Mineralogical investigation and some sedimentary phenomena of Ubhur formation, North Jeddah, Saudi Arabia

الملخص EN

Clay and evaporite components of Ubhur Formation are investigated.

Their mineralogy is studied by X-ray diffraction, differential thermal and thermogravimetric analyses.

The clay deposits are found to be made up mainly of montmorillonite (Ca++ and / or Mg++ rich variety) with lesser amounts of illite and kaolinite while chlorite is rarely recorded in minor amounts.

Spheroidal weathering believed to be formed as a result of expansion of montmorillonite by simply absorbing water, combined with leaching of some coarse ingredients and some mineralogical changes, characterizes the clay outcrops.

Evaporite deposits are represented by two gypsum varieties; one comprising massive, finely crystalline gypsum, the other pure selenite.

The uppermost exposed selenite shows diagenetically-formed desert roses.

It is suggested that they were formed as a result of dissolution of calcium sulphate in percolating groundwater, which was drawn to the surface and precipitated gypsum during very dry spells.

Results of thermogravimetric analysis of different prepared selenite concentrations gave a perfect linear relation between concentration and TG loss percent, which can be used to estimate precisely the gypsum content in any unknown sample.