Charnockite
Orthopyroxene-bearing granitic rock (granulite facies)
A granite-like rock containing orthopyroxene, formed at high temperatures and pressures and often classed with the granulites.
- Mohs hardness
- 6-7
- Color
- Greenish-grey to dark grey, often greasy-looking
- Type
- igneous
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Overview
Charnockite is a quartz- and feldspar-rich rock resembling granite but distinguished by the presence of orthopyroxene (typically hypersthene). It usually has a characteristic greenish-grey, slightly greasy or glassy appearance and forms under high-temperature, high-pressure granulite-facies conditions.
The rock is named after Job Charnock, founder of Kolkata, whose tombstone was carved from it. Geologists debate whether charnockites are truly igneous (crystallized from dry magma) or metamorphic (formed by recrystallization of older rocks), and in practice the "charnockite series" includes both origins.
For identification purposes it is treated here as an igneous-affiliated rock of the granite family.
Formation & geology
Charnockite forms deep in the continental crust under granulite-facies conditions, with temperatures often exceeding 700-900 degrees C and high pressure. Some charnockites crystallized directly from hot, water-poor (anhydrous) magmas, while others developed by dehydration and recrystallization of pre-existing rocks as carbon-dioxide-rich fluids drove off water.
It is characteristic of ancient Precambrian shields and high-grade metamorphic terranes, with classic occurrences in southern India and Sri Lanka, the Adirondacks of New York, Scandinavia, and parts of Africa and Antarctica.
How to identify it
Charnockite typically appears as a dark greenish-grey to brownish-grey rock with a greasy or glassy luster, abundant quartz and feldspar, and visible dark orthopyroxene. The greenish, oily look distinguishes it from ordinary pink or grey granite.
Hardness is about 6-7. The presence of orthopyroxene (hypersthene), often confirmed under a microscope, is the diagnostic feature. Distinguish it from normal granite by its color and pyroxene, and from gneiss by its generally massive (though sometimes foliated) texture.
Uses & significance
Charnockite is a prized dimension and building stone, widely quarried in India and sold internationally as a durable, attractive granite for countertops, flooring, cladding, and monuments. Its toughness and distinctive greenish hues make it commercially popular.
It has no traditional gemstone or metaphysical role, but it is of major scientific interest for understanding deep-crustal processes and the boundary between igneous and metamorphic rock formation.
Frequently asked questions
Is charnockite igneous or metamorphic?
Both origins exist; some charnockites crystallized from dry magma while others formed by high-grade metamorphism, so the term covers a series of rocks.
How is charnockite different from granite?
Charnockite contains orthopyroxene and has a greenish, greasy appearance, forming at higher temperatures and pressures than ordinary granite.
Where was charnockite named?
It is named after Job Charnock; his tombstone in Kolkata, India, was made from this rock.
Is charnockite a good countertop stone?
Yes, it is hard, durable, and attractive, and is widely sold commercially as granite for countertops and cladding.
Charnockite guides
In-depth guides for identifying, valuing, and understanding Charnockite.
