Crystal Biotite

Chemical Formula

The classic formula for Biotite is:
K(Mg,Fe²⁺₃)(Al,Fe³⁺)Si₃O₁₀(OH,F)₂

The group formula including all member minerals:
K(Mg,Fe²⁺)₃(Al,Fe³⁺)[(Al,Si)₃O₁₀](OH,F)₂

Individual members are:
Phlogopite: KMg₃AlSi₃O₁₀)(F,OH)₂
Siderophyllite: KFe²⁺₂Al(AlSi₂O₁₀)(OH)₂
Eastonite: KMg₂Al(AlSi₂O₁₀)(OH)₂
Annite: KFe²⁺₃AlSi₃O₁₀(OH)₂
Fluorannite: KFe²⁺₃AlSi₃O₁₀)F₂
Tetraferriannite: K(Fe²⁺₃Mg)(Fe³⁺,Al)Si₃O₁₀)(OH)₂
 

Composition

Basic fluoro potassium, magnesium, iron aluminum silicate
 

Color

Black, dark brown, dark green, reddish black. Individual group member minerals such as Phlogopite and Eastonite can be in lighter colors.

Streak

White

Hardness

2.5 - 3

Crystal System

Monoclinic

Crystal Forms 
and Aggregates

Crystals are in thick flakes, micaceous masses and groupings, and in tabular, foliated, flaky, and scalyforms. Crystals may also be elongated with one dimension flat, or stubby triangular or hexagonally shaped crystals. Also forms in prismatic barrel-shaped or tapered pyramid-shaped crystals composed of dense parallel plates, and as rounded nodules of dense crystals.
 

Transparency

Translucent to opaque. Thin flakes will always be translucent if held up to the light.

Specific Gravity

2.8 - 3.4

Luster

Pearly

Cleavage

1,1

Fracture

Uneven

Tenacity

Sectile, elastic

Other ID Marks

Tendency for small pieces or flakes or peel off.

In Group

Silicates; Phyllosilicates; Mica Group

Striking Features

Flaky habit, crystals, sectility, and mode of occurence.

Environment

Biotite is a common rock-forming mineral, and is especially noted in metamorphic rocks such as schistand gneiss. It is also found in igneous rock such as granites and rhyolites. Biotite is also the primary mica in rare earth pegmatites.
 

Rock Type

Igneous, Metamorphic

Popularity (1-4)

2

Prevalence (1-3)

1

Demand (1-3)

2