Lamprophyre Identification Guide
How to identify lamprophyre, a dark porphyritic dike rock with mafic phenocrysts in a feldspar groundmass.
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What Lamprophyre Looks Like
Lamprophyre is a dark, porphyritic igneous rock that typically occurs as dikes and sills. It is characterized by abundant euhedral (well-formed) phenocrysts of dark mica (biotite/phlogopite) and/or amphibole - and sometimes pyroxene or olivine - set in a fine-grained groundmass that contains feldspar (and/or feldspathoids) but, crucially, no feldspar phenocrysts. Colors are dark gray, greenish, to black, often with glinting biotite flakes. Weathering tends to be rapid, giving a crumbly, brownish surface.
Step-by-Step Field ID Checklist
- Confirm porphyritic texture - large dark crystals in a finer matrix.
- Identify the phenocrysts - shiny black biotite books and/or stubby amphibole prisms.
- Check that feldspar appears only in the groundmass, never as phenocrysts (defining feature).
- Note the occurrence - narrow dikes cutting other rock.
- Look for ocelli - rounded light-colored globules are common in some lamprophyres.
Key Diagnostic Tests
- Phenocryst mineralogy (diagnostic): Mafic minerals (biotite, amphibole) form the phenocrysts; feldspar is restricted to the matrix.
- Biotite/amphibole hardness: Mica ~2.5-3 (peels), amphibole ~5-6.
- Dark color, medium density.
- Generally not strongly magnetic; no acid reaction (unless secondary carbonate is present in ocelli).
- Subtypes by chemistry: minette, kersantite, vogesite, spessartite, camptonite - distinguished by feldspar/mica type and lab analysis.
Common Look-Alikes and How to Tell Them Apart
- Basalt: Has feldspar (plagioclase) phenocrysts and a more even texture; lamprophyre's phenocrysts are mafic only.
- Diabase/dolerite: Ophitic texture with plagioclase laths; lacks abundant mica phenocrysts.
- Lamproite: More ultrapotassic, contains leucite, and is phlogopite-dominated with little normal feldspar.
- Kimberlite: Olivine- and carbonate-rich, with mantle xenoliths; lamprophyre is mica/amphibole-phenocryst-dominated.
- Minette: Actually a lamprophyre subtype (orthoclase + biotite).
Where Lamprophyre Is Found
Lamprophyres occur worldwide as dikes, sills, and small intrusions associated with both alkaline and calc-alkaline magmatism, often near granitic plutons and in orogenic belts. Classic areas include the Vosges (France/Germany), the British Isles, Scandinavia, and many North American mountain belts.
Frequently asked questions
How do you identify lamprophyre?
Look for a dark porphyritic dike rock with abundant euhedral phenocrysts of biotite mica and/or amphibole set in a fine groundmass that contains feldspar only in the matrix - no feldspar phenocrysts. This mafic-phenocryst-only texture is the defining clue.
What is lamprophyre made of?
Mainly dark mica (biotite/phlogopite) and/or amphibole phenocrysts in a fine-grained groundmass of feldspar or feldspathoids, with possible pyroxene or olivine; subtypes include minette, kersantite, vogesite, and spessartite.
Lamprophyre vs basalt - what's the difference?
Basalt contains plagioclase feldspar phenocrysts and an even texture, while lamprophyre's phenocrysts are mafic minerals (mica/amphibole) only and feldspar is confined to the groundmass.
Where is lamprophyre found?
It typically occurs as narrow dikes and sills, often near granitic plutons and in mountain (orogenic) belts, with classic localities in the Vosges, the British Isles, and Scandinavia.
Lamprophyre identified by the community
Recent Lamprophyre specimens identified with Rock Identifier.
