Carbonates and Evaporites

, Volume 16, Issue 2, pp 210–223 | Cite as

The nature and possible origin of dolomite in Ar Rub’ Al Khali, the UAE

  • Mohamed I. El-Sayed


Surface and near-surface sediments in southeastern part of the United Arab Emirates show extensive development of dolomite that is characterized by whitish, dense rocks, partially dissected by veinlets of gypsum and well-developed coarse calcite crystals. The dolomite reveals rather monotonous aphanocrystalline to very finely crystalline crystals (2–20 μm) composed of idiotopic or planar dolomite fabric. Densely packed dolomite aggregates of clotted texture are observed. Moreover, several horizons of peloidal textures are recognized. The fossil content includes freshwater molluscs, foraminifera and gastropods embedded in very fine dolomite crystals., whereas dolomite forming the skeletal grains is considerably coarser (about 40 μm). The most common carbonate porosity type is fossil moldic followed by vug and channel porosity. Interparticle and intraparticle porosity, that may suggest possible dissolution by later diagenetic fluids, are only locally significant. Vari-sized blocky sparry calcite crystals are frequently observed as pore-filling cement.

Authigenic palygorskite of long and short-fiber variety is commonly associated with the dolomite. The fibers generally occur as compacted and dense bundles that show some orientation in the form of thick continuous mats composed of approximately parallel strips. The palygorskite does not show petrographic evidence of precursor carbonate or of any replacement feature.

Chemical analyses of dolomite show that the iron content (between 0.22 to 3.99%) is generally too high to have formed under oxygenated conditions, but are compatible with mixing-zone fluids. Na2O values are relatively low (0.13–0.88%), and Sr generally between 135 to 484 ppm. The Na and Sr values are similar to examples interpreted as non-evaporitic or mixing-zone in origin.

The dolomite described here clearly formed in a near-surface setting and due to the lack of evidence of replacement of precursor grains or minerals, it appears that the dolomite precipitated directly. Occurrence of very finely crystalline dolomite originated by concentration of Mgenriched brines through evaporation results from slow crystallization from highly dilute solutions. Presence of coarse sparry calcite filling pores in dolomite indicates predominance of freshwater conditions after precipitation of dolomite. Alternation between freshwater and hypersaline conditions may occur frequently.


Dolomite Aragonite United Arab Emirate Dolomitization Sparry Calcite 
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Copyright information

© Springer 2001

Authors and Affiliations

  • Mohamed I. El-Sayed
    • 1
  1. 1.Geology Department, Faculty of ScienceCairo University (Beni Suef)Egypt

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