Characterization of Anhydrite Soil Discovered in the Coastal Sabkha of Abu Dhabi Emirate

Soil inventory of the coastal sabkha of Abu Dhabi Emirate was completed using Landsat ETM image acquired on 2000 & 2002 and IRS image acquired on 2004 and 2006 and through field survey and classification. During this soil inventory, a soil rich in anhydrite (CaSO4) was discovered as a separate map unit with unique occurrence over a water table. The present study aims at characterizing chemical and mineralogical composition of anhydrite soil and linking it with the environmental conditions that maintains its stability. The study also suggests field and analytical procedures to differentiate anhydrite from gypsum. This soil has unique properties and occurs in the form of a well defined pure map unit (consociation) as well as minor component of other soil units (complexes). The solution chemistry confirmed that the soil is strongly saline and sodic (Na rich) and dominated by Na+ > Mg2+ > Ca2+ > K+ and Cl- > SO42- > HCO3-. The pH is in the neutral. X-ray diffraction analysis of the anhydritic layer confirmed anhydrite as dominant, with minor amounts of halite, calcite, and quartz, and no gypsum. A striking phenomenon observed is that the anhydrite mineral remains stable and does not convert to gypsum upon exposure to hypersaline water, this is possibly due to high salinity coupled with high temperature. Anhydrite could form from direct precipitation from solution, dehydration of gypsum and it is speculated that the salts in soil matrix with anhydrite has more attraction to water molecules than anhydrite, however, this is an area for further investigation. Anhydrite layer is calcareous and has high water holding capacity. The USDA Soil Taxonomy lacks anhydrite soil description; therefore, a proposal was submitted for addition of 1) anhydritic diagnostic horizon; 2) anhydritic soil mineralogy class prior to gypsic and 3) at subgroup level as anhydritic aquisalids.