**2.2.4.1 Anhydritization**

Petrichenko (1989) stated that the process of anhydritisation of gypsum began with its dissolution. This process is accompanied by the appearance of the nuclei of the new mineral phase – bassanite. During the second stage, bassanite transforms into anhydrite. The structural rearrangement of this mineral occurs, resulting in increase of thickness at the cost of length. Sheets (plates) of anhydrite crystals form with corroded edges. However in case of the presence of anhydrite "nuclei", the bassanite does not form, but anhydrite continues its crystallization at the cost of the calcium sulphate from dissolved gypsum. On the basis of examination of the inclusions in minerals, Petrichenko (1989) determined the conditions of the origin of anhydrite: this process takes place in the presence of concentrated brine solutions and under the conditions of high pressure and temperature, but not above 40- 50°C.

Depending on time and speed of the sulphates transformation there are three kinds of the process: syndepositional, early- and late-diagenetic. The syndepositional anhydritisation occurring during the deposit formation, in shallow basin, sabkhas, in the subsurface environment, causes the substitution of gypsum to take place so fast that the anhydrite remain in its primary form. Anhydritisation during the later stages, according to the solutions of lower salinity, causes the primary sedimentary structures to disappear and the nodular structures to form – gypsum is substituted by incohesive mass of fine anhydritic strips and water, whereas the anhydritisation under the influence of highly concentrated brines can lead to the preservation of the primary gypsum pseudomorphs (Peryt, 1996; Warren, 1999), especially apparent in the coarse-crystalline gypsum forming "the grass-like selenite".
