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Dissolution rate (K) of gypsum rocks of Fat'ha Formation (Middle Miocene) was determined at three different environments; closed system, that is still water and open system, that is flowing water and river water.In closed system, the dissolution rate is not a constant; it increases as the concentration (C) of the dissolving solution. In an open system, dissolution rate is a constant at constant flow velocity (V) and given temperature (T). It increases with an increase of either of them; however, the effect of (V) on (K) is much more pronounced than that of (T).Published data on the value of dissolution rate (K), based on laboratory experiments, are exaggerated by a factor of (4.1) to (4.6) compared with values of the present study. This is possibly due to the fact that these data represent dissolution through fissures in gypsum rocks and being far from natural environments for gypsum dissolution.

The mineralogy, petrography and chemistry of two local plasters (Rasheed and Madae'n) and a dental stone were studied. Mineralogical study by x-ray diffraction spectrometer proved that the three gypsum products consist mainly of bassanite or hemihydrate (CaSO 4 . 0.5 H 2 O) with minor amounts of gypsum and anhydrite. The petrography of the studied thin sections, reveals that the two local plasters consist of dispersed prismatic, monoclinic crystals with ill-defined edges (bassanite of the β-hemihydrate). Dental stone consists of more packed, prismatic monoclinic crystals with well-defined and sharp edges (bassanite of the α-hemihydrate). Compressed powders of the two local plasters improved their crystal shapes and edges and made their outlines similar to dental stone or (α-hemihydrate).Chemical analyses of Rasheed and Madae'n plasters suggest that they consist of more than (96%) bassanite with small amounts of gypsum and anhydrite. Based on these compositions, the two local plasters are suitable for moulding, casting and constructional purposes. They are unsuitable for surgical plasters.

A total number of (32) marl samples of Fat'ha Formation (Middle Miocene) was collected from four selected localities in northern Iraq. Geochemical study included the analysis of ( 13) major and minor oxides by x-ray fluorescence and wet chemical methods and ( 7) trace elements by (XRF). The study shows that the oxides (SiO