Scoria with secondary alteration

Description

Scoria is a dark-colored, highly vesicular, mafic to intermediate volcanic rock. It is typically reddish-brown, black, or dark gray. 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 the rock matrix, become partially or completely filled or coated with new minerals. Common secondary minerals include zeolites (e.g., analcime, chabazite, stilbite), calcite, chalcedony, quartz, chlorite, epidote, and various iron oxides. Lichens, while not minerals, can also colonize the surface, adding a biological component to the alteration. The presence of these secondary minerals or lichens can significantly change the appearance, color, and even the density of the scoria.

How to Identify

Color

Typically dark (black, dark gray, reddish-brown) for the scoria matrix. Secondary minerals can introduce a wide range of colors: green (chlorite, epidote), white/colorless (calcite, quartz, zeolites), red/yellow/brown (iron oxides), or various hues from lichens.

Luster

Scoria matrix is typically dull to earthy. Secondary minerals can exhibit vitreous (quartz, calcite), silky (some zeolites), or earthy (some clays, iron oxides) lusters.

Texture

Highly vesicular (spongy, frothy) with numerous gas bubbles. The vesicles may be partially or completely filled with secondary minerals, giving a more solid or crystalline appearance to the infillings. Lichens will appear as crusty, leafy, or powdery growths on the surface.

Crystal Form

Scoria itself is amorphous to microcrystalline. Secondary minerals will exhibit their characteristic crystal forms within the vesicles or fractures (e.g., rhombohedral calcite, prismatic zeolites, granular epidote, cryptocrystalline chalcedony).

Cleavage

Scoria lacks cleavage. Secondary minerals will exhibit their characteristic cleavage (e.g., rhombohedral cleavage in calcite, perfect cleavage in some zeolites).

Geological Environment

Volcanic terrains, cinder cones, lava flows, and volcanic ash deposits. Secondary mineralization occurs in environments where hydrothermal fluids or groundwater can circulate through the porous scoria, often in post-eruptive or diagenetic settings. Lichens are surface colonizers in various climates.

Key Facts

• Hardness: Scoria matrix is generally soft to moderately hard (Mohs 5-6 for constituent minerals like plagioclase, pyroxene, olivine). Secondary minerals vary widely: calcite (3), chlorite (2-2.5), epidote (6-7), quartz (7), zeolites (3.5-5.5).
• Specific Gravity: Scoria is typically low (1.0-2.0 g/cm³) due to high porosity, often floating on water when fresh. Secondary mineralization can increase the overall density, depending on the density and amount of infilling minerals (e.g., calcite ~2.7, epidote ~3.3-3.5, quartz ~2.65).
• Crystal System: Scoria is amorphous to microcrystalline. Secondary minerals exhibit their specific crystal systems (e.g., trigonal for calcite and quartz, monoclinic for chlorite and epidote, various for zeolites).
• Color: Dark (black, dark gray, reddish-brown) for the scoria matrix, with additional colors from secondary minerals (green, white, yellow, red) or lichens.
• Luster: Dull to earthy for scoria; variable for secondary minerals (vitreous, silky, earthy).
• Transparency: Opaque for scoria. Secondary minerals can be transparent to translucent (quartz, calcite, some zeolites) or opaque (chlorite, epidote, iron oxides).
• Fracture: Conchoidal to irregular for scoria. Secondary minerals exhibit their characteristic fracture (e.g., conchoidal for quartz, splintery for some zeolites).
• Cleavage: Absent in scoria. Present in many secondary minerals (e.g., rhombohedral in calcite, perfect basal in chlorite, one perfect in epidote).
• Composition: Scoria is typically mafic to intermediate, composed primarily of volcanic glass, plagioclase feldspar, pyroxene, and sometimes olivine. Secondary mineralization introduces new mineral phases such as hydrated silicates (zeolites, chlorite, epidote), carbonates (calcite), silica (quartz, chalcedony), and iron oxides/hydroxides.

Quick Check

• Color: Dark matrix with varied colors from secondary minerals (green, white, yellow, etc.) or lichens.
• Luster: Dull to earthy for scoria, varied for secondary minerals (vitreous, silky, earthy).
• Streak: Typically gray to brownish-black for scoria. Secondary minerals will have their characteristic streak (e.g., white for calcite, colorless for quartz).

Physical Characteristics

• Crystal Habit: Scoria is massive, vesicular. Secondary minerals can form drusy coatings, radiating aggregates, blocky crystals, or cryptocrystalline masses within vesicles and fractures.
• Cleavage Type: Not applicable for scoria. Variable for secondary minerals (e.g., rhombohedral, perfect basal, prismatic).
• Fracture Type: Conchoidal to irregular for scoria. Variable for secondary minerals (e.g., conchoidal, uneven, splintery).
• Tenacity: Brittle for scoria and most secondary minerals.
• Luster Type: Dull to earthy for scoria. Variable for secondary minerals (e.g., vitreous, silky, earthy).

Formation

Scoria forms during explosive volcanic eruptions when gas-rich magma is ejected and rapidly cools. The rapid exsolution of volcanic gases creates abundant vesicles (gas bubbles) within the solidifying lava. Secondary mineralization occurs when hydrothermal fluids, groundwater, or even biological agents (like lichens) interact with the porous scoria over time, depositing new minerals within the vesicles and fractures, or altering the existing rock matrix.

Usage

Scoria itself is used as lightweight aggregate, landscaping material, and in gas grills. Scoria with secondary mineralization is primarily of scientific interest for studying alteration processes, paleoclimate, and microbial activity in volcanic environments. Some specimens with aesthetically pleasing mineral infillings might be collected for display.

Age Distribution

Ranges from Cenozoic to recent, depending on the volcanic activity and subsequent alteration events. Secondary mineralization can occur over millions of years.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Not a single mineral; it's a rock. Secondary minerals belong to various classes: silicates (chlorite, epidote, zeolites, quartz), carbonates (calcite), oxides (iron oxides).

Group

Igneous rock (extrusive, vesicular). Secondary minerals belong to their respective mineral groups.

Crystal System

Amorphous to microcrystalline for scoria. Variable for secondary minerals.

Chemical Formula

Variable for scoria (complex silicate melt composition). Variable for secondary minerals (e.g., CaCO3 for calcite, SiO2 for quartz, complex for chlorite, epidote, zeolites).

Composition

Primarily volcanic glass, plagioclase, pyroxene, olivine for scoria. Secondary minerals introduce elements like Ca, Mg, Fe, Al, Si, O, H, C, etc., depending on the specific mineral phases.

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