Knorringite

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Knorringite
General and classification
other names

Hanleit, IMA 1968-010

chemical formula Mg 3 Cr 3+ 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.08-50)
51.4.3a.4
Similar minerals Pyrope, Majorit, Uvarowit
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  = 11.65 (natural)
synthetic: 11.5935  Å
Formula units Z  = 8
Physical Properties
Mohs hardness Please complete!
Density (g / cm 3 ) natural mixed crystal: measured: 3.756; calculated: 3.852 synthetic: 3.79
Cleavage not observed
colour blue green
Line color Please complete!
transparency Please complete!
shine Diamond luster
Crystal optics
Refractive index n  = 1.803 (natural mixed crystal); 1.83 (synthetic)
Birefringence δ = -

The mineral knorringite is a very rare island silicate from the upper group of the garnet and has the idealized chemical composition Mg 3 Cr 3+ 2 Si 3 O 12 . It crystallizes in the cubic crystal system with the structure of garnet.

Knorringite forms pink-red to blue-green crystals with a diamond luster , which are rarely larger than 1–2 mm.

Garnets rich in knurlingite are formed under the conditions of the upper mantle . The few finds come from kimberlites and inclusions in diamond and some meteorites . The type locality is the Kao kimberlite pipe in the Butha-Buthe district , Lesotho .

Etymology and history

In 1864, a tiny, green garnet was found near the 17th century Buddhist Drukpa monastery Hanle in the Hanle Valley in the Indian Union Territory of Ladakh and described by FR Mallet as being unusually rich in chrome. The material was lost, the reference was inaccessible and Lewis Leigh Fermor awarded in 1952 on the basis of discarded Mallet analyzes tentatively named Hanleit a magnesium equivalent of Uwarowit .

In the 1960s, GGK Sastri, then director of geology and mining for the state of Gujarat (India), came into possession of some tiny emerald-green, dodecahedral crystals, which were ascribed the origin "near Hanle Monastery". He was able to show that it is almost pure uvarowite.

Chromium-containing, pink to purple pyropes have been known from South African kimberlites since the beginning of the 20th century. Because of their origin in the upper mantle, the source of the diamond-bearing kimberlite magmas, they have been intensively investigated. The English group around Peter H. Nixon, Oleg von Knorring (1915-1994) and Joan M. Rooke from the University of Leeds noticed a particularly chromium-rich, blue-green garnet in heavy mineral concentrates of the Kao kimberlite, which had a high proportion of that of Fermor 1952 introduced “Hanleit” composition. It was not until 5 years later that Nixon and Hornung succeeded in using electron beam microanalysis, which was still little widespread at the time, an exact chemical analysis of the tiny crystal fragments and the description of Mg 3 Cr 3+ 2 Si 3 O 12 as a new mineral. They named it after their colleague von Knorring, who was involved in the initial examination of the samples.

classification

The structural classification of the International Mineralogical Association (IMA) is one of the Knorringit to Garnet supergroup, where he along with almandine , Andradite , Calderit , Eringait , Goldmanit , Grossular , Morimotoit , majorite , Menzerit- (Y) , Momoiit , pyrope , Rubinit , Spessartine and Uvarowite 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 Knorringit along with almandine, Andradite, Calderit, Goldmanit, Grossular, Henritermierit , Hibschite , Holtstamit , Hydrougrandit , katoite , Morimotoit, majorite, 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 within the department of " Island Silicates (Nesosilicates)".

The 9th edition of Strunz's mineral systematics, which has been in effect since 2001, also counts Knorringite to the “garnet group” with system no. 9.AD.25 within the department of "Island Silicates (Nesosilicates)". However, 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 , also classifies the knorringite in the category of "island silicate minerals ". Here it is together with pyrope, almandine, spessartine, majorite 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

Knorringite with the idealized composition [X] Mg 3 [Y] Cr 3+ 2 [Z] Si 3 O 12 is the Cr analogue of pyrope ( [X] Mg 3 [Y] Al 2 [Z] Si 3 O 12 ) or the Mg analogue of uwarowite ( [X] Ca 3 [Y] Cr 3+ 2 [Z] Si 3 O 12 ) with which it forms mixed crystals according to the exchange reactions

  • [Y] Cr 3+ = [Y] Al 3+ , (pyrope)
  • [X] Mg 2+ = [X] Ca 2+ (uwarowite).

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

  • [X] (Mg 1.91 Ca 0.66 Fe 2+ 0.41 Mn 0.04 ) [Y] (Cr 3+ 1.04 Al 0.86 Fe 3+ 0.07 Ti 4+ 0.005 ) [Z ] Si 3.01 O 12 ,

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

The iron content can be interpreted as the almandine content, according to the reaction

  • [Z] Mg 2+ + [Y] Cr 3+ = [Z] Fe 2+ + [Y] Al 3+ .

The chromium content of pyrope-knorringite mixed crystals increases with increasing pressure. From a pressure of approx. 10GPa the chromium and aluminum contents begin to decrease, in favor of a mixed crystal formation with majority corresponding to the exchange reaction

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

The combination of these two trends means that pure knorringite does not occur in mantle rocks. At lower pressure, the pyrope predominates in the grenades, at high pressure the compositions become increasingly rich in majorite.

Crystal structure

Knorringite 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  = 11.659  Å . For synthetic knorringite with 5 mol% majorite, a  = 11.5935 Å was measured. Template: room group / 230

The structure is that of garnet . Magnesium (Mg 2+ ) occupies the dodecahedral X positions surrounded by 8 oxygen ions, chromium (Cr 3+ ) the octahedral Y position surrounded by 6 oxygen ions and the tetrahedral Z position surrounded by 4 oxygen ions is exclusively silicon (Si 4+ ) occupied.

properties

colour

Chromium-containing pyropes show a pale pink to violet color, which turns into dark green with knorringite contents between 40 and 70 mol%. Both colors are caused by Cr 3+ on the octahedron position of the garnet structure. Spectroscopic investigations showed that the local environment of the Cr 3+ hardly changes at this position in the mixed crystal series pyrope - knorringite. What changes is the nature of the chromium-oxygen bonds. The covalent portion of the bonds decreases with increasing chromium content, without the formation lengths changing significantly. The change in color is attributed to this influence of the occupation of the neighboring octahedron positions on the character of the chromium bonds.

A comparable change in color was observed when the occupancy of the dodecahedral X position was changed. With the same chromium content, the color changes from pink to gray to green when the calcium content of the dodecahedral X position increases in mixed crystals with uwarowite / grossular.

Similar color changes were also observed in other mixed crystal rows with Cr 3+ terminal members. In the spinel group z. For example, the color of spinel - magnesiochromite and gahnite - zincochromite mixed crystals changes with increasing chromium content from pink-red to dark green.

The corundum - eskolaite mixed crystals are better known. In this row, too, the color changes from red ( ruby ) to green (eskolaite) with increasing chromium content .

Education and Locations

Garnets rich in cartilage form at high pressures and temperatures under the conditions of the upper mantle. Below 1600 ° C / ~ 7 GPa and 1000 ° C / 14.5 GPa, majoritic knorringite breaks down to enstatite (MgSiO 3 ) and eskolaite (Cr 2 O 3 ). With increasing aluminum content (pyrope content), the stability field of garnet increases to lower pressures up to ~ 2GPa. The breakdown products of pyrophic knorringites are enstatite, chromium-containing spinel and coesite .

There are only a few confirmed locations for knorringite worldwide. Chromium-containing pyropes, on the other hand, are found in many kimberlites and in inclusions in diamonds from deposits around the world.

Kimberlite and diamond inclusions

The first reports of chromium-rich pyropes come from South Africa. Garnet rich in gnarling occurs there in ultra-basic xenolites in kimberlites. In its type locality , the Kao Kimberlite Pipe in the Butha-Buthe District , Lesotho , knorringite was only found in heavy mineral concentrates that do not allow any direct conclusions to be drawn about its paragenesis . It is believed that the knorringite found there comes from tubers of mantle rock, where it occurs together with olivine, enstatite, chromium diopside and chromium-rich spinel.

In a diamond from the Akwatia Mine on the Birim River in Ghana , Africa , a knorringite inclusion with around 66 mol% knorringite was found.

Meteorites

In Ureilit L88774 from the Antarctic a Knorringgit-Uwarowit mixed crystal with 65-70 mol% Knorringit described. Knorringite occurs with SiO 2 -rich glass between chromite and olivine and was formed during the reaction of chromite-rich melt with olivine at ~ 4 - 4.5 GPa and temperatures above 2000 ° C, presumably through impact metamorphosis during the collision of asteroids in space.

Sikirdji and Warren describe a similar occurrence from a Ureilite from northwest Africa (NWA 766). When olivine came into contact with SiO 2 -rich glass, they observed garnets rich in uvarovite and knorringite, with the knorringite content increasing towards the glass. Here, too, the formation of knorringite through shock metamorphosis is suspected.

See also

Web links

Individual evidence

  1. ↑ a b c d e f g h i j k l Peter H. Nixon, George Hornung: A new chromium garnet end member, knorringite, from Kimberlite . In: American Mineralogist . tape 53 , no. 11-12 , 1968, pp. 1833–1840 ( minsocam.org [PDF; 516 kB ; accessed on February 7, 2018]).
  2. ↑ a b c d Amélie Juhin, Guillaume Morin, Erik Elkaïm, Daniel J. Frost, Michel Fialin, Farid Juillot, Georges Calas: Structure refinement of a synthetic knorringite, Mg 3 (Cr 0.8 Mg 0.1 Si 0.1 ) 2 (SiO 4 ) 3 . In: American Mineralogist . tape 95 , 2010, p. 59–63 ( rruff.info [PDF; 659 kB ; accessed on February 7, 2018]).
  3. ^ AE Ringwood: Synthesis Of Pyrope-Knorringite Solid Solution Series . In: Earth and Planetary Science Letters . tape 36 , 1977, pp. 443–448 ( rruff.info [PDF; 457 kB ; accessed on February 7, 2018]).
  4. ↑ a b c d Tetsuo Irifune, Yu Hariya: Phase relationships in the system Mg 3 Al 2 Si 3 O 12 - Mg 3 Cr 2 Si 3 O 12 at high pressure and some mineralogical properties of synthetic garnet solid solutions . In: Mineralogical Journal . tape 11 , no. 6 , 1983, pp. 269–281 ( jst.go.jp [PDF; 1.1 MB ; accessed on February 7, 2018]).
  5. ↑ a b c Yongtao Zou, Tetsuo Irifune: Phase relations in Mg 3 Cr 2 Si 3 O 12 and formation of majoritic knorringite garnet at high pressure and high temperature . In: Journal of Mineralogical and Petrological Sciences . tape 107 , 2012, p. 197–205 ( jst.go.jp [PDF; 1.5 MB ; accessed on February 7, 2018]).
  6. ↑ a b List of sites for Knorringite in the Mineralienatlas and Mindat
  7. ↑ Lewis Leigh Fermor: On a new chrome-garnet . In: Geological Magazine . tape 89 , no. 2 , 1952, pp. 145–147 ( geoscienceworld.org [accessed February 10, 2018]).
  8. ^ GGK Sastri: Note on a chrome and two manganese garnets from India . In: Mineralogical Magazine . tape 33 , 1963, pp. 508–511 ( rruff.info [PDF; 178 kB ; accessed on February 7, 2018]).
  9. ^ Peter H. Nixon: Oleg von Knorring . In: Mineralogical Magazine . tape 58 , 1994, pp. 693-694 ( minersoc.org [PDF; 756 kB ; accessed on April 26, 2020]). Oleg von Knorring ( Memento from August 19, 2014 in the Internet Archive )
  10. ^ A b Peter H. Nixon, Oleg von Knorring, Joan M. Rooke: Kimberlites and associated inclusions of Basutoland: A mineralogical and geochemical study . In: American Mineralogist . tape 48 , 1963, pp. 1090–1132 ( minsocam.org [PDF; 2.9 MB ; accessed on February 7, 2018]).
  11. ^ Edward S. Grew, Andrew J. Locock, Stuart J. Mills, Irina O. Galuskina, Evgeny V. Galuskin, Ulf Hålenius: IMA Report - Nomenclature of the garnet supergroup . In: American Mineralogist . tape 98 , 2013, p. 785–811 ( Online [PDF; 2,3 MB ; accessed on July 8, 2017]).
  12. ^ EA Sirotkina, AV Bobrov, Yu. A. Litvin LS Dubrovinsky: Experimental study of the system MgO-SiO 2 -Cr 2 O 3 at 7-16 GPa and 1200-1800 ° C . In: Vestnik Otdelenia nauk o Zemle RAN . tape 4 , 2012 ( wdcb.ru [PDF; 344 kB ; accessed on February 10, 2018]).
  13. ↑ Elena A. Bykova, Andrey V. Bobrov, Ekaterina A. Sirotkina, Luca Bindi, Sergey V. Ovsyannikov, Leonid S. Dubrovinsky, Yuriy A. Litvin: X-ray single-crystal and Raman study of knorringite, Mg 3 (Cr 1:58 Mg 0:21 Si 00:21 ) Si 3 O 12 , Synthesized at 16 GPa and 1600 ° C . In: Physics Chemistry of Minerals . tape 41 , 2014, p. 267-272 ( researchgate.net [PDF; 325 kB ; accessed on February 7, 2018]).
  14. ↑ MN Taran, K. Langer, Irmgard Abs-Wurmbach, DJ Frost, AN Platonov: Local relaxation around [6] Cr 3+ in synthetic pyrope – knorringite garnets, [8] Mg 3 [6] (Al 1 - X Cr X 3+ ) 2 [4] Si 3 O 12 , from electronic absorption spectra . In: Physics and Chemistry of Minerals . tape 31 , no. 9 , 2004, p. 650-657 , doi : 10.1007 / s00269-004-0424-9 .
  15. ↑ Alexej N. Platonov, Klaus Langer, Stanislav S. Matsyuk: Crystal field and covalency of octahedral chromium in natural [8] (Mg 1 - x Ca x ) 3 [6] (Al 0.67 Cr 0.33 ) 2 Si 3 O 12 garnets from upper mantle rocks . In: Physics and Chemistry of Minerals . tape 35 , no. 6 , 2008, p. 331-337 , doi : 10.1007 / s00269-008-0226-6 .
  16. ↑ Ulf Hålenius, Giovanni B. Andreozzi, and Henrik Skogby: Structural relaxation around Cr 3+ and the red-green color change in the spinel (sensu stricto) -magnesiochromite (MgAl 2 O 4 -MgCr 2 O 4 ) and gahnite-zincochromite (ZnAl 2 O 4 -ZnCr 2 O 4 ) solid-solution series . In: American Mineralogist . tape 95 , 2010, p. 456-462 ( researchgate.net [PDF; 1.1 MB ; accessed on February 11, 2018]).
  17. ↑ Emilie Gaudry, Philippe Sainctavit, Farid Juillot, Federica Bondioli, Philippe Ohresser, Isabelle Letard: From the green color of eskolaite to the red color of ruby: an X-ray absorption spectroscopy study . In: Physics Chemistry of Minerals . tape 32 , 2006, pp. 710-720 ( researchgate.net [PDF; 441 kB ; accessed on February 11, 2018]).
  18. ^ T. Stachel, Jeffrey W. Harris: Syngenetic inclusions in diamond from the Birim field (Ghana) - a deep peridotitic profile with a history of depletion and re-enrichment . In: Contributions to Mineralogy and Petrology . tape 127 , no. 4 , 1997, p. 336-352 , doi : 10.1007 / s004100050284 .
  19. ↑ CA Goodrich, GE Harlow: Knorringite-Uvarowite Garnet and Cr-Eskola Pyroxene in Ureilite LEW 88774 . In: Meteoritics and Planetary Science . tape 36 , no. 9 , 2001, p. 68 , bibcode : 2001M & PSA..36R..68G ( harvard.edu [PDF; 209 kB ; accessed on February 24, 2018]).
  20. ↑ M. Sikirdji, PH Warren: Northwest Africa 766: A New Ferroar Ureilite with Cr-Spinell, Cr-Rich Garnet (?) And Associated Si, Al-richt Glasses. In: Meteoritics and Planetary Science . tape 36 , no. 9 , 2001, p. 198 ( harvard.edu [PDF; 263 kB ; accessed on February 11, 2018]).