Authigenesis of trioctahedral smectite in magnesium-rich carbonate speleothems in Carlsbad Cavern and other caves of the Guadalupe Mountains, New Mexico

Trioctahedral smectite is a constituent of Mg-rich carbonate crusts and moo nmilks (pasty deposits) in caves of the Guadalupe Mountains of southeastern New Mexico. Energy dispersive X-ray microanalysis of individual crystallit es and their aggregates along with the X-ray diffraction analysis indicates that the smectite is probably stevensite. Saponite is likely present in so me samples also. The smectite is intimately associated with dolomite crusts and huntite moonmilks in Carlsbad Cavern, Lechuguilla Cave, and other dolo stone caves. Clay particles appear as fibers and films, with aggregates com prising decimicron-sized filamentous masses that envelop crystals of dolomi te, huntite, and magnesite. The occurrence of smectite is related to the ge nesis of the Mg-rich carbonate minerals. In water films, progressive evapor ation and carbon dioxide loss results in the sequential precipitation of Mg -rich calcite, aragonite, dolomite, huntite, and magnesite. This sequence o f carbonate precipitation removes Ca and greatly increases the Mg/Ca ratio in the solutions. Silica is commonly available probably because of high pH conditions, and consequently, smectite forms in the Mg-rich alkaline enviro nment. Along with the Mg-rich carbonate minerals, opal, quartz, and uranyl vanadates may precipitate with the smectite.