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Amygdaloidal basalt is a dark-colored, fine-grained extrusive igneous rock characterized by the presence of numerous almond-shaped or spherical cavities (amygdales) filled with secondary minerals. The matrix of the rock is typically basaltic, composed predominantly of plagioclase feldspar (labradorite to bytownite), pyroxene (augite), and often olivine, with minor amounts of iron-titanium oxides. The texture is aphanitic to porphyritic, meaning individual mineral grains in the matrix are generally too small to be seen with the naked eye, though larger phenocrysts may be present. The amygdales themselves can vary significantly in size, from a few millimeters to several centimeters, and their infilling minerals provide a striking contrast to the dark basaltic host rock.
How to Identify
- Color
- 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).
- Luster
- Dull to earthy in the basaltic matrix; infilling minerals can exhibit vitreous, silky, or waxy luster.
- Texture
- Aphanitic to porphyritic matrix with distinctive amygdaloidal texture due to mineral-filled vesicles. The amygdales are typically ovoid or spherical.
- Crystal Form
- 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).
- Cleavage
- 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).
- Geological Environment
- 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.
Quick Check
- Color: Dark gray to black matrix with lighter-colored (white, pink, colorless, banded) rounded inclusions.
- Luster: Dull to earthy matrix; vitreous, waxy, or silky inclusions.
- Streak: Gray to grayish-black (for the basaltic matrix).
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.
Formation
Amygdaloidal basalt forms from the rapid cooling of basaltic lava flows. As the lava erupts and flows, dissolved gases (primarily water vapor, carbon dioxide, and sulfur dioxide) exsolve from the magma due to decreasing pressure. These gas bubbles become trapped within the solidifying lava, forming vesicles. Subsequently, during diagenesis or low-grade metamorphism, these vesicles are filled with secondary minerals precipitated from circulating hydrothermal fluids. Common infilling minerals include zeolites (e.g., stilbite, heulandite, analcime, natrolite), chalcedony, quartz, calcite, chlorite, and various clay minerals. The term 'amygdale' (from Greek 'amygdalea' meaning almond tree, referring to the almond-like shape of the filled vesicles) specifically refers to these filled vesicles.
Usage
Historically, amygdaloidal basalts have been used as building materials and road aggregate, similar to other basalts. However, their primary significance lies in their role as host rocks for economically important mineral deposits, particularly zeolites, agates, and other secondary minerals. These secondary minerals are collected for their aesthetic value (e.g., agates for lapidary work, zeolites for mineral specimens) and for industrial applications (e.g., zeolites as molecular sieves, catalysts, and ion exchangers).
Age Distribution
Ranges from Precambrian to Cenozoic, common in large igneous provinces and oceanic crust.
Where to Find
Deccan Traps, India
Extensive flood basalt province, renowned for abundant amygdaloidal basalts containing a wide variety of zeolites, agates, and other secondary minerals.
Keweenaw Peninsula, Michigan, USA
Part of the Midcontinent Rift System, famous for copper mineralization hosted in amygdaloidal basalts, often with associated zeolites and other secondary minerals.
Columbia River Basalt Group, Pacific Northwest, USA
Another large igneous province with widespread basaltic flows, where amygdaloidal textures are common, often containing chalcedony and zeolites.
Iceland
Active volcanic island with numerous basaltic lava flows, frequently exhibiting vesicular and amygdaloidal textures.
Brazil (especially Rio Grande do Sul)
Known for large agate geodes and amethyst-filled amygdales within basaltic flows.
Finding Tips
Look for Volcanic Terrains
Amygdaloidal basalts are found in regions with past or present volcanic activity, particularly large igneous provinces and areas of extensive lava flows.
Examine Outcrops and Road Cuts
Freshly exposed surfaces in road cuts, quarries, and stream beds often reveal the characteristic textures. Look for dark, fine-grained rocks with lighter-colored, rounded inclusions.
Check for Weathering
Weathering can sometimes highlight the amygdales, as the infilling minerals may be more resistant or erode differently than the basaltic matrix.
Search for Associated Minerals
The presence of zeolites, chalcedony, quartz, or calcite in the field can be a strong indicator of amygdaloidal basalt, as these are common infilling minerals.
Safety Precautions
When collecting, always wear appropriate safety gear, including eye protection and gloves. Be aware of unstable slopes and falling rocks in quarry or outcrop environments. Basalt itself is generally safe, but some associated minerals might contain trace elements; however, the primary concern is physical safety during collection.
Similar Rocks
Vesicular Basalt
Vesicular Basalt
Also known as: Scoria (if highly vesicular and frothy)
Basalt
Basalt
Also known as: Trap Rock
Andesite
Andesite
Also known as: Porphyritic Andesite
Scientific Classification
- Mineral Class
- Not a single mineral, but a rock. The matrix is igneous. The infilling minerals belong to various classes (e.g., silicates for zeolites and quartz, carbonates for calcite).
- Group
- Igneous Rock (Extrusive, Volcanic)
- Crystal System
- Polycrystalline; matrix minerals are typically triclinic (plagioclase) and monoclinic (pyroxene). Infilling minerals vary.
- Chemical Formula
- Variable, reflecting the complex mineralogical composition of basalt and the diverse infilling minerals. Basalt is primarily (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 (pyroxene) and (Ca,Na)(Al,Si)4O8 (plagioclase).
- Composition
- Mafic igneous rock with secondary mineral infillings. Major elements in the basaltic matrix include SiO2 (45-52 wt%), Al2O3 (14-18 wt%), FeO (8-12 wt%), MgO (6-12 wt%), CaO (8-12 wt%), Na2O (2-4 wt%), K2O (<1 wt%), TiO2 (1-3 wt%). Infilling minerals introduce additional elements depending on their specific chemistry.
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