Granulite

Granulite is a coarse-grained, high-grade metamorphic rock that forms under extreme temperatures (roughly 700-900 degrees C) and high pressure deep within the continental crust. It belongs to the granulite metamorphic facies, the hottest stable conditions short of melting.

The rock is dominated by anhydrous minerals because water-bearing micas and amphiboles become unstable at such heat. Felsic granulites are rich in quartz and feldspar, while mafic granulites carry pyroxene and garnet. A faint streaky or granoblastic texture is common, but strong foliation is usually weak or absent.

Granulites record some of the deepest crustal conditions accessible at the surface and are prized by geologists for what they reveal about the lower crust.

Granulite forms during regional metamorphism at the granulite facies, where temperatures exceed about 700 degrees C and pressures reflect mid- to lower-crustal depths of 20-40 km. These conditions arise in continental collision zones, in the roots of ancient mountain belts, and in deep crust heated by underplating mantle magmas.

The defining process is dehydration: as rocks heat, hydrous minerals like biotite and hornblende break down, releasing fluids and leaving behind dry assemblages of pyroxene, garnet, and feldspar. Many granulites are exhumed within ancient shield areas (Archean and Proterozoic cratons) after billions of years of uplift and erosion.

Classic granulite terrains occur in southern India, Sri Lanka, Norway (Lofoten), Scotland, and the Adirondacks of New York.

Look for a coarse, granular, evenly crystalline rock with a sugary granoblastic texture and only weak banding. Colors range from pale pinkish-grey felsic types to dark greenish mafic varieties, often with red garnet specks.

Hardness is effectively that of its quartz and feldspar (6-7), and it lacks the easy splitting of schist or slate. Key tells are the presence of orthopyroxene (giving a satiny bronze sheen, as in charnockite) and the near absence of mica.

Look-alikes: gneiss shows much stronger compositional banding; charnockite is essentially an orthopyroxene-bearing granulite and grades into it. Unlike schist, granulite does not flake or show shiny mica sheets.

Granulite is used mainly as a durable construction and dimension stone for paving, facing, and aggregate, valued for its hardness and resistance to weathering. Some attractive garnet-bearing or charnockitic granulites are polished as decorative slabs.

Its greatest value, however, is scientific: granulites are natural samples of the deep continental crust, and geologists study them to reconstruct ancient pressures, temperatures, and tectonic histories. Garnet and pyroxene compositions act as geothermobarometers.

Granulite has little role in jewelry or metaphysical practice, though garnets extracted from it are occasionally cut as gems.