DOLOMITE Abdulkader M. Abed University of Jordan

General General Formula: CaCO3.MgCO3 Trigonal crystal system Ideal dolomite (stoichiometric): Ca:Mg ions are 1:1 Recent dolomites are called protodolomite where it is not ideal and less crystalline, Ca>>Mg Ancient dolokites Ca:Mg 58:42 Ferron dolomite: few moles of FeCO3 Ankerite: CaCO3.Mg0.5Fe0.5CO3

Dolomite is a rock and mineral name Dolostone may be used for the rock Limestone = 0-10% dolomite Dolomitic limestone = 10 – 50% dolomite Calcitic dolomite = 50-90% dolomite Dolomite = % dolomite. On base of grain size: Dolorudite, doloarenite, dolosparite, and dolomicrite Classification: add the prefix: dolo or dolomitic to Dunham or Folk classification

Under the microscope Dolomites are usually characterized by their rhombic crystal. Fine grained: dolomicrite Coarse grained: dolosparite Dolomite mosaic is either - xenotopic: anhedral with curved crystal boundaries -idiotopic: rhombic See the following table for texture description

Baroque or saddle dolomite Sometimes called pearlspar -Crystals are large, many mm - Conspicuous, curved crystal facies - in thisn sections, curved cleavage, & undulose extiction -Contain inclusions -Associated with sulphide mineralization, hydrthermal activities and hydrocarbons -Characteristic of burial depth

Dedolomitization Is the change of dolomite into calcite. Usually takes place in contact with meteoric water Silicification Is the replacement of dolomite by silica

the origin of dolomites & the dolomitization models Sea water is supersaturated with respect of dolomite. But it is highly ordered, thus inhibiting its precipitation directly from sea water. However, limestones are dolomitized by the Mg rich fluids perculating through the rocks. Following are the dolomitization models

All models rely on 1. Evaporation 2. Higher Mg/Ca ration than seawater 3. Precipitation of aragonite, gypsum/anhydrite. However, there are dolomites that are not associated with evaporites and need another model, this model is dolomitization by sea water itself through the pumping of seawater into the limestones.