الملخص

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 (CaSO4. 0.5 H2O) 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.