Cathodoluminescence (CL) features of the Anatolian agates, hydrothermally deposited in different volcanic hosts from Turkey

Abstract

Two different types of multi-colored gem-quality agate samples were investigated. They are both found in the same area in the Çubuk–Ankara region of Turkey although the first group is morphologically and geologically distinct from the second, being nodular-shaped agates occurring in cavity-spaces of a rhyolite host rock with an acidic character. They generally do not have any macroscopic inclusions, but the second group of rather block-shaped agates occurs in the fracture-spaces of an andesite host rock with a more neutral character, i.e. of lower free silica content, and they may display pseudomorphic bar-like macroscopic inclusions. Cathodoluminescence results at room temperature were obtained using measurements with alternating current (AC) (at energies of 14 and 24 keV) as well as direct current (DC) (at 14 keV energy), and they display remarkably different patterns between the two types of agates. It reveals a relation between the CL emissions and the presence of some transition metal elements. It is obvious that all trace elements do not play a direct role. Gaussian fitting of the cathodoluminescence AC experimental data at 14 keV energy obtained from the agates of rhyolite host indicates that there are three major spectral emissions, the dominant one being in the longer-visible wavelength region (red region) at about 690 nm. Additionally, two lesser emission lines occur in the middle-visible wavelength region (yellow region) at about 590 nm, and in the smaller-visible wavelength region (blue region) at about 430 nm. In spite of these, the same data from the agates of andesite host indicate that there is only one remarkable spectral emission which is in the in the middle-visible wavelength region (yellow region) at about 590 nm. On the other hand, Gaussian fitting of the cathodoluminescence AC experimental data at 24 keV energy obtained from the agates of rhyolite host indicates that these initial spectral emissions shift from the red and yellow regions to the orange and green regions respectively, even though the emission in the blue region is nearly constant. In spite of these, Gaussian fitting of the cathodoluminescence AC experimental data at 24 keV energy obtained from the agates of andesite host indicates that the initial spectral emission shifts from the yellow region to the green region, but also that a new minor emission develops in the blue region at about 430 nm. It is interpreted that these changes represent a maturation reaction in the microcrystalline quartz structure consisting of a condensation reaction eliminating water between neighboring paired silanol (Si–OH) groups to develop a strained Si–O–Si bond.