature structures of the selenate with related sulphates, and orthophosphates with a view to extending structural stability via synthesis of solid-solutions.