Osumilite (Mg)

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Osumilite (Mg)
Osumilite, thick tabulars with Mullite - Ochtendung, Eifel, Germany.jpg
Light blue Osumilite (Mg) panels together with mullite (white, needle-like crystals). Image size: 1.5 mm - Location: tub heads, Ochtendung, Eifel , Germany
General and classification
chemical formula K Mg 2 Al 3 Si 10 Al 2 O 30
Mineral class
(and possibly department) 		Silicates and germanates - ring silicates
System no. to Strunz
and to Dana 		9.CM.05 ( 8th edition : VIII / E.22)
63.02.01a.07
Crystallographic Data
Crystal system hexagonal 
Crystal class ; symbol dihexagonal-dipyramidal; 6 / m  2 / m  2 / m
Space group P 6 / mcc (No. 192)Template: room group / 192 
Lattice parameters a  = 10.0959  Å ; c  = 14.3282 Å
Formula units Z  = 2
Frequent crystal faces {0001}, {11 1 0}, subordinate {10 1 1}, {11 2 0}
Physical Properties
Mohs hardness 6.5
Density (g / cm 3 ) 2.59 (measured), 2.595 (calculated)
Cleavage not observed
Break ; Tenacity shell-like
colour colorless to light blue, brown 
Line color White 
transparency transparent 
shine Glass gloss 
Crystal optics
Refractive indices n ω  = 1.539 
n ε  = 1.547 
Birefringence δ = 0.008 
Optical character uniaxial positive sometimes slightly biaxial
Pleochroism weak, colorless; e too pale blue; O

The mineral Osumilith- (Mg) is a rarely occurring ring silicate from the milarite group and has the simplified chemical composition K Mg 2 Al 3 Si 10 Al 2 O 30 . It crystallizes in the hexagonal crystal system and develops light blue or brownish, six-sided, platy or short prismatic crystals of less than one millimeter in size.

Osumilith- (Mg) forms at high temperatures of around 900 ° C and is found in silicate-rich rocks that are either superficially shaped by hot alkali-rich magmas ( sanidinite-facial contact metamorphosis ) or under conditions of the lower earth's crust ( granulite ).

Etymology and history

The first osumilites, discovered in 1953, contained roughly as much magnesium as iron, and in the years that followed, magnesium-rich osmilites were discovered in numerous igneous rocks and the adjacent contact-metamorphic modified rock as well as in metamorphic granulites.

In 1962, the first experimental investigations into the stability of Mg-Osumilith led to the result that pure Mg-Osumilithe were not stable under the test conditions and the natural Osumilithe found were possibly formed metastable.

In 1973 Chinner and Dixon described almost iron-free K-Mg-Osumilite from rocks in contact with dolerite , which was apparently formed in a thermodynamically stable manner . They suggested the name Osumilith- (K, Mg), but it was never officially recognized as a mineral name.

In 2008 Seryotkin et al. also almost pure Mg-Osumilithe, which were formed when burning the spoil pile of a coal mine in the southern Urals. Since substances that were formed in mine fires do not meet the definition of a mineral , this report also did not lead to the recognition of Osumilith- (Mg) as a mineral.

It was not until 2009 that Chukanov and colleagues provided a complete chemical and structural characterization of a Mg-Osumilite of natural origin, a gneiss - Xenolite from Ettringer Bellerberg in the Eifel , Germany , which led to the recognition of Osumilith- (Mg) as a mineral name by the CNMNC of the IMA .

classification

In the outdated, but still partially common 8th edition of the mineral classification by Strunz Osumilith- (Mg) is one with Almarudit , Armenit , Berezanskit , Brannockit , Chayesit , Darapiosit , Dusmatovit , Emeleusit , Faizievit , Merrihueit , Oftedalit , Osumilith , Poudretteit , Roedderit , Shibkovite , Sogdianite , Sugilite , Trattnerite and Yagiite to the general division of " Ring Silicates (Cyclosilicates)" into the " Milarite Osumilith Group " with the system no. VIII / E.22 .

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 Osumilith- (Mg) in the "ring silicates" section. This is, however, further subdivided according to the structure of the rings, so that the mineral can be found in the sub-section "[Si 6 O 18 ] 12− -six double rings" according to its structure . With Almarudit, Armenit, Berezanskit, Brannockit, Chayesit, Eifelit , Darapiosit, Dusmatovit, Friedrichbeckeit , Klöchit , Merrihueit, Milarit, Oftedalit, Osumilith, Poudretteit, Roedderit, Shibkovite, Sogdianite with “ Yagitiit ” and Milarit , Trattner group belongs the system no. 9.CM.05 .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns Osumilith- (Mg) to the class of "silicates and germanates", but there in the more finely divided division of "ring silicates: condensed rings". Here it is in the " Milarite Osumilith Group (Milarite Osumilith Subgroup) " with the system no. 63.02.01a can be found in the subsection “ Ring Silicates : Condensed, 6-membered Rings”.

Chemism

Osumilith- (Mg) is the Mg analogue of Osumilith and the measured composition from the type locality is [C] (K 0.72 Na 0.03 Ca 0.01 ) [A] (Mg 1.97 Mn 0.04 ) [T2] (Al 2.53 Fe 3 + 0.45 ) [T1] [Si 10.32 Al 1.68 ] O 30 , where the position in the crystal structure is indicated in the superscript, square brackets.

Mg at the A position can be completely replaced by Fe 2+ and Osumilite- (Mg) forms a gapless mixed crystal row with Osumilite .

The potassium content of all natural Osumilithe is well below the ideal value of 1 potassium per formula unit. This is compensated for by the replacement of Mg / Fe by Al and of Al in the double circlips by Si in accordance with the coupled substitutions

  • [C] K + + [A] Mg 2+ = [C] □ + [A] Al 3+
  • [C] K + + [T1] Al 3+ = [C] □ + [T1] Si 4+

Both exchange reactions occur together in a ratio of 3: 1 and a hypothetical K-free Osumilite- (□, Mg) would have the composition □ □ 2 (Mg 1.65 Al 0.35 ) Al 3 [Si 10.4 Al 1.6 ] O 30 .

Crystal structure

Osumilith- (Mg) crystallizes in the hexagonal crystal system in the space group P 6 / mcc (space group no. 192) with the lattice parameters a  = 10.0959  Å and c  = 14.3282 Å as well as two formula units per unit cell .  Template: room group / 192

Osumilith- (Mg) is isotypic to milarite , ie it crystallizes with the same structure as milarite. The 12-fold coordinated C position is not completely occupied by potassium (K + ). The 9-fold coordinated B position is empty. Magnesium (Mg 2+ ) and small amounts of manganese (Mn 2+ ) fill the 6-fold coordinated A position and the tetrahedrally coordinated T2 position contains the trivalent cations aluminum (Al 3+ ) and iron (Fe 3+ ). The T1 position, which builds up the 6 double rings, contains 10 silicon (Si 4+ ) and 2 Al 3+ .

Education and Locations

stability

Osumilith- (Mg) is only formed at high temperatures above 750 ° C. In the presence of water, Osumilith- (Mg) is only stable at pressures below 1 kbar and temperatures between 765 and 800 ° C. These are conditions that occur in volcanic rocks close to the earth's surface and the contact metamorphism that takes place there . With increasing temperatures, Osumilite- (Mg) is formed in the reaction of cordierite + potassium feldspar + phlogopite + quartz . At high temperatures above 800 ° C, Osumilite- (Mg) is broken down into cordierite + potassium feldspar + phlogopite + melt.

Drier conditions, e.g. B. by the dilution of the water in the gas phase by carbon dioxide or the complete absence of a gas phase, lead to an expansion of the stability range of Osumilith- (Mg) to higher and slightly lower temperatures and above all to higher pressures up to ~ 12 kbar. These are conditions as they occur in anhydrous granulites in the lower crust of the earth at a depth of ~ 30-35 km. At pressures above 12 kbar, Osumulite- (Mg) reacts between 950 and 1100 ° C to form cordierite + enstatite + K-feldspar + quartz or sapphirin + enstatite + K-feldspar + quartz.

This is the maximum stability range of pure Osumilite (Mg). Natural Osumilithe always contain iron, which reduces their upper pressure and temperature stability. Other minerals in the rock can also react with Osumilite and lead to the degradation of Osumilite in the rock within the stability range of pure Osumilite (Mg).

Contact metamorphosis: Sanidinitfazielle Hornfelse

At low pressures near the earth's surface, magnesium-rich osmilites form at temperatures around 900 ° C in the presence of a water-rich vapor phase in contact metamorphic form in sediments or foreign rock inclusions ( xenolites ) in igneous melts. The high temperatures required for the formation of Osumilith are only achieved by low-silicon, basic melts, e.g. B. basalt melt achieved. Oxidizing conditions such as those found in e.g. B. by the occurrence of hematite (Fe 3+ 2 O 3 ), favor the formation of very magnesium-rich Osumilithen.

Already in 1973, 38 years before Osumilith-Mg was recognized as an independent mineral, magnesium-rich Osumilite was described in a sandstone from Tieveragh, County Antrim , Northern Ireland , which was metamorphically changed at contact with an olivine - dolerite vein. Osumilite occurs here with tridymite , enstatite , cordierite , feldspar, hematite and magnetite .

The occurrence of magnesium-rich Osumilite at several sites in the Vulkaneifel , Rhineland-Palatinate , Germany is well documented . It occurs here mostly as a deposition from a gas phase in the geothermal areas of contact-metamorphic altered secondary rock inclusions in basaltic magmas.

One of the deposits, the A. Casper quarry on Ettringer Bellerberg , 2 km north of Mayen , in the Laacher See region of the Eifel , is the type locality of Osumilith-Mg. He appears here with fluorine phlogopite , sanidine , cordierite , mullite , sillimanite , topaz , Pseudobrookit and hematite on.

In Newfoundland unusual pink Osumilith-Mg in the Kontaktaureole an anorthosite - pluton found. Accompanying minerals there are quartz, hypersthene , potassium feldspar, cordierite, plagioclase , graphite and pyrrhotite . The estimated pressure of 5 kbar (700-900 ° C) for this occurrence assumes anhydrous conditions.

Pyrometamorphosis: Metapelitic clinker

In fires on coal heaps, Osumilith- (Mg) forms at a bar pressure and temperatures between 870 ° C and 1000 ° C with exclusion of oxygen in deeper areas of the burning heaps during the conversion of sediments contained in the overburden.

The spoil heaps from coal mining in the Chelyabinsk coal basin in the Southern Urals, Russia , which were burned down by spontaneous combustion, have been well examined . The clinker formed in this way contains osmilite (Mg) together with K-Na feldspar, tridymite, mullite, graphite , amorphous carbon and , to a lesser extent, corundum , metallic iron , pyrrothine, writersite (Fe 3 P) and cohenite (Fe 3 C) . This paragenesis , especially the appearance of Schreibersite, suggests extremely low-oxygen conditions that are more like those in iron meteorites. Other minerals of the milarite group have previously been found in such meteorites, e.g. B. Roedderite in the Indarch meteorite.

Regional metamorphosis: pelitic granulites

Another occurrence of Osumilith- (Mg) are quartz-containing sediments that have been metamorphically changed under the conditions of the lower earth's crust at very high temperatures (800-1000 ° C, 7-12 kbar). The occurrence of osumilite in these rocks indicates very low H 2 O partial pressure , i.e. H. very dry conditions. At higher water contents, Osumilith- (Mg) is replaced by parageneses containing cordierite .

In the Late Archaic Napier Complex, Enderbyland (50-52'E), Antarctica , magnesium-rich Osumilite occurs in granulites with various accompanying minerals:

  • Osumilite, garnet , sillimanite + - spinel , quartz, plagioclase
  • Osumilite, sapphirin, sillimanite, quartz, plagioclase
  • Osumilite, orthopyroxene , garnet, sillimanite, quartz, plagioclase
  • Osumilite, orthopyroxene, biotite , quartz, plagioclase

The formation conditions are estimated at 900-980 ° C and 7-10 kbar.

Comparable Al – Mg-rich granulites can be found e.g. B. also in the Khanfous area (Tekhamalt, in Ouzzal, Hoggar , Algeria ). Osumilith occurs here together with sapphirin, biotite, orthopyroxene and sillimanite or orthopyroxene, sillimanite, quartz, biotite and feldspar. Here, too, formation in the absence of H 2 O at 8.5 to 9 kbar and 930 to 980 ° C is assumed. Retrograde, d. H. During the cooling of the rocks on the ascent, osumilite was partially mined to cordierite, orthopyroxene, potassium feldspar and quartz.

See also

literature

Web links

Individual evidence

  1. ↑ a b c d e f g h i j k l m n o p q r s t u v w NV Chukanov, IV Pekov, RK Rastsvetaeva, SM Aksenov, DI Belakovskiy, KV Van, W. Schüller, B. Ternes: Osumilite- (Mg): Validation as a mineral species and new data . In: Geology of Ore Deposits . tape 55 , no. 7 , January 14, 2014, p. 587-593 , doi : 10.1134 / S1075701513070064 .
  2. ↑ Webmineral - Osumilith- (Mg) (English)
  3. ^ A. Miyashiro (1956): Osumilite, a new silicate mineral, and its crystal structure , In: American Mineralogist , Volume 41, pp. 104–116 ( PDF, 736 kB )
  4. ↑ W. Schreyer AND JF Schairer (1962): METASTABLE OSUMILITE AND PETALITE-TYPE PHASES IN THE SYSTEM MgO-Al2O3-SiO , In: American Mineralogist , Volume 47, pp. 90-104 ( PDF, 1.0 MB )
  5. ↑ a b GA Chinner and PD Dixon (1973): Irish osumilite , In: Mineralogical Magazine , Volume 39, pp. 189–192 ( PDF, 736 kB )
  6. ^ EH Nickel: The Definition of a Mineral. In: The Canadian Mineralogist . tape 33 , 1995, pp. 689-690 ( PDF, 270 kB ). PDF, 270 kB ( Memento from September 5, 2014 in the Internet Archive )
  7. ↑ a b c Yurii V. Seryotkin, Ella V. Sokol, Vladimir V. Bakakin, Anna Y. Likhacheva: Pyrometamorphic osumilite: occurrence, paragenesis, and crystal structure as compared to cordierite . In: European Journal of Mineralogy . tape 20 , no. 2 , March 2008, p. 191–198 , doi : 10.1127 / 0935-1221 / 2008 / 0020-1805 ( PDF ).
  8. ↑ Webmineral - New Dana Classification of Cyclosilicates cyclosilicates Condensed ring
  9. ↑ T. Armbruster, R. Oberhänsli (1988): Crystal chemistry of double-ring silicates: Structural, chemical, and optical variation in osumilites. In: American Mineralogist , Volume 73, pp. 585–594 ( PDF (1.3 MB) )
  10. ↑ a b M. Olesch, F. Seifert (1981): The restricted stability of osumilite under hydrous conditions in the system K2O-MgO-Al2O3-SiO2-H2O , In: Contributions to Mineralogy and Petrology , Volume 76 (3), p 362-367. doi : 10.1007 / BF00375463
  11. ^ Y. Motoyoshi, BJ Hensen, M. Arima (1993): Experimental study of the high-pressure stability limit of osumilite in the system K2O-MgO-Al2O3-SiO2: implications for high-temperature granulites , In: European Journal of Mineralogy , Volume 5 (3), pp. 439–445 ( summary ( Memento from May 31, 2016 in the Internet Archive ))
  12. ↑ N. Audibert, P. Bertrand, BJ Hensen (1995): Experimental study of phase relations involving osumilite in the system K2O-FeO-MgO-Al2O3-SiO2-H2O at high pressure and temperature. , In: Journal of Metamorphic Geology , Volume 13, pp. 331-344. doi : 10.1111 / j.1525-1314.1995.tb00223.x
  13. ^ DP Carrington, SL Harley (1995): The stability of osumilite in metapelitic granulites. , In: Journal of Metamorphic Geology , Volume 13, pp. 613-625. doi : 10.1111 / j.1525-1314.1995.tb00246.x
  14. ↑ Kaushik Das, Somnath Dasgupta, Hiroyuki Miura (2003): Calculated phase equilibria in K2O-FeO-MgO-Al2O3-SiO2-H2O for sapphirine-quartz-bearing mineral assemblages , In: J. Petrology , Volume 44 (6), p . 1055–1075 ( (PDF, 505kB )
  15. ↑ DE Kelsey, RW White, JB Holland, R. Powell (2004): Calculated phase equilibria in K2O-FeO-MgO-Al2O3-SiO2-H2O for sapphirine-quartz-bearing mineral assemblages , In: Journal of Metamorphic Geology , Volume 22 , Pp. 559–578 ( (PDF, 839kB) )
  16. ^ W. Schreyer, G. Hentschel, K. Abraham (1983): Osumilith in the Eifel and the use of this mineral as a petrogenetic indicator , In: Tschermaks mineralogische und petrographische Mitteilungen , Volume 31, pp. 215-234. doi : 10.1007 / BF01081370
  17. ↑ JH Berg, EP Wheeler (1976): Osumilite of deep-seated origin in the contact aureole of the anorthositic Nain complex, Labrador , In: American Mineralogist , Volume 61, pp. 29-37 ( PDF, 903 kB )
  18. ↑ a b D. J. Ellis, JW Sheraton, RN England, WB Dallwitz: Osumilite-sapphirine-quartz granulites from Enderby Land Antarctica - Mineral assemblages and reactions . In: Contributions to Mineralogy and Petrology . tape 72 , no. 2 , April 1980, p. 123-143 , doi : 10.1007 / BF00399473 .
  19. ↑ a b Z. Adjerid, G. Godard, KH. Ouzegane and J.-R. Kienast: Multistage progressive evolution of rare osumilite-bearing assemblages preserved in ultra-high-temperature granulites from In Ouzzal (Hoggar, Algeria) . In: Journal of Metamorphic Geology . tape 31 , no. 5 , February 2013, p. 505-524 , doi : 10.1111 / jmg.12031 ( PDF, 4.8 MB ).
  20. ↑ ES Grew (1982): Osumilite in the sapphirin-quartz terrane of Enderby Land, Antarctica: implications for osumilite petrogenesis in the granulite facies , In: American Mineralogist , Volume 67, pp. 762-787 ( PDF, 2.9 MB )