A mineralogical study and crystal-structure determination of nonmetamict ekanite, ThCa2Si8O20

Nonmetamict ekanite, ideally ThCa2Si8O20 chemical composition very similar to the metamict gemstone ekanite originally found in Sri Lanka (Ceylon), has been discovered in the Tombstone Mountains, Yukon Territory. The physical and optical properties are described and the X-ray powder pattern is given. Crystalline ekanite is tetragonal, space group I422 with a 7.483(3), c 14.893(6) A. The structure has been solved and refined to R = 3.57% from 1319 independent reflections obtained from multiple data sets with MoKα radiation. The structure is closely related to that of the family with general composition ThK(Na,Ca)Si8O20, which crystallizes in space group P4/mcc and which has been misnamed 'ekanite' for many years. True crystalline ekanite has a body-centred unit cell whose dimensions agree closely with that of the material obtained on heating metamict ekanite to above 6500C. The metal co-ordination is remarkably similar in the two types of structures; Th is 8-co-ordinated in a square antiprism of oxygen atoms at 2.405(5) A, and Ca has four nearest oxygen neighbours [21.342(5) A] in a very distorted tetrahedron and four second-nearest neighbours [2.688(5) A] near the midpoints of the faces of the tetrahedron. Sheets of metals at z = 0, 1/2 are separated by a puckered silicate layer that extends infinitely in x, y. The structure is characterized by zeolite-like channels through the silicate layers, where non-structural water can become entrapped