Granulite
Granulite is a dense, high-temperature metamorphic rock of the deep continental crust, made of feldspar, quartz, pyroxene and often garnet, with almost no water-bearing minerals. It forms at around 750 to 900 degrees Celsius and 7 to 12 kilobars.
Granulite is a dense, high-temperature metamorphic rock of the deep continental crust, made of feldspar, quartz, pyroxene and often garnet, with almost no water-bearing minerals. It forms at around 750 to 900 degrees Celsius and 7 to 12 kilobars.
🪨 Geology Guide Specimen: Open Garnet–Pyroxene Granulite in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 6 subgroups of Metamorphic, and test your eye in LithoClade.
A Dry Rock: Why Are There No Micas?
At granulite temperatures the hydrous minerals, biotite, muscovite and hornblende, break down, releasing water and often a melt. Much of that liquid is drawn off, so the leftover rock is anhydrous, dominated by orthopyroxene, plagioclase, quartz and garnet.
Lower Crust: Where Does Granulite Come From?
Granulite is thought to be a main component of the lower continental crust. Pieces reach the surface as xenoliths carried up in volcanic rocks and in terranes uplifted and exposed by erosion. Its high density and dryness make the lower crust strong.
Name and Texture: Why Is It Called Granulite?
The name comes from the granular texture of the recrystallized grains, which are often stretched into streaks, with ribbon-like quartz. It is the final step in the series that begins with slate, and it often follows migmatite. Compare eclogite, a rock of similar depth made under different conditions.
Granulite at a Glance
| Property | Value |
|---|---|
| Type / variety | High-Temperature Lower-Crustal Granulite |
| Composition | Plagioclase + Quartz + Orthopyroxene + Red Garnet (Anhydrous assemblage; micas dehydrated) |
| Geology Guide group | Metamorphic Rocks (Foliated, Granoblastic & High-P) |
| Rock Classification | High-Grade Foliated (Gneiss & Migmatite) |
| Mohs hardness | Mohs 6.5–7.5 |
| Texture / habit | Dense, streaky granoblastic-elongate fabric with ribbon quartz |
| Density | 2.85–3.15 g/cm³ |
| Lustre and cleavage | Streaky light feldspathic bands with red garnet and bronzy pyroxene |
| Geological setting | Deep continental lower crust at 750–900 °C and 7–12 kbar |
| Diagnostic feature (Geology Guide) | Dense, deep-crustal rock with streaky ribbon quartz, feldspar, red garnet, and bronzy orthopyroxene formed at temperatures so extreme (750–900 °C) that hydrous micas broke down and drove off their water. |
| Typical temperature | About 750-900 °C |
| Key feature | Anhydrous mineral assemblage |
See Also
- Migmatite — the partly molten rock just before
- Banded Orthogneiss — a gneiss of lower grade
- Eclogite — a dense high-pressure metamorphic rock