Norite Identification Guide
How to identify norite, a dark plagioclase-orthopyroxene plutonic rock, and distinguish it from gabbro and other mafic intrusives.
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What Norite Looks Like
Norite is a coarse-grained, dark intrusive (plutonic) igneous rock — a member of the gabbro family in which the dominant pyroxene is orthopyroxene (such as hypersthene/enstatite) rather than clinopyroxene. Hand specimens are dark gray to greenish-black, speckled "salt-and-pepper" rocks built mainly of calcium-rich plagioclase (dull gray-white) interlocked with abundant dark pyroxene. Olivine and minor oxides (magnetite, ilmenite) may be present. Crystals are coarse and visible to the naked eye.
- Color: dark gray, greenish-gray to nearly black with pale plagioclase specks
- Luster: dull to vitreous; pyroxene cleavage faces glint
- Texture: holocrystalline, medium- to coarse-grained, equigranular
- Mafic mineral: orthopyroxene with bronzy submetallic sheen on cleavage
Step-by-Step Field ID Checklist
- Confirm intrusive + mafic: coarse interlocking crystals, dark overall, no glass or vesicles.
- Identify the plagioclase: gray-white blocky grains with straight cleavage and sometimes fine striations (twinning lines).
- Identify the pyroxene: dark stubby grains with two cleavages near 90°; look for a bronzy/coppery schiller on cleavage faces typical of orthopyroxene.
- Estimate proportions: roughly subequal plagioclase and pyroxene = gabbroic; the orthopyroxene dominance defines norite.
- Check for olivine: green glassy grains, if present (olivine norite).
Key Diagnostic Tests
- Hardness: plagioclase ~6, pyroxene ~5.5–6; scratches glass.
- Cleavage: pyroxene two cleavages ~87–93°; plagioclase two cleavages with striations on one set.
- Streak: white to gray.
- Acid: no reaction.
- Density: high (~2.9–3.1 g/cm³) — norite feels heavy in the hand, heavier than granite.
- Magnetism: weak to moderate if magnetite is present.
Common Look-Alikes and How to Tell Them Apart
- Gabbro: the key distinction is pyroxene type — gabbro is dominated by clinopyroxene (augite), norite by orthopyroxene. Without a microscope, look for the bronzy schiller of orthopyroxene; field identification of norite vs gabbro is often tentative and confirmed in thin section.
- Diorite: lighter and more "salt-and-pepper" with sodic plagioclase and hornblende; less dense and less dark than norite.
- Basalt: same composition but fine-grained (extrusive) — if you cannot see individual crystals, it is basalt, not norite.
- Anorthosite: dominated almost entirely by plagioclase with little pyroxene; much paler and less mafic than norite.
- Pyroxenite: nearly all pyroxene with little plagioclase; norite has substantial plagioclase.
Where Norite Is Found
Norite occurs in large layered mafic intrusions and gabbroic plutons. Classic localities include the Bushveld Complex (South Africa), the Sudbury Igneous Complex (Ontario, Canada — the famous "Sudbury norite" host to Ni-Cu-PGE ores), the Stillwater Complex (Montana), and many Precambrian shield terrains. It is commonly associated with nickel, copper, and platinum-group ore deposits, and is quarried as a durable dark dimension stone (often sold commercially as "black granite").
Frequently asked questions
How can you tell norite from gabbro?
Both are coarse, dark, heavy plutonic rocks of plagioclase and pyroxene. The difference is the pyroxene: norite is dominated by orthopyroxene (often with a bronzy schiller on cleavage faces), while gabbro is dominated by clinopyroxene (augite). Reliable separation usually needs a thin section, but the bronzy sheen hints at norite.
What does norite look like?
It is a coarse-grained, dark gray to greenish-black salt-and-pepper rock made of dull gray-white plagioclase and abundant dark orthopyroxene, with all crystals visible to the eye. It feels noticeably heavy.
Is norite the same as black granite?
Geologically no. Norite is a mafic gabbroic rock, while true granite is felsic and quartz-rich. However, the stone trade often sells polished norite and gabbro as 'black granite' for countertops and monuments.
Where is norite found?
In large layered mafic intrusions such as the Bushveld Complex (South Africa), the Sudbury Complex (Canada), and the Stillwater Complex (Montana), where it is often associated with nickel, copper, and platinum-group ore deposits.