How to identify Basalt with Secondary Minerals

To correctly identify Basalt with Secondary Minerals (Basalt (igneous rock) with secondary minerals (e.g., zeolites, calcite, or iron oxides)), 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

   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).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Basalt matrix is dull to sub-vitreous. Secondary minerals can be vitreous (quartz, calcite), silky (fibrous zeolites), pearly (some zeolites), or earthy (iron oxides).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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).

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.

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