Abstract

This is the first study of the gypsum crystals habits in the famous, large, inland Dukhan Sabkha in the Qatar Peninsula. The study seeks to identify the various habits and vertical and horizontal distribution of the crystals in the sediments of the Sabkha. Forty four sediment samples rich in crystals, six shallow cores and twenty two brine samples were collected for the study. The factors that affected the crystals formation and shape are discussed in the study.Crystal formation speed and the reasons for the increase or decrease the growth rate identified. Gypsum crystals of various habits and sizes are formed in the surface crust and at shallow depths in the Sabkha sediments. The crystals are of acicular, prismatic, lenticular and sub-lenticular, inter-grown sub-lenticular, pyramidal,elliptical and semi-elliptical and pseudo-tetragonal shapes.The proportion and habits of fine gypsum crystals in the Sabkha is large. Lenticular and Sub-lenticular crystals are dominant in the sediments, especially in the northeastern part of the Sabkha.The proportion of sandy desert rose crystals (about 5 cm length) is rare and only found in the northeastern part of the Sabkha, to the west from the salt lake area; the crystals formed above groundwater level and at shallow depth, few centimeters from the surface.Shallow groundwater level, saturated of sediments by brines, high temperature, high evaporation and saturated of brines by SO4 -2 and CaO3 2- are provide suitable condition for formation of gypsum crystals in the Dukhan Sabkha sediments. In addition, the general elevation of Dukhan Sabkha below sea level (-1) is lead to greater rates of evaporation over groundwater recharge in the Sabkha. The crystals are formed either as a result of evaporation of sewage brines within the Sabkha or as a result of dissolved previous gypsum in the rocks bed within the Sabkha and re- concentrated SO4-2 and Ca2+ in the Sabkha brines. In some locations, where anhydrite dominant, gypsum crystals could also formed as a result of anhydrite dehydration. The main conclusion of this study are: (1) The conditions for formation of gypsum in the Dukhan Sabkha are available and accordingly various shapes of gypsum crystals formed in the surface and at various depths in the Sabkha deposits; (2) This Sabkha is suitable place for establishing a project for cultivation of this important mineral and exploiting it economically; and (3) This work can be used as model for the study and utilization of the inland Sabkha in Qatar and neighboring countries.

Keywords: Gypsum crystals; Habits; Fluid loss; Dukhan sabkha; Brine chemistry