Basalt with secondary mineralization (likely calcite or quartz)

Igneous (extrusive) with secondary minerals

Also known as: Amygdaloidal Basalt (if vesicles are filled), Vesicular Basalt with infillings

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Description

Basalt with secondary mineralization is a dark-colored, fine-grained to aphanitic igneous rock characterized by the presence of infillings of other minerals within its primary vesicles or fractures. The basaltic matrix is typically black, dark gray, or greenish-black. The secondary minerals, commonly calcite (white, clear, or yellowish) or quartz (clear, white, gray, or amethystine), fill pre-existing voids, creating distinctive patterns and textures. These infillings can range from microscopic to several centimeters in size, often forming amygdules (almond-shaped fillings) or veinlets. The presence and type of secondary mineralization significantly alter the rock's appearance and, to some extent, its physical properties.

How to Identify

Color: Basalt matrix is typically dark gray to black. Secondary minerals vary: Calcite is usually white, clear, or yellowish; Quartz is clear, white, gray, or purple (amethyst).
Luster: Basalt matrix is dull to sub-vitreous. Calcite is vitreous to pearly. Quartz is vitreous.
Texture: Aphanitic to fine-grained basaltic matrix with visible infillings (amygdules, veinlets) of secondary minerals. Vesicular texture is common in the basalt host.
Crystal Form: Basalt minerals are typically anhedral to subhedral, microscopic. Secondary minerals can exhibit euhedral to subhedral crystal forms within cavities (e.g., rhombohedral calcite, hexagonal prismatic quartz) or massive/botryoidal habits (e.g., chalcedony).
Cleavage: Basalt minerals (pyroxene, plagioclase) have distinct cleavages but are rarely visible without magnification. Calcite exhibits perfect rhombohedral cleavage (3 directions, not at 90 degrees). Quartz has no cleavage, only conchoidal fracture.
Geological Environment: Volcanic flows, sills, dikes, and oceanic crust where basaltic magma has erupted and subsequently undergone hydrothermal alteration or diagenesis.

Key Facts

Hardness: Basalt matrix: 5-6 (Mohs). Calcite: 3 (Mohs). Quartz: 7 (Mohs).
Specific Gravity: Basalt: 2.8-3.0 g/cm³. Calcite: 2.71 g/cm³. Quartz: 2.65 g/cm³.
Crystal System: Basalt minerals: Monoclinic (pyroxene, plagioclase). Calcite: Trigonal. Quartz: Trigonal.
Color: Dark gray to black (basalt); white, clear, yellow (calcite); clear, white, gray, purple (quartz).
Luster: Dull to sub-vitreous (basalt); vitreous to pearly (calcite); vitreous (quartz).
Transparency: Opaque (basalt); transparent to translucent (calcite, quartz).
Fracture: Conchoidal to uneven (basalt); conchoidal (quartz); uneven (calcite).
Cleavage: Basalt minerals: 2 directions (pyroxene, plagioclase). Calcite: Perfect rhombohedral (3 directions). Quartz: None.
Composition: Basalt: Primarily plagioclase feldspar, pyroxene, olivine, and iron oxides. Calcite: CaCO₃. Quartz: SiO₂.

Quick Check

Color: Dark gray to black matrix with white, clear, or colored infillings.
Luster: Dull to sub-vitreous basalt, vitreous to pearly secondary minerals.
Streak: Basalt: Grayish-white. Calcite: White. Quartz: White.

Physical Characteristics

Crystal Habit: Basalt: Fine-grained, interlocking. Calcite: Rhombohedral, scalenohedral, massive, granular. Quartz: Prismatic, massive, cryptocrystalline (chalcedony).
Cleavage Type: Basalt: Imperfect to good in constituent minerals. Calcite: Perfect rhombohedral. Quartz: None.
Fracture Type: Basalt: Conchoidal to uneven. Calcite: Uneven to conchoidal. Quartz: Conchoidal.
Tenacity: Basalt: Brittle. Calcite: Brittle. Quartz: Brittle.
Luster Type: Basalt: Dull to sub-vitreous. Calcite: Vitreous to pearly. Quartz: Vitreous.

Formation

Basalt forms from the rapid cooling of basaltic lava at or near the Earth's surface. During or after cooling, gases trapped within the lava can create vesicles (cavities). Subsequently, circulating hydrothermal fluids, groundwater, or meteoric water can deposit secondary minerals (such as calcite, quartz, zeolites, or chalcedony) within these vesicles, fractures, or along grain boundaries.

Usage

Basalt itself is widely used as aggregate for construction, road building, and as dimension stone. Basalt with attractive secondary mineralization can be sought by collectors, used in lapidary arts, or as decorative stone. Specific secondary minerals like zeolites have industrial applications (e.g., adsorbents, catalysts).

Age Distribution

Ranges from Precambrian to Cenozoic, depending on the basaltic host rock. Secondary mineralization can occur at any time after the basalt's formation.

Where to Find

Columbia River Basalt Group, USA

Extensive flood basalts in Washington, Oregon, and Idaho, known for abundant zeolites, agates, and other secondary minerals in amygdales.

Deccan Traps, India

One of the largest volcanic provinces globally, rich in basaltic flows with numerous secondary mineral occurrences, including zeolites, apophyllite, and various forms of quartz.

North Atlantic Igneous Province (e.g., Iceland, Faroe Islands, Scotland)

Basaltic terrains with common occurrences of secondary minerals in vesicles and fractures.

Brazil (especially Rio Grande do Sul)

Known for world-class amethyst and agate geodes hosted within basaltic flows.

Various oceanic islands (e.g., Hawaii, Reunion)

Younger basaltic flows can exhibit early-stage secondary mineralization.

Finding Tips

Look for Vesicular Basalt

Secondary mineralization primarily occurs in vesicles. Look for basaltic outcrops with visible holes or cavities, as these are prime locations for infillings.

Examine Weathered Surfaces

Weathering can sometimes expose or highlight the secondary minerals, making them more visible against the darker basalt matrix.

Check Fractures and Veins

Secondary minerals can also precipitate along fractures and fault lines within the basalt, forming veinlets.

Utilize Acid Test (for Calcite)

A drop of dilute hydrochloric acid (HCl) will effervesce on calcite, confirming its presence. Always exercise caution with acids.

Observe Hardness (for Quartz)

Quartz is significantly harder than calcite (Mohs 7 vs. 3). A steel knife or common nail will scratch calcite but not quartz.

Similar Rocks

Unmineralized Basalt

Basalt

Also known as: Volcanic rock, Trap rock

Andesite with secondary mineralization

Also known as: Amygdaloidal Andesite

Scoria

Also known as: Volcanic clinker

Pumice

Also known as: Volcanic foam

Scientific Classification

Mineral Class: Igneous rock (basalt) with carbonate (calcite) or silicate (quartz) minerals.
Group: Igneous rock (basalt); Carbonate mineral (calcite); Tectosilicate mineral (quartz).
Crystal System: Basalt: Polycrystalline, constituent minerals are monoclinic, orthorhombic, etc. Calcite: Trigonal. Quartz: Trigonal.
Chemical Formula: Basalt: Complex silicate mixture. Calcite: CaCO₃. Quartz: SiO₂.
Composition: Basalt: Mafic, rich in Fe, Mg, Ca. Calcite: Calcium carbonate. Quartz: Silicon dioxide.

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