Amphibolite

Hardness: 5-6 (variable due to mineral composition). Color: Dark green, black, or grey. Luster: Vitreous to dull. Crystal structure: Typically massive to foliated, with elongated prismatic crystals of amphibole visible. Often exhibits a schistose or gneissic texture. Cleavage: Amphibole minerals have two distinct cleavages at 56 and 124 degrees. Specific Gravity: 2.9-3.2.

Amphibolite forms under conditions of medium to high-grade regional or contact metamorphism of mafic igneous rocks (like basalt or gabbro) or some impure limestones. This typically occurs at temperatures between 500-750C and pressures of 4-12 kbar. The protolith is subjected to burial and tectonic forces, leading to recrystallization of original minerals into amphibole, plagioclase, and other accessory minerals. Geological Age: Varies widely, depending on the age of the protolith and the metamorphic event, ranging from Precambrian to Cenozoic.

Amphibolite is used as crushed stone for road construction, railway ballast, and as aggregate in concrete. Due to its resistance to weathering and attractive dark color, it can be used as dimension stone for building facings, floor tiles, and countertops. Some varieties, especially those with interesting textures or mineral banding, are collected as ornamental stones. Historically, tough amphibolites were used for tools by ancient civilizations.

Amphibolite facies is a metamorphic facies typified by the presence of amphibole and plagioclase, indicating a specific range of temperature and pressure conditions during metamorphism. It is an important indicator rock for understanding regional metamorphic histories. The term 'amphibolite' can refer to a rock type and also to a metamorphic facies.

To identify amphibolite in the field, look for a dark-colored, medium- to coarse-grained metamorphic rock. It often has a speckled appearance due to the dark amphibole minerals (like hornblende) and lighter plagioclase feldspar. A key identification feature is the presence of elongated, prismatic amphibole crystals which may show a preferred orientation (foliation) or appear randomly oriented in a massive texture. It does not react to acid. Commonly found in ancient shields and mountain ranges worldwide, such as the Canadian Shield, Scandinavian Caledonides, and various metamorphic belts in the Appalachians and Rockies. Tips for collectors: Look for samples that clearly display the characteristic amphibole crystals and metamorphic texture. Break a fresh surface to better observe mineral grains and cleavage. Distinguish it from gabbro (an igneous rock) by its metamorphic texture (foliation or recrystallized appearance) and from dark schists by the dominance of amphibole over micas.