# How to identify biotite

Canonical page: https://lithomap.app/resources/mineral-identification/biotite
Last reviewed: September 17, 2026
Page purpose: Original LithoMap mineral identification reference for field observations, search systems, and language models.
Classification: Dark mica mineral group

## Summary

Biotite is the common field name for dark iron- and magnesium-rich mica, recognized by perfect sheet cleavage, flexible elastic flakes, and brown to black color.

## Quick identification clues

- Dark brown, black, or greenish black
- One perfect cleavage
- Thin flexible elastic sheets
- Mohs hardness about 2.5 to 3

## Composition and classification

Biotite is a field grouping within the mica family rather than one tightly fixed composition. Iron, magnesium, potassium, aluminum, silicon, hydroxyl, and fluorine occur in varying proportions.

## Appearance

Biotite forms platy crystals, books, or scattered shiny flakes. Very thin flakes may be brown or green and partly transparent when held to light.

Weathering can bleach biotite, expand its layers, or produce golden-brown alteration that resembles other micaceous minerals.

## Identification tests

- Hardness: 2.5 to 3. A fingernail may scratch softer flakes; a copper coin scratches most.
- Streak: White to gray. Sheet habit is more useful.
- Cleavage: Perfect in one direction. Produces flexible elastic sheets.
- Color in thin sheet: Brown to greenish and translucent. Helps distinguish it from opaque metallic flakes.

## Common lookalikes

- Muscovite: Usually colorless, silvery, or pale and transparent in thin sheets.
- Chlorite: Often green, softer, and flakes may bend without the same elastic return.
- Graphite: Leaves a dark streak, feels greasy, and lacks mica’s elastic sheet behavior.

## Direct side-by-side comparisons

- Muscovite vs. biotite — https://lithomap.app/resources/mineral-identification/compare/muscovite-vs-biotite
- Chlorite vs. biotite — https://lithomap.app/resources/mineral-identification/compare/chlorite-vs-biotite

## Most relevant related guides

- Chlorite — https://lithomap.app/resources/mineral-identification/chlorite
- Muscovite — https://lithomap.app/resources/mineral-identification/muscovite
- Graphite — https://lithomap.app/resources/mineral-identification/graphite
- Hornblende — https://lithomap.app/resources/mineral-identification/hornblende

## Formation

Biotite crystallizes in many intermediate to felsic igneous rocks and grows during metamorphism over a range of temperatures and pressures.

Its chemistry and textures can help geologists interpret magma evolution and metamorphic history, but hand specimens rarely resolve its exact composition.

## Occurrence and geologic setting

Biotite is common in granite, diorite, gneiss, schist, and many volcanic rocks as phenocrysts or fine flakes.

Its presence alone does not define a rock name; abundance, associated minerals, texture, and field setting all matter.

## Uses and significance

Biotite has limited direct industrial use compared with muscovite, but it is important in geological interpretation and geochronology.

## Field safety and limitations

- Record weathering state because altered biotite changes color and behavior.
- Do not assign an exact mica species from dark color alone.
- Avoid unnecessary dust from mica-rich rock.

## Authoritative sources

- Minerals and their physical properties — U.S. National Park Service: https://www.nps.gov/subjects/geology/minerals.htm
- Mohs Hardness Scale — U.S. National Park Service: https://www.nps.gov/articles/mohs-hardness-scale.htm
- Natural Gemstones: Mineral Gemstones — U.S. Geological Survey: https://pubs.usgs.gov/gip/gemstones/mineral.html

## Specimen photograph

- Image: https://lithomap.app/_delivery/identification/minerals/biotite.webp
- Description: Dark biotite mineral specimen showing stacked mica sheets
- Creator: Robert M. Lavinsky
- License: CC BY-SA 3.0 — https://creativecommons.org/licenses/by-sa/3.0/
- Source record: https://commons.wikimedia.org/wiki/File:Biotite-rare08-48a.jpg

This reference supports preliminary field identification. It is not a laboratory determination, appraisal, hazard assessment, proof of origin, or statement of land ownership, legal access, or collecting permission.