Clay minerals in hydrothermally altered volcanic rocks, Eastern Pontides, Turkey

Extensive hydrothermal alteration is observed around volcanogenic massive s ulfide deposits. These deposits are related to Late Cretaceous volcanism in various parts of the Eastern Pontide province. Mineral assemblages resulti ng from alteration consist of mostly clay minerals and silica polymorphs, s ome sulfate minerals, and scarce zeolite minerals. The clay minerals are ka olinite, illite, and smectite. These minerals were examined using X-ray dif fraction (XRD), scanning electron microscopy (SEM)energy dispersive spectro metry (EDS), X-ray fluorescence spectroscopy (XRF), and differential therma l analysis (DTA)-thermal gravimetry (TG) techniques. The illite and the tos eki deposits are a result of hydrothermal alteration of dacitic-andesitic v olcanites. Two groups of bentonite deposits occur; the first mainly formed by hydrothermal solution whereas the second group resulted from halmyrolysi s. The smectite in these alteration zones is generally montmorillonitic in com position and the interlayer cation is mostly Ca and lesser amounts of Na. T he SiO2 and Fe2O3 contents of the hydrothermal bentonites are higher than t hose of the halmyrolysis smectites: however, the MgO content of both groups is similar. The Na2O and K2O contents of both groups are generally <0.5%. The hydrothermal bentonites are not plastic and have open honeycomb microte xtures, although the halmyrolitic smectites are plastic with ultrafine and rod-shaped textures. Illite, which contains some smectite layers, is a 1M p olymorph, and has an asymmetry to the low-angle side of the XRD peaks. The impure illite deposits contain various combinations of smectite, kaolinite and gypsum, galena, sphalerite, pyrite, goetite, and quartz. The illite has >35 wt. % Al2O3. The toseki raw material, which may be possibly useful as a porcelain raw material, is composed mainly of illite, kaolinite and quart z, or illite and quartz. The crystallinity of the kaolinite is poor.