Granulite Identification Guide
A field guide to granulite, a high-grade metamorphic rock, focusing on its granoblastic texture, garnet and pyroxene, and how to separate it from gneiss.
Read the full Granulite encyclopedia entry →
What Granulite Looks Like
Granulite is a high-grade (high-temperature) metamorphic rock formed deep in the crust. Its defining look is a granoblastic texture: roughly equal-sized, interlocking, polygonal grains that give the rock a sugary, even-grained appearance. Colors are variable but often pale to medium — grey, tan, pinkish, or greenish — sometimes with a faint banding. Diagnostic accessory minerals include red garnet, green to brown pyroxene (orthopyroxene/clinopyroxene), and feldspar; mica and amphibole are typically scarce or absent because conditions were too hot for them.
Step-by-Step Field-ID Checklist
- Examine the texture. Look for even-sized, tightly interlocking polygonal grains (granoblastic) rather than aligned flakes.
- Hunt for garnet and pyroxene. Rounded red garnets and stubby green/brown pyroxene crystals strongly suggest granulite-grade metamorphism.
- Check for the absence of micas. Little or no shiny mica/amphibole is a key clue — high grade destroyed them.
- Note any faint banding. Granulite may show weak compositional layering but lacks strong, flaky foliation.
- Test hardness. Quartz and feldspar make it scratch glass; garnet and pyroxene are also hard.
Key Diagnostic Tests
- Texture: Granoblastic (equigranular, polygonal grains) is the hallmark.
- Mineralogy: Feldspar + quartz with pyroxene and/or garnet; mica and hydrous amphibole minimal.
- Hardness: Scratches glass overall (6.5–7+).
- Acid test: No reaction (unless calc-silicate variety with carbonate).
- Density: Moderately high (~2.7–3.0+) where garnet and pyroxene are abundant.
Common Look-Alikes and How to Tell Them Apart
- Gneiss: Gneiss shows strong, continuous light/dark banding and often contains mica; granulite has weaker banding, a more even granoblastic texture, and anhydrous minerals (pyroxene, garnet) instead of mica.
- Granite: Granite is igneous with random interlocking grains but contains no metamorphic garnet/pyroxene assemblage; granulite's pyroxene-garnet content and granoblastic fabric betray its metamorphic origin.
- Charnockite: Closely related; charnockite is an orthopyroxene-bearing granitoid that can grade into granulite. Distinguishing them often needs thin-section work.
- Quartzite/marble: Granoblastic too, but those are monomineralic (all quartz or all carbonate) and quartzite won't show garnet/pyroxene; marble fizzes in acid.
Where Granulite Is Found
Granulites occur in deeply eroded ancient terranes — Precambrian shields and the roots of old mountain belts. Classic regions include the Indian granulite terranes (southern India), the Adirondacks (USA), Scandinavia, southern Norway, Scotland's Lewisian complex, and Antarctica. Search exhumed lower-crustal terrains and high-grade metamorphic belts.
Frequently asked questions
How can you identify granulite?
Look for a sugary, even-grained granoblastic texture with feldspar and quartz, plus red garnet and green-brown pyroxene, and little or no mica. That anhydrous, high-grade assemblage is diagnostic.
What does granulite look like?
A pale to medium grey, tan, pink, or greenish rock with roughly equal-sized interlocking grains, often dotted with red garnet and pyroxene, and only weak banding.
Granulite vs gneiss: what's the difference?
Gneiss shows strong light/dark banding and contains mica, while granulite has a more even granoblastic texture and anhydrous minerals like pyroxene and garnet instead of mica.
Is granulite igneous or metamorphic?
Granulite is a high-grade metamorphic rock formed at very high temperatures deep in the crust.
Where is granulite found?
In deeply eroded ancient shields and mountain roots, such as southern India, the Adirondacks, Scandinavia, the Lewisian of Scotland, and Antarctica.
Granulite identified by the community
Recent Granulite specimens identified with Rock Identifier.
