by Formation and Composition
Dolomite is a common carbonate sedimentary rock that forms when lime mud and sediment are lithified into solid rock under pressure over time. Chemically, dolomite comprises calcium magnesium carbonate in the form of the mineral dolomite (CaMg(CO3)2). For dolomite to form, calcium carbonate in lime mud and sediment must be replaced with magnesium in alkaline, magnesium-rich fluids during diagenesis as the sediments are buried. This process of magnesium replacement is known as dolomitization. Though rare today, conditions were more favorable for dolomitization during the Proterozoic and Paleozoic eras in Earth’s history.
Occurrence and Distribution
Dolomite deposits are widespread on all continents and occur in marine environments from the Proterozoic to recent Precambrian and Phanerozoic eras. Major dolostone formations can be found throughout the world including: the Cambrian deposits of the Canadian Shield, the Mississippian deposits of the United States, the Devonian deposits of Germany, the Carboniferous deposits of the Russian Platform, and scattered occurrences throughout Africa and South America. In many formations, dolomite can make up over 50% of the total rock volume representing a major dolomitization event during sediment lithification and diagenesis.
Physical Properties
Texturally, Dolomite commonly displays sucrosic texture where calcite and dolomite crystals are interlocked in a mosaic pattern. The mineral dolomite is usually white, gray or pink in color with a white streak. It has a hardness of 3.5-4.0 on the Mohs hardness scale and has a refractive index of 1.483-1.489. Dolomite has a rhombohedral crystalline structure and breaks with a conchoidal fracture producing smooth curved surfaces. In hand sample it looks similar to limestone but will react more slowly with dilute hydrochloric acid due to its magnesium content.
Porosity and Permeability
Depending on the depositional environment, texture and post-depositional alteration, dolomite can display variably high porosity and permeability characteristics. Primary porosity resulting from voids between grains and fossils is common in dolomites formed from lime muds and sands. Secondary dissolution porosity forms when acids preferentially dissolve beds, along fractures and boundaries enhancing permeability. Karst features can form in structures dolomites increasing localized porosity. Well-developed solution pores and cavities make dolomite good reservoir rocks for oil and gas deposits.
Industrial and Commercial Uses
As a widely available natural resource, dolomite has many industrial and commercial applications:
– Construction – Used as a decorative facing stone on buildings and monuments, a popular dimension stone. Finely ground dolomite is also used as an aggregate in concrete and asphalt.
– Agriculture – Agricultural lime and dolomitic lime are commonly spread on acidic soils to raise pH levels making nutrients more bioavailable for plant growth.
– Metallurgy – As a flux and refractory material, dolomite is used in iron, steel and non-ferrous metal production.
– Mining – A cooling agent and flux in mining beneficiation processes, especially for talc and magnesite mining.
– Environment – An additive in wastewater treatment to regulate pH, used in flue gas desulfurization to reduce sulfur dioxide emissions.
– Landscaping – Rockeries, terraces, pathways built from dolostone slabs for ornamental gardens and hardscaping.
– Chemistry – A source material for the production of magnesium metal and chemicals. Refined dolomite lime is also used as a food additive.
– Medicine – Finely powdered dolomite used as an antacid or calcium supplement due to its calcium and magnesium carbonate content.
In summary, dolomite serves diverse purposes due to its chemical composition and sedimentary rock properties including construction, agriculture, industry and environmental applications. Its widespread natural abundance and versatility make dolomite an economically important non-metallic mineral commodity globally.
