Hornblende Schist

Hornblende schist is a dark green to black, foliated metamorphic rock dominated by hornblende, a common amphibole mineral that grows as elongated, needle- or prism-shaped crystals. The aligned hornblende gives the rock its foliation and often a slightly fibrous or streaky look, with pale plagioclase feldspar between the dark crystals.

It is closely related to amphibolite, differing mainly in having a more pronounced schistose fabric. The rock forms at medium to high metamorphic grade from mafic (iron- and magnesium-rich) parent rocks.

Hornblende schist is a hard, tough rock and a common component of metamorphic terrains derived from ancient volcanic and intrusive rocks.

Hornblende schist forms by medium- to high-grade regional metamorphism of mafic igneous rocks such as basalt, gabbro, and dolerite, or of mafic volcanic sediments. At amphibolite-facies temperatures (roughly 500-700 degrees C), original pyroxenes and other minerals recrystallize into hornblende and plagioclase.

Directed pressure aligns the prismatic hornblende crystals, producing the schistose foliation. With increasing grade, hornblende schist grades into massive amphibolite and eventually toward granulite as conditions become hotter and drier.

It is common in the deeper parts of mountain belts and in ancient greenstone and gneiss terrains, including the Scottish Highlands, the Appalachians, and many Precambrian shields.

Look for a dark green to black, foliated rock with visible elongated or needle-like hornblende crystals aligned in one direction, set among white or grey plagioclase. The rock is hard (about 5-6) and tougher than mica or chlorite schist.

Hornblende shows good cleavage at roughly 56 and 124 degrees and a glassy to silky luster on crystal faces, which helps distinguish it. The streaky, fibrous-looking dark fabric is characteristic.

Look-alikes: amphibolite is nearly identical in composition but more massive and less foliated; chlorite schist is softer and lighter green; biotite schist is shiny and flaky rather than fibrous. Hardness and needle-like amphibole separate hornblende schist from softer schists.

Hornblende schist is used mainly as a construction material, crushed for aggregate, road base, and railway ballast, and sometimes cut as a tough, dark dimension or facing stone for its durability and attractive dark color.

Its hardness and toughness make it suitable where resistance to wear is needed, though its foliation can limit some structural uses. Decorative slabs showing aligned crystals are occasionally produced.

Geologically it is valuable as an indicator of medium- to high-grade metamorphism of mafic rocks. It has little gemstone value and no major metaphysical tradition.