Wyomingite Identification Guide
A field guide to wyomingite, a rare potassium-rich volcanic lamproite from Wyoming, covering its fine-grained texture, phenocrysts, and how to distinguish it.
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What Wyomingite Looks Like
Wyomingite is a rare, exotic volcanic rock—a type of lamproite—named for the Leucite Hills of Wyoming. It is an ultrapotassic, silica-poor lava with an unusual mineralogy dominated by phlogopite mica, leucite, and diopside. In hand it looks like a dark, fine-grained volcanic rock studded with small shiny mica flakes.
- Color: dark grey to brownish-grey or greenish-grey groundmass
- Luster: dull groundmass with bright, reflective phlogopite mica flakes
- Transparency: opaque
- Habit/texture: porphyritic — small phlogopite and diopside phenocrysts set in a fine vesicular groundmass; often appears spongy or pumice-like in places
Step-by-Step Field ID Checklist
- Note the volcanic texture — fine-grained, sometimes vesicular (gas holes) groundmass.
- Spot the mica — abundant tiny, bronzy, glittering phlogopite flakes are characteristic of lamproites.
- Look for pale leucite — small rounded whitish-grey crystals may dot the rock.
- Check the setting — Wyomingite forms small flows, dikes, and plugs, not large bodies.
- Consider locality — genuine wyomingite is essentially restricted to the Leucite Hills.
Key Diagnostic Tests
- Mineralogy: phlogopite + diopside + leucite; this combination is diagnostic of lamproite.
- Hardness: mixed; the mica flakes are soft (~2.5–3), pyroxene harder (~6).
- Magnetism: weak to moderate where accessory iron-titanium oxides are present.
- Acid: generally non-reactive (silicate rock, not carbonate).
- Density: moderate, typical of mafic-to-ultramafic volcanics.
Common Look-Alikes and How to Tell Them Apart
- Basalt: also dark and fine-grained, but lacks abundant phlogopite mica flakes and leucite; basalt is plagioclase + pyroxene.
- Lamprophyre: related mica-rich rock, but lamproites like wyomingite are ultrapotassic with leucite, a distinguishing mineral assemblage best confirmed in thin section.
- Kimberlite: another exotic potassic rock, but kimberlite carries olivine and mantle xenoliths; wyomingite is leucite/phlogopite-dominated.
- Phonolite/tephrite: feldspathoid-bearing but lack the dominant phlogopite of wyomingite.
Where It Is Typically Found
Wyomingite is named after and almost exclusively found in the Leucite Hills volcanic field of southwestern Wyoming (Sweetwater County), one of the world's classic localities for ultrapotassic lamproite volcanism. Because it is a named rock from a single region, confident identification combines the phlogopite-leucite-diopside assemblage with that Wyoming provenance; lab petrography is usually needed for a definitive call.
Frequently asked questions
What is wyomingite?
Wyomingite is a rare ultrapotassic volcanic rock (a lamproite) from the Leucite Hills of Wyoming, characterized by phlogopite mica, leucite, and diopside in a fine-grained groundmass.
How can you tell if it's real wyomingite?
Look for a dark, fine-grained, often vesicular volcanic rock packed with bronzy phlogopite mica flakes plus pale leucite crystals, ideally from the Leucite Hills; thin-section petrography confirms the lamproite mineralogy.
Wyomingite vs basalt: how are they different?
Both are dark volcanic rocks, but basalt is built from plagioclase and pyroxene and lacks mica, while wyomingite is rich in glittering phlogopite mica and contains leucite, giving it a distinctive mineral assemblage.
Where is wyomingite found?
Almost exclusively in the Leucite Hills volcanic field of Sweetwater County, southwestern Wyoming, the type locality after which it is named.