How to identify Biotite Mica

To correctly identify Biotite Mica (Biotite), 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, duller on other surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Flaky, platy, or scaly aggregates. Individual crystals are often tabular or pseudohexagonal.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Pseudohexagonal tabular crystals, often forming books or aggregates of thin sheets. Can also be found as disseminated flakes.

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 (e.g., granite, granodiorite, diorite, gabbro, syenite, pegmatite), and a widespread mineral in medium- to high-grade metamorphic rocks (e.g., schist, gneiss, hornfels). Also found in some hydrothermal veins and as detrital grains in sediments.

Key Facts

Hardness

2.5-3 on the Mohs scale

Specific Gravity

2.7-3.3 (varies with Fe/Mg content)

Crystal System

Monoclinic

Color

Dark brown, black, dark green

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

Physical characteristics

• Crystal Habit: Tabular, pseudohexagonal crystals; often in books, flakes, or disseminated grains.
• Cleavage Type: Perfect basal cleavage (one direction), yielding thin, flexible, 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.

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