Spinel

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Spinel
Spinel2.jpg
1.83 ct blue spinel and 4.13 ct red spinel
General and classification
other names
  • Magnesium aluminate
  • Magnesia spinel
chemical formula MgAl 2 O 4
Mineral class
(and possibly department)
Oxides and hydroxides
System no. to Strunz
and to Dana
4.BB.05 ( 8th edition : IV / B.01)
02/07/01/01
Similar minerals Magnesia ferrite, hercynite, magnetite
Crystallographic Data
Crystal system cubic
Crystal class ; symbol cubic hexakisoctahedral; 4 / m  3  2 / m
Space group Fd 3 m (No. 227)Template: room group / 227
Lattice parameters a  = 8.09  Å
Formula units Z  = 8
Frequent crystal faces {111}, {110}, {221}
Twinning according to {111} (spinel law)
Physical Properties
Mohs hardness 8 (occasionally 7.5 to 8)
Density (g / cm 3 ) measured: 3.6 to 4.1; calculated: 3.578
Cleavage indistinct after {111}
Break ; Tenacity shell-like to uneven; splintery, brittle
colour colorless, red, orange, yellow, green, blue, violet, brown, black
Line color White
transparency transparent to almost opaque
shine Glass gloss, matt
magnetism non-magnetic
Crystal optics
Refractive index n  = 1.719

The spinel (specifically, magnesian spinel ) is a common mineral of the mineral class of "oxides and hydroxides" with the idealized chemical composition MgAl 2 O 4 and is thus chemically seen a magnesium - aluminate .

Spinel crystallizes in the cubic crystal system . The spinel structure is one of the most important and most common structure types , in which 30 other known minerals crystallize. Spinel is therefore also the namesake for the spinel supergroup newly defined by the International Mineralogical Association (IMA) .

Spinel develops predominantly octahedral , rarely also dodecahedral and cubic crystals and twins , which can be up to 30 centimeters tall. But it also occurs in the form of granular to massive mineral aggregates . Due to the formation of mixed crystals and the frequent addition of various metal ions such as iron , chromium , zinc , cobalt or manganese , natural spinels have a wide variety of colors. Since clear and transparent spinels also have a strong, glass-like shine on polished surfaces and are relatively insensitive to damage due to their high Mohs hardness of 8, these so-called "noble spinels" are among the valuable gemstones .

Many color varieties are now manufactured synthetically and, in addition to being used as gemstones, are also used as a base material for technical ceramics and pigments such as Thénards blue as a synthetic cobalt spinel.

Etymology and history

The origin of the name has not been fully clarified. It is assumed, however, that it originally comes from the ancient Greek σπίν (ν) ος [spín (n) os] for "spark" or "sparkle" and thus refers to its shine or to the typical thorn-like, sharp-edged crystal octahedron from the Latin spina , spinus or spinula for "thorn" or "cone" or spinella for "little thorn" or "small thorn".

The name Spinell has been handed down in various spellings in Europe since at least the 16th century, including as Spynell in England (1528), as Spinella by Georgius Agricola (1546) and as Spinellus by Anselmus de Boodt (1609). Similar to the name carbuncle or carbuncle stone, which has been known since ancient times , spinel does not specifically refer to the mineral known today as magnesia or noble spinel, but generally red gemstones in every shade from crimson to red violet ( hyacinth colors ) to yellowish white or whitish yellow. It was not until 1800 that it was recognized, on the one hand, that the red ruby and the blue sapphire were only color varieties of the same mineral corundum and, on the other hand, the spinel and the similarly colored and crystallographically similar minerals of the garnet group formed their own mineral types and learned to differentiate them.

classification

The current classification of the International Mineralogical Association (IMA) is spinel namesake of the spinel supergroup , where together with chromite , cochromite , coulsonite , cuprospinel , franklinite , gahnite , galaxite , hercynite , jacobsite , magnesiochromite , magnesiocoulsonite , magnesioferrite , magnetite , manganochromite , Trevorit , Vuorelainenit and Zincochromit forming the spinel subgroup within the Oxispinelle.

Already in the outdated, but partly still in use 8th edition of the mineral classification according to Strunz , the spinel belonged to the mineral class of "oxides and hydroxides" and there to the department of "oxides with a molar ratio of metal: oxygen = 3: 4 (spinel type M 3 O 4 and related compounds) ”, where together with gahnite, galaxite and hercynite he created the group of“ aluminate spinels ”with system no. IV / B.01 .

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), also classifies the spinel in the department of "Oxides with the molar ratio of metal: oxygen = 3: 4 and comparable". However, this is further subdivided according to the relative size of the cations involved , so that the mineral can be found according to its composition in the sub-section "With only medium-sized cations", where the "spinel group" with the system no. 4.BB.05 and the other members Brunogeierit , chromite, Cochromit, Coulsonit, Cuprospinell, Filipstadit , franklinite, gahnite, galaxite, Hercynit, Jakobsit, Magnesiochromite, Magnesiocoulsonit, Magnesioferrit, magnetite, Manganochromit, Nichromit (N), Qandilit , Trevorit, Ulvöspinell , Vuorelainenit and Zincochromit forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns spinel to the class of "oxides and hydroxides" and there into the category of "multiple oxides". Here it can be found together with galaxite, hercynite and gahnite in the " aluminum subgroup " 07.02.01 within the subdivision "multiple oxides (A + B 2+ ) 2 X 4 , spinel group".

Crystal structure

Unit cell of magnesium aluminate

Spinel crystallizes isotypically with magnetite in the cubic crystal system in the space group Fd 3 m (space group no. 227) with the lattice parameter a = 8.09  Å and eight formula units per unit cell . Template: room group / 227

Modifications and varieties

Pure spinel is colorless. However, the stoichiometry of the chemical formula is variable within wide limits, that is, magnesium or aluminum can be present in different proportions or can also be replaced by cations that are actually not related to the formula . Particularly large excesses of aluminum cations can be dissolved in it. An excess of magnesium is only possible at extremely high temperatures (from approx. 1500 ° C).

This results in a wide range of possible colors, ranging from purple to red to pink, yellow and from green to blue to brown to black. Some color varieties given their own names and are also in the gemstone - trading spreads.

  • “Noble spinel” or “ruby spinel” gets its strong light to dark red color from the diadochronous replacement of Al 2 O 3 with traces of Cr 2 O 3 .
    • "Almandine spinel" is a name from the 19th century for dark red spinels with a tinge of blue or purple.
    • "Balas ruby" or "balas ruby" is an outdated name for a pink to pale red gemstone, which spread around 1200 AD and served as a delimitation for a "carbuncle" or ruby ​​of lesser beauty. Since this designation is misleading, it has meanwhile been rejected by the CIBJO and, in contrast to the alternative designation “Balas spinel”, is hardly used in the gemstone trade.
    • "Rubicell" or "Rubacell" as a diminutive from the French rubis or rubace (ruby) have been used since the 17th century to denote yellow-orange to yellow-red hyacinth-like varieties.
  • Blue spinel or “sapphire spinel” gets its blue color by replacing MgO with FeO up to 3.5%.
  • In the case of green spinel or “chlorospinel” (from ancient Greek χλωρός chlōrós “light green, fresh”, according to Gustav Rose 1840), parts of MgO and Al 2 O 3 are replaced by traces of CuO and Fe 2 O 3 of up to 15%.

Education and Locations

Spinel forms as an accessory component in ultrabasic rocks of the upper mantle such as basalt and peridotite . It can also be metasomatic contact through displacement of dolomite and limestone or metamorphic contact in gneiss and marble . In the rocks concerned, spinel is usually found in the form of ingrown, well-formed, octahedral crystals and less often as twins . It is also found in unrolled , more or less loose form in gemstone soaps . Accompanying minerals include andalusite , chondrodite , forsterite , corundum , phlogopite , sillimanite and scapolite .

As a frequent mineral formation, spinels can be found at many localities, whereby so far (as of 2014) around 1,600 localities are known. One of the world's largest known spinel crystals so far is a reddish-purple-red, flat twin with a diameter of 17.8 cm, which was found in 2005 near An Phu in the Lục Yên mining district in Yên Bái , Vietnam .

The Siberian Aldan highlands with crystal finds of the variety Pleonast of up to 15 cm in size also became known due to unusual spinel finds . Crystals weighing up to 14 kg and 12 cm in size were found at Amity and Sterling Hill (New Jersey) in the USA. Also been known Ratnapura in Sri Lanka and Mogok in Myanmar, as well as Kukh-i-Lal ( Tadschikistan ) in Pamirgebirge cm for their crystal violet and red discoveries of particularly high quality and gemstone sizes 2 to 5 One of the world's largest reddish-violet-red spinels is also a flat crystal twin with a diameter of 17.8 cm, which was found in 2005 in An Phu in the Lục Yên mining district in the Vietnamese province of Yên Bái . The black and gray-blue to violet star spinels with four- and six-pointed asterism , which have so far mainly been found in Sri Lanka, are a particular rarity .

In Germany, the mineral has so far occurred in several quarries near Bötzingen , Horben , Immendingen , Schelingen , Scharnhausen and from Katzenbuckel in Baden-Württemberg; in many places in the Franconian Forest and Lower Bavaria ; near Hochstädten and Kilsbach in Hessen; at several sites in the vicinity of Bad Harzburg in Lower Saxony; in the Siebengebirge of North Rhine-Westphalia; in many places in the Eifel in Rhineland-Palatinate; near Waldheim , Dresden , Hinterhermsdorf , Pöhla (Schwarzenberg) as well as on the Greifensteinen and on Löbauer Berg in Saxony and on Kammberg near Joldelund in Schleswig-Holstein.

In Austria, spinel is known from Lölling and Kollnitz (municipality of Sankt Paul im Lavanttal ) in Carinthia; from several locations in the Dunkelsteinerwald and the Waldviertel in Lower Austria; from the skull (Hohe Tauern) in Salzburg; from Stradner Kogel , from a quarry near Klöch , the "Grube Breitenau" on the Hochlantsch and from a quarry near Stubenberg in Styria; a slag dump near St. Gertraudi in Tyrol and from several locations in the Mühlviertel in Upper Austria.

In Switzerland, the mineral has so far been mainly found in the cantons of Graubünden , Ticino and Valais .

Other locations are in Afghanistan, Egypt, Albania, Algeria, the Antarctic, Argentina, Azerbaijan, Australia, Belgium, Bolivia, Bosnia and Herzegovina, Botswana, Brazil, Bulgaria, China, the Democratic Republic of the Congo (Zaire), on Fiji, Finland, France, French Guiana and French Polynesia, Greece, Greenland, India, Iran, Iraq, Ireland, Israel, Italy, Japan, Yemen, Cambodia, Canada, Kazakhstan, Kenya, Lesotho, Libya, Madagascar, Malawi, Morocco , Mexico, Namibia, Nepal, the Netherlands, New Caledonia, New Zealand, Nigeria, North Korea, Norway, Oman, Pakistan, Palestine, Papua New Guinea, Peru, Poland, Romania, Russia, Sweden, Zimbabwe, Slovakia, Spain, South Africa, Sudan, Suriname, Taiwan, Tanzania, Thailand, the Czech Republic, Turkey, Uganda, Ukraine, Hungary, Uzbekistan, Venezuela, the United Kingdom (UK), the United States of America (USA) and the Central African Republic.

Spinel could also be detected in rock samples from the East Pacific Ridge (ultramafic complex “Hess Depth”) as well as outside of the earth in the comet dust from Wild 2 and in rock samples from the moon brought with them by the Apollo 14 and Apollo 16 missions.

In the relevant literature, the γ phase of olivine , which arises at high pressures and is the dominant mineral phase in the lower part of the transition zone of the earth's mantle between approx. 520 and 660 km depth, is more often referred to as spinel because its crystals also have the Have spinel structure. Due to the different chemical composition, however, this designation is mineralogically incorrect.

use

As a gem

Facet cut green spinel

Flawless spinels are coveted but rare gemstones . Red spinels have an outward resemblance to rubies . Thus, for example, long thought to be a ruby "Black Prince's Ruby" presented ( Rubin of the Black Prince ) in the Imperial State Crown and the "Timur Ruby" in a necklace from the British crown jewels and some drop-shaped gems in the Wittelsbach crown as Spinels out.

Big and famous spinels

Surname Weight raw / ground
in carats
Found year Found land comment
"Black Prince's Ruby" about 140 probably Badakhshan , Afghanistan polished and drilled, part of the British Crown Jewels, Imperial State Crown
Catherine spinel 146.43 Late 1980s on Pyanj south of Chorugh in the Pamir Mountains , Tajikistan faceted, privately owned
Samaria Spinel 500 probably Badakhshan , Afghanistan polished and drilled, part of the Persian Crown Jewels
"Timur Ruby" 352.50 probably Badakhshan , Afghanistan polished and engraved with the names of the previous owners, part of the British Crown Jewels , necklace
Two unnamed rough stones (unrolled or octahedron) 520 / - Exhibited in the British Museum
Tip of the Russian tsar's crown 398.72 Exhibited in the Diamond Fund of the Armory of the Moscow Kremlin

Further use

Due to its high hardness and chemical resistance as well as its high melting point of 2135 ° C, synthetic spinels are used for refractory and gas-tight technical ceramics .

Manipulations and imitations

In contrast to corundum (sapphire and the red variety ruby), spinels are only heated in rare cases because their colors either change insignificantly or do not change in the desired direction. Due to the fact that typical gemstone qualities of spinel have comparatively less opacifying inclusions, a further effect of the heating, namely the transparency improvement by melting the enclosed foreign crystals, is only documented for a few cases. In the specialist trade, natural spinels are still generally considered untreated. However, there are exceptions in the premium sector for certain color variants of certain sources. High-priced spinels are mostly certified by a gemological laboratory, where both heated and synthetic spinels are reliably differentiated from untreated spinels by means of Raman spectroscopy due to the accompanying structural change that already begins at 750 ° C.

Synthetic spinels for the jewelry industry have been manufactured using the Verneuil process since the 1920s , in which aluminum oxide ( corundum , Al 2 O 3 ) and magnesium oxide ( periclase , MgO ) react to form spinel (MgAl 2 O 4 ). With the help of other additives, the desired color varieties can be created, such as Co 2 O 3 for blue and Ni 2 O 3 for light green spinels, for example to imitate aquamarine , garnet, ruby, sapphire, emerald , various tourmalines and even moonstone . Since the 1950s, it has also been possible to imitate lapis lazuli using synthetic sintered spinel.

Under the polarization microscope , however, spinels synthesized with the Verneuil process always show a typical abnormal stress birefringence and can thus be distinguished from natural spinels.

See also

literature

  • Helmut Schrätze , Karl-Ludwig Weiner : Mineralogy. A textbook on a systematic basis . de Gruyter, Berlin; New York 1981, ISBN 3-11-006823-0 , pp. 354-358 .
  • 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. 116 .
  • Hellmuth Bögel: Knaurs mineral book. The house and handbook for friends and collectors of minerals . Droemer Knaur, Munich 1972, ISBN 3-426-00292-2 , p. 111, 112 .

Web links

Commons : Spinel  - collection of images, videos and audio files
Wiktionary: Spinell  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ^ A b c d Helmut Schrätze , Karl-Ludwig Weiner : Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin; New York 1981, ISBN 3-11-006823-0 , pp.  355 .
  2. David Barthelmy: Spinel Mineral Data. In: webmineral.com. Retrieved May 15, 2019 .
  3. ^ A b c Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables. Chemical-structural Mineral Classification System . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  188 .
  4. a b c d e f 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. 116 .
  5. a b c d e f Hans Jürgen Rösler : Textbook of Mineralogy . 4th revised and expanded edition. German publishing house for basic industry (VEB), Leipzig 1987, ISBN 3-342-00288-3 , p.  380 .
  6. Spinel . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 (English, handbookofmineralogy.org [PDF; 72  kB ; accessed on May 15, 2019]).
  7. Spinel. In: mindat.org. Hudson Institute of Mineralogy, accessed May 15, 2019 .
  8. Malcolm Back, William D. Birch, Michel Blondieau and others: The New IMA List of Minerals - A Work in Progress - Updated: March 2019. (PDF 1703 kB) In: cnmnc.main.jp. IMA / CNMNC, Marco Pasero, March 2019, accessed May 20, 2019 .
  9. a b c d e f Hans Lüschen: The names of the stones. The mineral kingdom in the mirror of language . 2nd Edition. Ott Verlag, Thun 1979, ISBN 3-7225-6265-1 , p. 323-324 .
  10. Cristian Biagioni, Marco Pasero: The systematics of the spinel-type minerals: An overview . In: American Mineralogist . tape 99 , no. 7 , 2014, p. 1254–1264 , doi : 10.2138 / am.2014.4816 (English, degruyter.com [PDF; 795 kB ] accessed via De Gruyter Online).
  11. Localities for Spinel. In: mindat.org. Hudson Institute of Mineralogy, accessed May 15, 2019 .
  12. Records in the mineral sector. In: Mineralienatlas Lexikon. Stefan Schorn u. a., accessed on February 7, 2014 .
  13. Petr Korbel, Milan Novák: Mineral Encyclopedia (=  Dörfler Natur ). Edition Dörfler im Nebel-Verlag, Eggolsheim 2002, ISBN 978-3-89555-076-8 , p. 76 .
  14. Mineral Atlas: Mineral Records
  15. A. Ruppenthal: The world of precious stones - Spinel (PDF 404.6 kB) ( Memento from June 6, 2013 in the Internet Archive )
  16. a b List of sites for spinel in the Mineralienatlas and Mindat
  17. a b c d gemmologie.at - Brief information from the Austrian Gemmological Society ( Memento from March 6, 2014 in the Internet Archive ) (PDF 1.5 MB; pp. 4–5)
  18. Russia Beyond: Lost, Sold, Remained in Russia: The Fate of the Romanov Jewels. February 27, 2019, accessed on March 3, 2020 (German).
  19. ^ A b Helmut Schrätze , Karl-Ludwig Weiner : Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin; New York 1981, ISBN 3-11-006823-0 , pp.  358 .
  20. Sudarat Saeseaw, Wuyi Wang, Kenneth Scarratt, John L. Emmett and Troy R. Douthit: Distinguishing Heated Spinels from Unheated Natural Spinels. A short review of on-going research. (PDF 554 kB) In: gia.edu. Gemolocical Institute of America (GIA), January 28, 2019, accessed May 15, 2019 .
  21. a b Bernhard Bruder: Decorated stones. Recognizing imitations and manipulations in gemstones and minerals . Neue Erde, Saarbrücken 2005, ISBN 3-89060-079-4 , p. 100 .
  22. Gemstone Lexicon: Spinel. In: edelsteine.at. Wiener Edelstein Zentrum, accessed on May 15, 2019 .