GEOLOGICAL AND MINERALOGICAL CHARACTERISTICS OF THE TEBINBULOQ DEPOSIT: COMPOSITION AND STRUCTURE OF VERMICULITE

Abstract

The Tebinbuloq vermiculite deposit in north-western Uzbekistan represents one of the few large occurrences of hydromica-group minerals in Central Asia. Although the site has been the focus of reconnaissance surveys since the late Soviet period, a detailed petro-mineralogical description that links field geology to the crystal-chemical features of vermiculite has remained unavailable. The present study integrates new field observations, petrographic microscopy, powder X-ray diffraction, scanning electron microscopy coupled with energy-dispersive spectroscopy, and ICP-OES chemical analysis in order to clarify the lithological setting, structural position, and physico-chemical parameters governing vermiculite formation. Results show that vermiculite occurs as lenticular bodies within an alkaline basalt–pyroxenite sequence of Late Jurassic age that has been overprinted by low-temperature hydrothermal alteration. The mineral is characterized by a trioctahedral structure with an average basal spacing of 1.425 nm, a Mg/(Mg + Fe_tot) ratio of 0.68, and a hydration coefficient that stabilizes a three-layer water complex under ambient conditions. Textural relationships point to in-situ biotite alteration under oxidizing, slightly acidic fluid regimes. These findings refine the genetic model of the deposit, highlight the role of pneumatolytic processes in vermiculite genesis, and provide baseline data for assessing technological properties such as thermal expansion and cation-exchange capacity.