Opal mineralization from Cigel’ locality (Central Slovakia)
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Besides the famous precious opals from Eastern Slovakia, the Neogene volcanic field in Central Slovakia shows numerous localities with various types of opals. Unfortunately these localities contain only common opal (or potch opal), but moreover, there are only few information on mineralogical composition or genesis of such opal types in Slovakian geological literature. Common opals are bound to all types of volcanic rocks and their pyroclastics. They can be found in fissures and cracks in basaltic, andesitic as well as in rhyolitic rocks. The Cigel' locality was chosen from the number of different opal localities based on the well preserved cross sections through the andesitic volcanic rocks and redeposited pyroclastics of Vtacnik Formation (Śimon et al. 1997) Opals can be found in irregular nodules of variable size in weathered redeposited pyroclastic materials as well as in fissures and cracks in fresh andesitic rocks. Opals are often associated with greenish clays, which are often intimately overgrown with opal, or they create a thin crust around the opal nodules. Also they create infilling of fissures in weathered pyroclastic material. Selected samples of opal were studied using optical microscopy, powder X-ray diffraction analyses (PXRD), scanning electron microscopy (SEM), infrared spectroscopy (IR) and electron microprobe (EMPA) in order to determine their exact mineralogical and chemical composition. Both types of opals samples (nodules and infilling of cracks in fresh andesite) shows typical PXRD pattern for opal-CT (Floerke et al. 1991, Graetsch 1994) with presence of expandable layer silicates. Etched surfaces of samples show presence of lepispheres (Floerke et al. 1975) of opal-CT in SEM. Based on IR analyses the associated clays consist mainly of nontronite, kaolinite was detected in lower amount. Chemical composition of opals is quite variable. Samples are not homogenous, they show two different phases which are either poor or rich on trivalent compounds. With increasing amount of trivalent compounds the content of SiO2 is getting lower, but the concentration of divalent and monovalent impurities is significantly getting higher. As for comparison phases poor on trivalent compound shows concentration of A12O3 up to 0.01 wt. % and Fe2O3 up to 0.2 wt. %. The concentration of SiO2 in this phase reaches up to 99.2 wt. %. Phase rich on trivalent compounds shows concentration of A12O3 up to 2.7 wt. % and 23.3 wt. % for Fe2O3. SiO2 concentration reaches 64.3-83.4 wt. %. Besides the opal phases, also impurities with high concentrations of MnO2 and Fe2O3 were identified. Sums over 100 wt. % belong to moganite, which was subsequently identified by the means of Raman spectroscopy. Based on the field observations, detected mineralogical features and chemical composition, an infiltration-weathering formation of opal was proposed. The proposed process includes low-temperature hydration of volcanic glass (smeetititzation) in solid state (Śamajova et al. 1992, Velde & Meunier 2008). Access of silica is transported by descending fluids and subsequently the opal can be precipitated in fissures or in cracks (Koivula et al. 1983, Horton 2002) between the weathered pyroelastic materials or in cracks and fissures in fresh andesite.
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