Biotite
“The dark mica that's everywhere”
Biotite is a common dark brown to black mica mineral found in a huge range of igneous and metamorphic rocks. Like muscovite, it splits into thin flexible sheets, but biotite's high iron and magnesium content gives it a much darker color and makes it an important indicator of rock formation conditions.
potassium iron magnesium aluminum silicate (K(Mg,Fe)₃AlSi₃O₁₀(OH)₂)
Virtually worldwide in granites, gneisses, schists, and volcanic rocks; notable crystals from Bancroft (Ontario, Canada), Vesuvius (Italy)
2.5-3
Biotite is a key mineral for radiometric dating — its potassium content allows geologists to use the potassium-argon method to date rocks from thousands to billions of years old.
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Why is biotite particularly valuable for potassium-argon radiometric dating?
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