How to identify Scoria with secondary alteration

To correctly identify Scoria with secondary alteration (Scoria (volcanic rock) with secondary alteration/mineralization (e.g., chlorite, epidote, or lichen)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   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.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

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

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

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

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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).

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