Majority

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Majority
General and classification
other names

IMA1969-018

chemical formula Mg 3 SiMg 2+ Si 3 O 12
Mineral class
(and possibly department)
Silicates and Germanates
System no. to Strunz
and to Dana
9.AD.25 ( 8th edition : 8 / A.08-060)
51.4.3a.5
Crystallographic Data
Crystal system cubic
Crystal class ; symbol cubic hexakisoctahedral; 4 / m  3  2 / m
Space group Ia 3 d (No. 230)Template: room group / 230
Lattice parameters a  = 11.524  Å
Formula units Z  = 8
Physical Properties
Mohs hardness 7.5
Density (g / cm 3 ) natural mixed crystal: measured: ~ 4; calculated: 4.00
Cleavage Please complete!
colour pink
Line color Please complete!
transparency Please complete!
shine Please complete!
Crystal optics
Refractive index n  = synthetic: 1.771
Birefringence δ = -

The mineral majorite (IMA1969-018) is a very rare island silicate from the upper group of the garnet and has the idealized chemical composition Mg 3 SiMg 2+ Si 3 O 12 . Natural majority crystallizes in the cubic crystal system with the structure of garnet.

Majorit forms colorless to pink crystals that are only a few µm in size.

The transition zone from the upper to the lower mantle at a depth of 400–700 km consists of around 40% garnet rich in majorite. On the other hand, majority is very rarely found on the earth's surface. In addition to its type locality , the Coorara meteorite , found near Rawlinna in Dundas Shire , Western Australia in Australia , there are only a few documented sites worldwide for the majority, mostly meteorites .

Etymology and history

In the 1960s, the experimental basis was developed that made it possible to carry out experiments under the high pressures of the earth's mantle. The working group around Alfred Edward Ringwood from Australia was one of the first to experimentally investigate the structural changes in the compound types A 2 BO 4 (olivine, spinel) and ABO 3 (pyroxene, ilmenite, perovskite, garnet) that dominate the earth's mantle . It thus laid the mineralogical basis for understanding seismic measurements and the ideas of the structure of the earth derived from them. In addition to the transition from olivine to the spinel structure, Ringwood and Major also observed the transition from (Fe, Mg) SiO 3 from the pyroxene structure to the garnet structure in 1966 , with part of the silicon being incorporated in the octahedron position.

The first finding of such a high-pressure grenade outside of a laboratory was made four years later by Smith and Mason in the Coorara meteorite. In cracks caused by impact metamorphosis , they found a garnet in addition to ringwoodite, the composition of which they stated as (Mg 2.86 Na 0.10 ) (Fe 1.02 Al 0.23 Cr 0.03 Si 0.78 ) Si 3 O 12 . They named the new mineral after Alan Major, who had previously synthesized this compound with Ringwood.

The first syntheses of pure majorite were achieved by Ringwood and Major in 1971 after they were able to achieve pressures of 25-30 GPa with improved high pressure presses.

Mössbauer spectroscopic investigations showed that iron as Fe 2+ sits predominantly on the dodecahedral position and as Fe 3+ on the octahedral position, whereupon Grew and co-workers determined the end link composition of Majorit to be [X] Mg [Y] (MgSi) [Z] SiO 12 .

classification

The structural classification of the International Mineralogical Association (IMA) is one of the majorite to Garnet supergroup, where he together with Menzerit- (Y) , pyrope , Grossular , almandine , Eringait , Goldmanit , Spessartin , Momoiit , Knorringit , Rubinit , Uwarowit , Andradite , Calderite and morimotoite form the garnet group with 12 positive charges on the tetrahedrally coordinated lattice position.

The obsolete, but still used the eighth edition of the mineral classification by Strunz takes the majorite along with almandine, Andradite, Calderit, Goldmanit, Grossular, Henritermierit , Hibschite , Holtstamit , Hydrougrandit , katoite , Knorringit, Morimotoit, pyrope, Schorlomit , Spessartin, Uwarowit, Wadalite and Yamatoite (discredited because they are identical to Momoiite) in the "garnet group" with the system no. VIII / A.08 in the department of " Island Silicates (Nesosilicates)".

The 9th edition of Strunz's mineral systematics, which has been in effect since 2001, also counts the majority in the “garnet group” with system no. 9.AD.25 within the department of "Island Silicates (Nesosilicates)". This is further subdivided according to the possible presence of further anions and the coordination of the cations involved , so that the mineral is classified according to its composition in the subsection “Island silicates without additional anions; Cations in octahedral [6] and usually greater coordination ”can be found.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , classifies the majority in the department of "island silicate minerals ". Here it is together with pyrope, almandine, spessartine, knorringite and calderite in the "garnet group (pyralspite series)" with the system no. 51.04.03a to be found in the subsection “ Island silicates: SiO 4 groups only with cations in [6] and> [6] coordination ”.

Chemism

Majorite with the idealized composition [X] Mg 3 [Y] (Si 4+ Mg 2+ ) [Z] Si 3 O 12 is the Mg-Si analog of morimotoite ( [X] Ca 3 [Y] (Ti 4+ Fe 2+ ) [Z] Si 3 O 12 ) and forms mixed crystals mainly with pyrope according to the exchange reaction

  • [Y] Si 4+ + [Y] Mg 2+ = 2 [Y] Al 3+ .

The following composition is given for the majority from the type locality:

  • [X] (Mg 2.31 Fe 2+ 0.30 Na 0.09 ) [Y] (Si 4+ 0.71 Mg 0.62 Fe 3+ 0.41 Al 0.22 Cr 0.04 ) [Z ] Si 3 O 12 ,

where [X], [Y] and [Z] indicate the positions in the garnet structure.

The iron contents can be used as a mixture with hypothetical end links Skiagit

  • 3 [X] Mg 2+ + [Y] Si 4+ + [Y] Mg 2+ = 3 [X] Fe 2+ + 2 [Y] Fe 3+

and cohesion, corresponding to the exchange reaction

  • [Y] Si 4+ + [Y] Mg 2+ = 2 [Y] Fe 3+

to be discribed. There are also small amounts of knorringite for chromium (Cr)

  • [Y] Si 4+ + [Y] Mg 2+ = 2 [Y] Cr 3+

and a hypothetical Na garnet about the reaction

  • 2 [X] Mg 2+ + [Y] Mg 2+ = 2 [X] Na + + [Y] Si 4+ .

Crystal structure

Natural majority-rich garnets crystallize with cubic symmetry in space group Ia 3 d (space group no. 230) with 8 formula units per unit cell . The natural mixed crystal from the type locality has the lattice parameter a  = 11.524  Å . Template: room group / 230

The structure is that of garnet . Magnesium (Mg 2+ ) occupies the X positions , which are dodecahedrally surrounded by 8 oxygen ions, and half of the Y position, which is surrounded by 6 oxygen ions in an octahedral manner . Silicon (Si 4+ ) occupies the other half of the Y position and the Z position, which is tetrahedrally surrounded by 4 oxygen ions.

Pressure-temperature phase diagram for the compound MgSiO 3

For synthetically produced, pure majority, tetragonal symmetry is given in the space group I 4 1 / a (space group no. 88) with the lattice parameters a  = 11.491 to 11.516 Å and b  = 11.406 to 11.480 Å and an extensive ordering of Si and Mg to two different octahedral positions. Syntheses of majorite pyrope mixed crystals at 2000 ° C and 19 GPa showed that majorite with more than 20 mol% pyrope is cubic. The type of twins of the tetragonal grenade suggests that the tetragonal majorites could also have been cubic under the test conditions and that the phase change to tetragonal majorites may not have occurred until the samples were cooled. Template: room group / 88

The compound MgSiO 3 undergoes a whole series of phase changes with increasing pressure . At temperatures above 1800 ° C, the structural types of orthopyroxene, clinopyroxene, tetragonal majorite and bridgmanite (silicate perovskite) follow one another with increasing pressure . At lower temperatures, this results in orthopyroxene, clinopyroxene, majorite, wadsleyite + stishovite , ringwoodite + stishovite, akimotoite (silicate ilmenite) and bridgmanite.

In the presence of calcium, majorite and calcium silicate perovskite are formed from diopside at temperatures above 1400 ° C and pressures above 14 GPa. With increasing pressure, majorite breaks down to Akimotoite from 20 GPa.

Education and Locations

Garnets rich in majority form under the conditions of the mantle transition zone at temperatures from ~ 1000 ° C and very high pressure from pyroxenes of similar composition. In the earth's mantle , majorite is an important component of the rocks at depths between 300 and 700 km and is therefore a very common mineral. However, material from these depths almost never reaches the surface of the earth, where majorite is extremely rarely found. Most of the finds come from meteorites in which the majority did not form upon impact on the earth, but through shock metamorphosis in collisions in space.

Meteorites

The type locality is the Coorara meteorite, an L6 chondrite that was found in 1966 by AJ Carlisle near Rawlinna in Dundas Shire , Western Australia in Australia . Majorit occurs here in fine corridors together with ringwoodite, olivine and kamacite .

The Tenham meteorite, which fell near Tenham Station near Windorah in Barcoo Shire , Queensland , Australia in 1879 , is an L6 chondrite and type locality of the high-pressure minerals ringwoodite, akimotoite and bridgmanite, which has been greatly modified by shock metamorphosis. It is criss-crossed by numerous melts whose base consists of majorite, magnesiowustite , iron , iron oxide and iron sulphide.

In the Catherwood meteorite, also an L6 chondrite, which got caught in the equipment of the Lorne E. Horton Farm near Catherwood in Saskatchewan , Canada in the mid-1960s , extremely fine-grained majorite occurs together with ringwoodite and maskelynite in fine tunnels and cracks.

In numerous other meteorites worldwide, predominantly L6 chondrites, majorite was found in similar structures and parageneses .

Kimberlite

Finds of terrestrial majorites are extremely rare and only known from kimberlites . They originate deep in the earth's upper mantle and the magmas rise quickly enough to transport entrained high-pressure minerals largely unchanged to the earth's surface.

The share of majority in the composition of shells of the earth's mantle increases with increasing pressure (depth) and is used as a geobarometer to estimate the formation conditions of mantle rocks and meteorites.

The kimberlite from the island of Malaita in the Malaita province of the Solomon Islands is known for its xenolites from the mantle transition zone at a depth of 400 to 670 km. The granitite xenolites contain red majorite with the highest Si content of terrestrial garnets measured to date, as well as Ca-Mg silicate perovskite, various aluminosilicate minerals and microdiamonds . A pressure of at least 22 GPa is assumed for the formation of the majority.

All other garnets containing majorite, mostly from inclusions in diamond, have lower Si contents and, with their compositions, less than 3.5 apfu Si, are no longer in the range of majorite. Garnets made from eclogitic or peridotitic diamonds indicate pressures of 7-10 GPa. Only a few grenades from some South African kimberlites show higher pressures of 14 GPa (Kankan, Monastery) or 13-15 GPa (Jagersfontein).

See also

Web links

Individual evidence

  1. a b c d e f Edward S. Grew, Andrew J. Locock, Stuart J. Mills, Irina O. Galuskina, Evgeny V. Galuskin and Ulf Hålenius: IMA Report - Nomenclature of the garnet supergroup . In: American Mineralogist . tape 98 , 2013, p. 785–811 ( main.jp [PDF; 2,3 MB ; accessed on July 8, 2017]).
  2. a b c d e f g h i Michael Fleischer: New Mineral Names: Majorite . In: The American Mineralogist . tape 55 , no. 12 , 1970, pp. 1815 ( minsocam.org [PDF; 600 kB ; accessed on January 27, 2018]).
  3. a b c d e f g h i JV Smith, Brian Mason: Pyroxene-Garnet Transformation in Coorara Meteorite . In: Science . tape 168 , no. 3933 , 1970, pp. 832-833 , doi : 10.1126 / science.168.3933.832 .
  4. ^ A b AE Ringwood, Alan Major: SYNTHESIS OF MAJORITE AND OTHER HIGH PRESSURE GARNETS AND PEROVSKITES . In: Earth and Planetary Science Letters . tape 12 , 1971, p. 411-418 ( htracyhall.org [PDF; 3.7 MB ; accessed on January 27, 2018]).
  5. ^ A b AE Ringwood: Phase transformations and their bearing on the constitution and dynamics of the mantle . In: Geochimica et Cosmochimica Acta . tape 55 , 1991, pp. 2083–2110 ( elte.hu [PDF; 3.9 MB ; accessed on January 27, 2018]).
  6. a b c d List of localities for majorite in the Mineralienatlas and Mindat
  7. ^ AE Ringwood, Alan Major: High-pressure transformations in pyroxenes . In: Earth and Planetary Science Letters . tape 1 , no. 5 , 1966, pp. 351-357 , doi : 10.1016 / 0012-821X (66) 90023-9 .
  8. ^ CA McCammonn, L. Ross: Crystal chemistry of ferric iron in (Mg, Fe) (Si, Al) O3 majorite with implications for the transition zone . In: Physics and Chemistry of Minerals . tape 30 , no. 4 , 2003, p. 206-216 , doi : 10.1007 / s00269-003-0309-3 .
  9. RJ Angel, LW finger RM Hazen, M. Kanzaki, DJ Weidner, RC Liebermann, DR Velben: Structure and twinning of single-crystal garnet MgSiO3 Synthesized at 17 GPa and 1800 ° C . In: American Mineralogist . tape 74 , no. 3-4 , 1989, pp. 509-512 ( minsocam.org [PDF; 510 kB ; accessed on February 2, 2018]).
  10. Yanbin Wang, Donald J. Weidner, Jianzhong Zhang, Gabriel D. Gwanrnesia c, Robert C. Liebermann: Thermal equation of state of garnets along the pyrope-Majorité join . In: Physics of the Earth and Planetary Interiors . tape 105 , 1998, pp. 59 - 78 ( anl.gov [PDF; 3.9 MB ; accessed on January 29, 2018]).
  11. S. Heinemann, TG Sharp, F. Seifert, DC Rubie: The cubic-tetragonal phase transition in the system majorite (Mg4Si4O12) - pyrope (Mg3Al2Si3O12), and garnet symmetry in the Earth's transition zone . In: Physics and Chemistry of Minerals . tape 24 , 1997, pp. 206-221 , doi : 10.1007 / s002690050034 .
  12. a b Hiroshi Sawamoto: Phase Diagram of MgSiO3 at Pressures up to 24 GPa and Temperatures up to 2200 ° C: Phase Stability and Properties of Tetragonal Garnet . In: High-Pressure Research in Mineral Physics: A Volume in Honor of Syun-iti Akimoto . 1987, p. 206-216 , doi : 10.1029 / GM039p0209 .
  13. a b c Naotaka Tomioka and Masaaki Miyahara: High-pressure minerals in shocked meteorites . In: Meteoritics & Planetary Science . tape 52 , no. 9 , 2017, p. 2017–2039 ( researchgate.net [PDF; 107 kB ; accessed on February 3, 2018]).
  14. AE Ringwood: The pyroxene-garnet transformation in the earth's mantle . In: Earth and Planetary Science Letters . tape 2 , 1967, p. 255-263 , doi : 10.1016 / 0012-821X (67) 90138-0 .
  15. Kei Hirose, Tetsuya Kornabayashi, and Motohiko Murakami, Ken-ichi Funakoshi: In Situ Measurements of the Majorite-Akimotoite-Perovskite Phase Transition Boundaries in MgSiO3 . In: Geophysical Research Letters . tape 28 , no. 23 , 2001, p. 4351-4354 ( wiley.com [PDF; 374 kB ; accessed on February 2, 2018]).
  16. ^ A b Leslie C. Coleman: RINGWOODITE AND MAJORITE IN THE CATHERWOOD METEORITE . In: Canadian Mineralogist . tape 15 , 1977, pp. 97–101 ( arizona.edu [PDF; 790 kB ; accessed on February 2, 2018]).
  17. Naotaka Tomioka and Kiyoshi Fujino: Akimotoite, (Mg, Fe) SiO, a new silicate mineral of the ilmenite group in the Tenham chondrite . In: American Mineralogist . tape 84 , 1999, pp. 267–271 ( minsocam.org [PDF; 107 kB ; accessed on February 3, 2018]).
  18. Kenneth D. Collerson, Quentin Williams, Balz S. Kamber, Soichi Omori, Hiroyoshi Arai, Eiji Ohtani: Majoritic garnet: A new approach to pressure estimation of shock events in meteorites and the encapsulation of sub-lithospheric inclusions in diamond . In: Geochimica et Cosmochimica Acta . tape 74 , no. 20 , 2010, p. 5939-5957 , doi : 10.1016 / j.g approx. 2010.07.005 .
  19. a b c CH Wijbrans, A. Rohrbach, S. Klemme: An experimental investigation of the stability of majoritic garnet in the Earth's mantle and an improved majorite geobarometer . In: Contributions to Mineralogy and Petrology . tape 171 , no. 50 , 2016 ( researchgate.net [PDF; 985 kB ; accessed on February 3, 2018]).
  20. Kenneth D. Collerson, Sarath Hapugoda, Balz S. Kamber, Quentin Williams: Rocks from the Mantle Transition Zone: Majorite-Bearing Xenoliths from Malaita, Southwest Pacific . In: Science . tape 288 , no. 5469 , 2000, pp. 1215-1223 , doi : 10.1126 / science.288.5469.1215 .