Charnockite Identification Guide

What Charnockite Looks Like

Charnockite is a coarse-grained, granite-like rock that contains orthopyroxene (hypersthene), formed under high-temperature granulite-facies conditions. It typically has a dark grey, greenish-grey, or brownish color with a characteristic slightly greasy or smoky appearance. The quartz often looks bluish-grey and glassy, and the rock can show a faint foliation or a massive, even granular texture. Mineral grains visible to the eye include feldspar (often dark or smoky), quartz, dark pyroxene, and sometimes garnet.

Step-by-Step Field ID Checklist

1. Overall look: Confirm a coarse, granitic-looking rock with a dark, greasy, smoky-grey tone.
2. Quartz color: Look for distinctive bluish-grey, glassy quartz.
3. Dark minerals: Identify stubby dark green-brown orthopyroxene grains (not just biotite or hornblende).
4. Texture: Note a massive to weakly foliated, even-grained (granoblastic) fabric.
5. Hardness: Quartz and feldspar scratch glass (Mohs 6–7).
6. Acid test: No fizz in dilute acid (silicate rock, no carbonate).

Key Diagnostic Tests

• Hardness: Quartz Mohs 7, feldspar 6; scratches glass.
• Key mineral: Presence of orthopyroxene (hypersthene) is diagnostic versus ordinary granite.
• Acid reaction: None.
• Density: Moderately high (~2.7–2.9 g/cm³) due to pyroxene content.
• Fracture: Tough, breaks across grains; no cleavage as a rock.

Common Look-Alikes and How to Tell Them Apart

• Granite: Lacks orthopyroxene and the greasy bluish quartz; granite is usually lighter and has biotite/hornblende as the main dark minerals.
• Granulite: Charnockite is essentially a granulite-facies granitoid; the term charnockite is reserved for orthopyroxene-bearing, quartz-feldspar-rich varieties.
• Gabbro / norite: Much darker, plagioclase-dominated, and quartz-poor; charnockite is quartz-rich and felsic overall.
• Gneiss: Strongly banded and foliated; charnockite is typically massive or only weakly foliated.
• Mangerite / monzonite: Lacks the abundant quartz of charnockite.

Where Charnockite Is Typically Found

Charnockite forms in deep continental crust under granulite-facies metamorphism and occurs in ancient cratons and high-grade terranes. Classic localities include southern India (named after Job Charnock's tombstone in Kolkata), Sri Lanka, the Adirondacks of New York, Scandinavia, and the Precambrian shields of Africa and Antarctica. Look for it as massive grey outcrops and dimension-stone quarries in old crystalline basement rock.