Biotite is a common group of phyllosilicate within the mica group, with the approximate chemical formula . It is primarily a solid solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more Aluminium end-members include siderophyllite and eastonite. Biotite was regarded as a mineral species by the International Mineralogical Association until 1998, when its status was changed to a mineral group.[ "Zur Bezeichnung des sogenannten einachsigen Glimmers ist hier der Name Biotit gewählt worden, um daran zu erinnern, daß Biot es war, der zuerst auf die optische Verschiedenheit der Glimmerarten aufmerksam machte." (For the designation of so-called uniaxial mica, the name "biotite" has been chosen in order to recall that it was Biot who first called attention to the optical differences between types of mica.)]
Members of the biotite group are . Iron, magnesium, aluminium, silicon, oxygen, and hydrogen form sheets that are weakly bound together by potassium . The term "iron mica" is sometimes used for iron-rich biotite, but the term also refers to a flaky micaceous form of haematite, and the field term Lepidomelane for unanalysed iron-rich Biotite avoids this ambiguity. Biotite is also sometimes called "black mica" as opposed to "white mica" (muscovite) – both may form in the same rocks, and in some instances side by side.
Properties
Like other
mica minerals, biotite has a highly perfect
basal cleavage, and consists of flexible sheets, or lamellae, which easily flake off. It has a monoclinic crystal system, with
tabular habit to prismatic crystals with an obvious termination. It has four prism faces and two pinacoid faces to form a pseudohexagonal crystal. Although not easily seen because of the cleavage and sheets, fracture is uneven. It appears greenish to brown or black, and even yellow when
weathering. It can be transparent to opaque, has a vitreous to pearly luster, and a grey-white streak. When biotite crystals are found in large chunks, they are called "books" because they resemble books with pages of many sheets. The color of biotite is usually black and the mineral has a hardness of 2.5–3 on the Mohs scale of mineral hardness.
Biotite Solvation in both acid and alkaline , with the highest dissolution rates at low pH.
File:Biotite mica 2 (31739438210).jpg|Flaky biotite sheets.
File:BiotitaEZ.jpg|Thick biotite sample featuring many sheets.
File:Biotite1.jpg|Biotite crystal exhibiting pseudohexagonal shape.
Optical properties
In
thin section, biotite exhibits moderate
Optical relief and a pale to deep greenish brown or brown color, with moderate to strong
pleochroism. Biotite has a high
birefringence which can be partially masked by its deep intrinsic color.
Under cross-polarized light, biotite exhibits extinction approximately parallel to cleavage lines, and can have characteristic bird's eye maple extinction, a mottled appearance caused by the distortion of the mineral's flexible lamellae during grinding of the thin section. Basal sections of biotite in thin section are typically approximately hexagonal in shape and usually appear
isotropic under cross-polarized light.
File:Muscovite and Biotite2a.jpg|Biotite (in brown) and muscovite in an orthogneiss thin section under plane-polarized light.
File:Thin Section of Biotite (test) (cropped to Biotite).jpg|Biotite in thin section under cross-polarized light.
File:Sagenitic biotite.JPG|Basal section of biotite, with needle-like rutile inclusions, in thin section under plane-polarized light.
Structure
Like other micas, biotite has a crystal structure described as
TOT-c, meaning that it is composed of parallel
TOT layers weakly bonded to each other by
(
c). The
TOT layers in turn consist of two tetrahedral sheets (
T) strongly bonded to the two faces of a single octahedral sheet (
O). It is the relatively weak ionic bonding between
TOT layers that gives biotite its perfect basal cleavage.
The tetrahedral sheets consist of silica tetrahedra, which are silicon ions surrounded by four oxygen ions. In biotite, one in four silicon ions is replaced by an aluminium ion. The tetrahedra each share three of their four oxygen ions with neighboring tetrahedra to produce a hexagonal sheet. The remaining oxygen ion (the apical oxygen ion) is available to bond with the octahedral sheet.
The octahedral sheet in biotite is a trioctahedral sheet having the structure of a sheet of the mineral brucite, with magnesium or ferrous iron being the usual cations. Apical oxygens take the place of some of the hydroxyl ions that would be present in a brucite sheet, bonding the tetrahedral sheets tightly to the octahedral sheet.
Tetrahedral sheets have a strong negative charge, since their bulk composition is AlSi3O105-. The trioctahedral sheet has a positive charge, since its bulk composition is M3(OH)24+ (M represents a divalent ion such as ferrous iron or magnesium) The combined TOT layer has a residual negative charge, since its bulk composition is M3(AlSi3O10)(OH)2−. The remaining negative charge of the TOT layer is neutralized by the interlayer potassium ions.
Because the hexagons in the T and O sheets are slightly different in size, the sheets are slightly distorted when they bond into a TOT layer. This breaks the hexagonal symmetry and reduces it to monoclinic symmetry. However, the original hexahedral symmetry is discernible in the pseudohexagonal character of biotite crystals.
Occurrence
Members of the biotite group are found in a wide variety of
igneous rock and metamorphic rocks. For instance, biotite occurs in the
lava of
Mount Vesuvius and in the Monzoni intrusive complex of the western
Dolomites. Biotite in
granite tends to be poorer in magnesium than the biotite found in its volcanic equivalent,
rhyolite.
Biotite is an essential
phenocryst in some varieties of
lamprophyre. Biotite is occasionally found in large cleavable crystals, especially in
pegmatite veins, as in
New England,
Virginia and
North Carolina USA. Other notable occurrences include Bancroft and
Greater Sudbury,
Ontario Canada. It is an essential constituent of many metamorphic
, and it forms in suitable compositions over a wide range of
pressure and
temperature. It has been estimated that biotite comprises up to 7% of the exposed continental crust.
An igneous rock composed almost entirely of dark mica (biotite or phlogopite) is known as a glimmerite or biotitite.
Biotite may be found in association with its common alteration product Chlorite group.
The largest documented single crystals of biotite were approximately sheets found in Iveland, Norway.
File:Biotite Granite student sample.jpg|Biotite-bearing granite samples (small black minerals).
File:Biotite Gneiss.JPG|Biotite-bearing gneiss sample.
File:Biotite and chlorite gneiss mg 7971.jpg|Gneiss sample bearing biotite and chlorite (green), a common alteration product of biotite.
File:Sheet mica, Namibia.jpg|Glimmerite from Namibia.
Uses
Biotite is used extensively to constrain ages of rocks, by either potassium-argon dating or argon–argon dating. Because
argon escapes readily from the biotite crystal structure at high temperatures, these methods may provide only minimum ages for many rocks. Biotite is also useful in assessing temperature histories of metamorphic rocks, because the partitioning of
iron and
magnesium between biotite and
garnet is sensitive to temperature.
Bibliography
External links
