Kyanite

Kyanite, kyanite, Sapparit

Kyanite, also called kyanite and kyanite or Sapparit, is a commonly occurring mineral from the mineral class of " silicates and Germanates ". It crystallizes in the triclinic crystal system with the chemical composition Al 2 [O | SiO 4 ], and is therefore chemically seen an aluminum silicate. Structurally, it is one of the island silicates.

Kyanite developed mainly prismatic to tabular crystals with glass-like sheen on the surfaces, but also occurs in the form of fibrous or granular to massive mineral aggregates. In pure form, kyanite is colorless and transparent. But it can also appear white and accept by foreign admixtures a light to dark blue, blue-violet, greenish to brownish and rarely reddish color by multiple refraction due to lattice defects or polycrystalline training, the transparency decreases accordingly.

Special Features

The outstanding property of kyanite is its extreme anisotropy with respect to its hardness. This varies from 6 to 7 in the direction of the b axis and 4.5 to 5.5 along the c - axis ( hardness data Mohs scale). The second special feature of the often intense blue color is mentioned. Both properties also led to the naming of the mineral in the sequence.

Kyanite is one with a density from 3.56 to 3.67 g/cm3 to the heavy minerals along with anatase, brookite, epidote and others. It is only sparingly soluble in hydrofluoric acid ( HFaq ) and weak red fluorescent.

Its mainly bluish color gets the mineral by incorporation of small amounts of up to 0.5 % iron (III ) oxide ( Fe2O3).

Etymology and history

The name kyanite is derived from Greek κύανος ( Dark metal, blue glass paste, enamel, lapis lazuli, azurite, mountain blue, ultramarine, according to Homer ) and refers to the predominantly blue color. The name kyanite also comes from the Greek δις σθένος = twice the thickness and refers to the strong anisotropy of the hardness properties.

The name kyanite or kyanite was given to the mineral in 1789 by Abraham Gottlob Werner. The name kyanite is derived from René -Just Haiiy (1801 ).

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the kyanite belonged to the mineral class of " silicates and Germanates " and then to the Department of " island silicates with tetrahedral foreign anions ( Neso - Subsilikate ) ", where he, together with andalusite, Kanonait, mullite, sillimanite, topaz and Yoderit was the unnamed group VIII/B.02.

The 9th edition valid since 2001 and of the International Mineralogical Association (IMA ) used the Strunz'schen Mineral classification assigns the kyanite also in the class " silicates and Germanates " and then in the Department of the " island silicates ( nesosilicates ) ". This division, however, is further subdivided according to the possible presence of other anions and the coordination of the cations involved, so that the mineral according to its composition in the subdivision of " island silicates with additional anions; Cations iner, he is and / or to find nurer coordination ", where it is the only member of the unnamed group 9.AF.15.

The commonly used in English-speaking classification of minerals according to Dana assigns the kyanite also in the class of silicates, there, however, in the department of " island silicates with SiO 4 Groups and O, OH, F, and H 2 O with cations in and > coordination " where he is the only member the " Al2SiO5 ( kyanite - subgroup ) " with the system no. 52.02.2c forms.

Modifications and varieties

Kyanite is a member of Al2SiO5 group and trimorph with the other members of andalusite and sillimanite, that is the chemical substance with the composition Al 2 [O | SiO 4 ] and appears similar to the carbon in three different forms ( modifications ) on. Andalusite and sillimanite, however, crystallize in the orthorhombic crystal system and the aluminum is different coordinates.

The rare green variety is called Chromkyanit.

Education and Locations

Kyanite forms in aluminum-rich metamorphic clastic sediments (mostly pelites ), the middle exposed to high temperatures and pressures were ( mesozonal metamorphosis from Barrow- type). These typically include schists, gneisses and granulites that have arisen from sediments. In green schists and eclogites kyanite appears only sporadically. He is an important facies Leitmineral For the pressure-temperature flow during metamorphism. Only rarely in the form of dark-blue crystals of gem quality in pegmatites it occurs. Kyanite can also occur as detritus in sediments.

The following minerals go parageneses with a kyanite: almandine, biotite, green hornblende, muscovite, quartz, rutile, and staurolite.

Kyanite is a typical Gesteinsbildner and could be detected as frequent mineral formation already at many localities, where so far (as of 2013) are approximately 1,300 localities known as.

Known due to exceptional Kyanitfunde among other Barra do Salinas in the Brazilian state of Minas Gerais, where up to 15 centimeters long ( and longer ), columnar aggregates were found. However, the largest known Kyanitkristalle reached a length of up to half a meter.

In Germany the mineral occurred among others Elzach ( Untereck ), Gaggenau and Holzschlägermatte near Horben in Baden- Württemberg; in several places in the Fichtelgebirge and the Upper Palatinate Forest in Bavaria; at Finkenberg, Drachenfels and Dächelsberg at Niederbachem in North Rhine -Westphalia; the mountain hut in Glees and on Kappiger Ley at the weir in the Rhineland-Palatinate Eifel; in Penig and Freiberg in Saxony and in the area around Buchholz Kuden, Niendorf and Schuby in Schleswig -Holstein.

In Austria they found kyanite, among other at several places on the hut Erzberg, in the Gurktaler Alps and the Kor, the Hohe Tauern Carinthia to Salzburg, in the Waldviertel in Lower Austria, the Fischbacher Alps in Styria, in North Tyrol and in Gaflunatal in Vorarlberg.

In Switzerland, the mineral mainly in the canton of Ticino could be found. Here is known primarily of Pizzo Forno in Val Piumogna. Some find sites but is also known in the cantons of Graubünden and Valais.

Other localities lie include Afghanistan, Egypt, Antarctica, Argentina, Ethiopia, Australia, Bangladesh, Bolivia, Botswana, Bulgaria, China, Democratic Republic of Congo ( Zaire), Ecuador, France, and French Guiana, Greece, Greenland, India, Ireland, Italy, Japan, Kazakhstan, Kenya, Colombia, Korea, Liberia, Madagascar, Macedonia, Mongolia, Mozambique, Myanmar, Namibia, Nepal, Netherlands, New Zealand, Norway, Pakistan, Poland, Portugal, Romania, Russia, Zambia, Sweden, Zimbabwe, Slovakia, Slovenia, Spain, South Africa, Sudan, Suriname, Tajikistan, Tanzania, Czech Republic, Turkey, Ukraine, Hungary, the United Kingdom ( UK) and the United States of America (USA).

Crystal structure

Kyanite crystallizes in the triclinic crystal system, space group P1 ( Raumgruppen-Nr. 2) with the lattice parameters a = 7.124 Å; b = 7.856 Å; c = 5.577 Å and α = 89.99 °, β = 101.12 ° and γ = 105.19 °, and four formula units per unit cell.

Al2O5 all modifications which are common to [ AlO6 ] octahedra are linked in parallel by common edges to the c- axis with each other. However, kyanite and andalusite has, in contrast to a high-pressure modification of sillimanite the closest packing of the compound. The coordination formula for kyanite is alal [O | SiO 4 ] with minor additions of Fe3 and Cr3 .

Use

As a raw material

Kyanite, andalusite and sillimanite serves as a basis for producing high- refractory products, as well as porcelain.

As a gemstone

As a gemstone kyanite rather rarely used because it is difficult to grind because of its unusual hardness properties and perfect cleavage. Because of its color, it can be confused with Aquamarine, Benitoite, cordierite, Dumortierite, sapphire and blue tourmaline ( Indicolite ).