What is the luster of the mineral biotite?
Biotite – This is a solid solution occupying an intermediate position between the annite-phlogopite and siderophyllite-istonite series. The most common micas are biotite, lepidolite, muscovite, phlogopite, zinnwaldite and Clintonite. Muscovite and phlogopite are used industrially, while biotite is considered the most common mica. The chemical formula of biotite is K(Mg, Fe)3[(OH, F)2|AISi3O10], but the ratio of iron and magnesium depends on the conditions of formation of the mineral. All micas are characterized by basal (very perfect) cleavage; they are easily split into thin, flexible leaves along weakly bound layers of potassium ions. That’s why they have such a layered structure. Biotite is characterized by a monoclinic crystalline system; it forms pseudohexagonal crystals, the color of which can range from green, brown to black (and sometimes even yellowish). Usually such crystals are called dark micas. Biotite was first described in detail by the German mineralogist Johann Friedrich Ludwig Hausmann in 1847, who named this substance in honor of Jean Baptiste Biot. Biotite crystals are found in igneous and metamorphic rocks and are of igneous origin. The largest of them are formed in pegmatites. In these relatively small granite intrusions, slow cooling promotes steady crystal growth. Often the crystals acquire enormous sizes – the largest individual biotite crystals have an area of several square meters. Smaller crystals usually occur as phenocrysts, large inclusions in fine-grained igneous rocks. They are also a typical component of metamorphic schists. Typically, biotite is associated with minerals such as quartz, apatite, feldspar, calcite, hornblende, garnet and schorl. In the official list of minerals, biotite is designated by IMA as a group of minerals with the composition K(Mg,Fe2+)3(Si3Al)O10(OH,F)2, thus officially the word biotite is not the name of a mineral.
Biotite series
- Annit (annite) K(Fe2+)3(Si3Al)O10(OH)2, described in 1868, more fully in 2007.
- Fluorannite (fluorannite) K(Fe2+)3(Si3Al)O10F2, registered by IMA in 1999 (IMA1999-048), described in 2000, 2001, 2007.
- Istonite (eastonite) was described in 1925, discredited in 1987 (it was considered a mixture of phlogopite and a new form of serpentine), redefined in 1998 as a mineral of the composition KAlMg2(Si2Al2)O10(OH)2.
- Siderophyllite (siderophyllite) K(Fe2+)2Al(Si2Al2)O10(OH)2, described in 1881, more fully in 2000.
- Phlogopite (phlogopite) KMg3(Si3Al)O10(OH)2 was described by A. Breithaupt in 1841, more fully described in 2008.
This mineral should not be confused with phlogopite, which was most fully described in 1998 and discredited by the IMA as a mineral of composition K(Mg,Fe)3(Si3Al)O10(OH)2. - Fluorophlogopite (fluorophlogopite) KMg3(Si3Al)O10F2, registered by IMA in 2006 (IMA2006-011), was observed several times previously, but was described as a new mineral only in 2007.
Do not confuse this mineral with fluorphlogopite, which was most fully described in 1982 and discredited by the IMA as a mineral of the composition K(Mg,Fe2+)3(Si3Al)O10(F,OH)2. - Tetraferrianite (tetraferriannite) K(Fe2+)3(Si3Fe3+)O10(OH)2, described in 1925, in 1998 renamed tetra-ferri-annite from monrepite due to the fact What is the mineral composition of Montrepite?
- K(Fe2+,Mg,Fe3+)3(Si,Al)4O10(OH)2 has been discredited by the IMA; renamed again in 2008 to comply with IMA requirements for the use of hyphens in mineral names.
According to IMA nomenclature, a solid solution is designated by its extreme members. For the biotite series, the extreme members are the trioctahedral mica minerals: phlogopite, siderophyllite, annite and eastonite, as well as two new minerals described already in the third millennium: fluorannite and fluorophlogopite ).
The name biotite is used by geologists as a general field term for dark micas that do not contain lithium.
- The mineral is elastic, flexible, splits into leaves.
- Behavior in acids: completely decomposes in concentrated sulfuric acid.
- Clarified biotite (bronze-colored) is sometimes called cat gold (bauerite).
Origin
Igneous: (a) plutonic: biotite is often found in pegmatites (granites, syenites and diorites), (b) volcanic: occasionally, in rocks ranging from rhyolite to basalt.
Metamorphic: (a) regional metamorphism: biotite is present in various schists and gneisses, (b) contact metamorphism.
Clastic-sedimentary.
Hydrothermal (metasomatic potassium alteration).
Place of Birth
Large foliated biotite aggregates are known from Norwegian pegmatites. Large biotite crystals were found in Russia in the Urals, as well as in France (Besne), Tanzania (Uluguru mountain range), Canada (Ontario), and Germany (Laacher See).
Application
Biotite was used as an electrical insulator in low-value products, in optical instrument making, the powder was used to make bronze paint and decorative cement.
Mica is used as a high-quality electrical insulating material; it has a high melting point and good resistance to deformation even at high temperatures. For industrial purposes, muscovite and phlogopite are more often used. In crushed form, all micas are suitable for the production of various building materials.
Biotite has the greatest value for science. It has the necessary properties to determine the age of rocks. Used to determine the age of rocks using isotope geochronological methods. Biotite contains a small amount of potassium (0,00117%), more precisely the radioactive isotope potassium-40, the atomic nuclei of which decay over time, turning into argon-40. The decay process is unpredictable, but on average, in a single sample, half of the potassium-40 decays in 1,25 million years (the half-life of this isotope). Argon, being a gas, can evaporate from hot rock, but when it solidifies, it remains “trapped” in it even after disintegration. Thus, by measuring the ratio of potassium and argon isotopes in a crystal, scientists determine the age of the rock. Biotite is considered one of the most suitable minerals for dating.
Properties of the Mineral
| Color | Black, dark brown, dark green (depending on iron content); bleached biotite (bronze-colored) is sometimes called cat gold (bauerite) |
| Line color | White gray |
| Origin of the name | named after the French physicist and mineralogist Jean Baptiste Biot (1774-1862), who studied the optical properties of micas |
| IMA status | discredited 1999 |
| Chemical formula | K(Mg,Fe2+)3(Si3Al)О10(OH,F)2 |
| Brilliance | metal glass pearl metal-like |
| Transparency | transparent shines through opaque |
| Cleavage | very perfect in |
| Kink | the mineral is elastic |
| Hardness | 2,5 3 |
| Electrical properties of the mineral | Dielectric |
| Thermal properties | Difficult to melt into gray or black glass (depending on iron content) |
| Point group | Prismatic – 2/m. |
| Density (measured) | 2,7-3,3 |
| Pleochroism | visible |
| Selection form | tabular pseudohexagonal crystals, leafy, scaly, dense aggregates, solid masses. |
| Classes on taxonomy of the USSR | Silicates |
| IMA classes | Silicates |
| Syngonia | monoclinic |
| Literature | Nomenclature of mica: final report of the Subcommittee on Mica of the Commission on New Minerals and Mineral Names of the International Mineralogical Association (CMNM MMA) // Zap.WMO, 1998 Part 127 Issue 5, p. 55-65. (PDF) Korinevsky V. G., Korinevsky E. V., Korinevskaya G. G. Barium biotite from Ilmen. – Zap. RMO, 2005, 134, No. 2, p.75-83 (Southern Urals) |
Biotite (named after the French scientist J.-B. Biot), a widespread mineral of the silicate class from the mica group, K(Fe 2+ /Mg)2(Al/Fe 3+ /Mg/Ti)([Si/Al/Fe]2Si2O10)(OH/F)2. Refers to silicates with a layered structure. In modern mineralogical nomenclature, it is recommended to use the name biotite for any minerals that are members of the isomorphic phlogopite series – Annite and siderophyllite, KFe 2+ 2Al(Al2Si2O10) (OH)2 – istonite, KMg2Al(Al2Si2O10) (OH)2, and should therefore no longer be considered a mineral species name. Traditionally, biotite is often referred to (in the field or petrologically) as any unanalyzed or incompletely analyzed dark, iron-rich micas. Contains impurities of Na, Rb, Cs, Ca, Ba, Mg, etc. The Fe-rich variety of biotite is called lepidomelane. Crystallizes in the monoclinic system.
Forms of location. Properties
Crystals are tabular or prismatic, pseudohexagonal in shape, twins are common. Characteristic are leafy and scaly aggregates and phenocrysts in the rock. Biotite is characterized by polytypia. Color reddish-brown, brown (to black). Transparent in thin plates. The luster is glassy (to slightly metallic or pearlescent). Hardness on the Mohs scale 2,5–3; density 2,8–3,0 g/cm3. The cleavage is very perfect. Biotite plates are flexible and elastic.
Biotite has very perfect cleavage and easily splits into thin elastic and transparent plates. Biotite has very perfect cleavage and easily splits into thin elastic and transparent plates.
Origin. Practical significance
A rock-forming mineral of many types of igneous and metamorphic rocks (granites, diorites, gneisses, crystalline schists, etc.), as well as pegmatites (the largest crystals). It is used to determine the geological age of rocks, in the production of electrical insulators, bronze paint, decorative types of cement, as well as in optical instrument making. Pekova Natalya Anatolevna
Published July 26, 2023 at 16:37 (GMT+3). Last updated July 26, 2023 at 16:37 pm (GMT+3). Contact the editors
Areas of expertise: Natural silicates and aluminosilicates System: Monoclinic Hardness on the Mohs scale (minimum value): 2,5 arb. units Density (minimum value): 2,8 g/cm³
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