Melteigite Identification Guide

What Melteigite Looks Like

Melteigite is a coarse- to medium-grained, intrusive (plutonic) feldspathoid-bearing igneous rock belonging to the ijolite series. It is melanocratic, meaning it is dark and mafic-rich: by definition mafic minerals (mostly dark green-black clinopyroxene, aegirine-augite) make up about 70-90% of the rock, with the balance being grey, greasy-looking nepheline. The overall impression is a heavy, dark green to greenish-black rock with a salt-and-pepper texture in which dark pyroxene dominates and pale nepheline appears as interstitial patches. There is no quartz and no ordinary feldspar.

• Color: dark green to black, locally with brownish or grey nepheline flecks
• Luster: dull to vitreous; pyroxene crystals can flash on cleavage faces
• Transparency: opaque rock
• Habit: granular, equigranular to slightly porphyritic; sometimes biotite, titanite, apatite, perovskite, or magnetite as accessories

Step-by-Step Field-ID Checklist

1. Confirm it is plutonic: look for interlocking, visible crystals (not glassy or fine volcanic groundmass).
2. Estimate the dark mineral fraction: if 70%+ is dark pyroxene, you are in melteigite territory (ijolite is ~50/50, urtite is light/nepheline-rich).
3. Look for nepheline: grey, greasy-lustered, slightly softer grains between the pyroxenes. It alters readily to dull cloudy material.
4. Check for absence of quartz and feldspar: scratch test and look for glassy quartz or blocky feldspar with cleavage steps — they should be absent.
5. Note geologic setting: found with carbonatites, ijolites, and nepheline syenites in alkaline ring complexes.

Key Diagnostic Tests

• Hardness: pyroxene ~5.5-6.5; nepheline 5.5-6 (knife barely scratches nepheline).
• Nepheline test: nepheline gelatinizes in HCl and may etch; it has a greasy luster, unlike glassy quartz.
• Density: relatively high (mafic-rich), feels heavy in hand.
• Streak/magnetism: weakly magnetic where magnetite is present; pyroxene gives a pale grey-green streak when crushed.

Common Look-Alikes and How to Tell Them Apart

• Ijolite: the same series but lighter — roughly equal nepheline and pyroxene. Melteigite is darker (more pyroxene).
• Urtite: the pale end-member, dominated by nepheline; melteigite is its dark opposite.
• Pyroxenite: also dark and pyroxene-rich, but contains no nepheline (and may have olivine). Find nepheline and you have melteigite, not pyroxenite.
• Basalt/gabbro: these contain plagioclase feldspar, which melteigite lacks. A feldspar-free, nepheline-bearing dark rock points to melteigite.

Where It Is Typically Found

Melteigite is rare and restricted to alkaline igneous provinces and carbonatite complexes. The type locality is the Fen Complex region of Norway; other occurrences include the Kola Peninsula (Russia), Oka and other Canadian carbonatite complexes, and East African Rift alkaline intrusions. Look in eroded ring dikes and zoned plutons associated with nepheline syenite and carbonatite.