The clay mineral geochemistry of one palygorskite clay deposit in east China

There are three kinds of palygorskite deposit, which are formed respectively from weathering, alteration and deposition. An important type is deposition by crystallization from solution in a shallow lake, for which the characteristic X-ray diffraction and infrared spectra indicate higher crystallinities than for palygorskites formed by weathering of basalts and alteration of volcanoclastic materials. The electron micrographs showed the particles of deposited type are elongated tubular fibres and the aggregates are spherules. The main oxides, trace elements and rare earth elements have diverged during weathering of basalts and formation of palygorskites, so that three kinds of geochem-ical environments can be distinguished during palygorskite formation : (I) a SiO₂-Al₂ O₃-MgO-Fe₂O₃-H₂O system for weathered palygorskite and montmorillonite in weathering basalts, (II) a SiO₂-MgO-Al₂O₃ H₂O system for deposited palygorskite associated with opal and quartz in the submarginal environment, (III) a CaO-MgO-SiO₂-CO₂-H₂O system for deposited palygorskite associated with dolomite in the farshore. Palygorskite formation and preservation is favoured as the contents of SiO₂ and MgO increase and Al₂O₃ decrease.