Basalt with secondary minerals

Description

Basalt with secondary mineralization refers to basaltic rocks that have undergone post-emplacement alteration, resulting in the formation of new minerals within the rock's matrix, vesicles, or fractures. The original basalt is typically fine-grained to aphanitic, dark-colored (black, dark gray, greenish-black), and composed primarily of plagioclase feldspar (labradorite to bytownite), pyroxene (augite), and often olivine. The presence of secondary minerals introduces a variety of colors (e.g., green for epidote and chlorite, white/colorless for quartz and calcite, various colors for zeolites) and textures. These secondary minerals often fill amygdales (infilled vesicles), veins, or replace primary minerals. The degree of alteration can range from minor vesicle infillings to pervasive replacement of the original rock fabric.

How to Identify

Color

The primary basalt matrix is typically dark gray to black. Secondary minerals introduce various colors: epidote (pistachio green to yellowish-green), quartz (colorless, white, gray), chlorite (dark green), calcite (white, colorless), zeolites (white, colorless, pale yellow, pink).

Luster

Basalt matrix is dull to sub-vitreous. Secondary minerals can exhibit vitreous (quartz, some zeolites), pearly (some zeolites), or dull (chlorite, epidote) lusters.

Texture

Fine-grained to aphanitic basaltic groundmass, often porphyritic with phenocrysts of plagioclase or pyroxene. The key identifying feature is the presence of amygdales (vesicles filled with secondary minerals), veins, or disseminated secondary mineral grains. Amygdales can be spherical, ovoid, or irregular.

Crystal Form

Secondary minerals often occur as euhedral to subhedral crystals filling voids (amygdales, veins) or as anhedral grains replacing primary minerals. Epidote typically forms prismatic crystals, quartz as anhedral to euhedral crystals (often drusy in vugs), zeolites as various habits (fibrous, tabular, radiating, blocky), and calcite as rhombohedral or scalenohedral crystals.

Cleavage

Basalt itself does not exhibit macroscopic cleavage. The secondary minerals will show their characteristic cleavage: epidote (one perfect), quartz (none), calcite (three perfect rhombohedral), chlorite (one perfect basal).

Geological Environment

Common in ancient and modern volcanic terrains, particularly in submarine basalts (oceanic crust), continental flood basalts, and rift zones. Alteration occurs during burial, hydrothermal circulation (e.g., near mid-ocean ridges, geothermal areas), or low-grade regional metamorphism (e.g., greenschist facies).

Key Facts

• Hardness: Basalt matrix: 5-6 on Mohs scale. Secondary minerals vary: Quartz (7), Epidote (6-7), Calcite (3), Zeolites (3.5-5.5), Chlorite (2-2.5).
• Specific Gravity: Basalt matrix: 2.8-3.0 g/cm³. Secondary minerals vary: Epidote (3.3-3.5), Quartz (2.65), Calcite (2.71), Zeolites (2.0-2.4), Chlorite (2.6-3.3).
• Crystal System: Basalt is polymineralic, not a single crystal system. Secondary minerals: Quartz (Trigonal), Epidote (Monoclinic), Calcite (Trigonal), Zeolites (Monoclinic, Orthorhombic, Tetragonal, Isometric), Chlorite (Monoclinic).
• Color: Dark gray to black (basalt) with various colors from secondary minerals (green, white, colorless, pink, yellow).
• Luster: Dull to sub-vitreous (basalt); vitreous, pearly, dull (secondary minerals).
• Transparency: Opaque (basalt); transparent to translucent (quartz, calcite, some zeolites); translucent to opaque (epidote, chlorite).
• Fracture: Conchoidal to uneven (basalt); conchoidal (quartz); uneven (epidote, calcite, zeolites, chlorite).
• Cleavage: None visible in basalt matrix. Secondary minerals: Epidote (one perfect), Quartz (none), Calcite (three perfect rhombohedral), Chlorite (one perfect basal).
• Composition: Primary basalt: Plagioclase feldspar (Ca-rich), pyroxene (augite), olivine, magnetite. Secondary minerals: Hydrous calcium aluminum iron silicate (epidote), silicon dioxide (quartz), calcium carbonate (calcite), hydrous aluminum silicates (zeolites), hydrous magnesium iron aluminum silicate (chlorite).

Quick Check

• Color: Dark gray to black matrix with green (epidote, chlorite), white/colorless (quartz, calcite, zeolites), or other colored infillings.
• Luster: Dull to sub-vitreous for basalt; vitreous, pearly, or dull for secondary minerals.
• Streak: White to grayish-white for the basalt matrix. Individual secondary minerals will have their characteristic streak (e.g., white for quartz and calcite, greenish-white for epidote).

Physical Characteristics

• Crystal Habit: Basalt: Aphanitic to fine-grained, sometimes porphyritic. Secondary minerals: Prismatic (epidote), anhedral to euhedral (quartz), rhombohedral/scalenohedral (calcite), various (zeolites), flaky/micaceous (chlorite).
• Cleavage Type: Basalt: Not applicable. Secondary minerals: Perfect (epidote, calcite, chlorite), none (quartz).
• Fracture Type: Conchoidal to uneven (basalt, quartz); uneven (epidote, calcite, zeolites, chlorite).
• Tenacity: Brittle (basalt, quartz, epidote, calcite, zeolites); flexible (chlorite).
• Luster Type: Dull to sub-vitreous (basalt); vitreous (quartz, some zeolites); pearly (some zeolites); dull to vitreous (epidote); dull to pearly (chlorite).

Formation

Basalt is an extrusive igneous rock formed from the rapid cooling of basaltic lava at or near the Earth's surface. Secondary mineralization occurs when these basalts are subsequently subjected to hydrothermal alteration, low-grade metamorphism (e.g., burial metamorphism, regional metamorphism, or contact metamorphism), or weathering processes. Fluids (often hot, chemically active water) circulate through fractures and vesicles (gas bubbles) within the basalt, dissolving primary minerals and precipitating new secondary minerals. Common secondary minerals include epidote, quartz, chlorite, calcite, various zeolites (e.g., analcime, stilbite, heulandite), prehnite, pumpellyite, and chalcedony.

Usage

While the primary basalt is used extensively as aggregate, road base, and in construction, basalt with significant secondary mineralization may have altered physical properties. Specimens with well-formed secondary minerals can be highly prized by mineral collectors. Epidote-rich altered basalts can be indicators of specific metamorphic conditions. Zeolite-rich basalts have industrial applications as adsorbents, catalysts, and in water purification.

Age Distribution

Globally distributed, ranging from Precambrian to Cenozoic, depending on the age of the original basaltic volcanism and subsequent alteration events.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Igneous rock with secondary minerals from various classes (silicates, carbonates, oxides).

Group

Basalt (igneous rock); Epidote group (silicates); Quartz group (silicates); Calcite group (carbonates); Zeolite group (silicates); Chlorite group (silicates).

Crystal System

Polymineralic rock. Secondary minerals: Trigonal (quartz, calcite), Monoclinic (epidote, chlorite, some zeolites), Orthorhombic (some zeolites), Tetragonal (some zeolites), Isometric (some zeolites).

Chemical Formula

Basalt: Complex mixture of (Ca,Na)(Al,Si)Si2O8 (plagioclase), (Ca,Mg,Fe)SiO3 (pyroxene), (Mg,Fe)2SiO4 (olivine), Fe3O4 (magnetite). Secondary minerals: Ca2(Al,Fe)3(SiO4)3(OH) (epidote), SiO2 (quartz), CaCO3 (calcite), various complex hydrous aluminosilicates (zeolites), (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6 (chlorite).

Composition

Mafic igneous rock (basalt) altered by hydrothermal fluids or low-grade metamorphism, leading to the precipitation of hydrous silicates (epidote, chlorite, zeolites), carbonates (calcite), and silica (quartz).

Explore Basalt with secondary minerals