metamorphic
Amphibolite with Hornblende
Amphibole-rich Metamorphic Rock (Ca2(Mg,Fe)5Si8O22(OH)2)
Hardness: 5-6 (Mohs scale); Color: Black, dark green, or gray with white/light feldspar matrix; Luster: Vitreous to sub-metallic on dark crystals; Crystal Structure: Monoclinic (hornblende crystals often bladed or prismatic); Cleavage: Perfect at 56 and 124 degrees; Specific Gravity: 3.0-3.5.
- Hardness
- 5-6 (Mohs scale)
- Color
- Black, dark green, or gray with white/light feldspar matrix
- Luster
- Vitreous to sub-metallic on dark crystals
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Black, dark green, or gray with white/light feldspar matrix; Luster: Vitreous to sub-metallic on dark crystals; Crystal Structure: Monoclinic (hornblende crystals often bladed or prismatic); Cleavage: Perfect at 56 and 124 degrees; Specific Gravity: 3.0-3.5.
Formation & geological history
Formed through the regional metamorphism of mafic igneous rocks (like basalt or gabbro) or impure limestone under high temperature and pressure conditions, typically found in orogenic (mountain-building) belts.
Uses & applications
Used primarily in the construction industry as crushed stone for roadbeds and railroad ballast. Some attractive specimens with large crystals are used as ornamental stone or for geological education.
Geological facts
Hornblende is a 'trash can' mineral because its chemical formula contains many different elements that can swap places. Amphibolite is a key rock for geologists to determine the metamorphic grade of a region.
Field identification & locations
Identify in the field by looking for dark, needle-like or prismatic crystals embedded in a lighter matrix. Common in areas with exposed Precambrian basement rock or mountain ranges like the Appalachians or Alps.
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