Orendite Identification Guide
How to recognize orendite, a rare potassium-rich lamproite volcanic rock from the Leucite Hills of Wyoming.
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What Orendite Looks Like
Orendite is a rare, ultrapotassic volcanic rock of the lamproite family, classically from the Leucite Hills of Wyoming. It is a fine-grained, dark to grayish igneous rock characterized by phlogopite mica and the potassium minerals leucite and sanidine, often with the rare mineral wadeite. It typically occurs as lava flows and dikes and is prized scientifically rather than as a gem.
- Color: dark gray to grayish-brown, often with golden-brown mica flecks
- Luster: dull groundmass with shiny mica plates
- Texture: fine-grained (porphyritic) with phenocrysts of mica and leucite
- Form: lava flows, dikes, volcanic necks
Step-by-Step Field ID Checklist
- Confirm the setting. Orendite is essentially restricted to the Leucite Hills; provenance is a major clue.
- Look for phlogopite mica. Bronze-to-golden mica flakes that peel in sheets are typical.
- Spot leucite. Pale, rounded, equant grains in the matrix suggest leucite.
- Note the dark fine matrix with scattered phenocrysts (porphyritic texture).
- Test groundmass hardness; silicate matrix scratches glass with effort.
Key Diagnostic Tests
- Hardness: mica 2-3 (peels), leucite/sanidine ~5.5-6.
- Streak: minerals give white to pale streaks.
- Acid: no reaction (silicate rock).
- Density: roughly 2.5-2.7 g/cm3.
- Mineralogy: phlogopite + leucite + sanidine (+ wadeite/diopside) defines it; thin-section or lab work is usually needed to confirm versus related lamproites.
Common Look-Alikes
- Wyomingite: the closely related Leucite Hills lamproite; orendite contains sanidine and wadeite in the groundmass while wyomingite is more glassy, so lab analysis separates them.
- Madupite: another Leucite Hills lamproite, richer in diopside and lacking sanidine.
- Lamprophyre/minette: mica-rich dark dike rocks, but lack leucite; distinguished by mineralogy.
- Basalt: plagioclase-bearing and not ultrapotassic; basalt lacks leucite and phlogopite phenocrysts.
Where It Is Found
Orendite is best known from the Leucite Hills volcanic field in Sweetwater County, Wyoming, USA, one of the world's classic ultrapotassic provinces. Similar ultrapotassic lamproites occur in places like Spain (Murcia) and Western Australia, but the name orendite is tied to the Wyoming type locality.
Frequently asked questions
What kind of rock is orendite?
Orendite is a rare ultrapotassic volcanic rock of the lamproite family, made largely of phlogopite mica, leucite, and sanidine, with wadeite, from the Leucite Hills of Wyoming.
How can you tell orendite from wyomingite?
Both are Leucite Hills lamproites, but orendite has sanidine and wadeite in its groundmass while wyomingite is more glassy. Reliable separation usually needs thin-section or chemical analysis.
Where is orendite found?
It is classically found in the Leucite Hills volcanic field in Sweetwater County, Wyoming, USA, its type locality, with comparable ultrapotassic rocks elsewhere in the world.
What does orendite look like?
It is a dark gray, fine-grained volcanic rock with golden-brown phlogopite mica flakes and pale leucite grains in a porphyritic matrix.