by Formation and Composition
Dolomite is a common rock-forming mineral. It is a calcium magnesium carbonate with a chemical composition of CaMg(CO3)2. Dolomite forms in sedimentary deposits where magnesium replaces some of the calcium in calcite or aragonite. For dolomite to form, magnesium ions in solution must be plentiful relative to calcium. Seawater has a high magnesium to calcium ratio and dolomite commonly forms in marine environments where limestone is being deposited. When limestone is later subjected to magnesian rich groundwater, dolomite can replace the original limestone via recrystallization.
Physical Properties
Dolomite has a distinctive pale white or pink color and is often streaked with gray. Its luster is generally dull or earthy. The mineral has a hardness of 3.5-4 on the Mohs hardness scale, slightly harder than calcite. In hand samples, dolomite can sometimes be confused with marble due to its white coloration. However, dolomite will effervesce very little if at all when hydrochloric acid is applied, unlike calcite which effervesces readily. This acid test can be used to distinguish dolomite from limestones which are primarily composed of calcite.
Crystallographically, Dolomite most commonly forms clear or cloudy crystal aggregates called “saccharoidal” in morphology. Rarer crystal forms include cubic and hexagonal scalenohedral shapes. Dolomite has a rhombohedral crystalline structure and when light passes through thin crystal fragments it displays a high order refractive index with double refraction.
Occurrence and Uses
Massive dolomite formations are widespread across the globe, with extensive deposits located in North America, Europe, Asia, and Africa. Notable exposures include the Dolomite Alps mountain range of northern Italy and the Cordillera Blanca of Peru. Dolomite is commonly used as an ornamental stone, a structural building material and roadbase aggregate. Where fractured and porous, dolomite acts as an important underground aquifer and storage medium for natural gas.
Dolomite’s high structural strength makes it useful as crushed stone for road construction, concrete aggregate, railroad ballast, filter stone and rubble. It is employed as a fluxing agent and source of magnesium in iron and steel production. Ground dolomite is used as an acid neutralizer, plumbing flux, soil conditioner and white pigment in products like paint, paper, rubber and plastics. Dolostone reservoirs also provide an important source of magnesium for industrial and agricultural needs.
Ecological Importance
Dolomite provides important habitat and buffers susceptible landscapes from erosion. Its porous nature allows rainwater infiltration which recharges underground aquifers. Coastal dolomite cliffs and barrens support special floral communities adapted to alkaline soils and sparse vegetation. The mineral acts as a buffering agent in terrestrial and marine sediments that moderates pH fluctuations.
Aquatic ecosystems, both freshwater and marine, often depend on the stability provided by dolomite sediments. Alkaline rivers, streams and groundwater fed by dolomite aquifers maintain suitable pH conditions for many organisms. Coral reef growth requires the magnesium supplied by dissolving dolomite deposits. Overall, dolomite plays a vital yet underappreciated role in sustaining productive ecological systems around the world.
Dolomite Alteration and Metamorphism
When dolomite is deeply buried and subjected to high pressures and temperatures during metamorphism, it undergoes recrystallization. Coarsely crystalline forms may develop with a white opaque appearance known as dolomitic marble. At higher grades, dolomite breaks down into magnesite (MgCO3) and calcium silicates. Contact metamorphism around igneous intrusions can cause dolomite to alter to ankerite (Ca(Fe,Mg,Mn)(CO3)2) depending on availability of iron.
Dolomite may also recrystallize during regional metamorphism, developing preferred orientations under directed stress. Textural changes characterize increasing metamorphic grade, evolving from planar-s to planar-e to granoblastic mosaic textures. Infiltration metasomatism through magnesium rich fluids during metamorphism can replace limestone with dolomite. Such late-stage dolomitization overprints original sedimentary textures.
Red Beds and Hydrothermal Settings
In some geological settings, dolomite forms under unusual conditions. Red beds containing oxidized iron minerals provide one environment where dolomitization occurred long after initial deposition. Here, upward circulating pore fluids enriched in magnesium leached calcium from lower siliciclastic rocks and redeposited dolomite in overlying limestone units.
Hydrothermal fluids associated with volcanism and plutonism also promoted dolomitization through focused fluid flow. Sites of massive dolomite occurrences near igneous intrusions testify to large scale alteration through hot fluids. Hot springs and geysers can still actively precipitate travertine and minor dolomite where groundwater discharges at the earth’s surface. These unusual depositional environments expand our understanding of dolomite genesis beyond normal marine sedimentary conditions.
In summary, dolomite is a widespread and economically important carbonate mineral that forms through diverse geological processes in sedimentary, metamorphic and hydrothermal contexts.
