How to identify Amygdaloidal Basalt

To correctly identify Amygdaloidal Basalt (Amygdaloidal Basalt), 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

   Typically dark gray to black, sometimes greenish-black due to alteration. The infilling minerals in the amygdales can be white, colorless, pink, red, green, or banded (e.g., agate).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy in the basaltic matrix; infilling minerals can exhibit vitreous, silky, or waxy luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Aphanitic to porphyritic matrix with distinctive amygdaloidal texture due to mineral-filled vesicles. The amygdales are typically ovoid or spherical.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Matrix minerals are anhedral to subhedral, fine-grained. Infilling minerals can show well-formed euhedral crystals within the amygdales (e.g., zeolite crystals, quartz crystals) or cryptocrystalline/massive forms (e.g., chalcedony, agate).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Basaltic matrix minerals (plagioclase, pyroxene) exhibit characteristic cleavage planes, but these are often not visible in the fine-grained groundmass. Infilling minerals will show their specific cleavage properties (e.g., calcite has perfect rhombohedral cleavage, quartz has no cleavage).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Formed in volcanic settings, specifically within the upper portions of basaltic lava flows, particularly flood basalts, shield volcanoes, and mid-ocean ridges. Also found in dikes and sills where gas exsolution occurred.

Key Facts

Hardness

Matrix: 5-6 on Mohs scale (due to plagioclase and pyroxene). Infilling minerals vary: Quartz (7), Calcite (3), Zeolites (3.5-5.5).

Specific Gravity

Matrix: 2.8-3.0 g/cm³. Overall rock density can be slightly lower if vesicles are not completely filled or if low-density minerals like zeolites are abundant.

Crystal System

Matrix minerals (plagioclase, pyroxene) are triclinic and monoclinic, respectively. Infilling minerals vary (e.g., quartz is trigonal, calcite is trigonal, zeolites vary).

Color

Dark gray to black matrix; infillings are variable (white, colorless, pink, red, green, banded).

Luster

Dull to earthy for the matrix; vitreous, waxy, or silky for infilling minerals.

Transparency

Opaque for the basaltic matrix; infilling minerals can be transparent to translucent.

Fracture

Conchoidal to uneven for the basaltic matrix. Infilling minerals vary (e.g., quartz is conchoidal, calcite is conchoidal to uneven).

Cleavage

Basaltic matrix minerals have cleavage but are often too fine-grained to observe. Infilling minerals exhibit their characteristic cleavage (e.g., calcite has perfect rhombohedral cleavage; quartz has none).

Composition

Predominantly plagioclase feldspar (labradorite to bytownite), pyroxene (augite), and often olivine in the matrix. Amygdales are filled with secondary minerals such as zeolites (e.g., stilbite, heulandite, analcime, natrolite), chalcedony, quartz, calcite, chlorite, and various clay minerals.

Physical characteristics

• Crystal Habit: Matrix: Fine-grained, anhedral to subhedral. Amygdales: Filled with massive, cryptocrystalline, or euhedral to subhedral crystals of secondary minerals.
• Cleavage Type: Matrix minerals have good cleavage (e.g., plagioclase: 2 directions at ~90°; pyroxene: 2 directions at ~90°). Infilling minerals vary (e.g., calcite: perfect rhombohedral; quartz: none).
• Fracture Type: Conchoidal to uneven for the basaltic matrix. Infilling minerals vary.
• Tenacity: Brittle.
• Luster Type: Dull to earthy for the matrix; vitreous, waxy, or silky for infilling minerals.

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