Steacyite, a new name, and a re-evaluation of the nomenclature of 'ekanite'-group minerals

Ekanite from the type locality (Sri Lanka), ThCa2Si8O20, is metamict and gives only a very poor Debye-Scherrer diagram; heated Sri Lanka ekanite is body-centred tetragonal, with a 7.46, c 14.96 A. Natural ekanite from the Tombstone Mountains, Yukon Territory (Symanski et al. 1982) has the same chemical composition and gives a powder pattern that matches that of heated ekanite from Sri Lanka. The crystal structure of ekanite is characterized by Si8O20 sheets, space group I422, a 7.483(3), c 14.893(6) A. Steacyite corresponds to Th(Na,Ca)sub(2)(Ksub(1-) blocksub(z))Sisub(8)0sub(20), z = 0.39, described by Perrault β Richard (1973) under the name 'ekanite'. The crystal structure of steacyite is characterized by discrete pseudocubic arrangments of Si8O20, space group P4/mcc, a 7.58(1), c 14.77(2) A, Z=2. Powder diagrams of ekanite and steacyite show significant diffrences. A Th-Ca-K silicate from central Asia was described by Ginzburg et al. (1965) under the name 'ekanite'; it corresponds to Th(Ca,Na)sub( 2)(Ksub(1-z)blocksub(z)) Sisub(8)Osub(20) with z between 0.23 and 0.40. Space group, cell dimensions and powder pattern are essentially identical to those of steacyite; this mineral should receive a name other than ekanite. These two minerals may define a continuous series. The name honours H.R. Steacy of the Geological Survey of Canada. Iraqite (Livingston et al. 1976) gives a powder pattern identical to that of steacyite and is presumed isotructural. It corresponds to (RE,Th)Casub(2)(Ksub(1-z)blocksub(z)) Sisub(8)0sub(20), with z = 0.47