How to identify Basalt with secondary minerals

To correctly identify Basalt with secondary minerals (Basalt with secondary minerals (possibly epidote and quartz)), 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

   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).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

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

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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).

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).

Explore Basalt with secondary minerals