Dolomite

DOLOMITE

Definition.

Ø     It is a carbonate rock of sedimentary origin and is made up chiefly more than 50 percent - of the mineral dolomite which is a double carbonate of calcium and magnesium with a formula of CaMg(C03h.

Ø       Ferrous iron is present in small proportions in some varieties.

Ø     Gypsum also makes appearance in some dolomites.

Ø       But the chief associated carbonate is that of calcium, in the form of calcite.

Texture

Ø          Dolomite  shows  textures  mostly  similar   to   limestones  to  which  it  is  very

often genetically related.

Ø In other varieties, dolomites may be coarsely crystalline, finely crystalline or showing interlocking crystals.

Formation.

Ø           Dolomites are formed in most cases from limestones by a simple process of replacement of Ca++

ions by Mg++ ions through the action of Mg++ ion rich waters.

Ø       This ionic replacement process is often termed dolomitization

Ø     The replacement may have started shortly after the deposition of limestone or quite subsequent to their compaction.

Ø     Direct precipitation of dolomites from magnesium rich waters is also possible.

Ø     Such directly precipitated deposits of magnesium carbonate occur in association with gypsum, anhydrite and calcite.

Ø     It is believed that in such cases, it is the calcite, which is precipitated first, depleting the seawater of CaC03 and enriching it with MgC03.

Ø     The CaMg(C03h precipitates at a later stage.

Ø           Dolomitization  by replacement  method,  however,  is  believed  to  be  the  most

common method of formation of dolomites.

Occurrence

Ø      Dolomite is a widespread sedimentary rock and is found commonly associated with

.limestones.

Ø     It forms intervening layers between limestone formations spread over wide areas.

Ø     Also, it may occur at the extended boundaries of many limestone deposits.

Ø      These indicate locations where' magnesium rich ground waters could have an easy

access for the replacement process to take place in an original limestone" rock.

Ø Dolomite is so closely related to limestone in composition, texture, structure and physical properties that it may not always be easily possible to differentiate between the two rocks in hand specimens.