Kersantite Identification Guide
Identify Kersantite, a mica-rich lamprophyre dike rock, by its dark color, abundant biotite, and plagioclase groundmass.
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What Kersantite Looks Like
Kersantite is a lamprophyre—a dark, mica-rich dike rock. It is defined as a lamprophyre in which biotite/phlogopite (dark mica) is the dominant mafic mineral and plagioclase is the dominant feldspar. In hand sample it is dark gray to greenish-black or brown, fine- to medium-grained, and characteristically porphyritic, with shiny black biotite flakes (and sometimes hornblende or augite) as phenocrysts in a finer feldspathic groundmass. Fresh surfaces glitter with mica.
Quick visual cues
- Dark, dense, fine-grained dike rock
- Abundant glittering black/brown mica flakes
- Porphyritic: mafic phenocrysts in finer matrix
- Often weathers to a brownish, crumbly surface
Step-by-Step Field ID Checklist
- Identify the field setting. Lamprophyres form narrow dikes or sills cutting other rocks—look for a dark, fine-grained tabular body.
- Spot the mica. Conspicuous biotite/phlogopite phenocrysts that flash in light are the key trait.
- Confirm porphyritic texture. Mafic phenocrysts only (feldspar stays in the groundmass)—a defining lamprophyre feature.
- Determine the feldspar. Plagioclase-dominant groundmass = kersantite (orthoclase-dominant would be minette).
- Test mica hardness. Mica peels in flexible sheets, Mohs ~2.5-3.
- Overall hardness/heft: tough, moderately dense (~2.8-3.0).
Key Diagnostic Tests
- Rock identification by texture + mineralogy, not a single streak.
- Mica: perfect basal cleavage, splits into elastic flakes.
- Acid: no fizz unless secondary carbonate (common in altered lamprophyres) is present—then localized fizzing.
- Density: ~2.8-3.0 g/cm^3.
- Distinguishing within lamprophyres: kersantite = biotite + plagioclase; minette = biotite + orthoclase; vogesite/spessartite = hornblende-dominant.
Common Look-Alikes and How to Tell Them Apart
- Minette: the closest relative—also mica-rich, but its dominant feldspar is alkali feldspar (orthoclase), whereas kersantite's is plagioclase (needs thin-section/staining to be sure).
- Basalt: basalt's phenocrysts are plagioclase and pyroxene/olivine, not abundant biotite; basalt forms flows, not just thin dikes.
- Diorite: coarser and fully crystalline; kersantite is finer and porphyritic with mica.
- Biotite-rich schist: metamorphic foliation versus igneous dike texture.
Where It Is Found
Kersantite (named for Kersanton, Brittany, France) occurs as dikes and sills in many orogenic and cratonic terranes worldwide, commonly intruding granites, gneisses, and sedimentary rocks. It is of geological interest rather than lapidary use.
Frequently asked questions
How do you identify Kersantite?
Identify it as a dark, fine-grained porphyritic dike rock in which biotite/phlogopite mica is the dominant dark mineral and plagioclase is the dominant feldspar, with mafic phenocrysts (mica) set in a finer feldspathic groundmass.
Kersantite vs minette—what's the difference?
Both are mica-rich lamprophyres, but kersantite's dominant feldspar is plagioclase while minette's is alkali feldspar (orthoclase); distinguishing them usually requires thin-section study or feldspar staining.
What does Kersantite look like in the field?
It appears as a dark gray to greenish-black, fine-grained tabular dike that glitters with abundant black or brown mica flakes and often weathers to a brownish crumbly surface.
Is Kersantite the same as basalt?
No. Basalt is a lava flow with plagioclase and pyroxene/olivine phenocrysts, while kersantite is a mica-rich lamprophyre dike rock dominated by biotite phenocrysts.