Wadsleyite

from Wikipedia, the free encyclopedia
Wadsleyite
General and classification
other names

IMA 1982-012

chemical formula
  • Mg 2 [SiO 4 ]
  • β- (Mg, Fe) 2 [SiO 4 ]
Mineral class
(and possibly department)
Silicates and germanates - group silicates (sorosilicates)
System no. to Strunz
and to Dana
09.BE.02 ( 8th edition : VIII / A.06)
51.03.04.01
Crystallographic Data
Crystal system orthorhombic
Crystal class ; symbol orthorhombic-dipyramidal; 2 / m  2 / m  2 / m (hydrous monoclinic prismatic; 2 / m )
Space group Imma (No. 74)Template: room group / 74
Lattice parameters a  = 5.71  Å ; b  = 11.47 Å; c  = 8.28 Å
Formula units Z  = 8
Physical Properties
Mohs hardness not defined
Density (g / cm 3 ) calculated: 3.84
Cleavage not defined
colour colorless to pale beige or light gray, dark green
Line color White
transparency transparent
shine not defined
Crystal optics
Refractive index n  = 1.76
Optical character biaxial

Wadsleyit is a mineral from the division of the group of silicates in the mineral class of "silicates and germanates" with the idealized chemical composition of Mg 2 [SiO 4 ] and is therefore chemically seen a magnesium - silicate .

Wadsleyite crystallizes in the orthorhombic crystal system , but only develops microcrystalline aggregates with grain sizes up to a maximum of 5  µm . With water absorption of more than 1.5%, wadsleyite changes its crystal system and becomes monoclinic .

In its pure form, wadsleyite is almost colorless and transparent. In naturally occurring wadsleyites, however, up to 30% of the Mg 2+ ions can be replaced ( substituted ) by Fe 2+ ions , whereby the mineral takes on a dark green color.

Etymology and history

The idea that olivine undergoes phase transformations at high pressure goes back to experiments of the crystal chemist Victor Mordechai Goldschmidt in the 1920s and based on considerations by Harold Jeffreys and John Bernal from the 1930s. The first of these phase transitions, i.e. that from olivine to wadsleyite, was not proven experimentally until 1966 by Ringwood and Major. Two years later, Akimoto and Sato confirmed wadsleyite, which was initially called the β-phase, as a stable phase.

The mineral is named after the mineralogist Arthur David Wadsley (1918–1969).

classification

In the outdated, but partly still in use, 8th edition of the mineral classification according to Strunz , the wadsleyite belonged to the mineral class of "silicates and germanates" and there to the department of "island silicates (nesosilicates)", where together with ringwoodite it belongs to the unnamed group VIII / A. 06 made.

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), classifies wadsleyite in the department of "group silicates (sorosilicates)". This is further subdivided according to the type of silicate group formation, the possible presence of further anions and the coordination of the cations involved , so that the mineral according to its composition and structure in the subsection “Si 2 O 7 groups with additional anions; Cations in octahedral [6] and greater coordination ”can be found, where it forms the unnamed group 9.BE.02 together with the previously only hypothetically known mineral wadsleyite II .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , classifies wadsleyite as the outdated 8th edition of the Strunz systematics in the section of " island silicates", where it is the only member of the unnamed group 51.03.04 within the sub-section " island silicates:" SiO 4 groups with all cations can only be found in octahedral [6] coordination ”.

Crystal structure

Molar volume as a function of pressure at room temperature

Wadsleyite crystallizes orthorhombically in the space group Imma (space group no. 74) with the lattice parameters a  = 5.71  Å ; b  = 11.47 Å and c  = 3.28 Å and 8 formula units per unit cell . Template: room group / 74

Wadsleyite has a modified spinel structure . The Mg and Si atoms are perfectly ordered. There are three different octahedral grid positions (M1, M2 and M3) and one tetrahedral position. Of the four different oxygen ions in the structure, the one in the O1 position is not bound to silicon , but lies between four Mg 2+ octahedra.

In the case of hydrogenation (water absorption), it can bond with a hydrogen ion, i.e. a proton , and thereby create a Mg vacancy at the M3 position. If the water content exceeds about 1.5%, the M3 defects are arranged in such a way that the normal orthorhombic symmetry of the crystal is reduced to a monoclinic one with an angle β of up to 90.4 °.

properties

Due to the anisotropy of the orthorhombic or monoclinic crystal structure , the crystals show birefringence .

Modifications and varieties

Wadsleyite is the high pressure modification of forsterite , the magnesium-containing end member of the olivine mixed series . In contrast to the α-modification forsterite and the γ-modification ringwoodite, there is no continuous mixed series with up to 100% iron instead of magnesium in the wadsleyite known as β-phase (see also Fayalite modifications and varieties ).

Education and Locations

Wadsleyite occurs on earth mainly in the upper part of the transition zone of the earth's mantle between about 410 and 520 km depth and forms the dominant mineral phase there. The presence of excess binding electrons of oxygen in the Si 2 O 7 groups of the wadsleyite crystal structure allows the relatively easy incorporation of up to three percent by mass of water into the actually anhydrous mineral. This makes wadsleyite potentially an important water reservoir in the earth's interior.

The mineral has also been found as a product of shock wave metamorphosis in meteorites . The first known natural specimen comes from a stone meteorite ( chondrite ) that was found near the Peace River in the Canadian province of Alberta.

Other meteorites in which wadsleyite could be detected are the Tenham stone meteorite in the Australian province of Queensland, the Chondrites Boxian and Sixiangkou in the Chinese provinces of Anhui and Jiangsu , the Chassigny - Martian meteorite in the French department of Haute-Marne, the carbonaceous chondrite Gujba im Nigerian state of Yobe and the chondrite Dhofar 922 in the region of the same name in the Sultanate of Oman.

In addition, wadsleyite was found in the ultramafic rocks of the Maowu complex in the Qianshan district in the Chinese province of Anhui and on the slag fields of the Richelsdorfer Hütte in the Hessian mountains of the same name in Germany.

See also

literature

  • GD Price, A. Putnis, SO Agrell, DGW Smith: Wadsleyite, natural β- (Mg, Fe) 2 SiO 4 from the Peace River Meteorite . In: The Canadian Mineralogist . tape 21 , no. 1 , January 2, 1983, p. 29–35 ( PDF [accessed January 3, 2016]).
  • Annette K. Kleppe, Andrew P. Jephcoat, Joseph R. Smyth: High-pressure Raman spectroscopic studies of hydrous wadsleyite II . In: American Mineralogist . tape 91 , 2006, p. 1102–1109 ( PDF 381.2 kB [accessed January 3, 2016]).
  • Takaaki Kawazoe, Johannes Buchen, Hauke ​​Marquardt: Synthesis of large wadsleyite single crystals by solid-state recrystallization . In: American Mineralogist . tape 100 , 2015, p. 2336–2339 , doi : 10.2138 / am-2015-5400 .

Web links

Individual evidence

  1. a b IMA / CNMNC List of Mineral Names; November 2015 (PDF 1.6 MB)
  2. ^ 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.  539 .
  3. Webmineral - Wadsleyite MineralData
  4. a b Wadsleyite , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF 63.6 kB )
  5. ^ Joseph R. Smyth: β-Mg 2 SiO 4 : A potential host for water in the mantle? In: American Mineralogist . tape 72 , no. 11-12 , January 12, 1987, pp. 1051-1055 ( PDF [accessed October 25, 2014]).
  6. ^ GD Price, A. Putnis, SO Agrell, DGW Smith: Wadsleyite, natural β- (Mg, Fe) 2 SiO 4 from the Peace River Meteorite . In: The Canadian Mineralogist . tape 21 , no. 1 , January 2, 1983, p. 29–35 ( PDF [accessed January 3, 2016]).
  7. a b List of sites for wadsleyite in the Mineralienatlas and Mindat