Mullite

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Mullite
Mullite-102482.jpg
White, fibrous tufts of mullite from Emmelberg near Üdersdorf , Eifel, Rhineland-Palatinate (field of view 4 mm)
General and classification
chemical formula
  • Al 2 Al 2 + 2x Si 2-2x O 10-x with x = oxygen vacancies per unit cell
  • Al 4 + 2x Si 2-2x O 10-x with x ≈ 0.4
  • Al [6] Al 1 + x [4] [O | Si 1-x O 4-x / 2 ] with x ≈ 0.2
Mineral class
(and possibly department) 		Silicates and Germanates - island silicates
System no. to Strunz
and to Dana 		9.AF.20 ( 8th edition : VIII / B.02)
52.02.02a.02
Crystallographic Data
Crystal system orthorhombic
Crystal class ; symbol orthorhombic-dipyramidal; 2 / m  2 / m  2 / m
Space group Pbam (No. 55)Template: room group / 55
Lattice parameters a  = 7.58  Å ; b  = 7.68 Å; c  = 2.89 Å
Formula units Z  = 2
Physical Properties
Mohs hardness 6 to 7
Density (g / cm 3 ) measured: 3.11 to 3.26; calculated: 3.170
Cleavage clearly after {010}
Break ; Tenacity brittle
colour colorless, white, gray, light pink, red, purple
Line color White
transparency transparent to translucent
shine Glass gloss
Crystal optics
Refractive indices n α  = 1.642 to 1.653
n β  = 1.644 to 1.655
n γ  = 1.654 to 1.679
Birefringence δ = 0.012 to 0.026
Optical character biaxial positive
Axis angle 2V = 20 to 50 °
Pleochroism Visible:
X = Y = colorless
Z = pink
Other properties
Chemical behavior insoluble in acids including hydrofluoric acid

Mullite is a seldom occurring mineral from the mineral class of "silicates and germanates" with the complex chemical composition Al [6] Al 1 + x [4] [O | Si 1-x O 4-x / 2 ], where x ≈ 0.2 corresponds. Structurally, mullite is one of the island silicates .

Mullite crystallizes in the orthorhombic crystal system and usually develops small, prismatic to needle-like crystals with a glass-like sheen on the surfaces. In its pure form, mullite is colorless and transparent. However, due to multiple refraction due to lattice construction defects or polycrystalline formation, it can also appear white and, due to foreign admixtures, take on a gray, light pink to red or violet color, the transparency decreasing accordingly.

Etymology and history

Mullite was first discovered near Seabank Villa on the Isle of Mull off the northwest coast of Scotland . It was first described in 1924 by NL Bowen, JW Greig and EG Zies, who named the mineral after its type locality .

Type material of the mineral is in the Natural History Museum in London under the catalog no. 1925, 432-437 kept.

Further differences in naming are related to the formation conditions of the mullite. Thus, in the solid phase formed granular mullite as Schuppenmullit referred to, while in the presence of a melt mullite needles forms. Mullite formed at low temperatures is also referred to as primary mullite and the mullite newly formed or recrystallized from this at high temperatures is referred to as secondary mullite .

classification

Already in the outdated, but partly still in use 8th edition of the mineral classification according to Strunz , mullite 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 , Kanonait , Krieselit , Kyanit , 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 mullite in the category of "island silicates (nesosilicates)". However, this is further subdivided according to the possible presence of further anions as well as the coordination of the cations involved , so that the mineral according to its composition and structure is classified 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.20 .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the mullite to the class of "silicates and germanates" and there in the section "island silicates: SiO4 groups and O, OH, F and H2O". Here it is together with boromullite and sillimanite in the " Al 2 SiO 5 (sillimanite subgroup) " with the system no. 52.02.02a within the subsection “ Island silicates: SiO 4 groups and O, OH, F and H 2 O with cations in [4] and> [4] coordination”.

Crystal structure

Comparison of the crystal structure of mullite (right) and sillimanite (left)

Mullite crystallizes orthorhombically in the space group Pbam (space group no. 55) with the lattice parameters a  = 7.58  Å , b  = 7.68 Å and c  = 2.89 Å as well as two formula units per unit cell . Template: room group / 55

The crystal structure consists of chains of edge-sharing AlO 6 - octahedra parallel to the c-axis [001]. These are double chains of SiO 4 - and AlO 4 - tetrahedra linked to each other. The structure and arrangement are similar to that of Sillimanit , although the ratio of Al: Si is greater than 1: 1 and the charge balance is maintained by a few free oxygen atoms.

properties

The thermal expansion of mullite with a high degree of purity shows a change in the coefficient of thermal expansion at around 1100 ° C, which is due to a phase transformation and the healing of defects . From 1830 ° C, the peritectic decomposition takes place. Due to this thermal expansion, mullite is a component which destroys the refractory structure and which is created when the fireclay bricks are fired from the binder component of clay.

At temperatures above 250 to 300 ° C, mullite has a higher microhardness than corundum .

Commercially available mullite has a glass phase content (recognizable in SEM images by fine lines at a 120 ° angle to one another), which lowers the melting point. According to the production process, synthetic mullite referred to the composition 3Al 2 O 3  • 2SiO 2 as sintered mullite and mullite composition 2Al 2 O 3 · 1SiO 2 as fused mullite .

Mullite and sillimanite are chemically similar and in all physical and optical properties.

Modifications and varieties

There is a metastable, pseudotetragonal modification of mullite which is attributed to the formation of domains and / or twinning. Above 1000 ° C this changes into the orthorhombic modification.

Education and Locations

Fibrous, radial-rayed mullite tufts with an unknown yellow coating from Henderson's quarry, Mount Ngongotaha, Rotorua , North Island, New Zealand (field of view 5 mm)
White, fibrous mullite in front of thick-tabular osmilite (image size: 1.5 mm)
Location: tub heads , Wannengruppe volcanic complex , Eifel, Germany

Mullite is formed by metamorphosis from kaolinite at normal pressure and around 1200 ° C or as a decay product of sillimanite or its polymorphs, thistene or andalusite at over 1000 ° C (sillimanite → mullite + SiO 2 ). Since these conditions correspond to the metamorphic sanidinite facies , which are rarely reached in nature and in limited rock volumes (e.g. in the side rock of basaltic volcanic vents), mullite usually only occurs there in very small quantities.

As a rare mineral formation, mullite could only be detected at a few sites, with a little more than 70 sites being documented so far. The mineral has been discovered in several places on the Isle of Mull in Scotland. Other locations in the UK are not known.

In Germany, mullite has so far mainly occurred in the area around Lake Laacher and at Ettringer Bellerberg in the Mayen-Koblenz district and in several places in the Vulkaneifel district in Rhineland-Palatinate. The mineral was also found on Katzenbuckel in Baden-Württemberg, on Teichelberg and on Schloßberg near Waldeck (municipality of Kemnath ) in Bavaria, near Georgsmarienhütte in Lower Saxony and in the Anna mine in North Rhine-Westphalia.

In Austria mullite has so far only in a basalt - quarry on Pauliberg in Burgenland and in Kloch and the Stradner Kogel be found in Styria.

Other sites are in Egypt, Australia, China, France, India, Italy, Japan, Kenya, the Netherlands, New Zealand, Poland, Russia, Slovakia, Spain, Tajikistan, the Czech Republic, Hungary and the United States.

Due to its rarity, mullite cannot usually be obtained directly by mining such deposits. A commercially used deposit with an annual production of 5000 t is named from the northern Transvaal in the Republic of South Africa .

Synthetic mullite

Technically, mullite is produced by melting a mixture of kaolinite and aluminum oxide in an electric arc furnace or by sintering a briquetted mixture of kaolinite, aluminum hydroxide and water in a tunnel furnace.

use

The starting mineral kaolinite creates mullite as an essential component in the production of porcelain and bricks as well as firebricks . Mullite is also used to manufacture filter elements for hot gas filtration . It is also used as an inert support material for coated catalysts . Another area of ​​application for mullite is high - temperature insulation .

See also

literature

  • NL Bowen, JW Greig, EG Zies: Mullite, a silicate of alumina . In: Journal of the Washington Academy of Sciences . tape 14 , 1924, pp. 183–191 (English, rruff.info [PDF; 415 kB ; accessed on January 5, 2019]).
  • H. Schneider and S. Komarneni: Mullite . Wiley-VCH, Weinheim 2005, ISBN 978-3-527-30974-0 .
  • Nicolas Bost, Shiyamala Duraipandian, Guillaume Guimbretière, Jacques Poirier: Raman spectra of synthetic and natural mullite . In: Vibrational Spectroscopy . tape 82 , 2016, p. 50-52 , doi : 10.1016 / j.vibspec.2015.11.003 (English).
  • Hans Jürgen Rösler : Textbook of Mineralogy . 4th revised and expanded edition. German publishing house for basic industry (VEB), Leipzig 1987, ISBN 3-342-00288-3 , p. 482-483 .

Web links

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

Individual evidence

  1. ^ IMA / CNMNC List of Mineral Names; November 2018 (PDF 1.7 MB)
  2. ↑ a b c d e f 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.  548 (English).
  3. ^ A b David Barthelmy: Mullite Mineral Data. In: webmineral.com. Retrieved January 5, 2019 .
  4. ↑ a b c d e Mullite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 (English, handbookofmineralogy.org [PDF; 66  kB ; accessed on January 5, 2019]).
  5. ↑ a b Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties. Status 03/2018 . 7th, completely revised and supplemented edition. Weise, Munich 2018, ISBN 978-3-921656-83-9 .
  6. ↑ a b c d e f Mullite. In: mindat.org. Hudson Institute of Mineralogy, accessed January 5, 2019 .
  7. ↑ Catalog of type mineral specimens - M. (PDF; 121 kB) In: smmp.net. Commission on Museums (IMA) June 1, 2007, archived from the original on September 2, 2018 ; Retrieved January 6, 2019 (English, Mullite p. 25).
  8. ↑ G. Brunauer: High temperature structural research using diffraction methods on the oxidic materials Mulit and Zirconia . University of Munich, Munich 2004 ( uni-muenchen.de [PDF; 8.4 MB ; accessed on January 5, 2019] dissertation).
  9. ↑ Jonathan Margalit: Thermal expansion of mullite to 1500 ° C . Technical University Aachen, Aachen 1993, ISBN 3-925714-96-0 (dissertation).
  10. ↑ a b Friedrich Klockmann : Klockmanns textbook of mineralogy . Ed .: Paul Ramdohr , Hugo Strunz . 16th edition. Enke, Stuttgart 1978, ISBN 3-432-82986-8 , pp.  676 (first edition: 1891).
  11. ↑ Hartmut Schneider, Tadeusz Rymon ‐ Lipinski: Occurance of pseudotetragonal mullite . In: Journal of the American Ceramic Society . tape 71 , no. 3 , 1988, pp. C ‐ 162 – C ‐ 164 , doi : 10.1111 / j.1151-2916.1988.tb05042.x (English).
  12. ↑ Number and distribution of the worldwide locations for mullite. In: mindat.org. Hudson Institute of Mineralogy, accessed January 5, 2019 .
  13. ↑ Find location list for mullite in the Mineralienatlas and Mindat
  14. ↑ a b D. Klose, W. Tufar: Silicates . In: Ullmann's Encyclopedia of Industrial Chemistry . 5th edition. A 23. VCH, Weinheim 1993, p. 694-696 .
  15. ↑ VDI 3677 sheet 3: 2012-11 Filtering separators; Hot gas filtration (filtering separators; high-temperature gas filtration). Beuth Verlag, Berlin, p. 15.
  16. ↑ VDI 3476 sheet 1: 2015-06 exhaust gas cleaning; Process of catalytic exhaust gas purification; Basics (Waste gas cleaning; Methods of catalytic waste gas cleaning; Fundamentals). Beuth Verlag, Berlin, p. 10.
  17. ↑ VDI 3469 sheet 1: 2016-09 emission reduction; Manufacture and processing of fibrous materials; Fibrous dusts; Basics, overview (Emission control; Production and processing of fibrous materials; Fibrous dusts; Fundamentals, overview). Beuth Verlag, Berlin, p. 6.