Andradite

• Allochroit
• Calcium - iron garnet
• Kalkeisengranat

Andradite, also known as calcium - iron garnet or Kalkeisengranat, is a commonly occurring mineral from the group of garnets within the mineral class of " silicates and Germanates ". It crystallizes in the cubic crystal system with the idealized composition Ca3Fe3 2 [ SiO4 ] 3, is so chemically speaking, a calcium -iron silicate which structurally belongs to the island silicates.

Andradite is the iron analogue of grossular ( Ca3Al2 [ SiO4 ] 3) and Uvarovite ( Ca3Cr2 [ SiO4 ] 3) and forms with them a series of solid solutions, the so-called " Ugrandit series". Since Andradit Schorlomit ( Ca3Ti24 [ Fe23 SiO12 ] ) and Kimzeyit ( Ca3Zr2 [ Al2SiO12 ] ) mixed crystals also forms with the other garnet minerals, it shows a correspondingly wide range of composition depending on the formation conditions more or less large amounts of titanium and zirconium. Since, in addition, other foreign admixtures may be included, it usually occurs in different colors, but with yellow-green to emerald green and brown to reddish brown colors predominate. Rarely, there are also colorless and black Andradite.

The mineral is transparent to translucent and typically developed rhombic dodecahedron or trapezohedron as well as combinations of these crystal forms, which can be as large as five inches and have a resin - to diamond-like luster. In addition, it also occurs in granular and massive mineral aggregates.

Special Features

Heated before the blowpipe, andradite forms a black, magnetic ball.

Etymology and history

Was first described in 1800 by the Brazilian mineral mineralogists and statesman José Bonifácio de Andrada e Silva, who discovered it on his journey through Norway in the pit " Wirum " near Drammen. Under the name Allochroit Silva described in his notes, some of the properties of the mineral, such as its yellowish gray to dark straw color (variety Topazolith ) relatively high hardness ( "just scratched by quartz", translated from quartz just be scratched ), but gave no chemical composition of.

His final name was the mineral in 1868 by James Dwight Dana, of all known up to that lime - iron garnets under the name Andradit summarized in his book " A System of Mineralogy ," thus honoring the first discoverer of these mineral species Andrada e Silva.

Classification

Already in the now outdated, but partly still in use 8th edition of the mineral classification by Strunz was one of andradite to mineral class of " silicates and Germanates " and then to the Department of " island silicates ( nesosilicates ) " where he along with Goldmanit, Grossularund Uvarovite the independent " garnet - Ugrandit series" with the system no. VIII/A.08 formed.

The 9th edition valid since 2001 and of the International Mineralogical Association (IMA ) used the Strunz'schen Mineral classification assigns the Andradit also in the department of " island silicates ". However, this is further subdivided according to the possible presence of additional anions and the coordination of the cations involved, so that the mineral according to its composition and its construction in the subdivision of " island silicates without other anions; Cations can be found in octahedral and usually greater coordination, " where, along with almandine, Blythit, Calderit, Goldmanit, grossular, Henritermierit, Hibschite, Holtstamit, Hydroandradit, katoite, Kimzeyit, Knorringit, majorite, Morimotoit, pyrope, Schorlomit, Spessartin Skiagit, Uwarovit and wadalite the " garnet " with the system no. 9.AD.25 forms.

The mainly common in English-speaking classification of minerals according to Dana assigns the andradite in the class of " silicates and Germanates " and then in the Department of the " island of silicate minerals " one. Here he is with Goldmanit, grossular, and Uwarovit Yamatoit in the " garnet group ( Ugrandit series) " with the system no. Find: " SiO4 groups with cations and > coordination island silicates " 51.04.03b within the subdivision.

Modifications and varieties

• Demantoid is a by foreign admixtures of chromium yellow-green to dark green colored andradite. It was named discovered in 1870 by Nils Nordenskiöld, who later identified as Andradites " greenish pebbles " in the Urals.
• Hydroandradit ( Ca3Fe3 2.0 ( SiO4) 2.71-2.81 ( H4O4 ) 0:29 to 0:19 ) does not count as a separate mineral, but as a variety of andradite.
• Melanite ( by Abraham Gottlob Werner, 1799) is regarded as titanium rich variety of andradite and after the Greek word for black μέλας named because it occurs predominantly in gray-black to jet-black crystals or coarse aggregates.
• Topazolith ( = topaz -like) is a light yellow to amber-colored andradite - variety.

Yellowish brown Topazolithe from the same locality (Total Size: Size: 7.7 cm × 6.5 cm × 2.6 cm )

Black Melanite from the Kayes region, Mali (size: 5.5 cm × 5.5 cm × 4.5 cm)

Education and Locations

Similar Grossular Andradite also forms by Kontaktmetasomatose ( material displacement of certain rock components) with intake of iron in skarns and in kontaktmetamorph converted iron ore deposits. Likewise, he finds himself in nepheline syenites as Nebengemengteil, phonolites and other alkaline rocks. Occasionally, andradite in alpine fissures when greenschist ( chlorite ) or similar iron-rich rocks are pending as host rock. Accompanying minerals include calcite, chlorite, dolomite, epidote, magnetite, spinel and vesuvianite.

As a frequent mineral formation Andradite is found in many localities, where so far (as of 2013) are considered to be known around 1,400 localities. Apart from its type locality Drammen the mineral in Norway yet joined, among others, in several places in the province Buskerud ( Kongsberg, Lier, Nedre Eiker ) and in some places in the provinces of Aust- Agder ( Arendal, Gjerstad ), Nordland, Oppland ( Grua, Nordmarka ) Telemark ( Porsgrunn, Skien ) and Vestfold ( Larvik, Sandefjord) on.

Known due to exceptional Andraditfunde among others, the pit " Kohse " is at Tenkawa in the district of Yoshino -gun ( Nara Prefecture ) on the Japanese island of Honshu, where several inches wide and partially iridescent crystal specimens came to light. These so-called "Rainbow grenade " (English Rainbow Garnet ) are however also found in Mexico. Up to four inches large crystals are known from the deposit Sinerechenskoye (administrative district Kawalerowo ) in the Russian Primorye and up to three inches wide Dematoidkristalle was found in Malencotal (Val Malenco ) in the Italian province of Sondrio.

In Germany Andradit has so far been demonstrated in many places: in the Black Forest in Baden- Württemberg; im Fichtelgebirge, Franconian and Bavarian Forest in Bavaria; Rachel Hausen, Hirzenhain and high cities ( Bensheim ) in Hesse; in Bad Harzburg and Saint Andreas mountain in Lower Saxony; in many places in the Eifel region ( Rhineland -Palatinate ), in the Saxon Erzgebirge and Vogtland and in Unterbreizbach and savings mountain in Thuringia.

In Austria, found the mineral among others Badersdorf and on Pauliberg in Burgenland, at St. Andrew's cross in the community hut mountain and Wollanig near Villach in Carinthia, at several places in Lower Austria's Waldviertel and the Salzburg Hohe Tauern mountains and in some places in Styria, Tyrol and Vorarlberg.

In Switzerland Andradit previously occurred mainly in the cantons of Graubünden and Valais, but also could be found near Upper Bargen, Schaffhausen.

Other localities lie include Afghanistan, Antarctica, Argentina, Ethiopia, Australia, Azerbaijan, Bolivia, Brazil, Bulgaria, Chile, China, the Democratic Republic of Congo, Ecuador, Finland, France, Greece, Greenland, Guinea, Honduras, Iceland, India, Iran, Israel, jeman, Canada, Kazakhstan, Kenya, Kyrgyzstan, Madagascar, Malawi, Mali, Morocco, Mexico, Mongolia, Myanmar, Namibia, New Zealand, Norway, Pakistan, Palestine, Papua New Guinea, Paraguay, Peru, the Philippines, Poland, Portugal, Romania, Solomon Islands, Sweden, Serbia, Slovakia, Spain, Sri Lanka, South Africa, South Korea, Taiwan, Tajikistan, Tanzania, Thailand, Turkey, Ukraine, Hungary, Uzbekistan, the United Kingdom ( UK), the United States of America (USA ) and Cyprus.

Yellow andradite and rhodochrosite, also from the " Wessels Mine " (size: 5.2 cm × 4.7 cm × 3 cm)

Iridescent " rainbow" andradite from the Pit " Kohse ", Honshu, Japan (size: 10 cm × 9 cm × 8 mm)

Topazolith from Wurlitz in the Fichtelgebirge. Exposed in the Mineralogical Museum, Bonn

Crystal structure

Andradit crystallizes in the space group Ia3d cubic ( Raumgruppen-Nr. 230 ) with the lattice parameters a = 12.06 Å, and eight formula units per unit cell.

Use as a gemstone

Like most grenade is used depends on the quality as a valuable gemstone and the andradite and demantoid and its varieties Topazolith. Due to the variety of colors, due to the extensive formation of mixed crystals of the grenade one is now gone in the gem trade to not assign the grenade after its often difficult to quantify, chemical composition, but according to their respective shade the individual garnet species which respectively serve only as color names. For example, the green garnets are referred to as either Demantoid, Hydrogrossular or as tsavorite or tsavolite, although the latter actually belongs chemically to the Grossularen.

Confusion possibilities exist depending on the color, among other things with ruby ​​and spinel ( red), peridot and emerald (green), Topaz and Hyacinth (yellowish to brownish ) and the multi-colored tourmaline.