Urtite Identification Guide
Identify urtite, a rare nepheline-rich plutonic rock of the ijolite series, by its grayish nepheline groundmass and minor pyroxene, with key look-alikes.
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What Urtite Looks Like
Urtite is a rare, coarse-grained plutonic igneous rock at the extreme nepheline-rich end of the ijolite series. It is dominated by nepheline (typically over ~70%) with only minor amounts of green-black aegirine/aegirine-augite pyroxene and sometimes apatite or titanite. The rock is generally pale - gray, greenish-gray, or whitish - with the nepheline giving a greasy, dull luster, peppered with sparse dark pyroxene grains. It is silica-undersaturated (no quartz) and feldspar-poor to feldspar-free.
Step-by-Step Field-ID Checklist
- Confirm coarse plutonic texture - interlocking visible crystals, not glassy or fine-grained.
- Check the dominant pale mineral. Nepheline is gray/greenish with a greasy, slightly cloudy luster and ~Mohs 5.5-6.
- Verify scarcity of dark minerals. Only a small fraction of green-black pyroxene; lots of dark = ijolite or melteigite instead.
- Rule out quartz. No glassy quartz grains (silica-undersaturated).
- Rule out feldspar dominance. Feldspar is minor/absent, separating it from syenite.
- Test hardness. Nepheline (~5.5-6) is scratched by quartz and barely scratches glass.
Key Diagnostic Tests
- Mohs hardness: Nepheline ~5.5-6; pyroxene ~5.5-6.
- Streak: White (nepheline).
- Cleavage/fracture: Nepheline has poor cleavage and greasy fracture; pyroxene shows two cleavages.
- Magnetism: None to weak (minor magnetite if present).
- Acid: Nepheline can be slowly attacked by strong acid and gel-etch (lab test); not a casual field test.
- Density: ~2.6-2.8 g/cm3.
Common Look-Alikes
- Ijolite: Same series but roughly equal nepheline and pyroxene - noticeably darker than urtite.
- Melteigite: Pyroxene-dominated, dark - the opposite end of the series.
- Nepheline syenite: Contains abundant alkali feldspar alongside nepheline; urtite is feldspar-poor.
- Granite/syenite: Contain quartz and/or dominant feldspar; urtite has neither in quantity.
- Marble: Pale but fizzes in acid and is softer (calcite Mohs 3).
The key is a pale, coarse, feldspar-poor, quartz-free rock made mostly of greasy nepheline with only sparse dark pyroxene.
Where It Is Found
Urtite occurs in alkaline igneous complexes. The classic locality is the Khibiny massif of the Kola Peninsula, Russia (where it hosts major apatite deposits), with the name derived from Lujavr-Urt (Lovozero). Other occurrences are in alkaline intrusions worldwide, but it remains a rare rock type.
Frequently asked questions
What is urtite?
Urtite is a rare coarse-grained plutonic rock at the nepheline-rich end of the ijolite series, composed mostly of nepheline with only minor pyroxene.
How do you tell urtite from ijolite?
Urtite is much paler because it is dominated by nepheline with few dark minerals, while ijolite has roughly equal nepheline and dark pyroxene, making it noticeably darker.
Does urtite contain quartz?
No. Urtite is silica-undersaturated, so it contains no quartz and little or no feldspar.
Where is urtite found?
The classic source is the Khibiny alkaline massif on the Kola Peninsula, Russia; it occurs rarely in other alkaline igneous complexes.