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.
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]
[HoM]
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 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).
CPL
PPL
⇔ slider. This is dolomite, not calcite. The identification is possible thanks to the presence of lamellar twins parallel to the short axis of the rhomb. Sample: high-pressure schist. Width: 2.5 mm. Photo by Atlas of Metamorphic Minerals (earth.ox.ac.uk).
CPL
PPL
⇔ slider. Euhedral (rhombohedral) crystal of dolomite surrounded by quartz. Width: 1.2 mm. Cavo, Island of Elba, Italy.
CPL
PPL
⇔ slider. Tiny euhedral crystals of dolomite (produced by dolomitization) surrounded by microcrystalline quartz in a very low-grade calcschist. Width: 1.2 mm. Cala dell’Alga, Cavo, Island of Elba, Italy.
CPL
PPL
⇔ slider. Lamellar twinning in deformed dolomite grains from a carbonate vein. Width: 3 mm. Capo Pini, Norsi, Island of Elba, Italy.
Examples of dolomite-bearing rocks
Dolomite vein
Massive dolomite vein with slightly deformed and recrystallized dolomite
Sample: dolomite vein in carbonated serpentinite
Assemblage: dolomite, minor calcite
Locality: Norsi, Island of Elba, Italy
Rhombohedral dolomite
Rhombohedral crystals of dolomite from a blocky quartz-carbonate vein
Sample: quartz-dolomite vein
Assemblage: dolomite, quartz, minor talc and chlorite
Locality: Cavo, Island of Elba, Italy
Dolomitization
Euhedral dolomite crystals overgrowing a very low-grade calcschist
Sample: calcschist
Assemblage: calcite, dolomite, quartz, sericite, chlorite
Locality: Cavo, 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.
Baker, P. A., & Kastner, M. (1981). Constraints on the formation of sedimentary dolomite. Science, 213(4504), 214-216.
Mazzullo, S. J. (1992). Geochemical and neomorphic alteration of dolomite: a review. Carbonates and evaporites, 7(1), 21.
Ross, N. L., & Reeder, R. J. (1992). High-pressure structural study of dolomite and ankerite. American Mineralogist, 77(3-4), 412-421.
Warren, J. (2000). Dolomite: occurrence, evolution and economically important associations. Earth-Science Reviews, 52(1-3), 1-81.
Resources
An introduction to the Rock-Forming Minerals. Deer, Howie & Zussmann.
Optical Mineralogy: Principles & Practice. Gribble & Hall.
Transmitted Light Microscopy of Rock-Forming Minerals: An Introduction to Optical Mineralogy (Springer Textbooks in Earth Sciences, Geography and Environment). Schmidt.
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