Kersantite
Lamprophyre; biotite/phlogopite + augite with plagioclase-dominant groundmass
A dark mica lamprophyre with biotite and augite phenocrysts in a plagioclase-dominated groundmass, the feldspar counterpart of minette.
- Mohs hardness
- 5-6
- Color
- dark gray to greenish black
- Type
- igneous
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Overview
Kersantite is a mica lamprophyre containing abundant biotite (or phlogopite) and augite phenocrysts in a groundmass in which plagioclase feldspar dominates over alkali feldspar. It is essentially the plagioclase-rich counterpart of minette within the calc-alkaline lamprophyre family.
It forms dark, porphyritic dikes and small intrusions and is named after the village of Kersanton in Brittany, France, where the local "kersanton stone" has long been used in sculpture.
Kersantite is primarily a petrological category, though its Breton type material has a notable history as a building and carving stone.
Formation & geology
Kersantite crystallizes from hydrous, mafic magma intruded as dikes and sills in the upper crust. Early growth of hydrous biotite and augite reflects the volatile-rich character of the melt, while the remaining liquid solidifies to a plagioclase-rich groundmass.
It is associated with late-orogenic magmatism, often emplaced along fracture systems after major granitic intrusions, in Hercynian and Caledonian belts. The classic Breton occurrences, where it forms dikes in the Armorican Massif, give the rock its name.
How to identify it
Look for a dark, fine- to medium-grained porphyritic dike rock speckled with shiny biotite mica flakes and augite in a darker matrix. It is tough and moderately hard (about 5-6).
Kersantite is distinguished within the lamprophyres by being a mica lamprophyre with a plagioclase-dominant groundmass — minette is the alkali-feldspar version. Hornblende lamprophyres (spessartite, vogesite) carry amphibole rather than mica as the main dark mineral.
Look-alikes include diorite porphyry, dark micaceous basalt, and other lamprophyres; separating kersantite from minette requires determining the dominant feldspar, usually under the microscope.
Uses & significance
Kersantite's chief practical use comes from the Breton "kersanton" stone, a fine-grained, weather-resistant variety historically quarried for sculpture, crosses, calvaries, and architectural carving in Brittany; its even texture and durability made it a favored monumental stone.
More broadly, kersantite has no major industrial value and is of scientific interest as a reference lamprophyre used in studies of enriched magma sources and late-orogenic dike systems.
It has no gemstone or metaphysical tradition.
Frequently asked questions
How is kersantite different from minette?
Both are mica (biotite) lamprophyres, but kersantite's groundmass is dominated by plagioclase while minette's is dominated by alkali feldspar.
Where does the name kersantite come from?
From Kersanton in Brittany, France, where the durable kersanton stone has been used in sculpture for centuries.
Has kersantite been used as a building stone?
Yes. The Breton kersanton variety is a prized carving and monumental stone known for its fine grain and durability.
Is kersantite a common rock?
It is an uncommon dike rock, found mainly in orogenic belts such as the Armorican Massif.
Kersantite guides
In-depth guides for identifying, valuing, and understanding Kersantite.
