Minette Identification Guide
A field guide to identifying Minette, a dark mica-rich lamprophyre dike rock, by its biotite and orthoclase mineralogy and porphyritic texture.
Read the full Minette encyclopedia entry →
What Minette Looks Like
Minette is a type of lamprophyre — a dark, fine-grained igneous dike rock rich in mica. By definition, minette contains abundant biotite/phlogopite mica as the main dark mineral and orthoclase (K-feldspar) as the dominant light mineral. It typically has conspicuous shiny mica flakes set in a darker groundmass.
- Color: Dark gray to brownish-black, often with a glittery look from mica.
- Texture: Porphyritic — visible biotite (and sometimes amphibole) phenocrysts in a fine groundmass.
- Luster: Mica flakes are bright/pearly; groundmass is dull.
- Form: Occurs as dikes, sills, and small intrusions.
Step-by-Step Field ID Checklist
- Spot the mica: Look for abundant dark, shiny biotite/phlogopite flakes — the hallmark of minette.
- Note the dark, dense matrix: Fine-grained and dark, but not glassy like obsidian.
- Look for porphyritic texture: Distinct dark crystals in a finer background.
- Check the geologic setting: Dike or sill, narrow intrusive body cutting other rocks.
- Rule out carbonate and confirm hardness (below).
Key Diagnostic Tests
- Mohs hardness: Variable by mineral; feldspar ~6, biotite ~2.5–3 (flakes peel easily). Bulk rock is hard.
- Streak: Non-diagnostic (silicate rock).
- Cleavage: Biotite shows one perfect cleavage (peels into thin sheets); feldspar shows blocky cleavage.
- Density: ~2.7–3.0 g/cm3.
- Acid: No fizz with dilute HCl (unless secondary carbonate is present).
- Magnetism: Weak if magnetite is present.
Common Look-Alikes and How to Tell Them Apart
- Kersantite (another lamprophyre): Mica-rich like minette, but its feldspar is plagioclase, not orthoclase — needs thin-section/chemistry to be sure.
- Basalt: Dark and fine-grained, but lacks abundant large biotite mica and contains plagioclase + pyroxene; minette is mica-dominated.
- Lamproite/minette confusion: Lamproites are more exotic, often leucite-bearing; minette is a more "ordinary" K-feldspar + biotite lamprophyre.
- Biotite-rich schist: Foliated and metamorphic with aligned mica; minette is an unfoliated igneous dike rock.
- Diorite/monzonite: Coarser, lighter overall, without the dense mica-rich dike character.
Where It Is Found
Minette occurs as dikes and small intrusions associated with continental and orogenic magmatism. Classic European localities are in the Vosges (France) and Germany; minettes are also reported in the western U.S. (e.g., Colorado Plateau) and other shield and mountain-belt regions.
Frequently asked questions
How do you identify minette?
Identify minette as a dark, fine-grained dike rock packed with shiny biotite/phlogopite mica flakes and orthoclase feldspar, usually with a porphyritic texture. The abundance of dark mica in an igneous (not foliated) rock is the key clue.
What is the difference between minette and kersantite?
Both are mica-rich lamprophyres. Minette's dominant feldspar is orthoclase (K-feldspar), whereas kersantite's is plagioclase; distinguishing them reliably usually requires thin-section or chemical analysis.
Is minette igneous or metamorphic?
Minette is an igneous rock — a lamprophyre dike rock — not metamorphic. Unlike a mica schist, it is unfoliated and forms intrusive bodies.
Minette vs basalt — how to tell them apart?
Basalt is a plagioclase-pyroxene volcanic rock with little or no large mica, while minette is mica-dominated (abundant biotite) and typically occurs as dikes rather than lava flows.