Biotite
| Biotite | |
|---|---|
 |
|
| General and classification | |
| chemical formula | K (Mg, Fe 2+ , Mn 2+ ) 3 [(OH, F) 2 | (Al, Fe 3+ , Ti 3+ ) Si 3 O 10 ] |
|
Mineral class (and possibly department) |
formerly layered silicates (phyllosilicates) |
|
System no. to Strunz and to Dana |
9.EC.20 ( 8th edition : VIII / H.11) 71.02.02b.02 |
| Similar minerals | Phlogopite |
| Crystallographic Data | |
| Crystal system | monoclinic (pseudohexagonal) |
| Crystal class ; symbol | monoclinic prismatic 2 / m |
| Physical Properties | |
| Mohs hardness | 2 to 3 |
| Density (g / cm 3 ) | 2.7 to 3.3 |
| Cleavage | perfectly |
| Break ; Tenacity | leafy |
| colour | black, dark brown, greenish black |
| Line color | White |
| transparency | translucent to opaque |
| shine | Glass gloss to pearlescent gloss |
| Crystal optics | |
| Refractive indices |
n α = 1.565 to 1.625 n β = 1.605 to 1.675 n γ = 1.605 to 1.675 |
| Birefringence | δ = 0.04 to 0.05 |
| Optical character | biaxial negative |
| Axis angle | 2V = 0 ° to 25 ° |
| Pleochroism | yellow-brown-red-brown only when transparent |
| Other properties | |
| Chemical behavior | Difficult to melt into black magnetic glass , soluble in concentrated H 2 SO 4 (a SiO 2 skeleton is formed) |
| Special features | Has not been considered a separate mineral since 1999 |
Biotite or dark mica (magnesium iron mica ) is a common sheet silicate . Since 1999, according to the decision of the International Mineralogical Association (IMA), it is no longer considered an independent mineral , but as a mixed crystal of the annite - phlogopite mixed series . With the “light mica” muscovite, biotite is one of the most common types of mica and gives the biotite series of the mica group its name .
The mineral crystallizes in flake form in the monoclinic crystal system with the general formula K (Mg, Fe 2+ , Mn 2+ ) 3 [(OH, F) 2 | (Al, Fe 3+ , Ti 3+ ) Si 3 O 10 ]. It develops translucent to opaque, very flexible and sometimes pseudo-hexagonal crystals with a metallic pearlescent sheen, which can be split into the finest platelets.
It was named after the French physicist Jean-Baptiste Biot (1774–1862), who was the first to recognize the optical diversity of mica .
Education and Locations
Biotite occurs to form rock in many igneous rocks , e.g. B. in granites . It crystallizes from a rising rock melt just before feldspar, quartz and pyroxene when the magma cools to around 800 to 700 ° C (phase of main crystallization ). Biotite is also widespread in metamorphic rocks such as gneiss and mica schist. It can be found in sand and sandstone in the form of small, gold-shining leaves.
The layer structure of many metamorphic rocks (e.g. gneiss ) is due to the different temperature behavior of the dark mica when the rock segregates at greater depths. Biotite and other phyllosilicates tend to recrystallize laterally when exposed to hot pressure, while quartz and feldspars tend to remain grainy. The cleavage areas correspond to the direction of the maximum tectonic shear forces .
Like muscovite, biotite forms large sheets in pegmatites . Light and dark mica can grow together, with the gap surfaces running through both mica without interruption. In addition, biotite forms parageneses with many other minerals , including amphiboles , andalusite , cordierite , garnets , potassium feldspar , nepheline , plagioclase , pyroxene , quartz and spinel .
The mineral is easily attacked by chemical weathering , loses shine and elasticity and becomes crumbly. In the final stage of decomposition, iron hydroxides and various clay minerals (hydrobiotite, vermiculite ), fool's gold or chlorite are formed . If biotite is deposited as fine platelets in the water on sand, it also undergoes changes there (e.g. oxidation ). The color takes on a golden brown tone, hence the name feline gold. Biotite is only used economically as an additive in cosmetics and as a filler (together with muscovite).
By 2010, biotite could be detected in around 4600 sites. Larger leaf crystals can be found in the area around Lake Laacher See in Germany, Bessnes in France, Ontario in Canada, Evje in Norway , and in the Uluguru Mountains in Tanzania . Countries with many sites (5 or more) include Argentina , Australia , Bolivia , Brazil , Bulgaria , Chile , China , Germany , Ecuador , Finland , France , India , Indonesia , Italy , Japan , Canada , Kazakhstan , Madagascar , Mexico , Mongolia , Namibia , North Korea , Norway , Austria , Papua New Guinea , Peru , Philippines , Poland , Paraguay , Portugal , Romania , Russia , Sweden , Switzerland , Slovakia , South Africa , South Korea , Spain , Czech Republic , Ukraine , Hungary , in the United Kingdom ( Great Britain) and in the United States of America (USA).
Petrology
Biotite occurs in almost all metasediments. In the amphibolites the mineral is in equilibrium with amphibole and plagioclase, while in the eclogites it is a decomposition product of phengite .
use
Rocks that contain biotite as a mineral component are only moderately suitable as a building material . Here, the cleavage of biotite proves to be unfavorable, since weathering solutions can penetrate into the rock on the resulting separation surfaces. Externally, such processes are documented, for example, with the formation of rust spots on the rock surface (e.g. granite curbs ). In addition, areas of separation, especially with small distances, are often areas of weakness in terms of rock mechanics . Due to its low hardness, biotite is not used in a concentrated form as a consumer material. In the cosmetic industry, biotite is used together with muscovite to provide cosmetic articles with the glittering effect typical of most mica minerals (see also use of mica ).
See also
Individual evidence
literature
- Petr Korbel, Milan Novák: Encyclopedia of Minerals . Nebel Verlag GmbH, Eggolsheim 2002, ISBN 3-89555-076-0 , p. 252 .
- Paul Ramdohr , Hugo Strunz : Klockmann's textbook of mineralogy . 16th edition. Ferdinand Enke Verlag, 1978, ISBN 3-432-82986-8 , pp. 748 .
- Armand Lévy : Description d'une collection de minéraux . 1837, among others in: VM Goldschmidt, Atlas der Krystallformen , 1913–1923 (mica group (biotite, muscowite, etc.))
Web links
- Mineral Atlas: Biotite (Wiki)
- Webmineral - Biotite
- MinDat - Biotite
- Handbook of Mineralogy - Biotite (English, PDF 78.5 kB)