Margaritasite, a Cs-rich analogue of carnotite, is newly discovered. It 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 fine-grained yellow mineral and, with the possible exception of the index of refraction, is optically indistinguishable from carnotite. Margaritasite is most easily recognized by X-ray diffraction through a shift in the (001) reflection representing an increase, relative to carnotite, in the c dimension. This increase is due to the large Cs atom in sites normally occupied by K in the carnotite structure. This Cs-K uranyl vanadate, with Cs:K about 5, is the natural equivalent of the compound Cs 2 (UO 2 ) 2 V 2 O 8 synthesized by fusion (Barton, 1958). Formula (Cs (sub 1.38) (H 3 O) (sub 0.35) K (sub 0.29) ) (sub Sigma 2.02) (UO 2 ) (sub 1.99) (V 2 O 8 ) (sub 1.00) .1.07H 2 O corresponding to the generalized formula (Cs,K,H 3 O) 2 (UO 2 ) 2 V 2 O 8 .nH 2 O where Cs>K,H 3 O and n nearly equal l. Unit cell parameters are a = 10.514, b = 8.425, and c = 7.25Aa,beta = 106.01 degrees (P2 1 /a,Z = 2). Microprobe analyses of margaritasite and Cs-enriched carnotites suggest a solid solution, 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 degrees (2theta ) while that of carnotite lies at 13.8 degrees (2theta ); 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. Local hydrothermal or pneumatolitic activity during or after uranium mineralization. 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 degrees C, but the same reaction does not occur or is too slow to be observed in 61 days at 80 degrees C. 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.--Modified journal abstract.