Trioctahedral micasTrioctahedral micas

By Dominic PapineauBy Dominic Papineau

Annite Annite ((Potassium Iron Aluminum Silicate HydroxidePotassium Iron Aluminum Silicate Hydroxide) )

AspidoliteAspidolite ( (Sodium Magnesium Aluminum Silicate HydroxideSodium Magnesium Aluminum Silicate Hydroxide) )

Biotite Biotite ((Potassium Magnesium Iron Aluminum Silicate Hydroxide FluoridePotassium Magnesium Iron Aluminum Silicate Hydroxide Fluoride ) )

EastoniteEastonite ( (Potassium Magnesium Aluminum Silicate HydroxidePotassium Magnesium Aluminum Silicate Hydroxide) )

EphesiteEphesite ( (Sodium Lithium Aluminum Silicate HydroxideSodium Lithium Aluminum Silicate Hydroxide) )

HendricksiteHendricksite ( (Potassium Zinc Aluminum Silicate HydroxidePotassium Zinc Aluminum Silicate Hydroxide) )

LepidoliteLepidolite ( (Potassium Lithium Aluminum Silicate Fluoride HydroxidePotassium Lithium Aluminum Silicate Fluoride Hydroxide ) )

MasutomiliteMasutomilite ( (Potassium Lithium Aluminum Manganese Silicate FluoridePotassium Lithium Aluminum Manganese Silicate Fluoride ) )

MontdoriteMontdorite ( (Potassium Iron Manganese Magnesium Aluminum Silicate FluoridePotassium Iron Manganese Magnesium Aluminum Silicate Fluoride ) )

NorrishiteNorrishite ( (Potassium Lithium Manganese SilicatePotassium Lithium Manganese Silicate) )

PolylithionitePolylithionite ( (Potassium Lithium Aluminum Silicate FluoridePotassium Lithium Aluminum Silicate Fluoride) )

PhlogopitePhlogopite ( (Potassium Magnesium Aluminum Silicate HydroxidePotassium Magnesium Aluminum Silicate Hydroxide) )

PreiswerkitePreiswerkite ( (Sodium Magnesium Aluminum Silicate HydroxideSodium Magnesium Aluminum Silicate Hydroxide) )

SiderophylliteSiderophyllite ( (Potassium Iron Aluminum Silicate HydroxidePotassium Iron Aluminum Silicate Hydroxide) )

TainioliteTainiolite ( (Potassium Lithium Magnesium Silicate FluoridePotassium Lithium Magnesium Silicate Fluoride) )

Tetra-ferri-anniteTetra-ferri-annite ( (Potassium Iron Silicate HydroxidePotassium Iron Silicate Hydroxide) )

Tetra-ferriphlogopiteTetra-ferriphlogopite ( (Potassium Magnesium Iron Silicate HydroxidePotassium Magnesium Iron Silicate Hydroxide) )

TrilithioniteTrilithionite ( (Potassium Lithium Aluminum Silicate FluoridePotassium Lithium Aluminum Silicate Fluoride) )

ZinnwalditeZinnwaldite ( (Potassium Lithium Iron Aluminum Silicate Fluoride HydroxidePotassium Lithium Iron Aluminum Silicate Fluoride Hydroxide ) )

The name of biotite comes from the French physicist J. B. BiotThe name of biotite comes from the French physicist J. B. Biot

The chemical and structural properties of biotiteThe chemical and structural properties of biotite

Other common substitutions include Ti, FeOther common substitutions include Ti, Fe3+3+ and Al for Mg in the octahedral and Al for Mg in the octahedral site and also Al for Si in the tetrahedral site.site and also Al for Si in the tetrahedral site.

The composition is similar to phlogopite but with considerable substitution of The composition is similar to phlogopite but with considerable substitution of FeFe2+2+ for Mg. In fact, biotite is an intermediate between phlogopite and annite. for Mg. In fact, biotite is an intermediate between phlogopite and annite.

Chemical formula of biotite: K(Mg,Fe)Chemical formula of biotite: K(Mg,Fe)33(AlSi(AlSi33OO1010)(OH))(OH)22

Substitutions can also occur in the site of the interlayer cation K+ where Substitutions can also occur in the site of the interlayer cation K+ where Na, Ca, NHNa, Ca, NH4+4+, Ba, Rb, and Cs can substitute., Ba, Rb, and Cs can substitute.

A complete solid solution exists between annite, biotite, and phlogopite. A complete solid solution exists between annite, biotite, and phlogopite.

Substitution of F and Cl for OH are also common.Substitution of F and Cl for OH are also common.

Biotite is more susceptible to chemical weathering Biotite is more susceptible to chemical weathering than muscovite, because the Fethan muscovite, because the Fe2+2+ can oxidize to can oxidize to FeFe3+3+ in the presence of an electron acceptor. As a in the presence of an electron acceptor. As a result, the mineral alters to other aluminosilicates result, the mineral alters to other aluminosilicates and ferric oxide. and ferric oxide.

The physical properties of biotiteThe physical properties of biotite

Luster: splendentLuster: splendent

Cleavage: perfect in one directionCleavage: perfect in one direction

Hardness: 2.5 to 3Hardness: 2.5 to 3

Density: 2.8 to 3.2 g/cmDensity: 2.8 to 3.2 g/cm33

Crystal habit: generally tabular crystals with a pseudohexagonal shape. Crystal habit: generally tabular crystals with a pseudohexagonal shape.

Colors: brown (high Fe + Mg and low Ti), green-brown (low Ti), blue-Colors: brown (high Fe + Mg and low Ti), green-brown (low Ti), blue-green (no Ti), or black.green (no Ti), or black.

The optical properties of biotiteThe optical properties of biotite

Biotite is biaxial negativeBiotite is biaxial negative

= 1.57 – 1.63= 1.57 – 1.63

= 1.61 – 1.70= 1.61 – 1.70

= 1.61 – 1.70= 1.61 – 1.70

Strongly pleochroicStrongly pleochroic

2V2VZZ = 0 = 0oo - 25° - 25°

= 0.040 to 0.080 (third order interference colors seen as a “bird’s eye = 0.040 to 0.080 (third order interference colors seen as a “bird’s eye texture”)texture”)

00oo to 9 to 9oo extinction angle extinction angle

The crystallographic properties of biotiteThe crystallographic properties of biotite

Crystal system: monoclinic Crystal system: monoclinic

Point Group: 2/mPoint Group: 2/m

Space group: C2/m is the most common (1M)Space group: C2/m is the most common (1M)

Unit cell parameters:Unit cell parameters: a = 5.31a = 5.31

b = 9.23b = 9.23

c = 10.18 c = 10.18

= 99.3° = 99.3°

Z = 2Z = 2

V = 479.79V = 479.79

Calculated density = 2.89 Calculated density = 2.89

The crystal structure of biotiteThe crystal structure of biotite

Silicon or aluminum Silicon or aluminum tetrahedratetrahedra

Iron or magnesium Iron or magnesium octahedronoctahedron

Oxygen atomOxygen atom

bbcc

aa

Potassium or interlayer Potassium or interlayer cationcation

The tetrahedral sheets are 2.271The tetrahedral sheets are 2.271ÅÅ thick for 1M biotite thick for 1M biotite

The octahedral sheets are 2.138The octahedral sheets are 2.138ÅÅ thick for 1M biotite thick for 1M biotite

The interlayer separation is of 3.334The interlayer separation is of 3.334Å Å for 1M biotite for 1M biotite

Optical spectroscopic properties of biotiteOptical spectroscopic properties of biotite

FeFe2+2+ is observed by absorption bands near: is observed by absorption bands near:

720 nm (14 000 cm720 nm (14 000 cm-1-1))

920 nm (11 000 cm920 nm (11 000 cm-1-1))1150 nm (9 000 cm1150 nm (9 000 cm-1-1))

Near UV, Near UV, Visible, and Visible, and

Near IRNear IR

CrCr3+3+ is observed by absorption bands near: is observed by absorption bands near:

600 nm (16 700 cm600 nm (16 700 cm-1-1))

400 nm (24 000 cm400 nm (24 000 cm-1-1))VisibleVisible

TiTi3+3+ is observed by absorption bands near: is observed by absorption bands near: 400 to 500 nm (24 000 to 400 to 500 nm (24 000 to 20 000 cm20 000 cm-1-1))

VisibleVisible

270 nm (37 040 cm270 nm (37 040 cm-1-1))

385 nm (25 970 cm385 nm (25 970 cm-1-1))

456 nm (21 930 cm456 nm (21 930 cm-1-1))

Infrared spectroscopic properties of biotiteInfrared spectroscopic properties of biotite

Infrared spectroscopy in Infrared spectroscopy in micas can be used for:micas can be used for:

Mica and polytype identificationMica and polytype identification

Estimation of chemical compositionEstimation of chemical composition

Determine ordering and orientation of small Determine ordering and orientation of small molecular unitsmolecular units

Derive thermodynamic parametersDerive thermodynamic parameters

Examine the interaction of a wide variety of Examine the interaction of a wide variety of chemical substances with micaschemical substances with micas

OH-stretching is observed by absorption bands near:OH-stretching is observed by absorption bands near:

2760 and 2900 nm 2760 and 2900 nm (3620 and 3450 cm(3620 and 3450 cm-1-1))

2700 nm (3710 cm2700 nm (3710 cm-1-1))1380 nm (7250 cm1380 nm (7250 cm-1-1))930 nm (10 750 cm930 nm (10 750 cm-1-1))

Wavenumber (cmWavenumber (cm-1-1))

PhlogopitePhlogopite

The crystallization of biotite in igneous rocksThe crystallization of biotite in igneous rocks

The crystallization of biotite from sedimentsThe crystallization of biotite from sediments

Biotite can crystallize from pelitic sediments Biotite can crystallize from pelitic sediments (metamorphosed argillaceous sediments).(metamorphosed argillaceous sediments).

This metamorphism occurs by a series of dehydration reactionsThis metamorphism occurs by a series of dehydration reactions

Biotite in pelitic schist thus originates from mineral reactions which include Biotite in pelitic schist thus originates from mineral reactions which include quartz, muscovite, biotite, chlorite, and K-feldspar.quartz, muscovite, biotite, chlorite, and K-feldspar.

Biotite Ti content increases as a function of metamorphic gradeBiotite Ti content increases as a function of metamorphic grade

The most basic requirements to obtain some biotite in metamorphic rocks are The most basic requirements to obtain some biotite in metamorphic rocks are a little Ka little K22O, sufficient AlO, sufficient Al22OO33 and the appropriate metamorphic conditions. and the appropriate metamorphic conditions.

Biotite is stable from greenschist to granulite faciesBiotite is stable from greenschist to granulite facies

The stability of biotite in metamorphic processesThe stability of biotite in metamorphic processes

Occurrences of biotiteOccurrences of biotite

Biotite occurs in a wide variety of environmentsBiotite occurs in a wide variety of environments

Granite, gabbro, diorite, peridotite, monzonite, syenite, etc.Granite, gabbro, diorite, peridotite, monzonite, syenite, etc.Igneous rocks:Igneous rocks:

Pegmatites:Pegmatites:

Metamorphic rocks:Metamorphic rocks: Found in a wide range of temperature and pressure Found in a wide range of temperature and pressure conditions characterizing the conditions characterizing the biotite zonebiotite zone. .

Where it can contain some rare-earth elements.Where it can contain some rare-earth elements.