DOLOMITEDOLOMITE
Abdulkader M. AbedAbdulkader M. Abed
University of JordanUniversity of Jordan
[email protected]@ju.edu.jo
GeneralGeneralFormula: CaCO3.MgCO3
Trigonal crystal systemIdeal dolomite (stoichiometric):
Ca:Mg ions are 1:1Recent dolomites are called protodolomite
where it is not ideal and less crystalline, Ca>>Mg
Ancient dolokites Ca:Mg 58:42Ferron dolomite: few moles of FeCO3
Ankerite: CaCO3.Mg0.5Fe0.5CO3
Dolomite is a rock and mineral nameDolomite is a rock and mineral nameDolostone may be used for the rockDolostone may be used for the rock
Limestone = 0-10% dolomiteLimestone = 0-10% dolomiteDolomitic limestone = 10 – 50% dolomiteDolomitic limestone = 10 – 50% dolomite
Calcitic dolomite = 50-90% dolomiteCalcitic dolomite = 50-90% dolomiteDolomite = 90-100% dolomite.Dolomite = 90-100% dolomite.
On base of grain size:On base of grain size:Dolorudite, doloarenite, dolosparite, and Dolorudite, doloarenite, dolosparite, and
dolomicritedolomicriteClassification: add the prefix: dolo or Classification: add the prefix: dolo or
dolomitic to Dunham or Folk classificationdolomitic to Dunham or Folk classification
Under the microscopeUnder the microscopeDolomites are usually characterized Dolomites are usually characterized
by their rhombic crystal.by their rhombic crystal. Fine grained: dolomicriteFine grained: dolomicrite
Coarse grained: dolospariteCoarse grained: dolospariteDolomite mosaic is eitherDolomite mosaic is either
- xenotopic: anhedral with curved - xenotopic: anhedral with curved crystal boundaries crystal boundaries -idiotopic: rhombic-idiotopic: rhombic
See the following table for texture See the following table for texture descriptiondescription
Baroque or saddle dolomiteBaroque or saddle dolomiteSometimes called pearlsparSometimes called pearlspar
--Crystals are large, many mmCrystals are large, many mm- Conspicuous, curved crystal facies- Conspicuous, curved crystal facies- in thisn sections, curved cleavage, - in thisn sections, curved cleavage,
& undulose extiction& undulose extiction-Contain inclusions-Contain inclusions
--Associated with sulphide Associated with sulphide mineralization, hydrthermal activities mineralization, hydrthermal activities
and hydrocarbonsand hydrocarbons-Characteristic of burial depth-Characteristic of burial depth
DedolomitizationDedolomitizationIs the change of dolomite into Is the change of dolomite into
calcite.calcite.Usually takes place in contact Usually takes place in contact
with meteoric waterwith meteoric waterSilicificationSilicification
Is the replacement of dolomite Is the replacement of dolomite by silicaby silica
the origin of dolomites & the the origin of dolomites & the dolomitization modelsdolomitization models
Sea water is supersaturated with Sea water is supersaturated with respect of dolomite.respect of dolomite.
But it is highly ordered, thus But it is highly ordered, thus inhibiting its precipitation directly inhibiting its precipitation directly
from sea water.from sea water.However, limestones are dolomitized However, limestones are dolomitized
by the Mg rich fluids perculating by the Mg rich fluids perculating through the rocks.through the rocks.
Following are the dolomitization modelsFollowing are the dolomitization models
All models rely on All models rely on 1. Evaporation 1. Evaporation
2. Higher Mg/Ca ration than seawater2. Higher Mg/Ca ration than seawater3. Precipitation of aragonite, 3. Precipitation of aragonite,
gypsum/anhydrite.gypsum/anhydrite.However, there are dolomites that However, there are dolomites that are not associated with evaporites are not associated with evaporites
and need another model,and need another model,this model is dolomitization by sea this model is dolomitization by sea water itself through the pumping of water itself through the pumping of
seawater into the limestones.seawater into the limestones.