Discussion of the Occurrence and Origin of Sedimentary Palygorskite-Sepiolite Deposits

Abstract

Abstract--Marine and non-marine palygorskite-sepiolite deposits are found throughout the world and occur interbedded with chert, dolomite, limestone, phosphates and other non-detrital sedimentary rocks. The origin of these high-magnesium clays has long been attributed to either alteration of volcanic ash or the structural transformation of smectite clays. More recently, others have argued origin by direct crystallization (neo-formation). Recent laboratory studies support this latter concept, particularly in environments where the concentration of alumina is low, the silica concentration high, and the pH alkaline. Such an origin is proposed for the Georgia-Florida deposits in southeastern United States, since major obstacles exist against formation by alteration of volcanic ash or by transformarion of smectites. Lateritic weathering during the Miocene would have favored direct precipitation of these clays in the shallow, marginal seas. The basinward increase in the MgO: AI~O3 ratio is further support.Deep weathering of crystalline rocks in northern British Honduras and Guatemala would have produced similar high silica, low alumina conditions on the adjacent submerged Yucatan Platform during the late Tertiary. The seaward increase in the MgO:Al203 ratio, the lack of associated detrital constituents, and the absence of associated smectites strongly indicate a similar origin by direct crystallization of these Yucatan palygorsldte-sepiolite clays.Some occurrences of palygorskite and sepiolite may well be related to the alteration of smectite clays or volcanic ash. However, many of the large sedimentary deposits are more probably the result of direct crystallization adjacent to areas undergoing tropical or subtropical weathering.

Engineering geological characterisation of the Barzaman Formation, with reference to coastal Dubai, UAE

“…Palygorskite is found with chert, dolomite, limestone and other non-detrital sedimentary rocks (Isphording 1973). It weathers to smectite when mean annual rainfall is greater than 300mm (Paquet & Millot 1972).…”

Clay mineralogy of Pleistocene Lake Tecopa, Inyo County, California

“…This would indicate that the presence of alumina in the mixture inhibits the formation of sepiolite from solution. Millot (1960), Isphording (1973, and Heron and Johnson (1966) have proposed an authigenic origin by direct precipitation for sepiolite in the natural sediments that they have studied. Precipitation could occur where the pH and MgO contents were high but A12O3 was low or absent.…”

Clay Mineralogy of the Pliocene and Pleistocene of Hole 653A, Western Tyrrhenian Sea (ODP Leg 107)

“…Palygorskite and sepiolite are formed mainly in marginalmarine and lacustrine, chemically restricted environments in arid and semiarid regions (Millot, 1970;Wiersma, 1970;Isphording, 1973;Weaver and Beck, 1977;Chamley et al, 1981;Singer and Galan, 1984). Previous research on the marine Pliocene and Quaternary of the Mediterranean has shown that palygorskite is best considered to be reworked from Paleogene North African sediments (Chamley and Millot, 1975;Chamley, 1975a andb, 1976;Chamley et al, 1977).…”