Mineralogy, geochemistry, and genesis of bentonites in Upper Cretaceous pyroclastics of the Bereketli member of the Res¸adiye Formation, Res¸adiye (Tokat), Turkey

Kadir S., Külah T., Erkoyun H., Önalgil Uyanık N., Eren M., Elliott W. C.

Applied Clay Science, no.204, pp.1-18, 2021 (SCI-Expanded)

  • Publication Type: Article / Article
  • Publication Date: 2021
  • Journal Name: Applied Clay Science
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus, Academic Search Premier, PASCAL, Aerospace Database, CAB Abstracts, Chemical Abstracts Core, Communication Abstracts, Index Islamicus, INSPEC, Metadex, Pollution Abstracts, Veterinary Science Database, Civil Engineering Abstracts
  • Page Numbers: pp.1-18
  • Eskisehir Osmangazi University Affiliated: Yes


Bentonites have been found widespread within the Bereketli member of the Cretaceous Res¸adiye Formation,

central Anatolia. These bentonites were hypothesized to have been formed by the in-situ alteration of parent

rhyolitic, dacitic, and trachyandesitic pyroclastics in a shallow marine environment during early diagenesis

shortly after deposition. Both Na- and Na-Ca-smectites were observed. The bentonites contained heulanditeclinoptilolite,

kaolinite, plagioclase/K-feldspar, mica, quartz, opal-CT, calcite, and in some samples, goethite.

The chemical analyses of smectite-rich clay fractions were montmorillonitic in composition. The average

structural formula is: (Ca0.13Na0.40K0.05)(Al2.98Fe0.41Mg0.50Ti0.04Mn0.002)(Si7.90Al0.10)O20(OH)4. Smectite was

formed from the devitrification of glass shards and dissolution of pyroclastic feldspars, and mica during early

diagenesis under alkaline conditions. Certain whole-rock major and trace element signatures supported the

formation of smectite from pyroclastic progenitors. The breakdown of pyroclastic hornblende and biotite was

inferred from the enrichment in Fe2O3 and MgO in the clay fractions. The devitrification of glass to clay and the

dissolution of K-feldspar and plagioclase contributed to the observed negative correlation of SiO2 vs. Al2O3,

Fe2O3 + TiO2, and MgO as well as the negative Eu anomaly. These bentonites originated from rhyolitic to trachyandesitic

pyroclastic rocks based on the Sc/Th and Co/Th, Th/U, Nd/Sm, Zr/Hf, La/Sc, Nb/Ta, Zr/Hf ratios.

The variabilities of the UCC-normalized elemental results were most pronounced for the elements whose solubilities

were high in the conversion of tuff to bentonite (alkali and alkaline earth elements). The variabilities seen

in the UCC normalized lanthanide elements likely reflected the lanthanide contents of the non-clay minerals in

the bentonite and pyroclastic parent materials. These bentonites are presently mined as important raw materials

for the drilling and molding industries in Turkey.