Pyrope

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Pyrope
Garnet, Madagascar.JPG
Pyrope from Madagascar
General and classification
other names

Bohemian garnet

chemical formula Mg 3 Al 2 [SiO 4 ] 3
Mineral class
(and possibly department) 		Island silicates (nesosilicates)
System no. to Strunz
and to Dana 		9.AD.25 ( 8th edition : VIII / A.08)
51.04.03a.01
Similar minerals other garnets, spinel , ruby
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  = 11.459  Å
Formula units Z  = 8
Frequent crystal faces Rhombic dodecahedron {110}
Physical Properties
Mohs hardness 7 to 7.5
Density (g / cm 3 ) measured: 3.582; calculated: 3.5591
Cleavage no
Break ; Tenacity shell-like, splintery
colour black red, purple red, blood red, orange-red, pink, colorless
Line color White
transparency transparent to translucent
shine Glass gloss
Crystal optics
Refractive index n  = 1.714
Birefringence none, often weakly birefringent with undetectable extinction

The mineral pyrope (from the Greek pyropos "fiery") is an island silicate from the garnet group and has the chemical composition Mg 3 Al 2 [SiO 4 ] 3 .

The mineral crystallizes in the cubic crystal system , often in (rounded) grains. It also occurs in aggregates. Pure pyrope z. B. from the white slate of the Dora Maira massif is colorless. By incorporating iron (Fe 2+ ) instead of magnesium (Mg), the color of pyrope ranges from pink to blood-red and black-red, often with a tinge of brown.

Etymology and history

One of the earliest mentions of grenades as a gemstone can be found in the Bible in the book of Exodus, 28.18 EU as a stone on the breastplate of the high priest Aaron. In his work Naturalis historia in Book 35, Chapter 25, Pliny the Elder summarized a number of red minerals under the term carbunculus , including garnets. A further differentiation of this group into three subgroups (ruby, spinel and garnet) took place by Albertus Magnus around 1250 in his work De mineralinus et rebus metallicus . The current name pyrope is derived from the Greek pyropos "of fiery appearance", which alludes to the red color.

In the 18th century, a wide variety of minerals were referred to as garnet based on their external characteristics (mainly crystal form), including z. B. also leucite . This changed when the systematic chemical analyzes of the minerals began. In the course of these investigations, in 1797 , Martin Heinrich Klaproth was the first to determine the composition of what was then known as the "Bohemian Garnet".

Georg Menzer clarified the crystal structure of the grenade in 1929 and Loring Coes junior succeeded in the first synthesis of pure pyrope at 30,000 bar and 900 ° C in the mid-1950s with newly developed high-pressure presses from the Norton Company ( Massachusetts , USA). Synthetic pyropes from Coe's laboratory were used by Skinner in 1956 to determine the physical properties (lattice constant, refractive index, density) of pure pyrope, and Anna and J. Zemann five years later for the first structural refinement of pyrope.

classification

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

In the meantime outdated, but still in use 8th edition of the mineral classification by Strunz of pyrope belonged to the department of the "island silicates (nesosilicates)" where he collaborated with almandine, Andradite, Calderit, Goldmanit, Grossular, Henritermierit , Hibschite , Holtstamit , Hydrougrandit , Katoit , Kimzeyit , Knorringit, Majorit, Morimotoit, Pyrope , Schorlomit , Spessartin, Uwarowit, Wadalit and Yamatoit (discredited because identical to Momoiit) the "Garnet Group" with the system no. VIII / A.08 .

The 9th edition of Strunz's mineral systematics, which has been in force since 2001 and is used by the International Mineralogical Association (IMA), also classifies pyrope in the category 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 can be classified according to its composition in the subsection “Island silicates without further anions; Cations in octahedral [6] and usually greater coordination ”can be found where it is found together with almandine, andradite, calderite, goldmanite, grossular, henritermierite, holtstamite, katoite, kimzeyite, knorringite, majorite, momoiite, morimotoite, schorlomite, spessartine and uwarowite the "garnet group" with the system no. 9.AD.25 forms. The garnet compounds blythite, hibschite, hydroandradite and skiagite, which are no longer regarded as minerals, were also included in this group. At that time, wadalite was still grouped with the grenades, proved to be structurally different and is now assigned to a separate group with chloromayenite and fluoromayenite . The garnets irinarassite , hutcheonite , kerimasite , toturite , menzerite (Y) and eringaite described after 2001 would have been classified in the garnet group.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , also classifies pyrope in the category of "island silicate minerals ". Here it is together with almandine, spessartine, knorringite, 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

Pyrope with the idealized composition [X] Mg 2+ 3 [Y] Al 3+ [Z] Si 3 O 12 is the magnesium analog of almandine ( [X] Fe 2+ 3 [Y] Al [Z] Si 3 O) 12 ) and forms mixed crystals with the other aluminum grenades almandine, spessartine and grossular, according to the exchange reactions

  • [X] Mg 2+ = [X] Fe 2+ (almandine)
  • [X] Mg 2+ = [X] Mn 2+ (Spessartine)
  • [X] Mg 2+ = [X] Ca 2+ (grossular)

With one exception, there is unlimited miscibility with all aluminum garnet end links, at least at geologically relevant temperatures. In the mixture series Pyrope - Grossular there is a miscibility gap at temperatures below approximately 600 ° C and 25-30 mol% grossular.

On the octahedrally coordinated Y position, Al 3+ can be replaced by Cr 3+ in accordance with the exchange reaction

  • [Y] Al 3+ = [Y] Cr 3+ (knorringite)

Under the high pressures and temperatures of the earth's mantle, Al 3+ in the Y position is replaced by magnesium and silicon, according to the exchange reaction

  • 2 [Y] Al 3+ = [Y] Mg 2+ + [Y] Si 4+ (majority)

Pyropes of this series with more than ~ 25 mol% majority are tetragonal .

Crystal structure

Pyrope crystallizes with cubic symmetry in the space group Ia 3 d (space group no. 230) and 8 formula units per unit cell . There are numerous determinations for the edge length of the cubic unit cell of natural mixed crystals as well as synthetic pyropes. For the pure pyrope terminal the lattice parameter z. B. given with a  = 11.459 Å, a  = 11.452 Å or a  = 11.450 Å for a natural pyrope from the white slate of the Dora Meira region (Italy). Template: room group / 230

The structure is that of garnet . Magnesium (Mg 2+ ) occupies the X position , which is dodecahedrally surrounded by 8 oxygen ions. This position is quite large for the small magnesium ion, which there carries out a clear, asymmetrical oscillation around the center of the position. Aluminum (Al 3+ ) occupies the Y position, which is surrounded by 6 oxygen ions in an octahedral manner, and the Z position, which is surrounded by 4 oxygen ions in an octahedral manner, is exclusively occupied by silicon (Si 4+ ).

properties

What is striking compared to other magnesium silicates is the relatively high hardness (7 - 7.5) and the high specific weight of 3.5 - 3.6 g / cm 3 . For comparison: Forsterite (Mg 2 SiO 4 ), also hardness 7, has a density of 3.3 g / cm 3 and enstatite (MgSiO 3 ) only has a hardness of 5–6 and a density of 3.2 g / cm 3 .

Natural pyropes often show weak, irregularly cloudy birefringence , which is attributed to lattice stresses .

Chemically pure pyrope is colorless. Natural pyropes are usually orange-red to purple-red and violet-red or green to blue due to their low iron (Fe 2+ ), manganese (Mn 2+ ), chromium (Cr 3+ ) and vanadium (V 3+ ) contents .

The color caused by chromium depends largely on the chromium content in the garnet. Low chromium contents lead to a wine-red color, which changes from gray to dark green with increasing chromium contents. This is caused by a change in the nature of the chromium-oxygen bonds . The covalent portion of the bonds decreases with increasing chromium content, which leads to a shift in the wavelengths of the absorbed light and ultimately to a change in color. In addition, the pyrope colored by chrome or vanadium show other unusual color effects.

Thermochromatic effect

Chromium-rich, violet-red colored pyrope from Koherab , Namibia change their color to green when heated from ~ 400 ° C, to be seen in short films on the mineralogy pages of the California Institute of Technology .

Alexandrite effect : color changes depending on the lighting

Pyropes with more than 3% by weight of Cr 2 O 3 show a color change for the human eye from blue-green in daylight to wine-red in the light of incandescent lamps or candlelight.

Pyrope-Spessartine mixed crystals can show a color change even with very low levels of chromium or vanadium. Spessartine-rich pyropes from Tanzania show a color change from blue-green to red. Low levels of almandine or grossular lead to a greater variance in the colors observed. Blue-green pyropes from a deposit near Bekily, Madagascar appear blue-green in daylight and pink in light bulb or candlelight.

Education and Locations

Pyrope is found in particular in ultramafic rocks such as serpentinites , peridotites or kimberlites , also secondarily in sands, gravel and soaps .

The most important deposits of pyrope are in Europe in the Czech Republic (Bohemian Central Mountains), in South Africa especially near Kimberley and in Tanzania, in Australia and the USA (Arizona). Another important site is in the village of Martiniana Po in Italy .

use

Pyrope was particularly valued as a gemstone in the 18th and 19th centuries ("Bohemian garnet").

literature

  • Jiří Kouřimský: The fire eye from the Bohemian volcanoes . In: garnet. The minerals of the garnet group: precious stones, jewelry and lasers (= Christian Weise [Hrsg.]: ExtraLapis . Volume 9 ). Christian Weise Verlag, 1995, ISBN 3-921656-35-4 , ISSN  0945-8492 , p. 76-82 .
  • Jaroslav Bauer, Vladimír Bouška: Gemstone Guide . Verlag Werner Dausien, Hanau / Main 1993, ISBN 3-7684-2206-2 , p. 100-106 .
  • Walter Schumann: Precious stones and gemstones. All kinds and varieties. 1900 unique pieces . 16th revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5 , pp. 120 .
  • Dana Stehlíková: The Bohemian Garnet (Carbunculus Granatus Zrnakoc) . 2nd Edition. Mucha Museum, Prague 2004.

Web links

Commons : Pyrope  - collection of images, videos and audio files

Individual evidence

  1. ↑ a b GV Gibbs and JV Smith: REFINEMENT OF THE CRYSTAL STRUCTURE OF SYNTHETIC PYROPE . In: The American Mineralogist . tape 50 , 1965, pp. 2023–2039 ( rruff.info [PDF; 1,2 MB ; accessed on May 5, 2018]).
  2. ^ A b GA Novak and GV Gibbs: The crystal chemistry of the silicate garnets . In: The American Mineralogist . tape 56 , 1971, p. 791–825 ( rruff.info [PDF; 2,3 MB ; accessed on May 4, 2018]).
  3. ^ A b c d e BJ Skinner: Physical properties of end-members of the garnet group . In: The American Mineralogist . tape 41 , 1956, pp. 428–436 ( minsocam.org [PDF; 522 kB ; accessed on May 5, 2018]).
  4. ↑ a b c d DK Teertstra: Index-of-refraction and unit-cell constraints on cation valence and pattern of order in garnet-group minerals . In: The Canadian Mineralogist . tape 44 , 2006, pp. 341–346 ( rruff.info [PDF; 197 kB ; accessed on May 5, 2018]).
  5. ^ A b Anne M. Hofmeister, Rand B. Schaal, Karla R. Campbell, Sandra L. Berry and Timothy J. Fagan: Prevalence and origin of birefringence in 48 garnets from the pyrope-almandine-grossularite-spessartine quaternary . In: The American Mineralogist . tape 83 , 1998, pp. 1293–1301 ( minsocam.org [PDF; 105 kB ; accessed on May 27, 2018]).
  6. ↑ Pliny the Elder: Naturalis historia. Book 35, 25.
  7. ^ Universität Karlsruhe, Wolfgang Wegner: Herzog Ernst ( Memento from April 21, 2009 in the Internet Archive ).
  8. ^ MH Klaproth: Chemical investigation of the Bohemian garnet . In: Contributions to the chemical knowledge of mineral bodies . tape 2 , 1797, pp. 16–21 ( rruff.info [PDF; 394 kB ; accessed on June 5, 2018]).
  9. ↑ G. Menzer: The crystal structure of the grenade . In: Journal of Crystallography - Crystalline Materials . tape 69 , 1929, pp. 300-396 , doi : 10.1524 / zkri.1929.69.1.300 .
  10. ↑ Loring Coes Jr .: High-pressure Minerals . In: Journal of the American Ceramic Society . tape 38 , 1955, pp. 298 , doi : 10.1111 / j.1151-2916.1955.tb14949.x .
  11. ↑ Anna Zemann, J. Zemann: Refinement of the crystal structure of synthetic pyrope, Mg3Al2 (SiO4) 3 . In: Acta Crystallographica . tape 14 , 1961, pp. 835-837 , doi : 10.1107 / S0365110X61002436 .
  12. ^ A b 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 ( rruff.info [PDF; 1.1 MB ; accessed on April 28, 2020]).
  13. ↑ a b c Thomas Armbruster, Charles A. Geiger, George A. Stock: Single-crystal X-ray structure study of synthetic pyrope almandine garnets at 100 and 293K . In: The American Mineralogist . tape 77 , 1992, pp. 512-521 ( rruff.info [PDF; 1,2 MB ; accessed on May 19, 2018]).
  14. ↑ Charles Geiger and Anne Feenstra: Molar volumes of mixing of almandine-pyrope and almandine-spessartine garnets and the crystal chemistry and thermodynamic-mixing properties of the aluminosilicate garnets . In: The American Mineralogist . tape 82 , 1997, pp. 571-581 ( minsocam.org [PDF; 213 kB ; accessed on May 21, 2018]).
  15. ^ Charles A. Geiger: A powder infrared spectroscopic investigation of garnet binaries in the system Mg3Al2Si30i2-Fe3Al2Si30i2-Mn3Al2Si30i2-Ca3Al2Si30i2 . In: European Journal of Mineralogy . tape 10 , 1998, pp. 407-422 ( researchgate.net [PDF; 4.0 MB ; accessed on June 6, 2018]).
  16. ↑ a b Karl Schmetzer and Heinz-Jürgen Bernhardt: GARNETS FROM MADAGASCAR WITH A COLOR CHANGE OF BLUE-GREEN TO PURPLE . In: GEMS & GEMOLOGY . 1999, p. 196–201 ( gia.edu [PDF; 245 kB ; accessed on June 6, 2018]).
  17. ↑ Jibamitra Ganguly, Weiji Cheng, Hugh St. C. O'Neill: Syntheses, volume, and structural changes of garnets in the pyrope-grossular join: Implications for stability and mixing properties . In: American Mineralogist . tape 78 , 1993, p. 583-593 ( rruff.info [PDF; 1,3 MB ; accessed on June 4, 2018]).
  18. ↑ a b Jibamitra Ganguly, Weiji Cheng, Massimiliano Tirone: Thermodynamics of alimosilicate garnet solid solution: new experimental data, an optimized model, and thermodynamic applications . In: Contributions to Mineralogy and Petrology . tape 126 , 1996, pp. 137–151 ( researchgate.net [PDF; 1.8 MB ; accessed on May 22, 2018]).
  19. ↑ a b LIPING WANG, ERIC J. ESSENE AND YOUXUE ZHANG: Direct observation of immiscibility in pyrope-almandine-grossular garnet . In: The American Mineralogist . tape 85 , 2000, pp. 41–46 ( researchgate.net [PDF; 344 kB ; accessed on May 22, 2018]).
  20. ↑ 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]).
  21. ^ 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]).
  22. ^ 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]).
  23. ↑ 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 .
  24. ↑ AKIHIKO NAKATSUKA, AKIRA YOSHIASA, TAKAMITSU YAMANAKA, OSAMU OHTAKA, TOMOO KATSURA, AND EIJI ITO: Symmetry change of majorite solid-solution in the system Mg3Al2Si3O12-MgSiO3 . In: The American Mineralogist . tape 84 , 1999, pp. 1135–1143 ( minsocam.org [PDF; 102 kB ; accessed on June 7, 2018]).
  25. ↑ Gilberto Artioli, Alessandro Pavese, Kenny steel, Richard K. McMullan: SINGLE-CRYSTAL NEUTRON DIFFRACTION STUDY OF THE TEMPERATURE RANGE pyrope IN 30-1173 K . In: The Canadian Mineralogist . tape 35 , 1997, pp. 1009-1019 ( rruff.info [PDF; 698 kB ; accessed on June 4, 2018]).
  26. ↑ Tetsuo Irifune, Yu Hariya: Phase relationships in the system Mg3Al2Si3O12 - 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]).
  27. ↑ MN Taran, K. Langer, Irmgard Abs-Wurmbach, DJ Frost, AN Platonov: Local relaxation around [6] Cr3 + in synthetic pyrope – knorringite garnets, [8] Mg3 [6] (Al1 - X CrX3 +) 2 [4] Si3O12, 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 .
  28. ↑ Short films of color changes on the mineralogy pages of the California Institute of Technology
  29. ↑ a b E. Gubelin and K. Schmetzer: GEMSTONES WITH EFFECT ALEXANDRITE . In: GEMS & GEMOLOGY . 1982, p. 197–203 ( gia.edu [PDF; 245 kB ; accessed on June 6, 2018]).