Trioctahedral micas
By Dominic Papineau
Annite (Potassium Iron Aluminum Silicate Hydroxide)
Aspidolite (Sodium Magnesium Aluminum Silicate Hydroxide)
Biotite (Potassium Magnesium Iron Aluminum Silicate Hydroxide Fluoride)
Eastonite (Potassium Magnesium Aluminum Silicate Hydroxide)
Ephesite (Sodium Lithium Aluminum Silicate Hydroxide)
Hendricksite (Potassium Zinc Aluminum Silicate Hydroxide)
Lepidolite (Potassium Lithium Aluminum Silicate Fluoride Hydroxide)
Masutomilite (Potassium Lithium Aluminum Manganese Silicate Fluoride)
Montdorite (Potassium Iron Manganese Magnesium Aluminum Silicate Fluoride)
Norrishite (Potassium Lithium Manganese Silicate)
Polylithionite (Potassium Lithium Aluminum Silicate Fluoride)
Phlogopite (Potassium Magnesium Aluminum Silicate Hydroxide)
Preiswerkite (Sodium Magnesium Aluminum Silicate Hydroxide)
Siderophyllite (Potassium Iron Aluminum Silicate Hydroxide)
Tainiolite (Potassium Lithium Magnesium Silicate Fluoride)
Tetra-ferri-annite (Potassium Iron Silicate Hydroxide)
Tetra-ferriphlogopite (Potassium Magnesium Iron Silicate Hydroxide)
Trilithionite (Potassium Lithium Aluminum Silicate Fluoride)
Zinnwaldite (Potassium Lithium Iron Aluminum Silicate Fluoride Hydroxide)
The name of biotite comes from the French physicist J. B. Biot
The chemical and structural properties of biotite
Chemical formula of biotite: K(Mg,Fe)3(AlSi3O10)(OH)2
The composition is similar to phlogopite but with considerable substitution of
Fe2+ for Mg. In fact, biotite is an intermediate between phlogopite and annite.
A complete solid solution exists between annite, biotite, and phlogopite.
Other common substitutions include Ti, Fe3+ and Al for Mg in the octahedral
site and also Al for Si in the tetrahedral site.
Substitution of F and Cl for OH are also common.
Substitutions can also occur in the site of the interlayer cation K+ where
Na, Ca, NH4+, Ba, Rb, and Cs can substitute.
Biotite is more susceptible to chemical weathering
than muscovite, because the Fe2+ can oxidize to
Fe3+ in the presence of an electron acceptor. As a
result, the mineral alters to other aluminosilicates
and ferric oxide.
The physical properties of biotite
Colors: brown (high Fe + Mg and low Ti), green-brown (low Ti), bluegreen (no Ti), or black.
Luster: splendent
Cleavage: perfect in one direction
Hardness: 2.5 to 3
Density: 2.8 to 3.2 g/cm3
Crystal habit: generally tabular crystals with a pseudohexagonal shape.
The optical properties of biotite
Biotite is biaxial negative
2VZ = 0o - 25°
 = 1.57 – 1.63
 = 1.61 – 1.70
 = 1.61 – 1.70
 = 0.040 to 0.080 (third order interference colors seen as a “bird’s eye
texture”)
Strongly pleochroic
0o to 9o extinction angle
The crystallographic properties of biotite
Crystal system: monoclinic
Point Group: 2/m
Unit cell parameters: a = 5.31
b = 9.23
c = 10.18
 = 99.3°
Z=2
V = 479.79
Calculated density = 2.89
Space group: C2/m is the most common (1M)
The crystal structure of biotite
b
c
a
Silicon or aluminum
tetrahedra
Iron or magnesium
octahedron
Oxygen atom
Potassium or interlayer
cation
The tetrahedral sheets are 2.271Å thick for 1M biotite
The octahedral sheets are 2.138Å thick for 1M biotite
The interlayer separation is of 3.334Å for 1M biotite
Optical spectroscopic properties of biotite
Fe2+ is observed by absorption bands near:
270 nm (37 040 cm-1)
385 nm (25 970 cm-1)
456 nm (21 930 cm-1)
720 nm (14 000 cm-1)
920 nm (11 000 cm-1)
1150 nm (9 000 cm-1)
Cr3+ is observed by absorption bands near:
400 nm (24 000 cm-1)
600 nm (16 700
Ti3+ is observed by absorption bands near:
cm-1)
400 to 500 nm (24 000 to
20 000 cm-1)
Near UV,
Visible, and
Near IR
Visible
Visible
Infrared spectroscopic properties of biotite
Infrared spectroscopy in
micas can be used for:
Mica and polytype identification
Estimation of chemical composition
Determine ordering and orientation of small
molecular units
Derive thermodynamic parameters
Examine the interaction of a wide variety of
chemical substances with micas
OH-stretching is observed by absorption bands near:
Phlogopite
Wavenumber (cm-1)
930 nm (10 750 cm-1)
1380 nm (7250 cm-1)
2700 nm (3710 cm-1)
2760 and 2900 nm
(3620 and 3450 cm-1)
The crystallization of biotite in igneous rocks
The crystallization of biotite from sediments
Biotite can crystallize from pelitic sediments
(metamorphosed argillaceous sediments).
This metamorphism occurs by a series of dehydration reactions
The most basic requirements to obtain some biotite in metamorphic rocks are
a little K2O, sufficient Al2O3 and the appropriate metamorphic conditions.
Biotite in pelitic schist thus originates from mineral reactions which include
quartz, muscovite, biotite, chlorite, and K-feldspar.
Biotite Ti content increases as a function of metamorphic grade
The stability of biotite in metamorphic processes
Biotite is stable from greenschist to granulite facies
Occurrences of biotite
Biotite occurs in a wide variety of environments
Igneous rocks:
Pegmatites:
Granite, gabbro, diorite, peridotite, monzonite, syenite, etc.
Where it can contain some rare-earth elements.
Metamorphic rocks:
Found in a wide range of temperature and pressure
conditions characterizing the biotite zone.