Mineralogical, geochemical and gemological investigation of Başibüyüklü (Yozgat) chalcedonies

The study area, which is located in the central part of the Kirsehir Massif, covers the chalcedony formations in the Basibuyuklu Village of the Central district of the Yozgat province and its immediate surroundings. It covers an area of about 80 km2 that falls into the map sheets of YOZGAT-I33-a3, b4, c1, and d2. The Eocene aged basaltic volcanic rocks exposed in the study area host secondary minerals such as chalcedony, quartz, zeolite, and calcite in their cracks and voids. The chalcedony formations in the study area are usually observed as veins that range from a few cm. to 30 cm. or reaching 15 cm. void fillings, with fine band structures, and they are blue and grey. However, the cathodoluminescence (CL) studies revealed that the size of these bands ranged between 150-500 µm. On the other hand, The XRD, FT-IR, and Raman spectroscopy analyses, and the OM examinations revealed that the blue and grey chalcedony samples consisted of silica grains with both length-fast chalcedony and length-slow chalcedony; also, they usually showed fibrous tissues in their microscopic structure. Also, according to the results of the SEM and micro-chemical analyses (SEM-EDS), the chalcedony samples were detected to consist of various silica (flint) particles with a size ranging between 30 nm and 150 nm. In addition, in XRF analysis, a decrease in SiO2 values and an increase in the amount of Al2O3, Fe2O3, MgO and CaO were detected in all chalcedony formations compared to crystalline quartz. Morphological appearances of quartz, epidote and zeolite minerals were observed in the characteristic crystal structures supporting the XRD data. According to the fluid inclusion studies, the formation temperatures of the chalcedony in the region was found to be between 150°C-180°C. To determine their gemological characteristics, the chalcedonies selected in different shades of grey and blue were polished by processing them in cabochon and slices forms, and they were turned into jewellery. The gemological examinations were carried out using a suitable refractometer, UV-VIS-NIR spectrophotometer, gemology microscope, and specific gravity meter. In addition, the current economic potential of the study area was tried to be determined by determining the visible reserve. Keywords: Chalcedony, Gemology, Gemstone, Yozgat