How to identify Biotite

To correctly identify Biotite (K(Mg,Fe)3AlSi3O10(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 greenish-black. Rarely yellowish-brown.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to pearly on cleavage surfaces, duller on other surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Foliated, flaky, or scaly aggregates. Individual crystals are platy.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Pseudohexagonal tabular crystals, often forming books or flakes. Can also be disseminated as small flakes within rocks.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect basal cleavage (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 igneous rocks (granites, diorites, syenites, pegmatites, some volcanic rocks), metamorphic rocks (schists, gneisses, hornfels), and detrital sediments.

Key Facts

Hardness

2.5-3 on the Mohs scale

Specific Gravity

2.7-3.3 g/cm³ (varies with Fe/Mg ratio)

Crystal System

Monoclinic

Color

Dark brown, black, greenish-black

Luster

Vitreous to pearly

Transparency

Transparent to translucent in thin sheets, opaque in thicker pieces

Fracture

Uneven to splintery (rarely observed due to perfect cleavage)

Cleavage

Perfect basal cleavage (001)

Composition

Potassium iron magnesium aluminum silicate hydroxide

Physical characteristics

• Crystal Habit: Pseudohexagonal tabular crystals, often forming 'books' or flakes; also massive, disseminated, or scaly aggregates.
• Cleavage Type: Perfect basal cleavage (one direction), producing thin, flexible, and elastic sheets.
• Fracture Type: Uneven to splintery, but rarely observed due to dominant cleavage.
• Tenacity: Flexible and elastic (sheets can be bent and will return to their original shape).
• Luster Type: Vitreous to pearly on cleavage surfaces, submetallic to dull on other surfaces.

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