Ijolite Identification Guide
How to identify ijolite, the dark nepheline-pyroxene plutonic rock of alkaline complexes and carbonatites.
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What Ijolite Looks Like
Ijolite is a coarse-grained, silica-undersaturated plutonic igneous rock made essentially of nepheline and clinopyroxene (aegirine-augite). It belongs to the alkaline rock family associated with carbonatites:
- Color: dark gray to greenish-gray to greenish-black, often speckled
- Texture: medium- to coarse-grained, holocrystalline, sometimes with eye-catching dark green to black pyroxene crystals
- Minerals: abundant nepheline (gray, greasy) plus dark green-black pyroxene; minor titanite, apatite, calcite, melanite garnet, biotite
Step-by-Step Field ID Checklist
- Confirm it is a coarse igneous rock with interlocking crystals, not layered or fragmental.
- Identify nepheline. Look for grayish, greasy-lustered grains with no good cleavage and a hardness around 5.5–6.
- Identify the dark pyroxene. Stubby to prismatic dark green-black aegirine-augite makes up roughly half the rock.
- Note the absence of quartz and feldspar. Ijolite is feldspar-free and quartz-free (silica-undersaturated).
- Check the geologic setting — alkaline intrusive complexes and carbonatites.
Key Diagnostic Tests
- Mineralogy is the main tool: roughly equal nepheline and clinopyroxene, no quartz, no feldspar.
- Nepheline test: greasy luster; gelatinizes with HCl (and can be stained for nepheline in the lab).
- Hardness: rock is moderately hard; pyroxene ~5.5–6, nepheline ~5.5–6.
- Density: moderately high (~2.9–3.1 g/cm3) owing to abundant pyroxene.
- No quartz: a knife-hard, glassy quartz grain should be absent.
Common Look-Alikes
- Nepheline syenite: contains abundant feldspar (alkali feldspar) along with nepheline; ijolite is feldspar-free with much more pyroxene.
- Pyroxenite: nearly all pyroxene with little nepheline; ijolite has substantial nepheline.
- Melteigite/urtite: members of the same series — melteigite is more pyroxene-rich, urtite more nepheline-rich; ijolite is the middle composition.
- Gabbro/diorite: contain plagioclase feldspar and no nepheline.
- Carbonatite: dominated by carbonate (fizzes strongly in acid); ijolite is a silicate rock often found alongside it.
Where It Is Found
Ijolite occurs in alkaline igneous provinces and carbonatite complexes: the Iivaara area of Finland (the type locality), the Kola Peninsula (Khibiny, Lovozero) in Russia, Fen in Norway, Oka in Quebec, Magnet Cove in Arkansas, and various East African Rift complexes.
Frequently asked questions
How can you tell if a rock is ijolite?
Ijolite is a coarse, dark greenish plutonic rock made of roughly equal nepheline and dark green-black pyroxene, with no quartz and no feldspar. Greasy-lustered nepheline grains plus abundant aegirine-augite in an alkaline/carbonatite setting confirm it.
What is the difference between ijolite and nepheline syenite?
Nepheline syenite contains abundant alkali feldspar with nepheline, whereas ijolite is feldspar-free and contains much more dark pyroxene. Both lack quartz, but the feldspar content is the key separator.
What does ijolite look like?
It looks like a dark gray to greenish-black, coarse-grained igneous rock speckled with grayish greasy nepheline and stubby dark green-black pyroxene crystals.
Where is ijolite found?
It occurs in alkaline igneous and carbonatite complexes such as Iivaara in Finland (type locality), the Kola Peninsula in Russia, Magnet Cove in Arkansas, and rift complexes in East Africa.