Orthoclase

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Orthoclase
Ortoclasio.jpg
White orthoclase with clear quartz
General and classification
other names
  • Adular
  • Adularia
chemical formula K [AlSi 3 O 8 ]
Mineral class
(and possibly department) 		Tectosilicates; Feldspar group (Buddingtonite-Orthoclase-Slawsonite series)
System no. to Strunz
and to Dana 		9.FA.30 ( 8th edition : VIII / J.06)
76.01.01.01
Crystallographic Data
Crystal system monoclinic
Crystal class ; symbol monoclinic prismatic; 2 / m
Space group C 2 / m (No. 12)Template: room group / 12
Lattice parameters a  = 8.56  Å ; b  = 12.96 Å; c  = 7.21 Å
β  = 116.1 °
Formula units Z  = 4
Twinning Bavenoer, Karlsbader, Manebach twins
Physical Properties
Mohs hardness 6 to 6.5
Density (g / cm 3 ) measured: 2.55 to 2.63; calculated: 2.563
Cleavage completely according to {001} and {010}
Break ; Tenacity uneven to scalloped
colour colorless, white, gray, brown, yellow, red, pink
Line color White
transparency transparent to translucent
shine Glass gloss, pearlescent gloss
radioactivity hardly measurable
Crystal optics
Refractive indices n α  = 1.518 to 1.520
n β  = 1.522 to 1.524
n γ  = 1.522 to 1.525
Birefringence δ = 0.004 to 0.005
Optical character biaxial negative
Axis angle 2V = 35 ° to 75 ° (measured), 52 ° to 70 ° (calculated)
Other properties
Chemical behavior soluble in HF and alkaline solutions

Orthoclase , also known synonymously as adular or adularia , is a frequently occurring mineral from the group of alkali feldspars within the mineral class of "silicates and germanates". It crystallizes in the monoclinic crystal system with the chemical composition of K [AlSi 3 O 8 ] and is therefore chemically seen a potassium - aluminum - silicate . Structurally orthoclase is one of the framework silicates (tectosilicates).

Orthoclase mostly develops prismatic to tabular crystals , but also occurs in the form of granular to massive mineral aggregates . The undamaged surfaces of the transparent to translucent crystals have a glass-like sheen , while cleavage surfaces shimmer more like mother-of-pearl.

Due to its mixed crystal formation with its sodium analogue albite and with the barium aluminosilicate Celsian , a portion of the potassium in orthoclase is often replaced ( substituted ) by sodium (up to several percent) or barium . Often there is also a small amount of iron and other foreign admixtures , which is why orthoclase is rarely colorless or, due to crystal twins or lattice construction defects, white, but often has a light yellow, red or gray to brown color.

Etymology and history

Orthoklas was named in 1823 by August Breithaupt , who named the mineral based on its good to perfect cleavage at right angles after the Greek words ὀρθός orthos for “straight” or “right” and κλάσις klas for “break”.

classification

Already in the now obsolete but still in use 8th edition of the mineral classification by Strunz orthoclase belonged to the mineral class of "silicates and Germanates" and then to the Department of " framework silicates (tectosilicates)" where he collaborated with Buddingtonit , celsian , Hyalophane , Kokchetavit , Mikroklin , Paracelsian , Rubiklin , Sanidin and Slawsonit the independent "Feldspar group (Buddingtonite-Orthoclase-Slawsonite series)" with the system no. VIII / J.06 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 assigns the orthoclase to the class of “silicates and germanates” and there to the “tectosilicates” section without zeolitic H 2 O “a. This section is, however, further subdivided according to the possible presence of further anions , so that the mineral can be found according to its composition in the sub-section "Tectosilicates (tectosilicates) without additional anions", where it can be found together with adulara , anorthoclase , buddingtonite, celsian, hyalophane, Kokchetavit, Mikroklin, Monalbit , Rubiklin and Sanidin the feldspar group with the system no. 9.FA.30 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the orthoclase to the class of "silicates and germanates" and there in the department of "framework silicates: Al-Si lattice". Here he is together with Anorthoklas, Celsian, Filatovit, Hyalophan, Mikroklin, Rubiklin and Sanidin in the group of "K (Na, Ba) -Feldspars" with the system no. 76.01.01 to be found in the subsection " With Al-Si grid ".

Crystal structure

Orthoclase crystallizes monoclinically in the space group C 2 / m (space group no. 12) with the lattice parameters a  = 8.56  Å ; b  = 12.96 Å; c  = 7.21 Å and β = 116.1 ° and 4 formula units per unit cell . Template: room group / 12

Modifications and varieties

The compound K [AlSi 3 O 8 ] is dimorphic and, in addition to the monoclinically crystallizing high-temperature modification orthoclase, also occurs as a triclinically crystallizing low-temperature modification microcline .

Various training and color varieties are known from the Orthoclase :

Adularia overgrown with some chlorite from the Felbertal ( Hohe Tauern ), Austria (size: 10.0 cm × 6.3 cm × 5.0 cm)
  • Adularia (Adularia) - pseudo-orthorhombic or pseudo-trigonal. Crystallized from hydrothermal solutions, mainly known from alpine fissures in the Adula Alps . White, less often colorless - transparent.
    • Paradoxite - flesh-red potassium feldspar in the adular habitus
    • Valencianite - milky, cloudy adulara
  • Moonstone - bluish-white, flat shimmer, similar to that of the moon (name!), Also known as adularize ; is particularly emphasized with gemstones by the cabochon cut

Education and Locations

Carlsbad twin (orthoclase) from the granite of Karlsbad (type locality)
Orthoclase crystal specimen (white crystals and coarse, yellowish matrix), associated with Spessartine (small, orange crystals) and smoky quartz (black) from the province of Livorno (Elba), Italy (size: 7.0 cm × 5.2 cm × 5 ,1 cm)

Orthoclase is a typical rock-forming mineral and forms either igneously in granite , pegmatite , rhyolite , syenite and trachyte or metamorphically in orthogneiss , migmatite and others. The accompanying minerals include albite , beryl , biotite , various horn blends , muscovite and schörl .

Orthoclase is one of the most common minerals in the earth's crust and can be found at very many sites around the world, with over 2200 sites known to date (as of 2015).

The Oberleidenberg near Bad Weißenbach in the Saualpe in the Austrian state of Carinthia with crystal finds of up to 70 centimeters in size and Twentynine Palms in the US state of California, where well-formed crystals and twins with a diameter of up to 20 centimeters, are worth mentioning due to extraordinary orthoclase finds were discovered. Adularia is known from the alpinotype corridors of the Gotthard massif in Switzerland.

Other sites with good crystal finds include the Pitwak mine in the Koktscha valley in the area of Kuran va Munjan in Afghanistan, Minas Gerais in Brazil, Hagendorf in Germany , Baveno in Italy, the Malosa massif in Malawi , the Skardu region in Pakistan , Strzegom in Poland , Loket and Karlovy Vary in the Czech Republic.

use

Oval cut orthoclase from the province of Toliara , Madagascar (size 2.0 cm × 1.4 cm × 1.0 cm; weight 15.03 ct)

Orthoclase is used in the glass , ceramic and pharmaceutical industries.

Orthoclase and its variety moonstone are used as gem stones . Due to the similarity of color and luster, orthoclase can be confused with chrysoberyl , citrine , golden beryl , prehnite , topaz and zircon .

See also

literature

Web links

Commons : Orthoclase  - collection of images, videos and audio files
Wiktionary: Moonstone  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ^ A b c Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables. Chemical-structural Mineral Classification System . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  693 .
  2. ↑ a b Orthoclase , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF 75.7 kB )
  3. ↑ Webmineral - Orthoclase
  4. ↑ a b c d e MinDat - Orthoclase (English)
  5. ↑ Stones and Minerals - Adular Mineralien-Briefbrief
  6. ↑ Origin of the name of Adular at karrer-edelsteine.de
  7. ↑ a b Stefan Weiß: The large Lapis mineral directory . 5th edition. Christian Weise Verlag, Munich 2008, ISBN 978-3-921656-70-9 , p. 197 .
  8. ↑ Mindat - Number of localities for Orthoclase
  9. ↑ Gerhard Niedermayr, Ingeborg Prätzel: Minerals Carinthia . Verlag Naturwissenschaftlicher Verein f. Carinthia, 1995, ISBN 978-3-85328-003-4 ( reference at mindat.org ).
  10. ↑ a b c Petr Korbel, Milan Novák: Mineral Encyclopedia . Nebel Verlag GmbH, Eggolsheim 2002, ISBN 3-89555-076-0 , p. 263 ( Dörfler Natur ).
  11. ↑ Find location list for Orthoclase in the Mineralienatlas and in 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. 180 .