Dolomite

Dolomite is a carbonate mineral made of calcium magnesium carbonate, and the term also names the rock (more precisely "dolostone") composed mostly of that mineral. It closely resembles limestone but contains magnesium.

The mineral forms rhombohedral crystals with a pearly to vitreous luster, often in white, gray, pink, or tan. Pink, curved-faced crystals known as "pearl spar" are a classic collector form.

Dolomite is slightly harder than calcite and, crucially, reacts only weakly to cold dilute acid (it fizzes briskly only when powdered or in warm acid). The famous Dolomite mountains of Italy are built from this rock and gave the mineral its name.

Most dolomite rock forms by dolomitization — the alteration of existing limestone when magnesium-rich fluids (typically seawater or brines) replace some of the calcium in calcite with magnesium. This usually happens during or after burial of carbonate sediments.

Primary dolomite can also precipitate directly in restricted, hypersaline lagoons and sabkhas, though this is less common. Hydrothermal dolomite forms from hot mineralizing fluids along faults and fractures.

Dolomite is widespread in ancient carbonate platforms; many limestones that are hundreds of millions of years old have been partly or fully converted to dolostone.

The key test distinguishes it from calcite: dolomite fizzes only weakly in cold dilute acid, whereas calcite reacts vigorously. Scratch or powder the dolomite first and it will fizz more readily.

It is harder than calcite (Mohs ~3.5-4 vs 3), shows rhombohedral cleavage, and often has curved, saddle-shaped crystal faces. Color is commonly white, gray, or pink with a pearly luster.

Look-alikes: Calcite (softer, strong acid reaction), magnesite (also weakly reactive but different habit), and limestone vs dolostone at outcrop scale (test with acid). The weak acid reaction plus rhombs is diagnostic.

Dolomite is an important industrial mineral. It is a source of magnesium and magnesium oxide, a flux in steelmaking, a raw material for refractory bricks, and an additive in glass, ceramics, and cement.

Crushed dolostone is used as construction aggregate, road base, and agricultural lime that supplies both calcium and magnesium to soils. It also hosts significant lead-zinc (Mississippi Valley-type) ore deposits and is an important petroleum reservoir rock.

Attractive crystal specimens are collected, and dolomite is sometimes used decoratively. Metaphysical claims of calming, centering energy exist but are not scientifically supported.