DOLOMITEDOLOMITE

Abdulkader M. AbedAbdulkader M. Abed

University of JordanUniversity of Jordan

aabed@ju.edu.joaabed@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.