Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Malachite-stained rock refers to any rock specimen where the mineral malachite (Cu2(CO3)(OH)2) is present as a coating, impregnation, or vein filling on a host rock. The malachite imparts a characteristic vibrant green color to the host rock, which can range from light green to dark emerald green, often with banding or concentric patterns. The host rock can be various types, including quartz, granite, sandstone, limestone, or other copper-bearing ore minerals. The malachite itself typically exhibits a silky, dull, or earthy luster, depending on its habit, and can form botryoidal, mammillary, stalactitic, or fibrous aggregates. The staining is a result of supergene alteration processes in copper deposits.
How to Identify
- Color
- Vibrant green, ranging from light green to dark emerald green, often with distinct banding or concentric patterns.
- Luster
- Malachite itself can have a silky (fibrous varieties), dull (massive varieties), or earthy luster. The host rock's luster will vary.
- Texture
- Malachite can form botryoidal (grape-like), mammillary (breast-like), stalactitic, fibrous, or massive crusts and coatings on the host rock. The host rock's texture will be characteristic of its type.
- Crystal Form
- Malachite rarely forms distinct crystals; it typically occurs as radiating fibrous aggregates, botryoidal masses, or dense, massive forms. When present, crystals are monoclinic, acicular, or tabular.
- Cleavage
- Malachite has perfect cleavage on {201} and good cleavage on {010}, but this is rarely observed in massive or fibrous forms. The host rock's cleavage will vary.
- Geological Environment
- Found in the oxidized zones of copper ore deposits, often associated with other secondary copper minerals like azurite, chrysocolla, cuprite, and native copper. Commonly occurs in carbonate-rich host rocks.
Key Facts
- Hardness: 3.5 to 4.0 on the Mohs scale (for malachite)
- Specific Gravity: 3.9 to 4.03 (for malachite)
- Crystal System: Monoclinic (for malachite)
- Color: Vibrant green, often banded
- Luster: Silky, dull, or earthy
- Transparency: Opaque to translucent
- Fracture: Conchoidal to uneven (for malachite)
- Cleavage: Perfect on {201}, good on {010} (rarely observed)
- Composition: Hydrous copper carbonate (Cu2(CO3)(OH)2)
Quick Check
- Color: Vibrant green (light to dark emerald green)
- Luster: Silky, dull, or earthy (for malachite)
- Streak: Light green
Physical Characteristics
- Crystal Habit: Typically botryoidal, mammillary, stalactitic, fibrous, or massive; rarely as acicular or tabular crystals.
- Cleavage Type: Perfect in one direction ({201}), good in another ({010}), but often not visible in common habits.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Silky (fibrous), dull (massive), earthy.
Formation
Malachite forms in the oxidized zone of copper ore deposits, typically above the water table. It is a secondary mineral, meaning it forms from the alteration of pre-existing copper minerals (such as chalcopyrite, bornite, or native copper) by carbonic acid-rich waters. The reaction involves copper ions, carbonate ions (from dissolved CO2 in water), and hydroxyl ions. This process often occurs in limestone or dolomite environments, where the host rock provides a source of carbonate. The malachite then precipitates as coatings, crusts, botryoidal masses, or fibrous aggregates on the surfaces and within fractures of the primary host rock.
Usage
Malachite-stained rocks are primarily collected for their aesthetic appeal as mineral specimens. While the malachite itself has historical uses as a pigment, gemstone, and ornamental material, the 'stained rock' typically refers to specimens where malachite is a coating or impregnation on a less valuable host rock. These specimens are valued by collectors for their vibrant green color and the contrast with the host rock. They can also be used in educational settings to demonstrate secondary mineral formation.
Age Distribution
Malachite forms as a secondary mineral, so its age is typically much younger than the primary copper deposits it originates from. It can form in geological environments ranging from Precambrian to Cenozoic, wherever copper mineralization and subsequent weathering have occurred.
Where to Find
Democratic Republic of Congo (DRC)
World-renowned for producing large, high-quality malachite specimens, often as botryoidal or stalactitic forms on various host rocks.
Russia (Ural Mountains)
Historically famous for massive malachite deposits, often found as large blocks or veins within host rocks.
Australia (various localities)
Significant copper deposits yield malachite, particularly in areas like Broken Hill and Mount Isa, often as coatings or veins.
United States (Arizona, New Mexico)
Copper mining districts in the southwestern US, such as Bisbee (Arizona) and Morenci (Arizona), produce malachite-stained rocks.
Zambia
Another major source of malachite, often found in association with other copper minerals.
Finding Tips
Target Copper Deposits
Focus your search on areas known for copper mineralization, especially the oxidized zones of these deposits. Look for old mine dumps or outcrops where weathering has occurred.
Look for Green Stains
Malachite's distinctive green color is a key indicator. Examine rocks for green coatings, crusts, or impregnations, particularly in fractures or vugs.
Associated Minerals
Malachite often occurs with other secondary copper minerals like blue azurite, turquoise-blue chrysocolla, or red cuprite. Finding these can indicate the presence of malachite.
Acid Test (Caution Advised)
Malachite is a carbonate and will effervesce (fizz) when a drop of dilute hydrochloric acid (HCl) is applied. This is a definitive test, but use caution and appropriate safety gear (gloves, eye protection) as HCl is corrosive. Always test on an inconspicuous area.
Safety Precautions
When collecting in old mine sites, be aware of unstable ground, shafts, and toxic residues. Malachite contains copper, which is toxic if ingested. Always wash hands thoroughly after handling specimens. Avoid inhaling dust if breaking specimens. Do not lick or ingest malachite.
Similar Rocks
Azurite-stained rock
Azurite (Cu3(CO3)2(OH)2) on host rock
Also known as: Azurite on matrix
Chrysocolla-stained rock
Chrysocolla ((Cu,Al)2H2Si2O5(OH)4·nH2O) on host rock
Also known as: Chrysocolla on matrix
Brochantite-stained rock
Brochantite (Cu4(SO4)(OH)6) on host rock
Also known as: Brochantite on matrix
Scientific Classification
- Mineral Class
- Carbonates
- Group
- Malachite Group
- Crystal System
- Monoclinic
- Chemical Formula
- Cu2(CO3)(OH)2
- Composition
- Copper (57.48% Cu), Carbon (5.43% C), Hydrogen (0.91% H), Oxygen (36.18% O). Contains 71.95% CuO and 19.90% CO2.
Explore Malachite-stained rock
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.