Opal Mineralization and Formation Conditions: Silicification Processes of Diatomites from Seydiler Basin (Afyonkarahisar, western Türkiye)

Authors : Evrim Çoban, Gürsel Yanık, Murat Hatipoğlu, Ahmet Yıldız

Pages : 176-201

Doi:10.17824/yerbilimleri.1751771

View : 93 | Download : 155

Publication Date : 2025-12-31

Article Type : Research Paper

Abstract :The Seydiler opal deposit (Afyonkarahisar, Türkiye) represents a rare example of gem-quality, lacustrine-hosted opal formation. This study was carried out to characterize gem quality opals in the Seydiler region using mineralogical, geochemical, isotope analysis and gemological methods. These stratiform, multicolored opals occur within a Miocene-aged, non-marine sedimentary succession influenced by distal volcaniclastic input but lacking direct hydrothermal or magmatic activity. Biogenic siliceous ooze, primarily derived from diatomaceous accumulation, served as the main precursor for opal precipitation through early-stage, low-temperature burial diagenesis. Mineralogical analyses (XRD, FTIR, Raman) confirm that the opals are predominantly composed of pseudo-crystalline silica phases, primarily opal-CT, with minor transitional opal-C and trace amounts of α-quartz. Diagnostic vibrational bands, such as Si–O–Si stretching (~777 cm⁻¹) and bending (~328 cm⁻¹), reflect the progressive structural reorganization of biogenic opal-A under mild burial conditions. Stable oxygen isotope compositions (δ¹⁸O = 23.86–25.29‰ V-SMOW) indicate formation temperatures of ~32–51 °C, consistent with the opal-A to opal-CT transition zone under suboxic, meteoric-fluid–dominated conditions. Geochemical data reveal high silica contents (>93 wt.%) and enrichments in trace elements (e.g., Zn, Cs, Rb, Sr, U), interpreted as products of biological uptake, diagenetic remobilization, and distal Neogene pyroclastic inputs. SEM imaging reveals an abundance of well-preserved diatom frustules, representing genera such as Ellerbeckia, Eunotia, Gomphonema, Halamphora, Pinnularia, and Staurosira/Straurosirella. Three new taxa, Staurosira construens, Straurosirella pinnata 3, and Straurosirella pinnata 6, are reported here for the first time. From a gemological perspective, Seydiler opals fulfill key criteria for gem-quality classification. They exhibit vivid natural color zoning, ranging from reddish-brown to yellowish-green, a vitreous to waxy luster, a refractive index of ~1.451, and specific gravity values that range from 2.18 to 2.30. Representative samples (e.g., S2-8, S2-9) display compact, inclusion-free microtextures. Although minor pigmentary zones linked to Fe–Mn oxides or U-bearing inclusions are present, they do not impair visual clarity. However, hydration-sensitive specimens may be prone to dehydration-induced cracking after extraction, warranting stabilization to preserve gem integrity.
Keywords : seydiler opal, biyojenik opal, göl diatomit

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