DELONEITE-(Ce) NaCa_2SrCe(PO_4)_3F - A NEW MINERAL WITH A BELOVITE-LIKE STRUCTURE

摘要

The mineral occurs in the hyperagpaitic pegmatites of Khibiny alkaline massif (Kola peninsula) in intimate intergrowths with belovite and a SrREE-rich apatite-like mineral. It is represented in sub-microscopic grains - 0.05-0.5 mm, rarely up to 1.0-1.5 mm in size. Bright-yellow, transparent, with strong vitrous lustre. Brittle. Fracture step-like. Hardness 5. Density 3.92 g/cm. Optically uniaxial, negative, Ne = 1.660, No = 1.682. Trigonal., space group P3, a =9.51, C =7.01 A, z =2. The composition (by the microprobe analysis data) is close to the formula NaCa_2SrCe(PO_4)_3F, which is isotypic to that of belovite NaSr_3Ce(PO_4)_3F. These minerals differ one from another not only stoichiometrically, but also in their crystal structure and some other features.