Clays and associated minerals in caves of the Guadalupe Mountains, New Mexico

The clay minerals occurring in caves of the Guadalupe Mountains of New Mexico include kaolinite, dickite, hydrated halloysite, illite, montmorillonite, trioctahedral smectite, and palygorskite. The minerals associated with these clays include alumimte, alunite, natroalunite, jarosite, calcite, dolomite, aragonite, huntite, gibbsite, nordstrandite, goethite, gypsum, hydrobasaluminite, metatyuyamunite, tyuyamunite, quartz, todorokite, and rancieite. The clay minerals found to be authigenic in the caves are hydrated halloysite, montmorillonite, trioctahedral smectite (probably saponite and stevensite), and palygorskite. Hydrated halloysite and alunite are by-products of the sulfuric acid-related speleogenesis that formed many of the larger caves; montmorillonite and illite were altered to hydrated halloysite and alunite. Montmorillonite was found to form in moistened floor and ledge sediments, but these occurrences are not common. Trioctahedral smectite forms in association with Mg-carbonates such as dolomite and huntite in crusts and moonmilks; evolution of the formation of these silicates follows the sequence of carbonate mineral formation. For instance, amorphous silica and poorly formed silicates precipitate with Mg-calcite and aragonite in stalagmites and crusts, while trioctahedral smectite forms in association with the Mg-carbonates. Palygorskite forms in smectite-rich sediments such as the brown laminated silts and green clays of Carlsbad Cavern. Dickite and illite are inherited in wall residues from the dissolution of the cavewall bedrock by condensate or carbon dioxide of the cave air, and/or by biochemical breakdown. Dickite and illite are not cave-authigenic.

40Ar/39Ar dating of alunite from these caves have yielded the formation age of five caves. The apparent ages ranged from 11.3 million years for Virgin Cave located at the higher elevations (2000 meters) to 3.9 million years for the Green Clay Room of Carlsbad Cavern at an elevation of 1090 meters. Alunite ages are strongly correlative with cave elevations. An apparent water table decline of approximately 1100 meters is shown to have occurred from 12 million years ago to the present.