Biotite Mineral Overview

Mineral name: Biotite
Mineral class: Silicate (phyllosilicate, mica group)

Known Facts

• Chemical formula: K(Mg,Fe)₃AlSi₃O₁₀(OH)₂ (variable)
• Crystal system: Monoclinic
• Mohs hardness: 2.5–3
• Color range: Black, dark brown, dark green
• Luster: Vitreous to pearly
• Streak: White to gray
• Cleavage: Perfect basal cleavage
• Fracture: Uneven
• Specific gravity: Approximately 2.7–3.4 (varies with iron content)
• Common locations: Worldwide (very common)
• Uses: Geological study, insulation materials (historically), collector specimens
• Similar minerals: Muscovite, phlogopite, chlorite

Definition and Classification

Biotite is a common mica mineral and part of the mica group of phyllosilicates, characterized by:

• Sheet-like (layered) crystal structure
• Perfect basal cleavage

It is the iron-rich member of the mica group, forming a series with:

• Phlogopite (magnesium-rich)

Physical and Optical Characteristics

Biotite typically occurs as:

• Thin, flexible sheets or flakes
• Platy crystals
• Disseminated grains in rocks

It is usually opaque to translucent in thin sheets.

Color is typically:

• Black to dark brown or greenish-black

The luster is vitreous on crystal faces and pearly on cleavage surfaces.

A defining feature is its perfect basal cleavage, allowing it to be split into:

• Thin, elastic sheets

With a hardness of 2.5–3, biotite is soft and easily scratched.

Chemical Composition and Structure

Biotite has a variable composition:

• K(Mg,Fe)₃AlSi₃O₁₀(OH)₂

Key components include:

• Potassium (K)
• Magnesium (Mg) and iron (Fe)
• Aluminum (Al)
• Silicate sheets (Si₄O₁₀ units)
• Hydroxyl (OH)

It is a phyllosilicate, meaning:

• It has a layered structure of silicate sheets
• Weak bonding between layers allows easy cleavage

The Fe:Mg ratio can vary, affecting:

• Color (more iron = darker)
• Density

Formation and Geological Occurrence

Biotite forms in a wide range of geological environments:

Igneous rocks

• Common in:
  • Granite
  • Diorite
  • Pegmatites

Metamorphic rocks

• Found in:
  • Schists
  • Gneisses

Hydrothermal environments

• Occurs in altered rocks and veins

It is commonly associated with:

• Quartz
• Feldspar
• Muscovite
• Hornblende

Biotite is a rock-forming mineral, present in many common rock types.

Common Locations

Biotite is found worldwide, with notable occurrences in:

• United States: New England, Colorado
• Canada: Shield regions
• Brazil: Pegmatites
• Russia: Ural Mountains
• India and Madagascar: Metamorphic terrains

It is one of the most abundant mica minerals.

Uses and Practical Significance

Biotite has limited industrial use compared to other micas:

Scientific importance

• Used in:
  • Geochronology (K–Ar dating)
  • Studying metamorphic conditions

Industrial use (historical/limited)

• As insulating material (less common than muscovite)

Collecting

• Collected for:
  • Crystal form
  • Occurrence in rocks

Similar and Related Minerals

Biotite may be confused with:

• Muscovite: Lighter color (colorless to silvery)
• Phlogopite: Brown and more magnesium-rich
• Chlorite: Softer and greener

Distinguishing features include:

• Dark color
• Perfect sheet cleavage
• Occurrence as a common rock-forming mineral

Identification Notes for Collectors

• Look for dark, platy flakes that split into thin sheets
• Check for perfect basal cleavage
• Note softness (2.5–3)
• Observe association with granite or metamorphic rocks
• Compare color with other mica minerals

Biotite is a common iron-rich mica, recognized for its sheet-like structure, dark coloration, and widespread occurrence in igneous and metamorphic rocks, making it an important mineral in geological studies.