Amygdaloidal Basalt

Description

Amygdaloidal basalt is a dark-colored, fine-grained extrusive igneous rock characterized by the presence of amygdules. The basaltic matrix is typically black to dark gray, aphanitic (fine-grained), and may contain phenocrysts of olivine, pyroxene, or plagioclase. The defining feature is the presence of numerous, often ovoid or almond-shaped (amygdaloidal) cavities filled with secondary minerals. These fillings can vary widely in size, shape, and mineralogy, creating a distinctive texture. The color and appearance of the amygdules depend entirely on the infilling mineral(s).

How to Identify

Color

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).

Luster

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

Texture

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.

Crystal Form

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).

Cleavage

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).

Geological Environment

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).

Quick Check

• Color: Dark gray to black matrix with variable colored infillings.
• Luster: Dull to sub-vitreous matrix; amygdules vary by mineral.
• Streak: White to grayish-white (for basalt matrix, if fine enough to streak).

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.

Formation

Amygdaloidal basalt forms from the cooling and solidification of basaltic lava flows. During eruption, dissolved gases (primarily water vapor, carbon dioxide, and sulfur dioxide) exsolve from the magma as pressure decreases, forming bubbles (vesicles) within the lava. As the lava cools and solidifies, these vesicles are preserved. Subsequently, during diagenesis or low-grade metamorphism, these empty vesicles are partially or completely filled by secondary minerals precipitated from circulating hydrothermal fluids. Common infilling minerals include zeolites (e.g., stilbite, heulandite, analcime, chabazite), chalcedony, quartz, calcite, chlorite, prehnite, and epidote. The term 'amygdule' specifically refers to these secondary mineral fillings.

Usage

Historically, amygdaloidal basalts have been used as local building materials and road aggregate. When the amygdules contain attractive minerals like agates, zeolites, or amethyst, the rock can be sought after by mineral collectors and lapidaries for ornamental purposes. Some zeolite-rich amygdaloidal basalts have been explored for their ion-exchange properties, though this is less common than for primary zeolite deposits.

Age Distribution

Ranges from Precambrian to Cenozoic, common in large igneous provinces and oceanic crust.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Igneous Rock (Extrusive)

Group

Volcanic Rocks

Crystal System

Polycrystalline (basalt matrix); Amygdule minerals vary.

Chemical Formula

Complex silicate (basalt matrix); Amygdule minerals have specific formulas (e.g., SiO2 for quartz, CaCO3 for calcite, various hydrated aluminosilicates for zeolites).

Composition

Mafic, low silica content (45-52% SiO2) for the basalt matrix, with secondary mineral infillings.

Explore Amygdaloidal Basalt