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Dolomite (mineral)

Trigonal

CaMg(CO3)2

Dolomite [CaMg(CO3)2] is the second most abundant carbonate in carbonate rocks after calcite. Dolomite is named after Déodat Gratet de Dolomieu (1750 – 1801), a French geologist who first described this mineral and the carbonate rocks of the Dolomites, in Northern Italy, which are also named after him. Except for some relatively rare primary dolomite, dolomite occurs mostly as a diagenetic mineral in carbonate sedimentary rocks (in particular: dolomite or dolostone).

Structure and chemistry
The structure of dolomite is similar to that of calcite. It has a face-centered rhombohedral cell with alternating Ca2+ and Mg2+ cations and (CO3)2- anions. Dolomite has a slightly lower trigonal symmetry than calcite. The chemical composition of natural dolomite is close to the CaMg(CO3)2 end-member, but it can incorporate some Fe2+, since a continuous solid solution with ankerite [CaFe(CO3)2] exists. Other elements that may enter the structure of dolomite are Mn, Zn, and Pb.

dolomite cell

The rhombohedral cell of dolomite, characterized by a regular arrangement of calcium – magnesium cations and bicarbonate anions, with a structure similar to that of calcite. Graphics: Samuele Papeschi/GW.

The trigonal symmetry of dolomite permits hundreds of different habits, which are combinations of rhombohedral (top), prismatic (lower left corner), scalenohedral (bottom, center), and tabular forms. Dolomite habit is virtually identical to those of calcite. Modified after Encyclopædia Britannica, 1911.


Group of dolomite crystals with rhombohedral habit and visible rhombohedral cleavage. Azcarate Quarry, Eugui, Navarre, Spain. Size: 8.0 x 4.9 x 4.4 cm. Photo by Robert M. Lavinsky.

Properties
Habit: rhombohedral, scalenohedral, prismatic, tabular, fibrous, acicular
Hardness: 3.5-4
Cleavage: {10-11} perfect rhombohedral cleavage
Twinning: {0001} {10-10} {11-20} lamellar twinning: common; {10-11}: rare; {02-21} glide twinning
Color: colorless to grey/white, alters to yellows and browns
Luster: vitreous, pearly
Streak: white
Alteration: dissolves in slightly acid waters
In thin section…
ε: 1.500-1.520
ω: 1.679-1.703
Color: colorless
Pleochroism: strong relief pleochroism
Birefringence (δ): 0.179-0.185 (fifth order colors)
Relief: high
Optic sign:
[Mindat]

Field features
Dolomite occurs in many carbonate rocks like the homonymous dolomite (alternatively known as dolostone) and dolomite marble. Dolomite often coexists with calcite, which has nearly identical rhombohedral habit, rhombohedral cleavage, and color (colorless of white). Differently from calcite, dolomite often contains some Fe, which may produce yellowish or brownish tints due to alteration. Moreover, dolomite does not produce a fizzy reaction with HCl and has a slightly higher hardness than calcite (3.5-4) that still makes it susceptible to scratch from glass and metal. A dolomite powder, however, still produces a fizzy reaction in contact with HCl, due to the increased reaction surface.

dolomite rock

Except for the lack of fizzy reaction in contact with HCl, dolomite-bearing rocks may appear very similar to other carbonates. Dolomite marble from Fauske, Norway. Width: 14 cm. Photo © Siim Sepp.

Dolomite in thin section
In thin section, dolomite shows very high relief and it is colorless at PPL, displaying strong birefringence with characteristic very high five order colors at CPL. It shows a perfect rhombohedral cleavage and very commonly shows lamellar twinning. These features make dolomite nearly identical to calcite [advice: check for dolomite in the hand sample before preparing the thin section]. The more straightforward way to distinguish calcite from dolomite is to use alizarin red staining, which colors calcite in pink and leaves dolomite unstained. Another way to identify dolomite from calcite is to look at lamellar cleavage patterns in euhedral crystals. Both calcite and dolomite show lamellar twins parallel to the rhombohedral edges ({0001} twins), but calcite only shows twinning parallel to the long axis of the rhomb ({10-12}), whereas in dolomite twin planes occurs along both the long and short axis of the rhombs ({11-20} and {02-21}). However, these twins are enhanced by deformation (therefore more common in metamorphic rocks) and these observations require euhedral grains (very rare in calcite and dolomite aggregates).

dolomite alizarin red

Rhombohedral dolomite crystals, highlighted by staining with Alizarin Red S (dolomite remains unstained). The red stuff surrounding them are largely ooids made of calcite. The blue material is ferroan calcite cement that has been stained with K ferricyanide. PPL image. Photo by Della Porta and Wright (2009), www.carbonateworld.com. Carbonateworld: a web tutorial for the petrographic analysis of carbonate rocks. A gallery of 85 images of dolomite is available at this link.

twins in calcite and dolomite

Twinning planes in calcite and dolomite. Modified after Tulane.edu (prof. Stephen A. Nelson).

CPL
CPL
CPL
PPL
PPL

This is dolomite, not calcite, because there are lamellar twins parallel to the short axis of the rhomb. Dolomite crystal from a high-pressure schist. Width: 2.5 mm. Photo by Atlas of Metamorphic Minerals (earth.ox.ac.uk).

CPL
CPL
CPL
PPL
PPL

Tiny euhedral crystals of dolomite surrounded by microcrystalline quartz in a very low-grade calcschist. Width: 1 mm. Cala dell’Alga, Cavo, Island of Elba, Italy.

CPL
CPL
CPL
PPL
PPL

Lamellar twinning in deformed dolomite grains from a carbonate vein. Width: 3 mm. Capo Pini, Norsi, Island of Elba, Italy.

Occurrence
Dolomite forms essentially as a secondary mineral in carbonate rocks [see dolostone]. Primary dolomite can form due to direct precipitation from saline waters in evaporitic environment, both coastal and continental. However, the vast majority of dolomite forms after deposition due to replacement of calcite and aragonite in the presence of Mg-rich waters. This process, called dolomitization, may occur shortly after deposition down to the diagenetic zone and it commonly affects only a part of the original carbonate rock producing dolomite rocks that commonly still contain calcite. Metamorphism transforms dolomite-bearing carbonate rocks in dolomite marbles, where dolomite is stable to high temperature conditions. In contact aureoles, dolomite breaks down at very high grade to brucite, periclase, and calcite. In metamorphic environment, new dolomite may form from reactions destroying talc or tremolite in marbles. Finally, dolomite can form in hydrothermal veins or from the alteration of mafic and ultramafic rocks, where it occurs in veins together with magnesite and serpentine.

References
Baker, P. A., & Kastner, M. (1981). Constraints on the formation of sedimentary dolomite. Science213(4504), 214-216.
Mazzullo, S. J. (1992). Geochemical and neomorphic alteration of dolomite: a review. Carbonates and evaporites7(1), 21.
Ross, N. L., & Reeder, R. J. (1992). High-pressure structural study of dolomite and ankerite. American Mineralogist77(3-4), 412-421.
Warren, J. (2000). Dolomite: occurrence, evolution and economically important associations. Earth-Science Reviews52(1-3), 1-81.

        

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