Mica (silicates): Biotite
Diagnostic card.
In the photo - black thin-plate biotite.
K (Mg, Fe) 3 (OH, F) 2 Al Si 3 O 10
Singonia monoclinic
Hardness 2,5-3
Specific weight 2.8-3.2
Cleavage is perfect
Crack irregular
Color from brown to black
Color in powder white
Glitter from glass to pearly
Biotite (magnesian-ferruginous mica) is named after the French physicist Biota. This is the most common of mica - aluminosilicate. The gloss is pearly, on the weathered surface it has a golden yellow color and a metallic gloss (in common parlance - "cat's gold"). It is transparent. Colors: dark green, dark brown, black. The line is white. Cleavage is very perfect. Elastic, flexible and splits into thin leaves. Crystals are rare. It is usually represented by plate aggregates. It occurs everywhere as a rock-forming mineral.
It is represented by tabular pseudohexagonal crystals, and also, more often, by elastic plates devoid of crystallographic outlines, in addition, by irregular scales and their aggregates. Coloration pinkish-brown, very light greenish or (in mass) black. Cleavage is perfect along the base, gloss is glassy, on the cleavage surface is pearly. By the ratio of the basic chemical elements in the mineral, there are varieties: lepidomelan, siderophyllite, annite, meroxene and others.
Black, dark brown, dark green (depending on the iron content); Clarified biotite (bronze color) is sometimes called cat gold (bauerite). Chemical composition. Very volatile; Potassium oxide (K2O) 4.5-8.5%, magnesium oxide (MgO) 0.3-28%, ferrous oxide (FeO) 2.8 - 27.5%, ferric oxide (Fe2O3) 0.3-20 , 5%, alumina (Al2O3) 9.5 to 31.5%, silica (SiO2) 33 to 45%, water (H2O) 6 to 11.5%; Impurities: oxide compounds of titanium, sodium, lithium, manganese, barium, strontium, cesium.
Chemically, biotite is "omnivorous": it contains numerous chemical elements. Varieties: meroxen (Greek "merox" - alien) - normal, iron-poor biotite; Lepidomelan (Greek "legaidos" - scales, "melas" - black); Siderophyllite (Greek "sider" - iron, "phyllos" - leaf) - very rich in iron and almost free of magnesium biotite. Rubellan is a brownish or brick-red variety of biotite. Occurs in effusive rocks.
Glauconite is a green hydromicum, the smallest dark green scales. The product of biotite conversion in seawater. Distributed in sedimentary rocks of marine origin.
Fracture. The mineral is flexible, flexible, split into leaves. The shape of the crystals. Columnar, pseudo-hexagonal, tabular. Crystal structure. Layered lattice. Class of symmetry. Prismatic - 2 / m. Cleavage. Very perfect in the basis (cleavage plane) (001), imperfect in the lateral faces and faces of the prism. Aggregates. Dense, leafy, scaly.
Diagnostic signs.
Is susceptible to sulfuric acid. In the flame, the candles melt easily around the edges. Under the influence of atmospheric factors it is easy to change, transforming into green chlorite; Can also pass into muscovite, becoming colorless or almost colorless. Hardly melts into gray or black glass (depending on the iron content). Behavior in acids. In concentrated sulfuric acid it completely decomposes.
Origin.
Biotite is a widespread mineral of almost all types of rocks; It occurs in intrusive, effusive, sedimentary and metamorphic conditions.
Deposits and applications.
Very beautiful crystals are found in the pegmatites of Russia, Greenland, Brazil and Scandinavia. On the Scandinavian Peninsula, in Norway, biotites of huge sizes, up to 7 m2 in area, were found. In Italy, magnificent specimens of crystals are represented in the geodes among the products of the eruptions of Vesuvius, as well as in Val di di Fassa (province of Trento). Siderophyllite found in Baveno (Verbania province), also found in Alaska.
Without wide industrial application, biotite retains a huge scientific value, allowing to judge the conditions of formation of rocks in which it occurs.
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Poisonous and radioactive dangerous stones and minerals
** - poisonous stones and minerals (mandatory check in the chemical laboratory + explicit indication of toxicity)
** - radioactive stones and minerals (mandatory check on the standard dosimeter + ban on open sales in case of radioactivity exceeding 24 milli / g / h + additional measures of population protection)
Catalog of minerals and semi-precious stones of the world by groups
** - poisonous stones and minerals
** - radioactive stones and minerals
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