Mayenit upper group

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The mayenite upper group is a group of nanoporous minerals from the class of oxides and silicates , which all have the same structural structure . Their composition obeys the general formula:

  • X 12 { IV T1 8-x VI T'1 x } IV T2 6 O1 24 O2 8-x (O2aH) 3 × [W 6-x ].

The simplified formula is used to define the individual minerals in this group

  • X 12 T 14 O 32-x (OH) 3 × [W 6-x ] is based.

In this structural formula , the capital letters X, T (T1, T'1, T2), O (O1, O2) and W represent different positions in the mayenite structure, with

  • X: Ca 2+
  • T: Al 3+ , Fe 3+ , Si 4+ , Mg 2+
  • O: O 2-
  • W: Cl - , F - , (OH) - , O 2- , H 2 O, □ (space)

The minerals of this group usually form inconspicuous, rounded grains or crystals with the surfaces of the triakis tetrahedron {211}, which are rarely larger than 0.1 mm. Depending on the composition, they are colorless, pale green, lime yellow or gray to black with a glassy sheen . They show no cleavage and, with a Mohs hardness of 5–6, are about as hard as window glass.

All minerals in this group are very rare and some are only known from one location. They occur in calcareous rocks that have been changed by gases containing chlorine and fluorine at low pressure and very high temperatures. These are mainly skarns and limestone inclusions in basaltic magmas or ignimbrites .

Synthetic compounds of this group are of great technical and industrial importance. The compound Ca 12 Al 14 O 32 [O □ 5 ] (mayenite) is a component of the cement clinker of alumina cements - that is corrosion-resistant and fast-curing special cements.

Mayenites, which contain two electrons in the W position instead of O 2− (C 12 A 7 : 2e), are temperature-stable compounds with electrons as anions ( electrides ) and are important for numerous applications in optoelectronics and as catalysts in the chemical industry . As a promoter for ruthenium - nanoparticles have the potential to one of the most important processes in the chemical industry, the synthesis of ammonia by the Bosch process Haber to revolutionize.

History, Etymology and Occurrence

From cement to ammonia

Calcium aluminate cements were developed in search of cements that are less susceptible to sulphate corrosoin, patented in 1908 by the Lafarge company in France. Calcium aluminate cements are still used today as corrosion-resistant and fast-hardening special cements or z. B. used as heat storage for solar thermal systems. In the course of systematic investigations into the compounds that are formed when the calaium aluminate cement clinker is fired, a cubic calcium aluminate was described in 1909, for which the composition 5CaO.3Al 2 O 3 was given at the time.

The structure of this compound was elucidated in 1936 by W. Büssem and A. Eitel at the Kaiser Wilhelm Institute for Silicate Research in Berlin-Dahlem . In the course of the structure elucidation, they corrected the composition to 12CaO · 7Al 2 O 3 , C 12 A 7 in the cement chemical notation .

Over 50 years later, interest in this compound shifted from cement research to the unusual electrical properties of this type of structure. In 1988 M. Lacerda from the Universidade Nova de Lisboa and colleagues from the University of Aberdeen discovered the good conductivity due to oxide anions O 2− , which until then was only known from a few oxides with a fluorite structure. How this ionic conduction takes place in the nanoporous structure of the mayenite was elucidated in the following 20 years. B. from the working group around Hans Boysen from the Ludwig Maximilians University in Munich .

Across the world, chemists from the Japan Science and Technology Agency and Tokyo University of Technology studied Mayenites that had been treated under very low-oxygen conditions and in which normally unstable anions such as the hydride anion (H - ) or electrons were trapped. In 2002, they reported permanent conversion of non-conductive hydride mayenite into electronically conductive mayenite by UV radiation. This is interesting e.g. B. for the development of translucent components into which electrical conduction paths can be written using photographic processes.

The following year they described the first electride that is stable at room temperature and ambient air, a mayenite that contains electrons as anions in the pores of its crystal structure. These electrides have technically interesting electrical and catalytic properties and are the subject of intensive research worldwide.

From theoretical calculations, Medvedeva and Freeman of Northwestern University in Evanston concluded in 2004 that the mechanism of electrical conductivity changes with increasing electron concentrations from an ionic to a metallic conductivity with delocalized electrons. Strictly speaking, such metallically conductive mayenites are no longer electrides, as they do not contain any isolated electrons at fixed crystal lattice positions of an anion. Three years later, Hideo Hosono's working group from the Tokyo University of Technology was able to confirm this metallic conductivity experimentally. In the same year they were able to show that mayenite-electrides at very low temperatures below 0.4 K, superconducting be. The designation of metallically conductive mayenite as electride has been used in the literature to this day (2018).

The following year, Luis Palacios' group from the University of Málaga , Spain, demonstrated for the first time that in mayenite electrides that are not yet metallically conductive, electrons are actually located in the center of the cavities of the nanoporous structure.

An international working group at the Sorbonne , Northwestern University and the Missouri University of Science and Technology documented the transition to metallic conductivity with increasing electron concentrations in the pore spaces in 2015.

In parallel to the electrical properties, the catalytic applications of mayenite electrides were investigated and in 2007 a Japanese research group demonstrated the suitability of mayenite as a catalyst for an organic reaction in aqueous solution.

In the same year Y. Toda and his colleagues determined the work function of the electrons from the Mayenitstruktur. At 2.4  eV , it is just as low as that of metallic potassium . Potassium and other alkali and alkaline earth oxides are used as promoter used the catalysts for ammonia synthesis, one of the most important processes of technical chemistry, currently 1-2% of global energy production needed.

With the combination of a mayenite electride as a promoter with ruthenium nanoparticles as a catalyst, Masaaki Kitano's group from the Tokyo University of Technology in Japan achieved a breakthrough in the synthesis of ammonia in 2012. They succeeded for the first time in catalytically reducing the activation energy of nitrogen splitting to such an extent that it is no longer the most energy-hungry and rate-limiting step in ammonia synthesis.

Another step in the large-scale implementation of these research results was taken by Dong Jiang and colleagues from Clemson University with the discovery of a cheap way to manufacture mayenite electrides.

About volcanoes, meteorites and burning coal heaps

The first finds of a natural, cubic calcium aluminate were made in 1963 by L. Heller in a Sprurrite rock in the Nalhal Ayalon outcrop of the Hatrurim Formation in Israel. It is a common mineral found in many outcrops of the Hatrurim Pyromethamorphic Formation.

It was described as a new mineral a year later by Gerhard Hentschel together with brownmillerite from limestone inclusions in lavas of the Ettringer Bellerberg with the composition Ca 12 Al 14 O 33 . He named the new mineral after the nearby town of Mayen Mayenite . Around 20 years later, Hentschel documented the chlorine content of the mayenites of the Bellerberg volcano, which he interpreted as mixed crystal formation of his mayenite (O 2− on the W position) and a chlorine-containing mayenite with 2Cl - on the W position. A renewed examination of the mayenite in 2012 showed that it did not contain any O 2− , but in addition to chlorine (Cl - ) (OH) - , which ultimately led to the material being renamed and redefined as chloromayenite. The old name Mayenite has been used as a group name and in the materials science literature and is reserved as a mineral name for a natural occurrence of the compound Ca 12 Al 14 O 32 [□ 5 O] (C 12 A 7 ).

Also in a skarn xenolite, this time from an andesite in Japan, a working group from the Geological Survey of Japan discovered a very chlorine-rich mineral of this structure type in 1993, which they named wadalite after the first general director of their facility, Tsunashiro Wada.

Also in the volcanic environment, in skarn xenolites from ignimbrites of the Chengen caldera , the minerals Eltyubyuit and Chlorkyuygenit were discovered.

An investigation of the minerals of the pyrometamorphic rocks of the Hatrurim formation, initially referred to as mayenite, led in 2015 to the discovery of the fluorine-rich members of the mayenite group fluoromayenite and fluororkyuygenite .

Chondritic meteorites can contain calcium-aluminum-rich inclusions (CAI). They consist of calcium-aluminum-silicon oxides, which have formed at high temperatures during the formation of the solar system, e.g. B. Åkermanite , anorthite , grossular . Some of these CAIs were later altered by chlorine-rich fluids, resulting in the formation of various minerals of the mayenite upper group. In 2010, Chi Ma et al. Described chloromayenite in the NWA 1934 meteorite , a CV3 chondrite from northwest Africa, which was initially recognized as a new mineral by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the IMA under the name Brearleyite . Four years later, Chi Ma and Alexander N. Krot described the mineral adrianite from a CAI of the Allende meteorite. and wadalite could also be detected in the CAIs of some meteorites.

Also in the period from 2010 to 2015, minerals of the mayenite group were found in the clinker of burned coal heaps , where conditions were similar to those in the case of carbonaceous chondrites : high temperatures, low pressure and the absence of oxygen. So minerals were the mayenite group in clinkers spent heaps of Rosice-Oslavany coal field in the Czech Republic , the Donezkkohlebeckens in Ukraine demonstrated and the Chelyabinsk coal basin in Russia.

Crystal structure

The symmetry of the minerals of the Mayenite upper group is cubic with the space group I 4 3 d (space group no. 220) and two formula units per unit cell . The lattice parameter is approximately a = 12  Å . The space group I 4 3 d (no.220) does not contain a center of symmetry and is a subgroup of Ia 3 d ( no.230 ) , the centrosymmetric space group of minerals of the garnet group, and all atomic positions of the mayenite structure can be assigned to positions of the garnet structure. A consequence of this is that powders from mayenites and grenades have almost identical X-ray diffraction patterns and are e.g. B. Grossular and Mayenit cannot be distinguished with X-ray powder methods. Another consequence of this structural affinity was that mayenite and wadalite were assigned to the garnet group for a long time , although their structures show considerable differences. Template: room group / 220Template: room group / 220Template: room group / 230

Similar to the minerals of the zeolite group , the mayenite structure is characterized by an aluminosilicate tetrahedral structure that encloses 6 cages per unit cell, in which ions or small molecules can be accommodated. This framework with the composition {(Al, Si) 14 O 32 } has, depending on the silicon content, a negative charge of -22 (mayenite group) to -18 (wadalite group). Each of the cages contains 2 Ca 2+ ions firmly bound to the framework (24 positive charges), which results in a total charge of the mayenite framework of +2 to +6. In contrast to the zeolites, which have a negative charge on the framework, the excess of positive charges in the mayenites is compensated for by the incorporation of exchangeable anions such as O 2− in the cages. The technologically interesting electrical and catalytic properties of compounds of this structure type are based on this structural peculiarity.

Tetrahedral framework: T1, T2

Mayenite structure: AlO 4 tetrahedron T1 in chloromayenite
Mayenite structure: AlO 4 tetrahedron T2 in chloromayenite
Mayenite structure: chlorine (green) and calcium (orange) in the cage of the tetrahedral structure

One oxygen position in the garnet structure is split into two positions in the mayenite structure.

The O1 oxygen is at Wyckoff position 48e, a general point location that occurs 48 times in the unit cell . They are surrounded by two calcium ions and one cation each on the two tetrahedral positions T1 and T2 and link the T1 and T2 tetrahedra to form a tetrahedral structure.

The O2 oxygen lies on a special point position (Wyckoff position 16c), of which there are 16 per unit cell. They are bound to three calcium ions and one cation at the T1 position. They lie on the threefold axis of rotation and correspond to a triplet of oxygen on general layers of the garnet structure.

The T1 position is tetrahedrally surrounded by 4 oxygen anions, three O1 and one O2 oxygen. The cation, predominantly Al 3+ , sits in the center and the oxygen anions on the corners of a tetrahedron. The T1 cation, like the O2 oxygen, lies on a 3-fold axis at Wykhoff position 16c (16 times per unit cell) and corresponds to the Al position in the garnet structure, which is octahedrally surrounded by 6 oxygen. Since three oxygens in the garnet structure correspond to one O2 oxygen in the mayenite structure, Al is tetrahedrally surrounded by only 4 oxygen in this position in the mayenite.

In natural chloromayenite, up to 10% of the cations on T1 are octahedrally coordinated as in garnet. When chloromayenite reacts with water, one O2 oxygen is replaced by three OH groups on the general O2a position and the coordination number of the T1 position is increased to 6.

If the center of symmetry is lost, the silicon position of the garnet structure splits into two positions and the T2 position of the mayenite structure on the point position 12a corresponds to one of these positions. The T2 position is also tetrahedrally surrounded by 4 oxygen anions on the O1 position.

The tetrahedron positions are linked to form a framework of rings of 8, which enclose zeolite-like pore spaces. The cations in the X position and the exchangeable anions in the W position are located in these cages.

Pore ​​spaces: positions X and W

Mayenite structure: CaO 6 octahedron (X position) in chloromayenite; Calcium (orange), O1 (blue), O2 (violet), Cl (green)

Half of the SiO 4 tetrahedra in the garnet structure are completely absent from the mayenite structure. The resulting voids are the W position of the Mayenite. There are 12 W positions per unit cell (Wyckhoff position 12b). They lie in the center of the cages of the tetrahedral framework and are either empty or mostly occupied by monovalent or divalent anions. The W anions form stretched bonds to two neighboring cations in the X position.

The X position of the mayenite (Wykhoff 24d) corresponds to the X position in the garnet (24c) and almost exclusively contains calcium. It is located on the inside of the pore spaces of the tetrahedral framework, two per cavity at opposite ends, and is surrounded by 4 O1-oxygen and two O2-oxygen in the form of a strongly distorted octahedron . If the adjacent W-position is occupied, the coordination number of calcium increases to 7.

Chemism

The various minerals of the mayenite upper group result mainly from variations in the occupation of the tetrahedral positions and the W position in the center of the pore spaces of the structure. The X position is always fully occupied with calcium.

The following table lists the minerals of the mayenite upper group. The upper group is divided into the maynite group (W 2- ) and the wadalite group (W 6- ) based on the negative charge on the W position .

The other columns show the idealized composition with an assignment to the cation positions.

Surname X 12 [4] T1 8-x [6] T'1 x T2 6 O1 24 O2 8-x O2AH 3x W 6-x annotation
unnamed group: 0 negative charges on W
X 2+ 12 [4] T1 3+ 6 [2] T'1 3+ 2 T2 3+ 6 O1 24 O2 6 A - 6 6
Ca 2+ 12 [4] Al 3+ 6 [6] Al 3+ 2 Al 3+ 6 O1 24 O2 6 (OH) - 6 6 hypothetical end link, ≈ 40% in chloromayenite
Ca 2+ 12 [4] Al 3+ 6 [6] Al 3+ 2 Al 3+ 6 O1 24 O2 6 F - 6 6 hypothetical end link, ≈ 3% in fluoromayenite
Mayenite group: 2 negative charges on W
X 2+ 12 T1 3+ 8 T2 3+ 6 O1 24 O2 8 6 5 W 2−
Mayenite (C 12 A 7 ) Approx 12 Al 3+ 8 Al 3+ 6 O1 24 O2 8 6 5 O 2− only known synthetically, discredited as a mineral name
X 2+ 12 T1 3+ 8 T2 3+ 6 O1 24 O2 8 6 4 W - 2
Chloromayenite Approx 12 Al 3+ 8 Al 3+ 6 O1 24 O2 8 6 4 Cl - 2
Sr 12 Al 3+ 8 Al 3+ 6 O1 24 O2 8 6 4 Cl - 2 hypothetical end link, ≈ 1% in chloromayenite
Approx 12 Fe 3+ 8 Fe 3+ 6 O1 24 O2 8 6 4 Cl - 2 hypothetical end link, ≈ 10% in chloromayenite
Fluoromayenite Approx 12 Al 3+ 8 Al 3+ 6 O1 24 O2 8 6 4 F - 2
Approx 12 Fe 3+ 8 Fe 3+ 6 O1 24 O2 8 6 4 F - 2 hypothetical end link, ≈ 2% in fluoromayenite
Approx 12 Al 3+ 8 Al 3+ 6 O1 24 O2 8 6 4 (OH) - 2 hypothetical end link, ≈ 5% in fluoromayenite
Approx 12 Fe 3+ 8 Fe 3+ 6 O1 24 O2 8 6 4 (OH) - 2 hypothetical end link, ≈ 1% in chloromayenite
X 2+ 12 T1 3+ 8 T2 3+ 6 O1 24 O2 8 6 (H 2 O) 4 W - 2
Chlorkyuygenit Approx 12 Al 3+ 8 Al 3+ 6 O1 24 O2 8 6 (H 2 O) 4 Cl - 2
Approx 12 Fe 3+ 8 Fe 3+ 6 O1 24 O2 8 6 (H 2 O) 4 Cl - 2 hypothetical end link, ≈ 2% in chloropyuygenite
Fluorocyuygenite Approx 12 Al 3+ 8 Al 3+ 6 O1 24 O2 8 6 (H 2 O) 4 F - 2
Approx 12 Fe 3+ 8 Fe 3+ 6 O1 24 O2 8 6 (H 2 O) 4 F - 2 hypothetical end link, ≈ 2% in fluorocyuygenite
Approx 12 Al 3+ 8 Al 3+ 6 O1 24 O2 8 6 (H 2 O) 4 (OH) - 2 hypothetical end link, ≈ 14% in fluorocyuygenite
Approx 12 Fe 3+ 8 Fe 3+ 6 O1 24 O2 8 6 (H 2 O) 4 (OH) - 2 hypothetical end link, ≈ 2% in chloropyuygenite
Wadalite group: 6 negative charges on W
X 2+ 12 T1 3+ 8 T2 3+ 6 O1 24 O2 8 6 W - 6
Wadalite Approx 12 Al 3+ 8 Al 3+ 6 O1 24 O2 8 6 Cl - 6
Eltyubyuit Approx 12 Fe 3+ 8 Fe 3+ 6 O1 24 O2 8 6 Cl - 6
Approx 12 Fe 3+ 8 Fe 3+ 6 O1 24 O2 8 6 Cl - 6 hypothetical final link, ≈ 2% in Eltyubyuit
X 2+ 12 T 2+ 5 T 4+ 9 O1 24 O2 8 6 W - 6
Adrianite Ca 2+ 12 Mg 2+ 5 Si 4+ 9 O1 24 O2 8 6 Cl - 6
X 3+ 4 X 2+ 8 T1 3+ 8 T2 3+ 6 O1 24 O2 8 6 W - 6
Y 4 Ca 8 Al 3+ 8 Al 3+ 6 O1 24 O2 8 6 Cl - 6 hypothetical end link, ≈ 10% in chloropyuygenite

Chemistry of the pore spaces (W position)

The cement compound C 12 A 7 (mayenite) reacts quickly and with heat generation with free water. Natural minerals of the mayenite upper group often contain water in the pore spaces of the structure in the form of molecular water (H 2 O) or OH groups, which are absorbed into the mayenite structure via various exchange reactions.

The conversion of chloromayenite and fluoromayenite into chlorine and fluorocyuygenite is based on the incorporation of molecular water:

  • W □ = W H 2 O (chlorine or fluorocyuygenite)

OH groups can be incorporated at different positions through different reactions. An O2 anion from the vicinity of a T1 tetrahedron can be replaced by three OH groups, whereby the coordination of the T1 position increases from tetrahedral (4 oxygen) to octahedral with 3 oxygen and 3 OH groups. The charge equalization takes place via additional vacancies in the adjacent W position according to the exchange reaction

  • [O2] O 2− + 3 [O2a] □ + [W] Cl - = [O2] □ + 3 [O2a] (OH) - + [W] □.

Previously, octahedrally coordinated aluminum was detected in OH-free, synthetic mayenite using infrared spectroscopy.

In the center of the cages of the aluminate framework, the halide ions can be replaced by individual OH groups, according to the reaction

  • W (Cl, F) - = W (OH) - .

In synthetic mayenites, other, often very unstable, anions such as H - , O 2 - , O - , O 2 2− , O 2− , S 2− , N 3− , NH 2− , NH 2 - , CN - , NO 2 - and free electrons e - are inserted and stabilized.

The structure of mayenite is closely related to that of garnet and the center of the pore spaces in the mayenite structure corresponds to an unoccupied position of an SiO 4 tetrahedron in the garnet structure. Spectroscopic investigations suggest that, similar to the hydrogrossular, (OH) 4 clusters can fill empty cage positions in mayenite.

The X-position on the inside of the pore spaces is chemically hardly variable and usually completely occupied by calcium. Replacement by strontium (Sr 2+ ) and yttrium (Y 3+ ) was observed , according to the exchange reactions

  • X Ca 2+ = X Sr 2+
  • X Ca 2+ + [W] □ = X Y 3+ + [W] (Cl, F, OH) - .

In adrianite , the substitution of calcium by sodium has been observed:

  • X Ca 2+ + [W] Cl - = X Na + + [W]

In synthetic mayenites, calcium could partially be replaced by magnesium.

Vacancies in the X position were also found on synthetic mayenite, coupled to additional vacancies in the W position corresponding to the substitution

  • X Ca 2+ + [W] O 2− = X □ + [W] □.

Chemistry of the aluminate framework

In natural mayenites only a few substitutions were observed at the tetrahedral positions T1 and T2. On the one hand, there is complete miscibility between aluminum (Al 3+ ) and iron (Fe 3+ )

  • [T1.2] Al 3+ = [T1.2] Fe 3+ (Eltyubyuit)

on the other hand, aluminum can be replaced by magnesium (Mg 2+ ) and silicon (Si 4+ ):

  • 2 [T1.2] Al 3+ = [T1.2] Mg 2+ + [T1.2] Si 4+ (adrianite)

The coupled incorporation of silicon in the framework and chlorine / fluorine in the pore spaces leads to a complete mixture of minerals of the mayenite group and the wadalite group:

  • [T1.2] Al 3+ + [W] □ = [T1.2] Si 4+ + [W] (Cl, F) -

The coupled incorporation of zinc (Zn 2+ ) and phosphorus (P 5+ ) is still known from synthetic mayenites .

  • 3 [T1.2] Al 3+ = 2 [T1.2] Zn 2+ + [T1.2] P 5+

In addition, small amounts of the cations Ga 3+ , Fe 3+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , V 5+ , Nb 5+ and Ta 5+ were incorporated into synthetic mayenites .

Web links

Commons : Mayenite group  - collection of pictures, videos and audio files

Individual evidence

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  12. Masashi Miyakawa, Sung Wng Kim, Masahiro Hirano, Yoshimitsu Kohama, Hitoshi Kawaji, Tooru Atake, Hiroki Ikegami, Kimitoshi Kono, and Hideo Hosono: Superconductivity in an Inorganic Electride 12CaO · 7Al2O3: e- . In: Journal of the American Chemical Society . tape 129 , 2007, p. 7270-7271 , doi : 10.1021 / ja0724644 .
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  20. Dong Jiang, Zeyu Zhao, Shenglong Mu, Vincent Phaneuf, and Jianhua Tong: Simple and Efficient Fabrication of Mayenite Electrides from a Solution-Derived Precursor . In: Inorganic Chemistry . tape 56 , 2017, p. 11702-11709 , doi : 10.1021 / acs.inorgchem.7b016 .
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