Tranquillityite

Characteristics of Tranquillityite

Tranquillityite forms thin stripes up to 15 by 65 micrometres in size in basaltic rocks, where it was produced at a late crystallization stage. It is associated with troilite, pyroxferroite, cristobalite and alkali feldspar. The mineral is nearly opaque and appears dark red-brown in thin crystals. The analyzed samples contain less than 10% impurities (Y, Al, Mn, Cr, Nb and other rare-earth element) and up to 0.01% (100 ppm) of uranium. Presence of a significant amount of uranium allowed scientists to estimate the age of tranquillityite and some associated minerals in Apollo 11 samples as 3710 million years using the uranium–lead dating technique. Irradiation by alpha particles generated by uranium decay is believed to be the origin of the predominantly amorphous metamict structure of tranquillityite. Its crystals were obtained by annealing the samples at 800 °C (1,470 °F) for 30 minutes. Longer annealing did not improve the crystalline quality, and annealing at higher temperatures resulted in spontaneous fracture of samples. The crystals were initially found to have a hexagonal crystal structure with the lattice parameters, a = 1.169 nm, c = 2.225 nm and three formula units per unit cell, but later reassigned a face-centered cubic structure (fluorite-like). A tranquillityite-like crystalline phase has been synthesized by mixing oxide powders in an appropriate ratio, determined from the chemical analysis of the lunar samples, and annealing the mixture at 1,500 °C (2,730 °F). This phase was not pure, but intergrown with various intermetallic compounds.