Kyanite

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Kyanite
Kyanite-1609.jpg
Crystal stage with bluish-white kyanite on coarse crystalline quartz; next to it on the left a sanded kyanite tablet (size 6.0 cm × 4.0 cm × 3.2 cm)
General and classification
other names

Cyanite, thistle, sapparite

chemical formula Al 2 [O | SiO 4 ]
Mineral class
(and possibly department)
Silicates and germanates - island silicates (nesosilicates)
System no. to Strunz
and to Dana
9.AF.15 ( 8th edition : VIII / B.02)
52.02.02c.01
Similar minerals Andalusite , sillimanite
Crystallographic Data
Crystal system triclinic
Crystal class ; symbol triclinic pinacoidal; 1
Space group P 1 (No. 2)Template: room group / 2
Lattice parameters a  = 7.124  Å ; b  = 7.856 Å; c  = 5.577 Å,
α  = 89.99 °; β  = 101.12 °; γ  = 105.19 °
Formula units Z  = 4
Frequent crystal faces {100}, {010} or {1 2 0}, {hk0}, rarely {001}
Twinning after (100)
Physical Properties
Mohs hardness 4.5 to 5.5 || [001]; 6 to 7 || [010]
Density (g / cm 3 ) 3.56 to 3.67
Cleavage completely after {100}; clearly after {010}, secretions after (001)
Break ; Tenacity fibrous according to (001), wavy according to (100)
colour colorless, white, gray, light to dark blue, blue-violet, greenish, brownish, reddish
Line color White
transparency transparent to translucent
shine Glass gloss, pearlescent, matt
Crystal optics
Refractive indices n α  = 1.712 to 1.718
n β  = 1.720 to 1.725
n γ  = 1.727 to 1.734
Birefringence δ = 0.015 to 0.016
Optical character biaxial negative
Axis angle 2V =
2v x = 82.5 °
Pleochroism weak: colorless-pale violet-blue-pale cobalt blue
Other properties
Chemical behavior Only sparingly soluble in HF
Special features strong, intersecting cracks, bends, sericization

Kyanite , and kyanite , kyanite or Sapparit called, 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 ], so from a chemical point of view it is an aluminum silicate. Structurally, it belongs to the island silicates .

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

Etymology and history

The name kyanite comes from the Greek κύανος (dark metal, blue glass flow, enamel, glaze stone, copper glaze, mountain blue, ultramarine; after Homer ) and refers to the predominantly blue color. The name thistle also comes from the Greek δις σθένος = double strength and refers to the strong anisotropy of the hardness properties.

The name kyanite or cyanite was given to the mineral in 1789 by Abraham Gottlob Werner . The name thistle comes from René-Just Haüy (1801).

classification

Already in the outdated, but partly still in use 8th edition of the mineral systematics according to Strunz , kyanite belonged to the mineral class of "silicates and germanates" and there to the department of " island silicates with non-tetrahedral anions (Neso-subsilicates)", where together with andalusite , Boromullite , Kanonait , Krieselit , Mullit , Sillimanit , Topas and Yoderit the "Topas group" with the system no. VIII / B.02 formed.

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA) , also classifies kyanite in the category of "island silicates (nesosilicates)". However, in addition to the possible presence of further anions, this is further subdivided according to the coordination of the cations involved, so that the mineral is classified according to its composition and structure in the subdivision of “island silicates with additional anions; Cations in [4] er, [5] er and / or only [6] er coordination ”can be found, where it is the only member of the unnamed group 9.AF.15 .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the kyanite to the class of "silicates and germanates", but there in the more finely divided division of "island silicates: SiO 4 groups and O, OH, F and H 2 " a. Here he is the only member of the "Al 2 SiO 5 (kyanite subgroup)" with the system no. 52.02.2c within the subsection “ Island silicates with SiO 4 groups and O, OH, F and H 2 O with cations in [4] and> [4] coordination ”.

Crystal structure

Structure of kyanite,
__ Si 4+   __ Al 3+   __ O 2−

Kyanite crystallizes in the triclinic crystal system in the space group P 1 (space group no. 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 ° as well as 4 formula units per unit cell . Template: room group / 2

All Al 2 SiO 5 modifications have in common the [AlO 6 ] octahedra , which are linked to one another via common edges parallel to the c-axis. In contrast to andalusite and sillimanite, however, as a high-pressure modification, kyanite has the closest packing of the compound. The coordination formula for kyanite is Al [6] Al [6] [O | SiO 4 ] with small amounts of Fe 3+ and Cr 3+ .

properties

Graphical representation of the hardness anisotropy in the direction of the crystal axes a, b and c

The outstanding property of kyanite is its extreme anisotropy in relation to its hardness . This varies between 6 and 7 in the direction of the b-axis and 4.5 to 5.5 in the direction of the c-axis ( hardness information according to Mohs ). The second special characteristic is the often intense blue color. Both properties subsequently led to the name of the mineral.

With a density of 3.56 to 3.67 g / cm 3, kyanite belongs to the heavy minerals together with anatase , brookite , epidote and others. It is only sparingly soluble in hydrofluoric acid (HF aq ) and has a weak red fluorescence .

The mineral gets its predominantly bluish color from the inclusion of small contents of up to 0.5% iron (III) oxide (Fe 2 O 3 ).

Modifications and varieties

Kyanite is a member of the Al 2 SiO 5 group and is trimorphic with the other members andalusite and sillimanite , i.e. the chemical substance with the composition Al 2 [O | SiO 4 ] appears in three different forms (modifications) similar to carbon . However, andalusite and sillimanite crystallize in the orthorhombic crystal system and the aluminum is coordinated differently.

The rare green variety is known as chrome kyanite .

Education and Locations

Kyanite specimen from Bahia, Brazil (size: 26.8 × 11.1 × 6.2 cm)
Rare reddish kyanite from Nani, Loliondo, Arusha , Tanzania (size: 5 × 1.7 × 0.9 cm)

Kyanite forms metamorphically in aluminum- rich clastic sediments (mostly pelite ) that have been exposed to medium to high temperatures and pressures (mesozonal metamorphosis of the Barrow type). Typical for this are slate , gneiss and granulite that have arisen from sediments . Kyanite appears only sporadically in green slates and eclogites . For the pressure-temperature process during metamorphosis, it is an important facies key mineral. It rarely occurs in the form of dark blue gem quality crystals in pegmatites . Kyanite can also occur as a detritus in sediments.

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

Kyanite is a typical rock former and has already been proven to be a frequent mineral formation at many sites, whereby so far (as of 2013) around 1300 sites are known.

Barra do Salinas in the Brazilian state of Minas Gerais , where up to 15 centimeters long (and longer), columnar aggregates were found, is known for its extraordinary kyanite finds . However, the largest known kyanite crystals reached a length of up to half a meter.

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

In Austria, kyanite was found in several places on the Hüttenberger Erzberg , in the Gurktal Alps and the Koralpe , the Hohe Tauern from Carinthia to Salzburg, in the Waldviertel in Lower Austria, the Fischbacher Alps in Styria, in North Tyrol and in the Gaflunatal in Vorarlberg.

In Switzerland, the mineral was found mainly in the canton of Ticino . The Pizzo Forno in Val Piumogna is best known here . However, some of the sites are also known in the cantons of Grisons and Valais .

Other locations include Afghanistan, Egypt, Antarctica, Argentina, Ethiopia, Australia, Bangladesh, Bolivia, Botswana, Bulgaria, China, Democratic Republic of the Congo ( Zaire ), Ecuador, France and French Guiana, Greece, Greenland, India, Ireland, Italy, Japan, Canada, 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 (Great Britain) and the United States of America (USA).

use

As a raw material

Like andalusite and sillimanite, kyanite serves as the basis for the manufacture of highly refractory products and porcelain .

As a gem

Oval cut kyanite from Nepal

Kyanite is rarely used as a gemstone , as it is difficult to grind due to its unusual hardness properties and the fact that it can be completely split . Because of its color, it can be confused with aquamarine , benitoite , cordierite , dumortierite , sapphire and blue tourmaline (indigolite).

See also

literature

  • Martin Okrusch, Siegfried Matthes: Mineralogy. An introduction to special mineralogy, petrology and geology . 7th, completely revised and updated edition. Springer, Berlin [a. a.] 2005, ISBN 3-540-23812-3 , pp. 85 .

Web links

Commons : Kyanite  - collection of images, videos and audio files

Individual evidence

  1. Webmineral - Kyanite (English)
  2. a b c d P. Comodi, PF Zanazzi, S. Poli, MW Schmidt: High-pressure behavior of kyanite: Compressibility and structural deformation . In: American Mineralogist . tape 82 , 1997, pp. 452–459 ( arizona.edu [PDF; 956 kB ; accessed on September 19, 2017]).
  3. a b c d e f Mindat - Kyanite
  4. Walter Ehrenreich Tröger : Optical determination of the rock-forming minerals . 4th, revised edition. E. Schweizerbart'sche Verlagbuchhandlung, 1971, ISBN 3-510-65011-5 , p.  51 .
  5. ^ Helmut Schrätze , Karl-Ludwig Weiner : Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin; New York 1981, ISBN 3-11-006823-0 , pp.  690-693 .
  6. a b Martin Okrusch, Siegfried Matthes: Mineralogie. An introduction to special mineralogy, petrology and geology . 7th, completely revised and updated edition. Springer, Berlin [a. a.] 2005, ISBN 3-540-23812-3 , pp. 84-85 .
  7. Hans Pichler, Cornelia Schmitt-Riegraf: Rock -forming minerals in thin sections . Ferdinand Enke Verlag, Stuttgart 1987, ISBN 3-432-95521-9 .
  8. Mindat - Number of localities for kyanite
  9. Petr Korbel, Milan Novák: Mineral Encyclopedia (=  Dörfler Natur ). Nebel Verlag, Eggolsheim 2002, ISBN 978-3-89555-076-8 , p. 202 .
  10. Kyanite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 ( handbookofmineralogy.org [PDF; 70  kB ; accessed on September 19, 2017]).
  11. Find location list for kyanite in the Mineralienatlas and Mindat
  12. ^ Walter Schumann: Precious stones and gemstones. All kinds and varieties. 1900 unique pieces . 16th revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5 , pp. 212 .