MECHANISM OF K-RHYOLITE AND NA-RHYOLITE ALTERATION AT SERGEEVSKOE (PRIMORSKII-KRAI, RUSSIA) AND PURUIO (CUBA) DEPOSITS OF THE PORCELAIN CLAYS

The mechanism of low-temperature hydrothermal alteration of K- and Na- rhyolites at the Sergeevskoe (Primorskii Krai, Russia) and Purnio (Cub a) deposits was studied by means of electron microscope techniques (va cuum decoration, study of translucent samples, microdiffraction, and e nergy dispersion). The mechanism of rhyolite alteration corresponds to metasomatic replacement and includes two simultaneous conjugate proce sses: the dissolution of rhyolitic minerals and the crystallization of new minerals stable in chan,oed environments in situ. The hydrotherma l alteration of rhyolites at the Sergeevskoe deposit causes the dissol ution of sanidine and the crystallization of iron-containing montmoril lonite and kaolinite. At the Purnio deposit the alteration process res ults in the dissolution of plagioclase (oligoclase-andesine) and the d eposition of Mg-containing montmorillonite and goethite. Quartz in rhy olites of both deposits is preserved unaltered, and the common product of alteration is montmorillonite. Newly formed minerals were deposite d from solutions and replaced magmatic minerals by epitaxial mechanism s without either intermediate amorphous or crystalline phases. The com positions of newly formed montmorillonites and some other minerals dep end not only on the composition of initial rock-forming minerals but a lso on composition of the solution and its circulation velocity as wel l. Despite the same deposition mechanism, montmorillintes from the two deposits studied differ in morphology, structure, and composition.