Melteigite
Plutonic feldspathoidal rock; clinopyroxene-dominant (aegirine-augite) with subordinate nepheline
A dark, pyroxene-dominated plutonic rock at the mafic end of the ijolite series, made mainly of aegirine-augite with subordinate nepheline.
- Mohs hardness
- 5.5-6
- Color
- dark green to black
- Type
- igneous
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Overview
Melteigite is a dark, coarse-grained feldspathoidal plutonic rock dominated by sodic clinopyroxene (aegirine-augite) with subordinate nepheline and accessory titanite, apatite, melanite garnet, and Fe-Ti oxides. It is feldspar-free and strongly silica-undersaturated.
Melteigite is the melanocratic (dark) end member of the ijolite series, with ijolite intermediate and urtite the pale, nepheline-rich end. It is named from Melteig, in the Fen alkaline complex of Norway.
It is a hallmark of alkaline-carbonatite complexes and is of mainly petrological and locally economic interest.
Formation & geology
Melteigite forms by crystallization and accumulation of sodic pyroxene from silica-undersaturated, alkali-rich magma in alkaline ring complexes, often spatially linked with carbonatites and the ijolite-urtite suite. Concentration of dense pyroxene produces the dark, mafic rock at the base or margins of these bodies.
It occurs in continental rift and intraplate alkaline complexes, with classic examples in the Fen complex (Norway), the Kola Peninsula, and East African Rift complexes. It is commonly found gradational with ijolite and associated feldspathoidal rocks and carbonatites.
How to identify it
Look for a dark green to black, coarse-grained, heavy rock dominated by aegirine-augite pyroxene with subordinate greasy gray nepheline and possibly bright titanite or black melanite garnet. Hardness is moderate.
The diagnostic features are the dominance of sodic pyroxene with some nepheline and no feldspar. This distinguishes melteigite from feldspar-bearing mafic rocks like gabbro.
Look-alikes include ijolite (more nepheline) and pyroxenite (no nepheline); melteigite is the dark, pyroxene-rich member of the ijolite series, with the presence of nepheline confirmed by microscopy.
Uses & significance
Melteigite has little direct commercial use itself, but the alkaline-carbonatite complexes it belongs to are economically important sources of niobium, rare earth elements, phosphate, and titanium; melteigite's accessory minerals (apatite, titanite, pyrochlore in associated rocks) contribute to that resource potential.
Its main role is scientific: as the mafic end of the ijolite series it helps geologists understand alkaline, silica-undersaturated magmatism and carbonatite associations.
It has no gemstone or metaphysical tradition.
Frequently asked questions
What is melteigite made of?
Mainly sodic clinopyroxene (aegirine-augite) with subordinate nepheline and accessories like titanite, apatite, and melanite garnet; it has no feldspar.
How does melteigite relate to ijolite and urtite?
It is the dark, pyroxene-rich end member of the ijolite series; ijolite is intermediate and urtite is the nepheline-rich pale end.
Where is melteigite found?
In alkaline-carbonatite complexes such as the Fen complex of Norway, the Kola Peninsula, and East African Rift intrusions.
Is melteigite associated with carbonatites?
Yes. Like the rest of the ijolite series it commonly occurs in alkaline ring complexes alongside carbonatites.
Melteigite guides
In-depth guides for identifying, valuing, and understanding Melteigite.
