Dolomite [CaMg(CO3)2], is a calcium and magnesium carbonate that constitutes approximately 2% of the earth’s crust of sedimentary rocks that can be found in massive beds or strata several meters thick throughout the world; it is relatively common with marble.
Its origin is due to the deposition of calcium carbonate muds that were postpositionally altered by magnesium-rich pore water and by the transformation of limestone to dolomites. It also occurs in an igneous rock known as Dolomite carbonatite.
In this article you will learn all the physical and chemical properties, origin and formation of the dolomite as a mineral, but there are also sections where it is classified to the dolomite as dolomitic rock (dolomite) based on the percentage of dolomite with respect to calcite.
Mineral or rock dolomite?
The dolomite is a term that can be taken as both a mineral and a rock, however, depending on the geological context, it can be distinguished dolomite clearly as a mineral and as a dolomitic rock (dolomite or dolostone).
When talking about the dolomite as a rock, that is to say as a dolomitic rock, geologists refer to a rock that has in its composition at least 90% of dolomite (calcium magnesium carbonate mineral)
When talking about the dolomite as a mineral it is understood that it is a carbonate of calcium and magnesium that is usually present in limestone rocks, marbles and, of course, dolomitic rocks.
Dolomite (ore) in thin sheet
In thin sheet the dolomite (mineral) has the following distinctive properties:
- It is colorless in natural light
- Relief: variable (high to low)
- Exfoliation: Excellent
- Birefringence colors: extreme, white to cream
- Extinction: symmetric to exfoliation
- Otic: uniaxic (-)
- Others: polysynthetic twinned and does not stain with alizarin red S.
Optically, the Dolomite It is characterized by being similar to calcite, but in sedimentary rocks it occurs with a rhombohedral habit, with quite distinctive parallelogram-shaped sections.
Dolomite Rock Classification (Dolomite)
As mentioned at the beginning, the dolomite It can be interpreted as a carbonate rock known as dolomitic rock or dolomite, which is enriched in the mineral dolomite.
Dolomitic carbonate rocks are classified according to their carbonate content. Dolomite compared to calcite:
| % dolomite (ore) | % calcite | rock type |
| 0 – 10 | 90 – 100 | Limestone |
| 10 – 50 | 50 – 90 | Magnesian limestone or dolomitic limestone |
| 50 – 90 | 10 – 50 | calcareous dolomite |
| 90 – 100 | 0 -10 | dolomite (dolomitic rock) |
Characteristics and physical properties of dolomite (mineral)
It is a colorless to white, pale brown, greyish, reddish or pinkish mineral. Its crystals are commonly rhombohedral or tabular, often with curved faces.
Chemical properties
| Dolomite | Characteristics and physical properties |
|---|---|
| chemical classification | Carbonate (calcium magnesium carbonate) |
| Formula and chemical composition | CaO=30.4%, MgO=21.7%, CO2=47.9% |
| common impurities | Fe, Mn, Co, Pb, Zn |
| Color | Colorless, white, pink, green, gray, brown, black |
| Stripe | White |
| gloss – luster | Vitreous, pearly |
| Exfoliation or cleavage | Perfect, rhombohedral, three directions |
| Fracture | conchoidal |
| diaphanousness | transparent to translucent |
| Mohs hardness | 3.5 to 4 |
| Tenacity | Fragile |
| Origin and geological environment | Deposition of calcium carbonate muds that were postpositionally altered by magnesium-rich pore water and transformation of limestones to dolomites. |
| Density | 2.84 – 2.86 g/cm3 (Measured) 2.876 g/cm3 (Calculated) |
| Associated minerals | Barite, calcite, fluorite, sphalerite, sulfides, quartz, and sometimes with gold. |
| associated rocks | Limestones (dolomitic limestone), dolomite, marble |
| Applications | Refractories, magnesium extraction for iron manufacturing. For construction, cement processing, lime production, oil and gas storage, magnesium source, agricultural products, metallurgical flow. |
The dolomite It is colorless, but it is also found with whitish, pink, greyish, greenish, brown or blackish tones.
This mineral is transparent to translucent, has a vitreous luster and in some cases pearly, the color of its stripe is white.
In addition, it has a perfect rhombohedral cleavage. Its crystals are normally formed by the fundamental rhombohedron and on other occasions by an acute rhombohedron and base. Also their faces are usually curved.
Three directions of perfect cleavage can be distinguished, only it is not distinguished when it is presented in a fine grain, unlike when it is thick, the cleavage angles are observed with a magnifying glass.
Regarding some physical properties, it is characterized by having a density of 2.85 g/cm3, Mohs hardness is between 3.5 and 4, and its fracture is conchoidal.
Origin, formation and geological environment
The Dolomite As a mineral it occurs as the main component of dolostones (dolomitic rock) and marbles.
Dolostones are found in the rock record, are geographically extensive deposits and hundreds to thousands of feet thick.
Therefore, the explanation for the formation of dolomitic rock (dolomite) is by conversion (dolomitization) of CaCO3 sediment (limestone) or calcareous sedimentary rocks to dolostone.
Dolomitization involves four main variables:
- Weather.
- Location with respect to the sediment-seawater interface.
- Composition and derivation of the solutions involved.
- Flow mechanisms.
The time ranges from dolomitization occurring contemporaneously with deposition to occurring after the relatively deep burial of the parent sediment.
The location varies from very close to the sediment-seawater interface to well below some overlying sediments that were deposited at a later time.
Solutions supplying magnesium must have the appropriate pH and concentrations of other necessary ions; these solutions are generally considered seawater (either “normal” seawater or evaporatively concentrated brines) or meteoric water, or some combination of these waters.
Another important variable is the presence of dissolved sulfate [(SO4)-2] in ions, since this delays the dolomitization process.
The flow mechanisms are generally attributed to the density differences of the solutions involved and the permeability characteristics available for filtration through the parent sediment.
Another of the occurrences of the dolomite is quite common, where it is included that: the dolostones that have undergone metamorphism, product of this marbles of dolomite and calcite.
The dolomite It is also found in hydrothermal veins, in certain igneous rocks and as an alteration of limestone or evaporites.
Much of the rocks that are rich in dolomite It is due to the deposition of calcium carbonate sludge that was postpositionally altered by magnesium-rich pore water that originated the mineral.
Another origin is as a lode mineral in the limestone rocks in the veins of zinc and lead.
It is also believed that it is formed by the transformation of limestone in the substitution of calcium for magnesium.
How to identify Dolomite in hand sample (mineral)
The best way to identify dolomite and differentiate it from calcite is in a thin layer by placing red alizarin on it, which makes the calcite turn pink and the dolomite maintains its color, for this check the section of the dolomite in a thin layer to understand it.
The pure samples of dolomite and calcite can look similar and share many properties, so the easiest way to tell them apart is by their reaction with dilute acid at room temperature.
Calcite (and aragonite) will readily react with hydrochloric acid (HCl) to form small bubbles (effervescence).
Instead, the dolomite it will only fizz if the mineral is crushed into a powder (or if the acid is heated).
Unfortunately, natural bulk samples often consist of a mixture of the two minerals, so it is sometimes difficult to distinguish whether the dolomite is present in a massive mixed sample.
Iron can also replace part of the magnesium in the dolomite, so dolomite can be converted to siderite, an iron carbonate (FeCO3), although the dolomite it is much more abundant than siderite.
In the dolomite three directions of perfect cleavage are evident. If the crystals are fine-grained, this characteristic cannot be appreciated. If the crystals are thick, the cleavage angles can be distinguished with a magnifying glass.
It is basically recognized by its generally white or pink color, vitreous or pearly luster, and by its perfect rhombohedral cleavage with rhombohedral habit with curved faces.
This mineral also shows a weak reaction when cold hydrochloric acid (HCl) is added to it, therefore it effervesces slightly. For a greater reaction to the dolomite, it is crushed to a powder and hot HCl is added.
The dolomite and calcite can make it difficult for them to be recognized and distinguished, whether in the field or in the classroom. Dolomites are composed of calcium and magnesium carbonates and calcites are made of calcium carbonate.
To distinguish between the two minerals, we first take into account the hardness, the hardness of calcite is 3, being less than the dolomite from 3.5 to 4.
The dolomite As mentioned before, it has a weak reaction with cold hydrochloric acid, unlike calcite, which presents a strong reaction or effervescence.
Dolomite exploration (ore and rock)
They are found all over the world and are quite common in sedimentary rock sequences.
The dolomite It is a common sedimentary rock-forming mineral and can be found in massive beds known as dolomites.
Dolomite also forms as sediment in mineral veins such as limestone.
Limestone where dolomite is present in more than a small amount is called dolomitic limestone.
Dolomite is rarely found in higher temperature metamorphic settings.
Dolomite Uses
- Dolomite is used to make certain types of refractory bricks used in steelmaking.
- The Dolomite is heated to a high temperature to expel the carbonate as carbon dioxide and the remaining material, a mixture of calcium and magnesium oxides, is mixed with carbon and other materials and pressed into blocks for the furnaces.
There are actually more uses, do you want to learn more?