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Gypsum and halite are the most common salts in reg soils developed on alluvial parent material under extremely arid conditions in the Negev Desert, Israel. The aim of this paper is to document the changes in the micromorphology of these salts at different stages of Reg soil development on two alluvial fan chronosequences. The micromorphological analyses included thin section observations and scanning electron microscope and electron microprobe analyses.

In this arid soil environment, gypsum and halite possess a variety of crystal forms which may change with depth in a single profile and/or between profiles of different ages. The variety of crystal forms results from changes in the microenvironmental conditions that occur in desert reg soils over time. Poikilitic lenticular gypsum is found in all Reg soils and is distributed throughout the profiles. The conditions needed for such crystals to form are high ionic impurities and deposition in a void system where space is not limiting. Microcrystalline alabastrine gypsum is only found in mature Reg soils and is crystallized when the profile has high amounts of fine material and a well-developed desert pavement. In a well-developed Reg soil, profile indicators, such as a well-developed desert pavement and high amounts of fine earth, limit the leaching depth and cause gypsum deposition from supersaturated soil solutions under high evaporation rates close to the surface. Prismatic and fibrous gypsum are less common. Low amounts of prismatic gypsum are found in young and mature soils while fibrous gypsum is found only in mature soils in re-cemented shattered gravel.

The halite crystal form is mainly cubic with low amounts of host material incorporated into the crystal. It occurs predominantly in mature Reg soil profiles through the crystallization from supersaturated soil solutions at the depth of maximum water penetration. Although the alabastrine, prismatic and fibrous gypsum and cubic halite are deposited in a displacive manner, no correlation was found between their occurrence and the distribution of shattered gravel in the soil profile.