Margaritasite, a Cs-rich analogue of camotite, is a newly discovered uranium mineral that is part of the ore at the Margaritas deposit in the Peña Blanca uranium district near Chihuahua, Mexico. The margaritasite occurs as disseminated pore fillings and relict phenocryst linings within a rhyodacitic tuff breccia of the lower Escuadra Formation (Oligocene) and provides significant reserves of both uranium and cesium. It is a finegrained yellow mineral and, with the possible exception of the index of refraction, is optically indistinguishable from camotite. Margaritasite is most easily recognized by X-ray diffraction through a shift in the (001) reflection representing an increase, relative to camotite, in the c dimension. This increase is due to the large Cs atom in sites normally occupied by K in the camotite structure.
Unit cell parameters are a = 10.514, b = 8.425, and c = 7.25Å, β = 106.01° (P21/a, Z = 2). Microprobe analyses of margaritasite and Cs-enriched carnotites synthesized by ion exchange of camotite in aqueous CsCl solution at 200°C suggest a solid solution between camotite and margaritasite, but X-ray powder patterns reveal that two discrete c dimensions exist with no intermediate value between them. The margaritasite has a (001) reflection at 12.7° (2θ) while that of carnotite lies at 13.8° (2θ); these peaks do not shift. It is likely that there are two distinct phases, perhaps as interlayered lamellae which are intergrown on a smaller scale than can be resolved by the electron microprobe beam.
The discovery of Cs-rich carnotite in the Peña Blanca uranium district provides important evidence for local hydrothermal or pneumatolitic activity during or after uranium mineralization. Data from the geochemical literature indicate that the high Cs:total alkali element ratios required to produce Cs-rich minerals can be generated and sustained only in high temperature environments. Synthesis experiments show that margaritasite can form by reaction of Cs-rich solutions with natural carnotite at 200°C, but the same reaction does not occur or is too slow to be observed in 61 days at 80°C. Nevertheless, it appears unlikely that margaritasite is part of the ore in any Colorado Plateau-type uranium deposit. Reported “carnotite” occurrences from uranium deposits of probable hydrothermal origin are likely sites for new discoveries of margaritasite.