How to identify Vesicular Basalt with Secondary Mineralization

To correctly identify Vesicular Basalt with Secondary Mineralization (Basalt with secondary infilling minerals (likely calcite or 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

   Basaltic matrix is typically dark gray to black. Infilling minerals vary: white, colorless, or yellowish for calcite; white, gray, or translucent for quartz/chalcedony; various colors for zeolites (e.g., white, pink, orange, green).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Basaltic matrix is dull to sub-vitreous. Infilling minerals can be vitreous (quartz, calcite), waxy (chalcedony), or pearly/vitreous (zeolites).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Porphyritic to aphanitic basaltic groundmass with numerous spherical to ovoid amygdules (filled vesicles). The amygdules can be massive, crystalline, or botryoidal.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Basaltic minerals are typically anhedral to subhedral, fine-grained. Infilling minerals exhibit characteristic crystal forms: rhombohedral/scalenohedral for calcite, hexagonal prisms for quartz, fibrous/radiating for some zeolites, botryoidal for chalcedony.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Basaltic minerals (plagioclase, pyroxene) have distinct cleavages, but are often too fine-grained to observe macroscopically. Calcite exhibits perfect rhombohedral cleavage (3 directions not at 90°). Quartz has no cleavage. Zeolites vary.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Formed in subaerial or shallow subaqueous basaltic lava flows, particularly in flood basalts, shield volcanoes, and mid-ocean ridge environments where subsequent hydrothermal alteration or diagenesis has occurred.

Key Facts

Hardness

Basaltic matrix: 5-6 (Mohs). Calcite: 3. Quartz: 7. Zeolites: 3.5-5.5.

Specific Gravity

Basaltic matrix: 2.8-3.0. Calcite: 2.71. Quartz: 2.65. Zeolites: 2.0-2.4.

Crystal System

Basaltic minerals are typically anhedral. Calcite: Trigonal. Quartz: Trigonal. Zeolites: Monoclinic, Orthorhombic, Tetragonal, Isometric (depending on species).

Color

Dark gray to black matrix; infillings vary widely (white, clear, yellow, pink, green, purple).

Luster

Dull to sub-vitreous for basalt; infillings can be vitreous, waxy, pearly.

Transparency

Basaltic matrix is opaque. Infilling minerals can be transparent to translucent.

Fracture

Basalt: uneven to conchoidal. Calcite: conchoidal to uneven. Quartz: conchoidal. Zeolites: uneven.

Cleavage

Basaltic minerals are fine-grained, difficult to observe. Calcite: perfect rhombohedral (3 directions). Quartz: none. Zeolites: good to perfect in 1 or 2 directions.

Composition

Primary basalt: predominantly plagioclase feldspar (Ca-rich), pyroxene, olivine, Fe-Ti oxides. Secondary infillings: CaCO3 (calcite), SiO2 (quartz, chalcedony, opal), various hydrated aluminosilicates (zeolites), chlorite, prehnite.

Physical characteristics

• Crystal Habit: Basaltic groundmass is typically aphanitic to fine-grained. Infilling minerals show diverse habits: rhombohedral/scalenohedral (calcite), prismatic (quartz), botryoidal/massive (chalcedony), fibrous/radiating/tabular (zeolites).
• Cleavage Type: Basaltic minerals have cleavage but are often too small to see. Calcite: perfect rhombohedral. Quartz: none. Zeolites: variable, good to perfect.
• Fracture Type: Basalt: uneven to conchoidal. Calcite: conchoidal to uneven. Quartz: conchoidal. Zeolites: uneven.
• Tenacity: Basalt: brittle. Calcite: brittle. Quartz: brittle. Zeolites: brittle.
• Luster Type: Basalt: dull to sub-vitreous. Calcite: vitreous. Quartz: vitreous. Chalcedony: waxy. Zeolites: vitreous to pearly.

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