New Data of the Carbonate Onyx's Mineral Composition From the Yevhen Lazarenko Mineralogical Museum's Collection (Gaurdatska Cave, Turkmenistan)

The results of complex mineralogical studies of carbonate onyx from the Yevhen Lazarenko Mineralogical Museum's collection are given. The structural and textural features of the dripstone type carbonate aggregate were established by the methods of optical and electron microscopy. Polarized light microscopy, chemical and X-ray structural methods revealed that the brown color of carbonate onyx probably is coursed by inclusions of organic material in calcite. Microprobe analysis determined the mineral composition and chemical composition of carbonate onyx minerals. The main mineral of carbonate onyx is calcite, which contains 1.0—7.8 wt.% ZnO. In the carbonate onyx, irregularly shaped Mg-Ca-Zn carbonate up to 50 μm in size were also found, the chemical composition of which does not correspond to calcite, dolomite, smithsonite, or minrecordite. It is assumed that based on the stoichiometric ratios of the main components, the unknown Mg-Ca-Zn carbonate may belong to the structural type AB3(CO3)4 and may be isostructural with hanthite CaMg3(CO3)4, and its probable formula is CaZn3(CO3)4.