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Dolomite is both a carbonate mineral and a sedimentary rock. The mineral dolomite is a calcium magnesium carbonate with the chemical formula CaMg(CO3)2. It crystallizes in the trigonal system and is structurally similar to calcite, but with alternating layers of calcium and magnesium ions. The rock dolostone (often simply called dolomite) is a sedimentary rock composed of more than 50% dolomite mineral. It typically forms from the alteration of limestone. It is generally white, gray, or pinkish, and can be fine-grained to coarsely crystalline. It is distinguished from limestone by its lower reactivity with dilute hydrochloric acid.
How to Identify
- Color
- Typically white, gray, pinkish, or yellowish. Can also be colorless, brown, or black.
- Luster
- Vitreous to pearly, especially on cleavage surfaces.
- Texture
- Fine-grained to coarsely crystalline. Often granular in dolostone. Individual mineral crystals can be rhombohedral.
- Crystal Form
- Commonly forms rhombohedral crystals, often with curved faces (saddle-shaped). Also occurs as granular, massive, or columnar aggregates.
- Cleavage
- Perfect rhombohedral cleavage in three directions, producing fragments with diamond-shaped faces.
- Geological Environment
- Found in extensive sedimentary beds, often associated with evaporite deposits, ancient reef complexes, and platform carbonates. Also occurs in hydrothermal veins and as a gangue mineral in some ore deposits. Forms through diagenetic replacement of limestone or, less commonly, primary precipitation.
Key Facts
- Hardness: 3.5 - 4 on Mohs scale
- Specific Gravity: 2.85 - 2.95
- Crystal System: Trigonal
- Color: White, gray, pinkish, yellowish, colorless, brown, black
- Luster: Vitreous to pearly
- Transparency: Transparent to translucent
- Fracture: Conchoidal to uneven
- Cleavage: Perfect rhombohedral in three directions at 73° and 107°
- Composition: Calcium magnesium carbonate (CaMg(CO3)2)
Quick Check
- Color: White, gray, pinkish, yellowish
- Luster: Vitreous to pearly
- Streak: White
Physical Characteristics
- Crystal Habit: Rhombohedral, often with curved faces (saddle-shaped), massive, granular, columnar
- Cleavage Type: Perfect rhombohedral {1011}
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous to pearly
Formation
Dolomite mineral forms primarily through the diagenetic alteration of limestone (calcium carbonate) by magnesium-rich fluids, a process known as dolomitization. This can occur in various environments, including sabkhas, hypersaline lagoons, and burial settings. Primary precipitation of dolomite is rare but can occur in specific hypersaline or microbial environments. Dolomite rock (dolostone) is composed predominantly of the mineral dolomite.
Usage
Dolomite is a versatile mineral and rock. It is used as an aggregate in construction, a source of magnesium oxide (magnesia) for refractories, a flux in metallurgy, an agricultural soil conditioner, and as an ornamental stone. High-purity dolomite is used in glass manufacturing and as a filler in various products. It is also a significant reservoir rock for oil and natural gas.
Age Distribution
Found throughout the geological record, particularly abundant in Proterozoic and Paleozoic strata, but also present in younger formations.
Where to Find
Niagara Escarpment, North America
Extensive dolostone formations, particularly in the Silurian Lockport Group, forming prominent cliffs and bluffs across the Great Lakes region.
Dolomite Alps, Italy
The mountain range is named after the mineral and rock, featuring vast exposures of Triassic dolostone.
Midcontinent Rift System, USA
Significant dolomite deposits are found in association with this ancient rift, particularly in states like Wisconsin and Michigan.
Permian Basin, USA
Important oil and gas reservoirs are hosted within dolomitized carbonates in this region of Texas and New Mexico.
Various ancient carbonate platforms worldwide
Dolomite is a common component of ancient marine platform sequences, found in numerous locations globally from the Proterozoic to the Cenozoic.
Finding Tips
Acid Test (Dilute HCl)
Dolomite reacts weakly or not at all with cold, dilute (5-10%) hydrochloric acid unless powdered. Calcite, in contrast, effervesces vigorously. This is the primary field test to distinguish it from limestone/calcite.
Crystal Shape
Look for characteristic rhombohedral crystals, often with curved or saddle-shaped faces, especially in vugs or cavities.
Hardness
Dolomite is slightly harder than calcite (3.5-4 vs. 3 on Mohs scale), which can be a subtle distinguishing feature if you have a known sample for comparison.
Geological Context
Dolomite often occurs in thick, massive beds within sedimentary sequences, frequently associated with evaporites or ancient shallow marine environments.
Similar Rocks
Calcite
CaCO3
Also known as: Calcium Carbonate
Limestone
Predominantly CaCO3
Also known as: Calcium Carbonate Rock
Magnesite
MgCO3
Also known as: Magnesium Carbonate
Scientific Classification
- Mineral Class
- Carbonates
- Group
- Dolomite Group
- Crystal System
- Trigonal
- Chemical Formula
- CaMg(CO3)2
- Composition
- Calcium magnesium carbonate
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