Scoria with secondary mineralization

Description

Scoria with secondary mineralization is a dark-colored, highly vesicular, mafic volcanic rock that has undergone post-emplacement alteration. The primary scoria is typically black, dark brown, or reddish-brown, characterized by abundant, irregularly shaped vesicles (gas bubbles) that give it a low density. The secondary mineralization manifests as coatings, infillings, or replacements within these vesicles and along fracture surfaces. These secondary minerals often impart distinct colors (e.g., yellow, orange, red, brown from iron oxides; white, green, or pale colors from clay minerals or zeolites) and textures, contrasting with the original scoria. The degree of alteration can range from minor coatings to complete infilling of vesicles, significantly changing the rock's overall appearance and properties.

How to Identify

Color

Original scoria is typically black, dark brown, or reddish-brown. Secondary mineralization introduces a range of colors: yellows, oranges, reds, and browns (iron oxides/hydroxides); whites, pale greens, or earthy tones (clay minerals); white or colorless (zeolites, calcite, quartz).

Luster

Original scoria is dull to earthy. Secondary minerals can exhibit various lusters: earthy (clays, iron oxides), vitreous (quartz, some zeolites), silky (some zeolites), or dull.

Texture

Highly vesicular (porous) with irregularly shaped, often interconnected vesicles. The secondary minerals will fill or coat these vesicles, creating a more solid or encrusted appearance within the pores. The overall texture remains rough and abrasive.

Crystal Form

The primary scoria is amorphous to microcrystalline. Secondary minerals may exhibit microcrystalline to macroscopic crystal forms within the vesicles, such as botryoidal, reniform, radiating, or drusy habits for zeolites, calcite, or quartz, or earthy/massive for clays and iron oxides.

Cleavage

No cleavage in the scoria matrix. Individual secondary minerals may exhibit cleavage (e.g., calcite, some zeolites), but this is typically not observable in the bulk rock.

Geological Environment

Volcanic terrains, particularly around cinder cones, stratovolcanoes, and basaltic lava flows. Found in areas where post-eruptive hydrothermal activity, groundwater circulation, or prolonged weathering has occurred.

Key Facts

• Hardness: Variable. Scoria matrix is generally 5-6 (Mohs), but the overall hardness can be reduced if the matrix is extensively altered to soft clay minerals. Secondary minerals vary widely (e.g., clays 1-2.5, zeolites 3.5-5.5, quartz 7, calcite 3).
• Specific Gravity: Variable, typically 1.0-2.0 for scoria (due to high porosity). Secondary mineralization can increase the bulk density if vesicles are filled with denser minerals, but it will still be relatively light compared to solid basalt.
• Crystal System: Scoria matrix is amorphous to microcrystalline. Secondary minerals can be various crystal systems (e.g., monoclinic for many zeolites and clays, trigonal for calcite, hexagonal for quartz).
• Color: Dark (black, brown, reddish-brown) with secondary colors (red, yellow, white, green, pale).
• Luster: Dull to earthy for scoria; variable for secondary minerals (earthy, vitreous, silky, dull).
• Transparency: Opaque for scoria matrix. Secondary minerals can be opaque, translucent, or rarely transparent (e.g., small quartz crystals).
• Fracture: Conchoidal to irregular for scoria. Secondary minerals may exhibit their own fracture patterns.
• Cleavage: None for scoria matrix. Some secondary minerals may exhibit cleavage.
• Composition: Primary scoria is mafic (basaltic) in composition, rich in pyroxenes, olivine, and plagioclase feldspar, with a glassy groundmass. Secondary mineralization involves the deposition of iron oxides/hydroxides (e.g., FeO(OH), Fe2O3), clay minerals (e.g., hydrated aluminosilicates), zeolites (e.g., hydrated tectosilicates of alkali and alkaline earth metals), and sometimes carbonates (e.g., CaCO3) or silica (SiO2).

Quick Check

• Color: Dark (black, brown, reddish-brown) with contrasting colors (red, yellow, white, green) in vesicles.
• Luster: Dull to earthy for scoria matrix; variable for secondary minerals (earthy, vitreous, silky).
• Streak: Typically brownish-black for scoria; secondary minerals may have their own streak (e.g., reddish-brown for hematite, yellowish-brown for goethite, white for clays).

Physical Characteristics

• Crystal Habit: Scoria is massive, highly vesicular. Secondary minerals can form coatings, infillings, botryoidal, reniform, radiating, or drusy aggregates within vesicles.
• Cleavage Type: Not applicable for the scoria matrix. Individual secondary minerals may have distinct cleavage (e.g., perfect rhombohedral in calcite, good in some zeolites).
• Fracture Type: Conchoidal to irregular for the scoria matrix. Secondary minerals may exhibit their own fracture types.
• Tenacity: Brittle for scoria. Secondary minerals can be brittle, earthy, or sectile depending on the mineral.
• Luster Type: Dull to earthy for scoria. Secondary minerals can be earthy, vitreous, silky, or dull.

Formation

Scoria forms during explosive volcanic eruptions from mafic (basaltic) magma. The rapid cooling and exsolution of volcanic gases create a highly vesicular, frothy texture. Secondary mineralization occurs post-formation, as hydrothermal fluids, groundwater, or weathering processes interact with the porous scoria. These fluids deposit new minerals within the vesicles and fractures, or alter the primary minerals of the scoria itself. Common secondary minerals include iron oxides/hydroxides (e.g., goethite, hematite, limonite), clay minerals (e.g., smectite, kaolinite), zeolites, and sometimes carbonates or silica.

Usage

While scoria itself is used as lightweight aggregate, landscaping material, and in gas grills, scoria with significant secondary mineralization is generally less desirable for these applications due to altered physical properties (e.g., increased density, reduced porosity, friability). However, it can be of interest to mineral collectors for the secondary mineral specimens it hosts. In some cases, extensive alteration can lead to economic deposits of clay minerals or zeolites.

Age Distribution

Variable, from recent to Cenozoic, depending on the volcanic activity and subsequent alteration events.

Where to Find

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Scientific Classification

Mineral Class

Not a single mineral, but a rock. Primary minerals are silicates (pyroxenes, olivine, feldspars). Secondary minerals can belong to various classes: oxides/hydroxides, silicates (clays, zeolites), carbonates.

Group

Igneous rock (extrusive, mafic) with secondary mineral assemblages.

Crystal System

Amorphous to microcrystalline for scoria matrix. Secondary minerals vary (e.g., monoclinic, trigonal, hexagonal, orthorhombic).

Chemical Formula

Highly variable. Primary scoria is broadly (Mg,Fe)2SiO4 (olivine), (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 (pyroxene), NaAlSi3O8-CaAl2Si2O8 (plagioclase). Secondary minerals include FeO(OH) (goethite), Fe2O3 (hematite), Al2Si2O5(OH)4 (kaolinite), (Na,Ca,K)x(AlxSiyO2(x+y))·nH2O (zeolites), CaCO3 (calcite), SiO2 (quartz).

Composition

Mafic volcanic rock (basaltic) composed primarily of volcanic glass, plagioclase, pyroxene, and olivine, with secondary infillings or alterations of iron oxides/hydroxides, clay minerals, zeolites, and sometimes carbonates or silica.

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