How to identify Vesicular Basalt with Secondary Infillings

To correctly identify Vesicular Basalt with Secondary Infillings (Basalt (vesicular) with secondary infillings (e.g., calcite, zeolites)), 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

   Dark gray to black basaltic matrix with infillings of various colors (white, colorless, pink, orange, green, purple) depending on the secondary minerals present.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Basaltic matrix is typically dull to earthy; secondary minerals can range from vitreous (quartz, calcite) to pearly (some zeolites) or silky.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Aphanitic to fine-grained basaltic groundmass with numerous spherical to elongated cavities (vesicles) filled with secondary minerals (amygdules). The texture is distinctly amygdaloidal.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Basaltic matrix shows interlocking anhedral to subhedral grains. Secondary minerals exhibit their characteristic crystal forms within the amygdules (e.g., rhombohedral calcite, tabular heulandite, acicular natrolite, botryoidal chalcedony).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Basaltic minerals (pyroxene, plagioclase) have characteristic cleavages, but these are rarely visible in the fine-grained matrix. Secondary minerals will show their specific cleavage (e.g., perfect rhombohedral in calcite, good in some zeolites).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in ancient and modern basaltic lava flows, flood basalts, and oceanic crust. Often associated with areas of hydrothermal alteration or groundwater circulation through porous volcanic rocks.

Key Facts

Hardness

Basaltic matrix: 5-6 (Mohs); Secondary minerals: variable, e.g., Calcite 3, Zeolites 3.5-5.5, Quartz 7.

Specific Gravity

Basaltic matrix: 2.8-3.0; Secondary minerals: variable, e.g., Calcite 2.71, Zeolites 2.0-2.4, Quartz 2.65.

Crystal System

Basaltic minerals: Monoclinic (pyroxene), Triclinic (plagioclase); Secondary minerals: variable, e.g., Calcite (Trigonal), Zeolites (Monoclinic, Orthorhombic, Tetragonal, Isometric, Triclinic).

Color

Dark gray to black (basalt); white, colorless, pink, orange, green, purple (infillings).

Luster

Dull to earthy (basalt); vitreous, pearly, silky (infillings).

Transparency

Opaque (basalt); transparent to translucent (infillings).

Fracture

Conchoidal to uneven (basalt); variable for infillings (e.g., conchoidal for quartz, uneven for calcite).

Cleavage

Poor to absent in fine-grained basalt; distinct cleavage in many secondary minerals (e.g., perfect rhombohedral in calcite).

Composition

Basalt: Predominantly plagioclase feldspar (labradorite to bytownite), pyroxene (augite), olivine (minor), and iron-titanium oxides. Secondary minerals: Hydrous aluminosilicates (zeolites), calcium carbonate (calcite), silicon dioxide (quartz/chalcedony), prehnite, clay minerals.

Physical characteristics

• Crystal Habit: Basalt: Aphanitic to fine-grained, sometimes porphyritic. Secondary minerals: Varied, including tabular, acicular, fibrous, botryoidal, rhombohedral, blocky, or granular forms within vesicles.
• Cleavage Type: Basalt: Not typically observed macroscopically due to fine grain size. Secondary minerals: Varies by mineral (e.g., perfect rhombohedral in calcite, good in some zeolites).
• Fracture Type: Basalt: Conchoidal to uneven. Secondary minerals: Varies by mineral (e.g., conchoidal in quartz, uneven in calcite).
• Tenacity: Basalt: Brittle. Secondary minerals: Varies (e.g., brittle for calcite and quartz, some zeolites can be more friable).
• Luster Type: Basalt: Dull to earthy. Secondary minerals: Vitreous, pearly, silky, waxy, or dull.

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