Lamproite Identification Guide
A guide to identifying lamproite, a rare ultrapotassic volcanic rock and occasional diamond host.
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What Lamproite Looks Like
Lamproite is a rare, ultrapotassic, magnesium-rich volcanic or subvolcanic rock. It is typically dark - gray, greenish-gray, brown, to nearly black - and commonly porphyritic, with visible phenocrysts of phlogopite mica (bronze-brown shiny flakes), olivine, leucite, or amphibole set in a fine-grained groundmass. Weathered surfaces often look soft and earthy. Famously, lamproite at Argyle in Western Australia is a primary host rock for diamonds, alongside kimberlite.
Step-by-Step Field ID Checklist
- Note the dark, fine-grained matrix with scattered larger crystals (porphyritic texture).
- Look for bronze phlogopite mica flakes - a hallmark of lamproite.
- Check for rounded olivine or leucite/amphibole phenocrysts.
- Assess the setting - small volcanic pipes, dikes, or lava flows, not large plutons.
- Test hardness of components - mica is soft (2-3), olivine harder (6.5-7).
Key Diagnostic Tests
- Mineral assemblage (diagnostic): Phlogopite + leucite/sanidine + Ti-rich amphibole (richterite) + olivine/diopside, with NO feldspathoid-free granitic minerals.
- High potassium/magnesium chemistry: Confirmed only by lab analysis, but the unusual mica-rich, feldspar-poor mineralogy is a strong field hint.
- Phlogopite hardness ~2.5-3: Mica flakes peel and are soft.
- Olivine hardness ~6.5-7.
- Not strongly magnetic (minor magnetite possible); no acid reaction.
Common Look-Alikes and How to Tell Them Apart
- Kimberlite: The other major diamond host; kimberlite is rich in serpentinized olivine and carbonate (often fizzes weakly in acid) and characteristically contains mantle xenoliths/xenocrysts; lamproite is more potassic and leucite/phlogopite-rich and typically lacks groundmass carbonate.
- Lamprophyre: Dark porphyritic dike rock with biotite/amphibole phenocrysts and feldspar groundmass; lamproite is more potassic, contains leucite, and lacks normal feldspar dominance.
- Basalt: Has plagioclase feldspar and pyroxene; lamproite is feldspar-poor and mica-rich.
- Minette (a lamprophyre): Similar K-rich mica-bearing rock; chemistry distinguishes them.
Where Lamproite Is Found
Lamproite occurs in small volcanic pipes, dikes, and lava flows in ancient cratons. Key localities include Argyle and Ellendale (Western Australia), the Leucite Hills (Wyoming, USA), Spain (Murcia/fortunite), and Smoky Butte (Montana).
Frequently asked questions
How do you identify lamproite?
Look for a dark, fine-grained, porphyritic volcanic rock rich in bronze phlogopite mica with olivine, leucite, or amphibole phenocrysts and little to no ordinary feldspar. Its potassium- and magnesium-rich character is confirmed by chemistry, but the mica-rich, feldspar-poor mineralogy is the field clue.
What is the difference between lamproite and kimberlite?
Both can host diamonds, but kimberlite is rich in serpentinized olivine and carbonate (often fizzes in acid) with abundant mantle xenoliths, while lamproite is more potassic and contains leucite and phlogopite, usually without groundmass carbonate.
Does lamproite contain diamonds?
Some lamproites do - the Argyle pipe in Western Australia was a major diamond producer hosted in lamproite - but most lamproite bodies are barren, so the rock type alone does not guarantee diamonds.
Lamproite vs lamprophyre - what's the difference?
Lamprophyre is a dark porphyritic dike rock with biotite or amphibole in a feldspar-bearing groundmass, whereas lamproite is more ultrapotassic, contains leucite, and is dominated by phlogopite with little normal feldspar.