Collection: Biotite

Biotite is a mineral of the mica group , very common in many igneous, metamorphic and sometimes sedimentary rocks. Here is a detailed presentation:

🔍 General characteristics

• Chemical formula : K(Mg,Fe) $_3$ AlSi $_3$ O $_10$ (OH) $_2$

• Crystal system : Monoclinic

• Color : Dark brown to black, sometimes dark green

• Luster : Pearly to vitreous

• Hardness : 2.5 – 3 (Mohs scale)

• Cleavage : Perfect in one direction (typical of micas)

• Density : Approximately 2.8 to 3.2

🧪 Composition

Biotite is a ferromagnesian mica , meaning it contains iron (Fe) and magnesium (Mg). It can be considered as a solid solution between two poles:

• Phlogopite (rich in Mg)

• Annite (rich in Fe)

It also contains potassium (K), aluminum (Al), silicon (Si) and hydroxyl (OH).

🪨 Training and attendance

Igneous rocks:

• Present in granites , diorites , gabbros , etc.

• Forms from magma by crystallizing at relatively high temperature.

Metamorphic rocks:

• Common in schists and gneisses

• Forms by regional metamorphism at medium or high temperature.

Sedimentary rocks:

• Rare, because it weathers easily, but it can be found in detrital form in certain sandstones .

🌍 Use and interest

• Petrographic interest : Allows us to determine the conditions of rock formation (pressure, temperature, composition).

• Dating : Often contains potassium, so can be used for K/Ar or Ar/Ar dating.

• Geological indicator : Its composition in Fe and Mg provides information on the chemistry of the crystallization environment.

☠️ Alteration

Biotite is relatively unstable at the surface. It can alter into:

• Chlorite

• Vermiculite

• Clays

This transformation is common in soils developed on granitic rocks.

📸 Thin-section appearance (polarizing microscope)

• Strong pleochroism (color change depending on orientation)

• Moderate color interference (2nd order)

• Medium relief, clearly visible cleavage