PETROGAPHIC CHARACTERIZATION OF DOLOMITIC ROCKS FROM EL HEFHUF FORMATION, WADI HENNIS, FARAFRA OASIS, WESTERN DESERT OF EGYPT MOHAMED I. EL-SAYED Dept. Geology, Fac Science Beni Suef Cairo Univ., Egypt El Hefhuf Formation in Wadi Hennis of Farafra Oasis is observed as ridges in both sides of the main wadi where it is completely eroded from the surface by the action of water and wind. Petrographically, El Hefhuf Formation is composed of dolomite occurs predominantly as euhedral rhombohedra scattered in iron oxy-hydroxidess and micrite matrix or clustered in patches. Dolomite textures include inclusions-free and inclusion-rich crystals containing eight textural types. Three post-depositional changes are observed in El Hefhuf Formation: dolomitization, dissolution and dedolomitization. The possible sources of Mg ions required to initiate dolomitization could be seawater or connate Mg bearing water circulating through sediment or both. Dissolution in dolomite is common and is an integral part for the development of Wadi Hennis. Extensive dissolution is seen as slight etching on crystal surfaces and is inferred from hollow-centered dolomite crystals. Dedolomitization is observed in cloudy core with clear rim dolomite that shows partial replacement in the inner cores of individual crystals. This phenomenon is observed near the top of the formation in which dolomite have dedolomitized cores, yet the rims are apparently unaffected. Oxidation of pyrite may be responsible for the dedolomitization due to its abundance in El Hefhuf dolomite as a whole and its scarcity in the dedolomitized areas, where the original pyrite was consumed during the course of dedolomitization.