Dolomitization

Dolomitization, process by which limestone is altered into dolomite; when limestone comes into contact with magnesium-rich water, the mineral dolomite, calcium and magnesium carbonate, CaMg(CO3)2, replaces the calcite (calcium carbonate, CaCO3) in the rock, volume for volume. Dolomitization involves recrystallization on a large scale.

This process is described by the stoichiometric equation:

2CaCO₃ (limestone) + Mg²⁺ ⇔ CaMg(CO₃)₂ (dolomite) + Ca²⁺

Dolomitization depends on specific conditions which include low Ca:Mg ratio in solution, reactant surface area, the mineralogy of the reactant, high temperatures which represents the thermodynamic stability of the system, and the presence of kinetic inhibitors such as sulfate.

The dolomite mineral grains often show distinct faces, are of more or less uniform size throughout, and are larger than the calcite crystals in the limestone. When the recrystallization is not complete, the dolomite crystals are scattered throughout a calcite matrix. Sometimes rocks are formed that show patches of dolomite mottling the limestone where the magnesium-rich waters are thought to have filtered through the rock;

In the process of dolomitization the dolomite crystals cut across original calcite grains, fossils, and oölites and sometimes include quartz grains within their boundaries. Dolomites do not preserve the textures of the original limestone nor the fossils therein. Sometimes the only evidence that the limestone was formed from grains of older rocks is scattered quartz grains. Fossils are uncommon in dolomites but sometimes remain as faint shadows outlining the original shape without showing internal detail or as molds with poor detail and filled with tiny dolomite crystals.

Bibliography

• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc
• M. J. Hibbard (1994): Petrography to Petrogenesis. Prentice Hall editore

Rhombohedral crystal of dolomite at the center of a Fusulinid fossil

Partially dolomitized Fusulinid fossils (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 1x (Field of view = 9mm)	Partially dolomitized Fusulinid fossils (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. XPL image, 1x (Field of view = 9mm)	Fusulinid fossils with different degree of dolomitization (to the right you can see a partially dolomitized fossil, in the middle a strongly dolomitized fossil and to the left a fossil completely replaced by dolomite rhombs). Fossiliferous limestone from Tunisia. PPL image, 1x (Field of view = 9mm)
Fusulinid fossils with different degree of dolomitization (to the right you can see a partially dolomitized fossil, in the middle a strongly dolomitized fossil and to the left a fossil completely replaced by dolomite rhombs). Fossiliferous limestone from Tunisia. XPL image, 1x (Field of view = 9mm)	Partially dolomitized Fusulinid fossils (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 1x (Field of view = 9mm)	Fusulinid fossils with different degree of dolomitization (to the right you can see a fossil completely replaced by dolomite rhombs and to the left a partially dolomitized fossil). Fossiliferous limestone from Tunisia. PPL image, 1x (Field of view = 9mm)
Partially dolomitized Fusulinid fossils (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 1x (Field of view = 9mm)	Partially dolomitized Fusulinid fossils (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 1x (Field of view = 9mm)	Partially dolomitized Fusulinid fossils (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 1x (Field of view = 9mm)
Partially dolomitized Fusulinid fossils (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 1x (Field of view = 9mm)	Partially dolomitized Fusulinid fossils (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 1x (Field of view = 9mm)	Partially dolomitized Fusulinid fossils (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 1x (Field of view = 9mm)
Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 2x (Field of view = 7mm)	Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. XPL image, 2x (Field of view = 7mm)	Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 2x (Field of view = 7mm)
Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 2x (Field of view = 7mm)	Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. XPL image, 2x (Field of view = 7mm)	Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 2x (Field of view = 7mm)
Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 2x (Field of view = 7mm)	Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. XPL image, 2x (Field of view = 7mm)	Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 2x (Field of view = 7mm)
Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 2x (Field of view = 7mm)	Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. XPL image, 2x (Field of view = 7mm)	Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 2x (Field of view = 7mm)
Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 2x (Field of view = 7mm)	Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. XPL image, 2x (Field of view = 7mm)	Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 2x (Field of view = 7mm)
Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 2x (Field of view = 7mm)	Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. XPL image, 2x (Field of view = 7mm)	Completely dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 2x (Field of view = 7mm)
Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 2x (Field of view = 7mm)	Partially dolomitized Fusulinid fossils (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 2x (Field of view = 7mm)	Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 2x (Field of view = 7mm)
Rhombohedral crystal of dolomite at the center of a Fusulinid fossil. Fossiliferous limestone from Tunisia. PPL image, 10x (Field of view = 2mm)	Rhombohedral crystal of dolomite at the center of a Fusulinid fossil. Fossiliferous limestone from Tunisia. XPL image, 10x (Field of view = 2mm)	Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 10x (Field of view = 2mm)
Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 10x (Field of view = 2mm)	Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 10x (Field of view = 2mm)	Partially dolomitized Fusulinid fossil (the rhombohedral crystals are dolimite). Fossiliferous limestone from Tunisia. PPL image, 10x (Field of view = 2mm)
Rhombohedral crystals of dolomite on a Fusulinid fossil. Fossiliferous limestone from Tunisia. PPL image, 10x (Field of view = 2mm)	Rhombohedral crystals of dolomite on a Fusulinid fossil. Fossiliferous limestone from Tunisia. PPL image, 10x (Field of view = 2mm)	Rhombohedral crystals of dolomite on a Fusulinid fossil. Fossiliferous limestone from Tunisia. PPL image, 10x (Field of view = 2mm)
Rhombohedral crystals of dolomite on a Fusulinid fossil. Fossiliferous limestone from Tunisia. PPL image, 10x (Field of view = 2mm)	Rhombohedral crystals of dolomite on a Fusulinid fossil. Fossiliferous limestone from Tunisia. XPL image, 10x (Field of view = 2mm)	Rhombohedral crystals of dolomite on a Fusulinid fossil. Fossiliferous limestone from Tunisia. PPL image, 20x (Field of view = 1mm)
Rhombohedral crystals of dolomite on a Fusulinid fossil. Fossiliferous limestone from Tunisia. XPL image, 20x (Field of view = 1mm)	Rhombohedral crystals of dolomite on a Fusulinid fossil. Fossiliferous limestone from Tunisia. PPL image, 20x (Field of view = 1mm)	Rhombohedral crystals of dolomite on a Fusulinid fossil. Fossiliferous limestone from Tunisia. PPL image, 20x (Field of view = 1mm)
Small, white rhombohedral Dolimite crystals. PPL image, 2x (Field of view = 7mm)	Small, white rhombohedral Dolimite crystals. XPL image, 2x (Field of view = 7mm)	Small, white rhombohedral Dolimite crystals. PPL image, 2x (Field of view = 7mm)
Small, white rhombohedral Dolimite crystals. PPL image, 2x (Field of view = 7mm)	Small, white rhombohedral Dolimite crystals. PPL image, 2x (Field of view = 7mm)	Small, white rhombohedral Dolimite crystals. XPL image, 2x (Field of view = 7mm)
Small, white rhombohedral Dolimite crystals. PPL image, 2x (Field of view = 7mm)	Small, white rhombohedral Dolimite crystals. PPL image, 2x (Field of view = 7mm)	Small, white rhombohedral Dolimite crystals. XPL image, 2x (Field of view = 7mm)
Small, white rhombohedral Dolimite crystals. PPL image, 10x (Field of view = 2mm)	Small, white rhombohedral Dolimite crystals. PPL image, 10x (Field of view = 2mm)	Small, white rhombohedral Dolimite crystals. PPL image, 10x (Field of view = 2mm)
Small, white rhombohedral Dolimite crystals. PPL image, 10x (Field of view = 2mm)	Small, white rhombohedral Dolimite crystals. XPL image, 10x (Field of view = 2mm)	Small, white rhombohedral Dolimite crystals. PPL image, 10x (Field of view = 2mm)
Small, white rhombohedral Dolimite crystals. PPL image, 10x (Field of view = 2mm)	Small, white rhombohedral Dolimite crystals. XPL image, 10x (Field of view = 2mm)	Small, white rhombohedral Dolimite crystals. PPL image, 10x (Field of view = 2mm)
Small, white rhombohedral Dolimite crystals. PPL image, 2x (Field of view = 7mm)	Small, white rhombohedral Dolimite crystals. PPL image, 2x (Field of view = 7mm)	Small, white rhombohedral Dolimite crystals. PPL image, 2x (Field of view = 7mm)
Small, white rhombohedral Dolimite crystals. PPL image, 10x (Field of view = 2mm)	Small, white rhombohedral Dolimite crystals. PPL image, 10x (Field of view = 2mm)	Small, white rhombohedral Dolimite crystals. PPL image, 10x (Field of view = 2mm)
Small, white rhombohedral Dolimite crystals. PPL image, 10x (Field of view = 2mm)	Small, white rhombohedral Dolimite crystals. PPL image, 10x (Field of view = 2mm)