metamorphic
Amphibolite (specifically Hornblende Schist/Gneiss)
Amphibole-rich Metamorphic Rock
Hardness: 5-7 (Mohs scale); Color: Black to dark green with white/gray flecks; Luster: Vitreous to sub-metallic on crystal faces; Structure: Foliated or lineated with visible needle-like or bladed crystals; Cleavage: Two directions at 56 and 124 degrees; Specific Gravity: 2.8–3.4.
- Hardness
- 5-7 (Mohs scale)
- Color
- Black to dark green with white/gray flecks
- Luster
- Vitreous to sub-metallic on crystal faces
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Physical properties
Hardness: 5-7 (Mohs scale); Color: Black to dark green with white/gray flecks; Luster: Vitreous to sub-metallic on crystal faces; Structure: Foliated or lineated with visible needle-like or bladed crystals; Cleavage: Two directions at 56 and 124 degrees; Specific Gravity: 2.8–3.4.
Formation & geological history
Formed at high temperature and pressure conditions (regional metamorphism) from the transformation of mafic igneous rocks like basalt or gabbro, or from clay-rich sedimentary rocks. Typically found in mountain belts and Precambrian shields.
Uses & applications
Used primarily in construction as 'crushed stone' for roadbeds and railroad ballast. Due to its durability and attractive dark color when polished, it is occasionally used as architectural facing stone or 'black granite' in countertops.
Geological facts
Amphibolite is a major constituent of the Earth's lower continental crust. The presence of large hornblende crystals can indicate the amount of water present during the rock's metamorphism, as hornblende is a hydrous mineral.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and the presence of elongated, shimmering black crystals often oriented in a parallel fashion. Commonly found in the Appalachian Mountains, the Canadian Shield, and the Scandinavian Peninsula.
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