How to identify Amygdaloidal Basalt

To correctly identify Amygdaloidal Basalt (Basalt with amygdules (secondary mineral fillings)), 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

   Matrix is typically dark gray to black. Amygdules can be white (calcite, some zeolites), colorless (quartz, chalcedony), green (chlorite, epidote, prehnite), pink/orange (stilbite, heulandite), or purple (amethyst).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Basaltic matrix is dull to sub-vitreous. Amygdules can range from vitreous (quartz, calcite) to silky (some zeolites) to dull (chlorite).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Aphanitic (fine-grained) to porphyritic basaltic matrix with distinctive amygdaloidal texture due to mineral-filled vesicles. Amygdules are typically ovoid, spherical, or irregular, ranging from millimeters to several centimeters in size.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Basalt matrix minerals are typically anhedral to subhedral. Amygdule minerals can exhibit well-formed euhedral crystals within the cavity (e.g., quartz, calcite, some zeolites) or be massive/botryoidal (e.g., chalcedony, chlorite).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Basalt matrix minerals (plagioclase, pyroxene) have characteristic cleavages, but these are often too small to observe without magnification. Amygdule minerals will exhibit their characteristic cleavage (e.g., calcite: rhombohedral; zeolites: variable).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Volcanic lava flows, particularly subaerial or shallow submarine flows, often associated with continental flood basalts, oceanic islands, and mid-ocean ridges. The amygdules form during post-emplacement alteration by hydrothermal fluids.

Key Facts

Hardness

Basalt matrix: 5-6 on Mohs scale. Amygdule minerals vary (e.g., calcite 3, quartz 7, zeolites 3.5-5.5).

Specific Gravity

Basalt matrix: 2.8-3.0 g/cm³. Amygdule minerals vary (e.g., calcite 2.7, quartz 2.65, zeolites 2.0-2.4).

Crystal System

Basalt matrix: Polycrystalline, typically fine-grained. Amygdule minerals: Varies depending on the mineral (e.g., quartz: trigonal; calcite: trigonal; zeolites: monoclinic, orthorhombic, tetragonal, cubic).

Color

Dark gray to black matrix; amygdules are highly variable.

Luster

Dull to sub-vitreous for basalt; amygdules vary from vitreous to silky to dull.

Transparency

Basalt matrix: Opaque. Amygdule minerals: Transparent to translucent to opaque.

Fracture

Basalt matrix: Conchoidal to uneven. Amygdule minerals: Varies (e.g., quartz: conchoidal; calcite: conchoidal to uneven).

Cleavage

Basalt matrix: Poorly developed or absent in hand sample. Amygdule minerals: Varies (e.g., calcite: perfect rhombohedral; zeolites: good to perfect in one or more directions; quartz: none).

Composition

Basalt matrix: Predominantly plagioclase feldspar (labradorite), pyroxene (augite), and often olivine, with minor iron-titanium oxides. Amygdules: Secondary minerals such as quartz, chalcedony, calcite, chlorite, and various zeolites (e.g., stilbite, heulandite, analcime, chabazite, laumontite).

Physical characteristics

• Crystal Habit: Basalt matrix: Granular, aphanitic. Amygdules: Massive, botryoidal, drusy, or well-formed crystals within cavities.
• Cleavage Type: Basalt matrix: Not typically observable in hand sample. Amygdules: Varies by mineral.
• Fracture Type: Basalt matrix: Conchoidal to uneven. Amygdules: Varies by mineral.
• Tenacity: Brittle for both basalt matrix and most amygdule minerals.
• Luster Type: Basalt matrix: Dull to sub-vitreous. Amygdules: Vitreous, silky, pearly, or dull depending on the mineral.

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