Experimental Synthesis of the Rare-Earth Fluoride Gagarinite-(Ce) by Fluid-Induced Alteration of Chevkinite-(Ce)

Gagarinite-(Ce) [Na(Ca,Ce)2F6] has been synthesized as a product in experiments designed to examine the fluid-induced alteration of chevkinite-(Ce). The experiments were conducted at 600 °C and 400 MPa for 21 days and at 550 °C and 200 MPa for 63 days. At 600 °C, a rim of gagarinite-(Ce) was seen to develop around chevkinite-(Ce), which was itself enclosed in a glassy, amorphous material containing inclusions of albite. At 550 °C, the chevkinite-(Ce) was observed to be surrounded by a rim of gagarinite-(Ce) and fluorbritholite-(Ce), which in turn was enclosed in massive narsarsukite with inclusions of albite. This assemblage was associated with a sodic pyroxene. In the 550 °C run both rims with gagarinite-(Ce) were subsequently overgrown by both Ce-bearing frankdicksonite and a lamprophyllite group mineral identified as delindeite. Electron probe microanalyses are given of all reactant phases. The stability conditions of gagarinite-(Ce) are in accord with those inferred from the only natural occurrence, Strange Lake, Canada. The formation of gagarinite-(Ce) is ascribed to NaF being the only component added in the experiments, resulting in a very high Na/Ca ratio in the system. New parageneses for narsarsukite, frankdicksonite, and delindeite are reported.