Origin of Magnesium Clays from the Amargosa Desert, Nevada

Abstract

Deposits of sepiolite, trioctahedral smectite (mixed-layer kerolite/stevensite), calcite, and dolomite, found in the Amargosa Flat and Ash Meadows areas of the Amargosa Desert were formed by precipitation from nonsaline solutions. This mode of origin is indicated by crystal growth patterns, by the low A1 content for the deposits, and by the absence of volcanoclastic textures. Evidence for low salinity is found in the isotopic compositions for the minerals, in the lack of abundant soluble salts in the deposits, and in the crystal habits of the dolomite. In addition, calculations show that modern spring water in the area can precipitate sepiolite, dolomite, and calcite following only minor evaporative concentration and equilibration with atmospheric COs. However, precipitation of mixed-layer kerolite/stevensite may require a more saline environment. Mineral precipitation probably occurred during a pluvial period in shallow lakes or swamps fed by spring water from Paleozoic carbonate aquifers.

Textural Transition and Genetic Relationship between Precursor Stevensite and Sepiolite in Lacustrine Sediments (Jbel Rhassoul, Morocco)

“…The 2nd mechanism is dissohition-crystallization during diagenesis. Many authors describe such a mechanism, as for example Eberl et al (1982) and Khoury et al (1982), who suggest that sepiolites crystallize after dissolution of kerolite/stevensite. In the case of the "Ballarat" formation in California, Post and Janke (1984) think that it is altered saponite that provides Mg and Si necessary for the formation of sepiolite.…”

“…To our knowledge, such TEM observations are uncommon. Nevertheless, according to Khoury et al (1982), SEM images show sepiolite growing from stevensite. Doval et al (1987) also present a genetic relationship between smectite and sepiolite by means of SEM observations.…”

Mg-Smectite Authigenesis in a Marine Evaporative Environment, Salina Ometepec, Baja California

“…The Mg-rich trioctahedral mixed-layer smectitechlorite, corrensite, and chlorite clay mineral assemblages, commonly observed in sediments, are interpreted as forming in ancient hypersaline marine or lacustrine environments (Droste, 1963;Dyni, 1976;Bodine and Standaert, 1977;Tettenhorst and Moore, 1978;Eberl et al, 1982;Khoury et al, 1982;Bodine and Madsen, 1987;Fisher, 1988;Turner and Fishman, 1991;Andreason, 1992;Hillier, 1993). Such Mg-rich assemblages were attributed to initial formation of mixed-layer trioctahedral corrensite or chlorite soon after deposition in hypersaline sediments by either neoformation or transformation of detrital minerals (e.g., Turner and Fishman, 1991;Hillier, 1993 and references therein).…”

Ground water of Yucca Mountain: How high can it rise?; Final report

“…The mineral assemblage--calcite, opal CT, and certain clay minerals--found at shallow levels along some faults exposed in trenches near Yucca Mountain is very similar to the mineral assemblage found in surficial deposits, incluuing pedogenic calcretes and caliches, throughout southwestern Nevada (Khoury et al, 1982;Jones, 1983;Vaniman et al, 1988). This supports an origin for the fault deposits in which wind-blown dust accumulates in fractures where it becomes cemented and crusted over by carbonate and silica precipitated from infiltrating and evaporating surface water.…”