Formation and stability conditions of carbonate minerals in some arid soils of Egypt were studied. The soils are generally calcareous with prominent carbonate accumulations at different depths. In some cases they are satisfying the requirements of calcic or petrocalcic horizons. The Northwestern coast at Mersa Matruh area is selected for the study. The parent rock of the soils is the upper Miocene limestone and the parent materials are transported along Wadis to the coastal plain and alluvial fans. Mineral composition of the carbonates of the soils include Calcite and Ankerite Ca(Mg.Fe)(CO3)2 in general, together with oxide minerals as quartz and CaSi2O5. Dolomite and Magnesite are suspected in another trace of profile 1 in the same region. The most important elemental composition of the soils has the following sequence Si > Ca > Al > Fe which is corresponding to the texture and parent materials. Important trace elements (ppm) were found in the analyses such as S, Cr, Zn, As, Zr, Ni and Rb.
Soil gels extracted were found of negligible amounts denoting poor weathering of silicate minerals in general.
Stability conditions as exemplified by dissolution-precipitation processes are greatly affected by acid and redox conditions. All samples are falling in the slightly alkaline- positive oxidation fields (aerobic condition) in the plotted diagrams.