Geology and Mineralogical Properties of Dereyalak Opal Occurences -Eskişehir, NW Türkiye

Çalik A.

76th Geological Congress of Türkiye with International Participation, Ankara, Turkey, 15 - 19 April 2024, no.155, pp.178

  • Publication Type: Conference Paper / Summary Text
  • City: Ankara
  • Country: Turkey
  • Page Numbers: pp.178
  • Çanakkale Onsekiz Mart University Affiliated: Yes


This study investigates the geology, mineralogy, petrography, and geochemical properties of opal formations situated in the northwest of Dereyalak village, in the southwest of Eskişehir province. While previous studies have examined the Dereyalak agates in the region, this study represents the first detailed investigation of the mineralogy and geochemical properties of opals in the area. Furthermore, the characteristics of opal nodules were compared to those of agate formations found in the region. Opal nodules are found within Pliocene conglomerates of volcanogenic and/or carbonate. Through field and petrographic studies, opals were found to be nodule in form, ranging in size from 2-10 cm, and generally white, yellowish, beige, and transparent in color. Dendritic manganese oxide (pyrolusite) inclusions are visible in certain opal nodules, such as those found in agate nodules in the region. The X-ray diffraction (XRD) analysis revealed that Dereyalak opals are primarily composed of opal-CT, along with quartz (chalcedony), moganite, and opal-C. Furthermore, sepiolite peaks were identified in the X-ray diffraction (XRD) patterns of the opal samples. In the scanning electron microscope (SEM) analysis, the tissue was identified as spherical and fibrous. The fibrous texture found in opal nodules resembles the fibrous texture observed in Dereyalak agates. Previous research, field observations, and mineralogical and geochemical analysis have indicated that opals in Dereyalak may have formed through processes similar to those of the agates found in the region. Opal and agate nodules found near fault and fracture systems are thought to have formed when silica-rich hydrothermal fluids at low temperatures replaced sepiolite and magnesite nodules. Keywords: Agate, Dereyalak, Eskişehir, opal.