How to identify Biotite

To correctly identify Biotite (K(Mg,Fe)3AlSi3O10(F,OH)2), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Typically dark brown, black, or dark green. Rarely yellowish-brown.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to pearly on cleavage surfaces, dull to submetallic on other surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Foliated, flaky, or platy aggregates. Can form pseudohexagonal crystals.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Pseudohexagonal tabular crystals, often with a short prismatic habit. Commonly found as irregular flakes or books (stacks of thin sheets).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect basal cleavage in one direction (001), allowing it to be split into very thin, flexible, and elastic sheets.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in felsic to mafic igneous rocks (granite, diorite, gabbro, pegmatite), a major constituent of many metamorphic rocks (schist, gneiss, hornfels), and occasionally found as detrital grains in sedimentary rocks.

Key Facts

Hardness

2.5-3 on the Mohs scale

Specific Gravity

2.7-3.3 (varies with Fe/Mg ratio)

Crystal System

Monoclinic

Color

Dark brown, black, dark green

Luster

Vitreous to pearly

Transparency

Transparent to translucent in thin sheets, opaque in thicker masses

Fracture

Uneven to splintery (rarely observed due to perfect cleavage)

Cleavage

Perfect basal cleavage on {001}

Composition

Potassium iron magnesium aluminum silicate hydroxide fluoride

Physical characteristics

• Crystal Habit: Pseudohexagonal tabular crystals, often in 'books' or irregular flakes; also massive, disseminated.
• Cleavage Type: Perfect basal cleavage (one direction), producing thin, flexible, and elastic sheets.
• Fracture Type: Uneven to splintery (rarely seen due to cleavage).
• Tenacity: Flexible and elastic (sheets bend and return to original shape).
• Luster Type: Vitreous to pearly on cleavage surfaces, submetallic to dull on other surfaces.

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