Scoria with Copper Mineralization

Igneous (extrusive) with secondary mineralization

Scoria (vesicular basalt) with copper mineralization (e.g., chrysocolla or malachite)

Also known as: Vesicular Basalt with Copper Infillings

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Description

Scoria is a dark-colored, highly vesicular, mafic extrusive igneous rock. It is typically reddish-brown to black due to the oxidation of iron-bearing minerals. The vesicles are often irregular in shape and size, giving the rock a spongy or frothy texture. When secondary mineralization occurs, these vesicles, as well as fractures and cracks within the scoria, become partially or completely filled with various minerals. In the case of copper mineralization, common secondary minerals include chrysocolla (blue-green hydrous copper silicate), malachite (green hydrous copper carbonate), azurite (blue hydrous copper carbonate), and sometimes native copper (metallic red-brown). These minerals provide vibrant color contrasts against the dark scoria matrix.

How to Identify

Color: Scoria itself is typically dark brown, reddish-brown, or black. The secondary copper minerals introduce vibrant colors: blue-green (chrysocolla), bright green (malachite), deep blue (azurite), or metallic reddish-brown (native copper).
Luster: Scoria has a dull to earthy luster. The secondary minerals can exhibit various lusters: waxy to vitreous (chrysocolla), silky to dull (malachite), vitreous (azurite), or metallic (native copper).
Texture: Highly vesicular, spongy, or frothy due to abundant gas bubbles. The vesicles are often filled or partially filled by the secondary minerals, creating a distinctive 'amygdaloidal' texture.
Crystal Form: Scoria is amorphous or microcrystalline. Secondary minerals can form botryoidal, mammillary, stalactitic, or encrusting habits within the vesicles and fractures. Individual crystals of malachite or azurite may be acicular or tabular, but are often cryptocrystalline in these settings. Native copper can occur as dendritic or massive fillings.
Cleavage: Scoria lacks cleavage. The secondary minerals have varying cleavage properties: chrysocolla (none), malachite (perfect on {001}, good on {010}), azurite (perfect on {011}, good on {100}), native copper (none).
Geological Environment: Volcanic terrains, particularly in areas with basaltic volcanism and subsequent hydrothermal alteration. Often found in lava flows, cinder cones, and volcanic ash deposits that have undergone post-emplacement fluid circulation.

Key Facts

Hardness: Scoria: 5-6 (Mohs). Secondary minerals vary: Chrysocolla (2.5-3.5), Malachite (3.5-4), Azurite (3.5-4), Native Copper (2.5-3).
Specific Gravity: Scoria: 1.0-2.0 (due to high porosity). Secondary minerals vary: Chrysocolla (2.0-2.4), Malachite (3.9-4.0), Azurite (3.77-3.89), Native Copper (8.94).
Crystal System: Scoria: Amorphous to microcrystalline. Secondary minerals: Chrysocolla (amorphous to cryptocrystalline, sometimes orthorhombic), Malachite (monoclinic), Azurite (monoclinic), Native Copper (isometric).
Color: Scoria: Dark brown, reddish-brown, black. Secondary minerals: Blue-green (chrysocolla), bright green (malachite), deep blue (azurite), metallic reddish-brown (native copper).
Luster: Scoria: Dull to earthy. Secondary minerals: Waxy to vitreous (chrysocolla), silky to dull (malachite), vitreous (azurite), metallic (native copper).
Transparency: Scoria: Opaque. Secondary minerals: Translucent to opaque.
Fracture: Scoria: Conchoidal to irregular. Secondary minerals: Conchoidal (chrysocolla, azurite), subconchoidal to uneven (malachite), hackly (native copper).
Cleavage: Scoria: None. Secondary minerals: Chrysocolla (none), Malachite (perfect on {001}, good on {010}), Azurite (perfect on {011}, good on {100}), Native Copper (none).
Composition: Scoria: Primarily basaltic (plagioclase, pyroxene, olivine, volcanic glass). Secondary minerals: Chrysocolla (CuSiO3·nH2O), Malachite (Cu2(CO3)(OH)2), Azurite (Cu3(CO3)2(OH)2), Native Copper (Cu).

Quick Check

Color: Dark brown/black scoria with bright blue, green, or reddish-brown infillings.
Luster: Dull to earthy for scoria; variable for infillings (waxy, silky, vitreous, metallic).
Streak: Scoria: brownish-black. Chrysocolla: white to pale blue-green. Malachite: pale green. Azurite: blue. Native copper: metallic reddish-brown.

Physical Characteristics

Crystal Habit: Scoria: Vesicular, frothy, massive. Secondary minerals: Botryoidal, mammillary, encrusting, stalactitic, massive, acicular, tabular, dendritic.
Cleavage Type: Scoria: Not applicable. Secondary minerals: Variable (see 'cleavage' in Key Facts).
Fracture Type: Scoria: Conchoidal to irregular. Secondary minerals: Variable (see 'fracture' in Key Facts).
Tenacity: Scoria: Brittle. Secondary minerals: Brittle (chrysocolla, malachite, azurite), ductile and malleable (native copper).
Luster Type: Scoria: Dull to earthy. Secondary minerals: Waxy, vitreous, silky, dull, metallic.

Formation

Scoria forms during explosive volcanic eruptions when gas-rich basaltic magma is ejected and rapidly cools. The rapid exsolution of volcanic gases (primarily H2O, CO2, SO2) creates abundant vesicles (gas bubbles) within the solidifying lava. Secondary mineralization occurs later, as hydrothermal fluids, often rich in dissolved copper, circulate through the porous scoria. These fluids precipitate copper-bearing minerals within the vesicles and fractures as they cool and react with the host rock or other dissolved species.

Usage

While scoria itself is used as lightweight aggregate, landscaping material, and in gas grills, scoria with significant secondary copper mineralization is primarily of interest to mineral collectors, geologists, and occasionally as a low-grade copper ore, though the economic viability is often limited by the disseminated nature of the mineralization.

Age Distribution

Variable, typically Cenozoic to recent, but can occur in older volcanic terrains.

Where to Find

Keweenaw Peninsula, Michigan, USA

Famous for its native copper deposits, often found in amygdaloidal basalts, which are essentially scoria or vesicular basalt filled with secondary minerals, including native copper, epidote, calcite, and zeolites.

Arizona, USA

Numerous copper mining districts, particularly in the southwestern US, where secondary copper minerals like chrysocolla and malachite are common in oxidized zones of copper deposits, sometimes infilling vesicular volcanic rocks.

Chile

A major copper producer, with many deposits exhibiting secondary copper mineralization in various host rocks, including volcanic sequences.

Democratic Republic of Congo (DRC)

Known for its vast copper-cobalt deposits, where secondary copper minerals are abundant and can be found in diverse geological settings.

Finding Tips

Look in Volcanic Regions

Focus on areas with recent or ancient basaltic volcanism. Cinder cones, lava flows, and volcanic ash beds are prime locations for scoria.

Identify Hydrothermal Alteration

Seek out areas that show signs of hydrothermal activity, such as hot springs (past or present), altered rock zones, or proximity to known copper deposits. Hydrothermal fluids are key to secondary mineralization.

Examine Vesicles and Fractures

Carefully inspect the vesicles (holes) and any fractures in scoria. Look for colorful infillings that are distinct from the dark host rock. A hand lens can be very useful.

Check for Green and Blue Colors

The presence of bright green (malachite) or blue-green (chrysocolla) colors is a strong indicator of copper mineralization. Metallic reddish-brown specks might indicate native copper.

Test Hardness (Carefully)

While scoria is relatively soft, the secondary minerals vary. Chrysocolla (2.5-3.5), malachite (3.5-4), azurite (3.5-4), native copper (2.5-3). This can help differentiate them from other infillings like zeolites or calcite.

Similar Rocks

Pumice

Also known as: Volcanic foam

Basalt

Also known as: Volcanic rock

Amygdaloidal Basalt

Also known as: Filled vesicular basalt

Scientific Classification

Mineral Class: Scoria: Igneous rock. Secondary minerals: Silicates (chrysocolla), Carbonates (malachite, azurite), Native Elements (native copper).
Group: Scoria: Volcanic rock. Secondary minerals: Hydrous copper silicates, hydrous copper carbonates, native metals.
Crystal System: Scoria: Amorphous to microcrystalline. Secondary minerals: Variable (see 'crystalSystem' in Key Facts).
Chemical Formula: Scoria: (Na,Ca)(Al,Mg,Fe)Si2O6 (general basaltic composition). Secondary minerals: CuSiO3·nH2O (chrysocolla), Cu2(CO3)(OH)2 (malachite), Cu3(CO3)2(OH)2 (azurite), Cu (native copper).
Composition: Scoria: Mafic, rich in iron and magnesium, with plagioclase feldspar, pyroxene, and often olivine. Secondary minerals: Hydrated copper silicate, hydrated copper carbonate, elemental copper.

Explore Scoria with Copper Mineralization

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