Urtite
Plutonic feldspathoidal rock; nepheline-dominant (>70%) with minor sodic pyroxene
A pale, nepheline-dominated plutonic rock at the leucocratic end of the ijolite series, sometimes associated with major apatite ore deposits.
- Mohs hardness
- 5.5-6
- Color
- light gray to white, greasy luster
- Type
- igneous
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Overview
Urtite is a pale, coarse-grained feldspathoidal plutonic rock dominated by nepheline (commonly more than about 70%), with only minor sodic clinopyroxene (aegirine-augite) and accessory minerals such as apatite and titanite. It is feldspar-free and strongly silica-undersaturated.
Urtite is the leucocratic (light-mineral-rich) end member of the ijolite series, with ijolite intermediate and melteigite the dark, pyroxene-rich end. It is named from Lujavr-Urt (Lovozero), Kola Peninsula, Russia.
In the Khibiny complex, apatite-rich urtites form some of the world's largest phosphate deposits, giving the rock notable economic value.
Formation & geology
Urtite forms by crystallization and accumulation of nepheline from silica-undersaturated, alkali-rich magma in alkaline ring complexes, commonly associated with carbonatites and the broader ijolite-melteigite suite. Concentration of nepheline (and locally apatite) produces the pale, feldspar-free rock.
It is characteristic of continental rift and intraplate alkaline complexes. The classic occurrences are the Khibiny and Lovozero massifs on the Kola Peninsula, Russia, where apatite-nepheline urtites are mined, along with other alkaline complexes worldwide.
How to identify it
Look for a pale gray to whitish, coarse-grained rock with a greasy or slightly resinous luster from abundant nepheline, sparse dark green aegirine-augite, and possibly greenish apatite. Hardness is moderate (about 5.5-6).
The diagnostic features are the dominance of nepheline and the absence of feldspar. Nepheline's greasy luster and moderate hardness help separate it from feldspar and quartz.
Look-alikes are ijolite (more pyroxene) and melteigite (pyroxene-rich, dark); urtite is the lightest, most nepheline-rich member. Confirming nepheline over feldspar usually requires microscopy or chemical testing.
Uses & significance
Urtite is economically important where it is apatite-rich: the Khibiny apatite-nepheline ores are a major source of phosphate for fertilizer, with the nepheline itself recovered as a feedstock for alumina, ceramics, and glass.
Elsewhere urtite is mainly of scientific interest as the nepheline-rich end member of the ijolite series and as evidence of extreme alkaline, silica-undersaturated magmatism.
It has no gemstone or metaphysical tradition.
Frequently asked questions
What is urtite mostly made of?
Predominantly nepheline (often over 70%), with only minor sodic pyroxene and accessories like apatite; it contains no feldspar.
How does urtite fit in the ijolite series?
It is the light, nepheline-rich end member; ijolite is intermediate and melteigite is the dark, pyroxene-rich end.
Why is urtite economically valuable?
In the Khibiny massif, apatite-rich urtite is a major phosphate (fertilizer) ore, and its nepheline is used for alumina and ceramics.
Where is urtite found?
Classically in the Khibiny and Lovozero alkaline complexes of the Kola Peninsula, Russia, and in other alkaline provinces.
Urtite guides
In-depth guides for identifying, valuing, and understanding Urtite.
