Basalt with Secondary Minerals

Igneous (Extrusive) with Secondary Mineral Infillings

Basalt (igneous rock) with secondary minerals (e.g., zeolites, calcite, or iron oxides)

Also known as: Amygdaloidal Basalt, Vesicular Basalt with Infillings, Zeolite Basalt, Calcite Basalt

Got a photo? Identify rocks instantly with the Rockby app

Description

Basalt with secondary mineralization is characterized by a dark, fine-grained to aphanitic (or sometimes porphyritic) basaltic matrix containing distinct infillings of lighter-colored or crystalline secondary minerals within vesicles (amygdales) or fractures. The basalt matrix is typically black, dark gray, or greenish-black. The secondary minerals can vary widely in color, crystal habit, and luster. Zeolites often form radiating aggregates, drusy coatings, or distinct crystals (e.g., tabular heulandite, fibrous natrolite, blocky chabazite). Calcite typically forms rhombohedral or scalenohedral crystals, or massive white infillings. Quartz and chalcedony can form botryoidal, banded (agate), or crystalline infillings. The size of the amygdales can range from microscopic to several centimeters in diameter. The presence and type of secondary minerals are indicative of the post-emplacement hydrothermal alteration history of the basalt.

How to Identify

Color: Basalt matrix is typically dark gray to black, sometimes greenish-black. Secondary minerals vary widely: zeolites (white, colorless, pink, orange, yellow), calcite (white, colorless), quartz/chalcedony (white, gray, brown, banded), iron oxides (red, brown, yellow).
Luster: Basalt matrix is dull to sub-vitreous. Secondary minerals can be vitreous (quartz, calcite), silky (fibrous zeolites), pearly (some zeolites), or earthy (iron oxides).
Texture: Basalt matrix is aphanitic to fine-grained, often porphyritic. The key identifying feature is the presence of amygdales (filled vesicles) or veins/fractures filled with secondary minerals. These infillings can be crystalline, massive, botryoidal, or radiating.
Crystal Form: Basalt matrix minerals (plagioclase, pyroxene, olivine) are typically anhedral to subhedral. Secondary minerals exhibit characteristic crystal forms: rhombohedral/scalenohedral for calcite, various tabular, fibrous, or blocky forms for zeolites, hexagonal prisms for quartz.
Cleavage: Basalt matrix minerals (pyroxene, plagioclase) have distinct cleavages, but these are often not visible in hand specimen due to fine grain size. Secondary minerals like calcite have perfect rhombohedral cleavage (3 directions, not at 90°). Some zeolites may have good cleavage (e.g., heulandite). Quartz has no cleavage.
Geological Environment: Formed in areas of basaltic volcanism, including oceanic crust, continental flood basalt provinces, volcanic islands, and rift zones. The secondary mineralization occurs in the post-emplacement stage due to hydrothermal alteration, often associated with burial, geothermal activity, or interaction with seawater.

Key Facts

Hardness: Basalt matrix: 5-6 on Mohs scale (due to pyroxene, plagioclase). Secondary minerals vary: Calcite (3), Zeolites (3.5-5.5), Quartz (7).
Specific Gravity: Basalt matrix: 2.8-3.0 g/cm³. Secondary minerals vary: Calcite (2.71), Zeolites (2.0-2.4), Quartz (2.65).
Crystal System: Basalt matrix minerals are typically monoclinic (pyroxene, plagioclase) or orthorhombic (olivine). Secondary minerals vary: Calcite (trigonal), Zeolites (monoclinic, orthorhombic, tetragonal, cubic), Quartz (trigonal).
Color: Dark gray to black basalt matrix; secondary minerals are highly variable (white, colorless, pink, orange, yellow, red, brown).
Luster: Basalt matrix: dull to sub-vitreous. Secondary minerals: vitreous, silky, pearly, earthy.
Transparency: Basalt matrix: opaque. Secondary minerals: transparent to translucent (calcite, quartz, many zeolites), opaque (iron oxides).
Fracture: Basalt matrix: conchoidal to uneven. Secondary minerals vary: Calcite (conchoidal to uneven), Quartz (conchoidal), Zeolites (uneven to conchoidal).
Cleavage: Basalt matrix minerals have cleavage but often not visible in hand specimen. Secondary minerals vary: Calcite (perfect rhombohedral), some Zeolites (good to perfect), Quartz (none).
Composition: Basalt: Predominantly plagioclase feldspar (labradorite), pyroxene (augite), and often olivine. Secondary minerals: Hydrated aluminosilicates (zeolites), calcium carbonate (calcite), silicon dioxide (quartz, chalcedony), iron oxides/hydroxides (goethite, hematite).

Quick Check

Color: Dark gray to black matrix with lighter-colored infillings (white, pink, colorless, red, brown).
Luster: Dull to sub-vitreous matrix; infillings can be vitreous, silky, pearly, or earthy.
Streak: Basalt: grayish-black. Secondary minerals vary: calcite (white), quartz (white), zeolites (white).

Physical Characteristics

Crystal Habit: Basalt matrix: fine-grained, aphanitic, sometimes porphyritic. Secondary minerals: varied, including radiating aggregates, fibrous, tabular, blocky, rhombohedral, scalenohedral, botryoidal, massive, drusy.
Cleavage Type: Basalt matrix minerals: typically two directions at ~90° (pyroxene) or ~60°/120° (amphibole, if present), or two directions at ~90° (plagioclase). Secondary minerals: Calcite (perfect rhombohedral), some zeolites (good to perfect), Quartz (none).
Fracture Type: Basalt matrix: conchoidal to uneven. Secondary minerals: conchoidal (quartz, calcite), uneven (many zeolites).
Tenacity: Basalt matrix: brittle. Secondary minerals: brittle (calcite, quartz, most zeolites).
Luster Type: Basalt matrix: dull to sub-vitreous. Secondary minerals: vitreous, silky, pearly, earthy.

Formation

Basalt is a fine-grained, mafic extrusive igneous rock formed from the rapid cooling of basaltic lava flows at or near the Earth's surface. During or after its formation, basalt often contains vesicles (gas bubbles) due to the exsolution of volatiles (e.g., water vapor, carbon dioxide) during eruption. Secondary mineralization occurs when these vesicles, as well as fractures and other pore spaces within the basalt, are subsequently filled by minerals precipitated from circulating hydrothermal fluids. These fluids, often heated by geothermal gradients or magmatic intrusions, leach elements from the surrounding rock and deposit new minerals as temperature, pressure, and chemical conditions change. Common secondary minerals include zeolites (e.g., stilbite, heulandite, analcime, chabazite, natrolite), calcite (CaCO3), quartz (SiO2), chalcedony (cryptocrystalline SiO2), various iron oxides/hydroxides (e.g., goethite, hematite), and sometimes prehnite, pumpellyite, or epidote, depending on the metamorphic grade and fluid chemistry.

Usage

Historically, basalt itself has been used as a construction material (e.g., aggregate, paving stones). Basalt with attractive secondary mineralization, particularly well-formed zeolite crystals or colorful agate/chalcedony, is highly prized by mineral collectors and lapidaries. Some zeolite-rich basalts have been explored for their ion-exchange properties, though this is less common than for sedimentary zeolites. Calcite-filled amygdales can be a source of decorative stone.

Age Distribution

Ranges from Precambrian to Cenozoic, depending on the age of the basaltic volcanism and subsequent hydrothermal activity. Most commonly observed in Cenozoic and Mesozoic flood basalts and oceanic crust.

Where to Find

Deccan Traps, India

World-renowned for abundant and diverse zeolite occurrences within amygdaloidal basalts, particularly in Maharashtra state. Famous for large, well-formed crystals of stilbite, heulandite, apophyllite, and cavansite.

Columbia River Basalt Group, USA

Extensive flood basalts in Washington, Oregon, and Idaho, containing various zeolites, calcite, and agate/chalcedony infillings.

Iceland

Active volcanic island with numerous occurrences of basalt with zeolite and other secondary mineral infillings, particularly along coastal exposures and in geothermal areas.

Nova Scotia, Canada

Triassic-Jurassic flood basalts along the Bay of Fundy are famous for their rich zeolite localities, including analcime, chabazite, natrolite, and stilbite.

Faroe Islands

Tertiary flood basalts with significant zeolite mineralization, similar to those found in Iceland and Scotland.

Brazil (Rio Grande do Sul)

Basalts of the Paraná Basin are known for amethyst and agate geodes, which are essentially large amygdales filled with quartz and chalcedony.

Finding Tips

Look for Vesicular Textures

Begin by identifying basaltic outcrops, especially those with a vesicular (bubbly) texture. These vesicles are the primary sites for secondary mineral deposition.

Examine Weathered Surfaces

Weathering can sometimes highlight the infillings, as secondary minerals may be more resistant or have different colors than the basalt matrix. Look for areas where the basalt has broken open to reveal the amygdales.

Check for Hydrothermal Alteration Zones

Areas with evidence of past or present hydrothermal activity (e.g., hot springs, altered rock, mineral veins) are prime locations for finding well-developed secondary mineralization.

Coastal and River Exposures

Erosion by water can expose fresh surfaces of basalt, making it easier to spot amygdales. Coastal cliffs and riverbeds in basaltic regions are often productive.

Safety Precautions

When collecting, always wear appropriate safety gear, including eye protection, gloves, and sturdy footwear. Be aware of unstable cliffs and falling rocks. Some secondary minerals, like quartz, can produce respirable crystalline silica dust if broken or cut, so use proper ventilation and dust masks if processing samples. Basalt itself is generally safe to handle.

Similar Rocks

Unaltered Basalt

Basalt

Also known as: Volcanic Rock, Trap Rock

Andesite

Also known as: Intermediate Volcanic Rock

Scoria

Also known as: Vesicular Basalt

Diabase (Dolerite)

Diabase

Also known as: Microgabbro

Scientific Classification

Mineral Class: Igneous rock with various secondary mineral classes (e.g., Tectosilicates for zeolites and quartz, Carbonates for calcite, Oxides/Hydroxides for iron minerals).
Group: Basalt is a mafic volcanic rock. Secondary minerals belong to various groups based on their chemistry and structure.
Crystal System: Basalt matrix minerals are typically monoclinic or orthorhombic. Secondary minerals exhibit various crystal systems (trigonal, monoclinic, orthorhombic, tetragonal, cubic).
Chemical Formula: Basalt: Complex silicate mixture (Ca,Na)(Al,Si)2Si2O8 (plagioclase), (Ca,Mg,Fe)SiO3 (pyroxene), (Mg,Fe)2SiO4 (olivine). Secondary minerals: e.g., NaAlSi2O6·H2O (analcime), CaAl2Si7O18·7H2O (heulandite), CaCO3 (calcite), SiO2 (quartz).
Composition: Basalt is rich in ferromagnesian minerals and calcic plagioclase. Secondary minerals are typically hydrous aluminosilicates (zeolites), calcium carbonate, or silica.

Explore Basalt with Secondary Minerals

Identification Localities & Collecting Look-alikes

Identify rocks anywhere

Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.

Download on theApp Store Get it onGoogle Play Continue onWeb

Open in app