A microscopic approach to the pedogenic formation of palygorskite associated with Quaternary calcretes of the Adana area, southern Turkey

Abstract

Quaternary calcretes in the Adana area (southern Turkey) are widespread and occur in a variety of forms such as nodular, columnar/tubular, fracture-infll, laminated hardpan, and conglomeratic crusts. Palygorskite associated with calcrete nodules, tubes, and fracture-inflls in the columnar horizon and adjacent host-rock mudstones suggests a cogenetic formation. Te β-fabric (biogenic) constituents and stable isotope values of calcretes support a pedogenic origin for calcretes and also palygorskite. Extensions of palygorskite fbre and fbre bundles from euhedral to subhedral calcite crystals in calcretes and from smectite fakes in adjacent host- rock mudstone suggest an authigenic origin related to calcretisation in which precipitation and dissolution/precipitation are the main processes for palygorskite formation, respectively. Te presence of palygorskite in subspherical aggregates containing calcite crystals, and with smectitic clay coatings around the palygorskite aggregate, similarly suggests a relationship of pedogenic evolution between the 2 clays. Palygorskite precipitated in a vadose zone from strongly evaporated alkaline water, rich in Si and Mg and low in Al and Fe, at elevated temperatures and at an advanced stage of calcrete formation. Development of palygorskite mainly on relict calcite crystals indicates its formation during or shortly following calcite formation in calcretes. Palygorskite precipitation on relative large crystal surfaces has resulted in encrusted and platy fan-like crystal morphologies, whereas its nucleation on the crystal edge relief triggers fbre- like morphologies from crystal edges.

Quaternary calcretes in the Adana area (southern Turkey) are widespread and occur in a variety of forms such as nodular, columnar/tubular, fracture-infll, laminated hardpan, and conglomeratic crusts. Palygorskite associated with calcrete nodules, tubes, and fracture-inflls in the columnar horizon and adjacent host-rock mudstones suggests a cogenetic formation. Te β-fabric (biogenic) constituents and stable isotope values of calcretes support a pedogenic origin for calcretes and also palygorskite. Extensions of palygorskite fbre and fbre bundles from euhedral to subhedral calcite crystals in calcretes and from smectite fakes in adjacent host- rock mudstone suggest an authigenic origin related to calcretisation in which precipitation and dissolution/precipitation are the main processes for palygorskite formation, respectively. Te presence of palygorskite in subspherical aggregates containing calcite crystals, and with smectitic clay coatings around the palygorskite aggregate, similarly suggests a relationship of pedogenic evolution between the 2 clays. Palygorskite precipitated in a vadose zone from strongly evaporated alkaline water, rich in Si and Mg and low in Al and Fe, at elevated temperatures and at an advanced stage of calcrete formation. Development of palygorskite mainly on relict calcite crystals indicates its formation during or shortly following calcite formation in calcretes. Palygorskite precipitation on relative large crystal surfaces has resulted in encrusted and platy fan-like crystal morphologies, whereas its nucleation on the crystal edge relief triggers fbre- like morphologies from crystal edges.