Elbrusite

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Elbrusite
General and classification
other names
  • Elbrusite (Zr)
  • IMA 2009-051
chemical formula Ca 3 U 5+ ZrFe 3 O 12
Mineral class
(and possibly department) 		Oxides and hydroxides
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  = 12.49 (natural mixed crystal)  Å
Formula units Z  = 8
Frequent crystal faces Rhombendodecahedron {110}, subordinate to deltoidicositetrahedron {211}
Physical Properties
Mohs hardness not determined
Density (g / cm 3 ) calculated: 4.801
Cleavage Please complete!
colour dark brown to black
Line color brown
transparency transparent to translucent (U-rich)
shine Glass gloss
radioactivity radioactive
Crystal optics
Refractive index n  = not determined

The mineral elbrusite is a very rare oxide from the upper group of the garnet with the idealized composition Ca 3 U 5+ ZrFe 3 O 12 . It crystallizes in the cubic crystal system with the structure of garnet. The maximum of 10 to 30 microns large crystals occur in aggregates with wadalite , spurrite and Lakargiit or zones and stains in Kerimasit on.

So far (2017), Elbrusite has only been detected at two sites. Type locality is a calcium silicate xenolite from an ignimbrite from Mount Lakargi and two other xenolites from Chegem Caldera in the North Caucasian Republic of Kabardino-Balkaria in Russia . The second site is an outcrop of the Hatrurim Formation in Jordan.

Etymology and history

The mineral elbrusite was described by Irina O. Galuskina and colleagues under the name Elbrusit- (Zr) with the idealized composition Ca 3 ZrU 6+ Fe 3+ 2 Fe 2+ O 12 and in 2009 by the International Mineralogical Association (IMA) recognized as a new mineral. It was named after the nearby mountain Elbrus , which at 5642 m is the highest mountain in Europe. When the garnet supergroup was reorganized in 2013, the idealized composition was changed from pure Elbrusite to Ca 3 Zr 1.5 U 6+ 0.5 Fe 3 O 12 and the mineral was renamed Elbrusite.

A hypothetical U 6+ Fe 2+ garnet end link was introduced to describe the measured compositions . High Fe 2+ contents are doubted because of the oxidizing formation conditions and after the investigation of further uranium garnets it was proposed to change the Elbrusite formula to Ca 3 U 5+ ZrFe 3+ 3 O 12 . A decision by the IMA is still pending (2017).

Garnets are often very resistant to weathering, can incorporate actinides into their crystal lattice and have therefore been investigated as a possible group of substances for the immobilization and storage of highly radioactive waste. In the course of this research, a synthetic equivalent of Elbrusite was found in 2002 with that of Grew et al. Composition Ca 3 Zr 1.5 U 6+ 0.5 Fe 3 O 12 introduced in 2013 . Investigations of synthetic, Si-free uranium garnets presented in 2016 confirmed that they only contain trivalent iron and that uranium is both 6- and 5-valent.

classification

The current classification of the International Mineralogical Association (IMA) counts the Elbrusite to the garnet upper group, where together with Dzhuluit , Bitikleit and Usturit it forms the Bitikleit group with 9 positive charges on the tetrahedrally coordinated grid position.

The 9th edition of the Strunz'schen mineral systematics, valid since 2001 , does not list the Elbrusite. Although elbrusite is not a silicate, like katoite , because of its formation of mixed crystals with silicate grenades, it would be classified in the garnet group with the order number 9.AD.25 in the class of “silicates and germanates”, division A (island silicates), subdivision “D . Island silicates without further anions; Cations in octahedral [6] and usually greater coordination ”.

Chemism

Uranium can be built into the garnet structure with different charges (U 3+ , U 4+ , U 5+ and U 6+ ) on different grid positions (X and Y). Elbrusite occurs in the type locality together with U 6+ -containing minerals and its composition was initially given as U 6+ : [X] (Ca 3.040 Th 0.018 Y 0.001 ) [Y] (U 6+ 0.658 Zr 4+ 1.040 Sn 4 + 0.230 Mg 0.004 Hf 0.009 ) [Z] (Fe 3+ 1.575 Al 0.539 Si 0.099 Ti 4+ 0.199 Fe 2+ 0.559 Sn 4+ 0.025 V 5+ 0.004 ).

The incorporation of uranium in garnet is favored by iron-rich compositions. This is indicated by the high iron content of the natural, uranium-containing garnets. Calculations were able to confirm this connection at least for the incorporation of U 3+ and U 4+ in the X position and U 5+ in the Y position and recent studies on natural Elbrusite show that uranium is present as U 5+ and all of the Fe than Fe 3+ .

Elbrusite forms complex mixed crystals , especially with iron-rich garnets of the schorlomite group according to the exchange reaction (R stands for any cation with the specified charge)

  • [Y] U 5+ + [Z] Fe 3+ = [Y] R 4+ + [Z] R 4+

and a Sn analog of Elbrusite according to the exchange reaction

  • [Y] Zr 4+ = [Y] Sn 4+

For synthetic uranium garnets, the incorporation of U 6+ in the octahedron position has been proven, at least under oxidizing conditions and in the absence of Si 4+ and Ti 4+ .

  • [Y] U 5+ = 0.5 [Y] U 6+ + 0.5 [Y] Zr 4+

Crystal structure

Elbrusite crystallizes with cubic symmetry in the 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  = 12.49 Å. Template: room group / 230

Uranate grenades of the series Ca 3 U 6+ 0.5 Zr 1.5 Fe 3+ 3 O 12 - Ca 3 U 5+ ZrFe 3+ 3 O 12 with a maximum of 0.7U apfu were investigated synthetically. An extrapolation from their lattice parameters to 1U apfu results in a  = 12.8 Å for the U 5+ terminal link.

The structure is that of garnet . Calcium (Ca 2+ ) occupies the dodecahedral X positions surrounded by 8 oxygen, uranium (U 5+ , U 6+ ) and zirconium (Zr 4+ ) the octahedral Y position surrounded by 6 oxygen and the tetrahedral surrounded by 4 oxygen Z-position is occupied by iron (Fe 3+ ).

Education and Locations

The type locality of Elbrusit is a calcium silicate xenolite from an ignimbrite from Mount Lakargi and two other xenolites from Chegem Caldera in the North Caucasian Republic of Kabardino-Balkaria in Russia. Elbrusit formed here kontaktmetamorph in the Sanidinit facies at temperatures above 800 ° C and low pressure in the spurrite zone of Kalksilikatskarnen . Elbrusite occurs here in fine-grained aggregates of wadalite or forms crusts around lakargiite . More Begleitminerale are Kimzeyit, spurrite, Larnite and Rondorfit .

The only other documented occurrence so far (2017) is an outcrop of the Hatrurim Formation in Jordan . Here, bituminous limes and marls were pyrometamorphically converted into a kind of natural Portland cement when the bitumen burned down , the mineral content of which is similar to the xenolites, which are characterized by contact metamorphosis.

See also

Web links

Individual evidence

  1. ↑ a b c d e f g I. Galuskina, E. Galuskin, S. Utsunomiya, Y. Nakamatsu, M. Murashko, & Y. Vapnik: Uranian garnet from pyrometamorphic rocks of the Hatrurim Complex, Jordan. Problem of crystal chemical formula of elbrusite. In: Abstracts of 21st General Meeting of IMA, South Africa . 2014, p. 378 .
  2. ↑ a b c d e IRINA O. GALUSKINA, BILJANA KRÜGER, EVGENY V. GALUSKIN, THOMAS ARMBRUSTER, VIKTOR M. GAZEEV, ROMAN WŁODYKA, MATEUSZ DULSKI, PIOTR DZIERŻANOWSKI: Mineral Flourchengemite, aw4 - Bearing Endoscarn Zone of an Altered Xenolith in Ignimbrites from Upper Chegem Caldera, Northern Caucasus, Kabardino-Balkaria, Russia: Occurrence, Crystal Structure, and new Data on the Mineral Assemblages . In: The Canadian Mineralogist . tape 53 , 2015, p. 325–344 ( researchgate.net [PDF; 1,2 MB ; accessed on November 15, 2017]).
  3. ↑ a b c d e f g h i j k l m n o p Irina O. Galuskina, Evgeny V. Galuskin, Thomas Armbruster, Biljana Lazic, Joachim Kusz, Piotr Dzierżanowski, Viktor M. Gazeev, Nikolai N. Pertsev, Krystian Prusik, Aleksandr E. Zadov, Antoni Winiarski, Roman Wrzalik, and Anatoly G. Gurbanov: Elbrusite- (Zr) - A new uranium garnet from the Upper Chegem caldera, Kabardino-Balkaria, Northern Caucasus, Russia . In: American Mineralogist . tape 95 , no. 7 , 2010, p. 1172–1181 ( unibe.ch [PDF; 2.0 MB ; accessed on July 29, 2017]).
  4. ↑ a b List of localities for Elbrusite in the Mineralienatlas and Mindat
  5. ^ A b c 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]).
  6. ↑ S. Utsunomiya, LM Wang, S. Yudintsev, RC Ewing: Ion irradiation-induced amorphization and nano-crystal formation in garnets . In: Journal of Nuclear Materials . tape 303 , no. 2 , 2002, p. 177–187 ( researchgate.net [PDF; 775 kB ; accessed on July 29, 2017]).
  7. ↑ a b c d e Xiaofeng Guo, Alexandra Navrotsky, Ravi K. Kukkadapu, Mark H. Engelhard, Antonio Lanzirotti, Matthew Newville, Eugene S. Ilton, Stephen R. Sutton, Hongwu Xu: Structure and thermodynamics of uranium-containing iron garnets . In: Geochimica et Cosmochimica Acta . tape 189 , 2016, p. 269–281 ( researchgate.net [accessed November 15, 2017]).
  8. ↑ Zs. Rak, RC Ewing, and U. Becker: Role of iron in the incorporation of uranium in ferric garnet matrices . In: Physical Review B . tape 84 , no. 155128 , 2011, p. 1–10 ( researchgate.net [PDF; 653 kB ; accessed on July 30, 2017]).
  9. ↑ Zs. Rak, RC Ewing, and U. Becker: Electronic structure and thermodynamic stability of uranium-doped yttrium iron garnet . In: Journal of Physics: Condensed Matter . tape 25 , no. 495502 , 2011, p. 1–10 ( researchgate.net [PDF; 517 kB ; accessed on July 30, 2017]).