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Dolomite (mineral)

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Dolomite is an anhydrous carbonate mineral composed of calcium magnesium carbonate. This means it is a chemical compound that does not contain water within its structure. It is also the name for a sedimentary rock, often called dolostone, that consists mostly of this mineral.

Thin section microscopy Siilinjärvi 501M3 etched
Thin section microscopy Siilinjärvi 501M3 etched

Understanding dolomite is important because it helps geologists understand how rocks form and how water moves through the Earth. It can also act as a reservoir for oil and gas or host valuable metal deposits.

The internal structure of dolomite is a double carbonate. It features an alternating structural arrangement of calcium and magnesium ions. This specific arrangement is part of the trigonal-rhombohedral crystal system.

Thin section microscopy Siilinjärvi 501M3 etched
Thin section microscopy Siilinjärvi 501M3 etched

Because of its chemistry, dolomite reacts differently to acid than calcite does. While calcite fizzes quickly in cold, dilute hydrochloric acid, dolomite does not react rapidly unless it is in a fine powder form. This property is often used by scientists to tell the two minerals apart under a microscope.

Dolomite crystals can appear in many different colors. Most commonly, they are white, tan, gray, or pink. The specific color often depends on small amounts of other elements substituting into the crystal structure. For instance, small amounts of iron can create a yellow or brown tint. If manganese substitutes into the structure up to about three percent, the crystals can take on a rosy pink color. Other elements like lead, zinc, or cobalt can also substitute for magnesium in the structure.

Thin section microscopy Siilinjärvi 501M3 etched
Thin section microscopy Siilinjärvi 501M3 etched

The history of dolomite involves several famous naturalists. Carl Linnaeus likely first described the mineral in 1768. He noted it was a white, transparent mineral that was as hard as quartz. In 1791, the French geologist Déodat Gratet de Dolomieu described it as a rock type. He observed these rocks in the old buildings of Rome and the Tyrolean Alps. In March 1792, Nicolas-Théodore de Saussure officially named the mineral after Dolomieu.

Faloria Cortina d'Ampezzo 12
Faloria Cortina d'Ampezzo 12

Dolomite forms through several complex geological processes. In modern environments, it can form in anaerobic conditions within supersaturated saline lagoons. Examples include the Lagoa Vermelha and Brejo do Espinho near the coast of Brazil. It also forms in places like the Persian Gulf or in hypersaline lakes. Some scientists believe that sulfate-reducing bacteria, such as Desulfovibrio brasiliensis, help the mineral nucleate. Other microbial metabolisms may also play a role in its formation.

Faloria Cortina d'Ampezzo 12
Faloria Cortina d'Ampezzo 12

Another way dolomite forms is through a process called dolomitization. This occurs when high diagenetic temperatures affect sedimentary rocks. This can happen when groundwater flows through deeply rooted fault systems or affects deeply buried limestone. In continental saline lakes, such as those in Australia, dolomite forms when there is high salinity and high magnesium to calcium ratios. In some cases, the subsurface biosphere may help drive this process due to long-term climate changes.

Faloria Cortina d'Ampezzo 12
Faloria Cortina d'Ampezzo 12

Humans use dolomite for many industrial and scientific purposes. It is used as an ornamental stone and a concrete aggregate. In industry, it serves as a source of magnesium oxide and is used in the Pidgeon process to produce magnesium. It is also used as a flux for smelting iron and steel when calcite is too expensive. Large amounts of processed dolomite are required to produce float glass. In agriculture, it is added to soil as a pH buffer and a source of magnesium.

Thin section microscopy Siilinjärvi 501M3 etched
Thin section microscopy Siilinjärvi 501M3 etched

Dolomite also plays a unique role in high-level science and nature. Because it contains low levels of radioactive materials, it can act as an insulator. Particle physics researchers build detectors under layers of dolomite to shield them from cosmic rays. This allows them to detect exotic particles more effectively. In nature, dolomite can dissolve in slightly acidic water, which helps create karst terrain and important aquifers.

Faloria Cortina d'Ampezzo 12
Faloria Cortina d'Ampezzo 12
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File:Faloria Cortina d'Ampezzo 12.jpg
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