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Dolomite (mineral)

Trigonale

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).

Struttura e chimica
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.

Proprietà
Abito: rhombohedral, scalenohedral, prismatic, tabular, fibrous, acicular
Durezza: 3.5-4
Sfaldatura: {10-11} perfect rhombohedral cleavage
Geminazione: {0001} {10-10} {11-20} lamellar twinning: common; {10-11}: rare; {02-21} glide twinning
Colore: colorless to grey/white, alters to yellows and browns
Lucentezza: vitreous, pearly
Struscio: bianco
Alterazione: dissolves in slightly acid waters
In sezione sottile...
ε: 1.500-1.520
ω: 1.679-1.703
Colore: incolore
Pleocroismo: strong relief pleochroism
Birifrangenza (δ): 0.179-0.185 (fifth order colors)
Rilievo: alto
Segno ottico:
[Mindat]

Caratteristiche di terreno
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

Dolomite-bearing rocks appear very similar to calcite-bearing marbles and limestones. The main difference is the lack of an effervescent reaction with HCl 10%. 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 Alizarin Red S, which stains calcite and leaves dolomite unstained. The red material overgrown by dolomite are calcite ooids and their primary matrix/cement. 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.

twins in calcite and dolomite

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

CPL
CPL
NX
PPL
N//

⇔ 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
CPL
NX
PPL
N//

⇔ slider. Euhedral (rhombohedral) crystal of dolomite surrounded by quartz. Width: 1.2 mm. Cavo, Island of Elba, Italy.

CPL
CPL
NX
PPL
N//

⇔ 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
CPL
NX
PPL
N//

⇔ 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
Campione: dolomite vein in carbonated serpentinite
Associazione mineralogica: dolomite, minor calcite
Località: Norsi, Island of Elba, Italy

dolomite vein

Columnar basalt? Nope, twinned crystals of dolomite from a deformed carbonate vein. CPL: Width: 3 mm. Norsi, Island of Elba, Italy.

Rhombohedral dolomite
Rhombohedral crystals of dolomite from a blocky quartz-carbonate vein
Campione: quartz-dolomite vein
Associazione mineralogica: dolomite, quartz, minor talc and chlorite
Località: Cavo, Island of Elba, Italy

rhombohedral dolomite

Rhombohedral crystals od dolomite surrounded by quartz. CPL. Width: 1.2 mm. Dolomite-quartz vein. Cavo, Island of Elba, Italy.

Dolomitization
Euhedral dolomite crystals overgrowing a very low-grade calcschist
Campione: calcschist
Associazione mineralogica: calcite, dolomite, quartz, sericite, chlorite
Località: Cavo, Island of Elba, Italy

Ritrovamento
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.

Bibliografia
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.

        

Proprietà dei Minerali
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