Abstract: The fracture systems of Eocene age seem to be revealed volcanic materials and hydrothermal fluids byreactivating possibly in Upper Miocene. Thus hydrothermal neoformation minerals, the products of interactionof thermal water and volcanogenic compounds, are occurred in the west of Sivas. Kaolinite and I-S are dominantly developed in the white-yellow altered volcanic rocks in the Şeyhhalil, Canabdal, Kiremitli and Kavakareas. Besides jarosite, alunite, goyazite, gypsum, barite, hematite, goethite, pyrite, marcasite, gibbsite, sideriteand tridymite minerals are determined in these rocks. Alunite and goyazite from Canabdal area are derived fromK-feldspar. Jarosite appears in the areas observed no alunite and accompanies usually goethite. The alterationsofbiotite and/or opaque minerals are necessary for formation of this mineral. A formation type similar to tonsteins can be proposed for kaolinite and I-S. Here first stage is the hydrolysis of volcanic glass by acidichydrothermal solutions and then the synthesis ofkaolinite or I-S from an intermediate product such a hydratedAl-silicate gel is possibly postulated.