Dolomite

Description

Dolomite is a carbonate mineral composed of calcium magnesium carbonate, CaMg(CO3)2. It is also the name of a sedimentary carbonate rock, dolostone, which is composed predominantly of the mineral dolomite. It is a common rock-forming mineral, often found in extensive beds, particularly in ancient marine sedimentary sequences. It is chemically and structurally similar to calcite but contains magnesium, which gives it distinct properties.

How to Identify

Color

Typically white, gray, pink, or tan; can also be colorless, green, brown, or black due to impurities.

Luster

Vitreous to pearly.

Texture

Often granular in dolostone; individual crystals can be rhombohedral with curved faces.

Crystal Form

Usually forms rhombohedral crystals, often with curved or saddle-shaped faces. Also occurs as granular masses, massive, or columnar aggregates.

Cleavage

Perfect rhombohedral cleavage in three directions, producing fragments with angles of 73° and 107°.

Geological Environment

Found in extensive sedimentary beds, often associated with evaporites, chert, and gypsum. Also occurs in hydrothermal veins, metamorphic rocks (dolomitic marbles), and as a gangue mineral.

Key Facts

• Hardness: 3.5-4 on Mohs scale
• Specific Gravity: 2.85 g/cm³
• Crystal System: Trigonal
• Color: White, gray, pink, tan, colorless, green, 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, pink, tan (common)
• Luster: Vitreous to pearly
• Streak: White

Physical Characteristics

• Crystal Habit: Typically rhombohedral, often with curved or saddle-shaped faces; also massive, granular, columnar.
• Cleavage Type: Perfect rhombohedral {1011}
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Vitreous to pearly

Formation

Dolomite 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 in modern environments but is thought to have been more common in the geological past under specific conditions (e.g., high Mg/Ca ratios in seawater).

Usage

Dolomite is used as an aggregate in construction, a source of magnesia (MgO) for refractories, a flux in metallurgy, a soil conditioner in agriculture, and as an ornamental stone. 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.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Carbonates

Group

Dolomite Group

Crystal System

Trigonal (R-3)

Chemical Formula

CaMg(CO3)2

Composition

Calcium magnesium carbonate

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