Malachite Stalactites

Malachite stalactites  Locality: L'Etoile du Congo Mine (Star of the Congo Mine; Kalukuluku Mine), Lubumbashi (Elizabethville), Southern area, Katanga Copper Crescent, Katanga (Shaba), Democratic Republic of Congo (Zaïre) Photo Copyright © Rob Lavinsky

Stalactitic Habit:

Malachite stalactites grow downward from the ceilings of open spaces such as mine voids, vugs, or small cave-like pockets. They are elongated, often tapering, and can range in diameter from a few millimeters to several centimeters. In particularly copper-rich environments, lengths may exceed 10–20 cm.

Crystal Structure and Morphology:

On a microscopic scale, these stalactites exhibit a radiating, acicular (needle-like) or fibrous crystal habit. Malachite layers accumulate concentrically around a central drip channel, often forming a hollow or semi-hollow core—similar to carbonate stalactites, but with malachite replacing calcite.

Stalactitic malachite from Star of Congo Mine, Katanga, DRC.

Color Zoning:

Distinct growth bands in shades of dark to light green are often visible in cross-section. These color variations reflect fluctuations in copper concentration, pH, and other environmental factors during mineral deposition.

About Malachite

Malachite is a common secondary copper carbonate hydroxide mineral formed through the weathering and oxidation of primary copper sulfides. It displays a wide range of habits, including:

• Crystalline aggregates and crusts with concentric banding resembling agates
• Botryoidal clusters composed of radiating crystals
• Mammillary (rounded) aggregates commonly seen in oxidized copper zones

Mineral Properties:

• Formula: Cu₂CO₃(OH)₂
• Crystal System: Monoclinic
• Color: Bright green
• Hardness: 3½ to 4 on the Mohs scale
• Streak: Light green

Stalactitic malachite specimen from Shilu Mine, Guangdong, China

Host Rock & Oxidation Zone: Primary copper sulfide ore undergoes weathering near the surface. Oxygenated, slightly acidic groundwater converts sulfides → soluble Cu²⁺.

Carbonate Interaction: That Cu²⁺-rich water migrates downward until it encounters carbonate host rock (limestone/dolomite) or mixes with CO₂-rich air in a cavity, which raises pH and causes Cu²⁺ to combine with CO₃²⁻/OH⁻ → malachite.

Drip Deposition: As each drop of copper solution hangs and degasses CO₂, it precipitates a ring of malachite. Repetition builds a stalactite. Temperature, CO₂ pressure, and fluid composition control growth rate, fiber orientation, and banding.

Associated Minerals:

• Commonly found with azurite [Cu₃(CO₃)₂(OH)₂], particularly deeper in the oxidation zone.
• Chrysocolla (Cu₂–xAlₓ(H₂–xSi₂O₅)(OH)₄·nH₂O) may overgrow or form near stalactites in silica-rich zones.
• Brochantite [Cu₄SO₄(OH)₆] or malachite/azurite intergrowths appear if sulfate is abundant.
• Calcite/Dolomite Host: The underlying carbonate provides CO₃²⁻ for malachite precipitation; sometimes small calcite speleothems coexist nearby.

Localities:

Tsumeb Mine, Namibia: Renowned for vivid green, drusy malachite stalactites reaching up to ~15 cm; often associated with exceptional mineral paragenesis.

Bisbee District, Arizona, USA: Found in old Copper Queen and Lavender Pit workings; frequently accompanied by azurite and chrysocolla.

Majuba Hill Mine, Nevada, USA: Historic locality known for slender, fibrous stalactitic malachite lining volcanic rock cavities.

Shilu Mine, Guangdong, China: Produces striking chatoyant “Cat’s Eye” malachite stalactites (5–15 cm), formed in oxidized supergene zones of this open-cast copper mine.

Katanga (Shaba), D.R. Congo: Hosts malachite stalactites in weathered copper-rich dolomitic zones beneath oxidized chalcocite and chalcopyrite layers.