Garnet

The garnet group is an important group of rock-forming minerals from the mineral class of " silicates and Germanates " and the Department of the island silicates ( nesosilicates ). The garnet minerals crystallize mostly in the cubic crystal system and form predominantly isometric crystals with the characteristic forms of the rhombic dodecahedron (deprecated also Granatoeder ) Ikositetraeders and combinations thereof.

The general garnet formula is: X3Y2 [ ZO4 ] 3 or A32 B23 [ RO 4 ] 3, however, where 'X', 'Y' and 'Z ' or ' A', ' B' and 'R' represent any chemical elements represent, but defined sites in the crystal lattice. The respective lattice sites can thereby be occupied by different elements or ions better:

• X: divalent cations surrounded by eight oxygen anions, usually Mg2 , Fe2 , Mn2 and Ca2
• Y: mainly trivalent cations surrounded by six oxygen anions, usually Al3 , Fe3 , Cr3 and V3 , but also Ti3 , Zr 4 , Sn 4
• R: mainly tetravalent cations surrounded by four oxygen anions, mostly Si4 , as well as Al3 , Fe3 , Ti4 , P5 , As 3 , 5

In addition, at the Hydro grenades so-called one [ ZO4 ] - module is still replaced by four [ OH] - modules.

Garnets are transparent to translucent in general, with many foreign admixtures and coarse mineral aggregates also opaque. Uninjured or unweathered crystal faces have a glass - to -fat -like shine. The color of the shell is very variable, even if reddish color varieties predominate. The palette ranges from light green to yellow -green to dark green, light yellow to yellow-orange and orange-red as well as from a bright pink to a dark red almost black acting. More rarely, there are colorless and brown varieties, and very rarely also color-changing ( Iridescent ) and blue grenade. The stroke color is, however, always white.

Their relatively high density ( 3.5 to 4.5 g/cm3), Mohs hardness (6.5 to 7.5) and refraction (n = 1.61 ( katoite ) to n = 1.96 ( Calderit ) make them both interesting as a gemstone and for industrial applications.

• 4.1 metapelites ( mica schist, gneiss )
• 4.2 metabasites ( granulite, eclogite )
• 4.3 Decomposition of garnet ( Retrograde conversions)

• 5.1 Structure of the garnet structure
• 5.2 symmetry reduction

• 6.1 The abrasive
• 6.2 In the technique
• 6.3 As a gemstone

Etymology and history

The name garnet was only coined in the Middle Ages, but has its origin in the Latin word granum for grain or core or granatus for grainy or nuclear rich and refers on one hand to the occurrence of the mineral in grains which resemble the seeds of the pomegranate ( Punica granatum ) have, on the other hand also on the orange -red to red-violet color of flower, fruit and seeds of the pomegranate.

Even in ancient grenade were used as gemstones. In the Middle Ages they were (also carbuncle ) is known along with rubies and spinels under the name carbuncle - most of that time came from India. Especially popular but they were in the 19th century, were in great demand as Bohemian Pyrope so that they were shipped to America.

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the garnet Group belonged to the general ward of the " island silicates ( nesosilicates ) " carries the system no. VIII/A.08 and consisted of the members almandine, andradite, Calderit, Goldmanit, grossular, Henritermierit, Hibschite, Holtstamit, Hydrougrandit ( discredited in 1967 as unnecessary group name), katoite, Kimzeyit, Knorringit, majorite, Morimotoit, pyrope, Schorlomit, Spessartin, Uvarovite, wadalite and Yamatoit ( discredited, since it is identical with Momoiit ).

The 9th edition valid since 2001 and of the International Mineralogical Association (IMA ) used the Strunz'schen Mineral classification assigns the garnet group also in the department of " island silicates ". However, this is further subdivided according to the possible presence of other anions and the coordination of the cations involved, so that the garnet group with the system no. 9.AD.25 according to the composition of the members almandine, andradite, Blythit, Calderit, Goldmanit, grossular, Henritermierit, Hibschite, Holtstamit, Hydroandradit, katoite, Kimzeyit, Knorringit, majorite, Momoiit (IMA 2009-026 ), Morimotoit, pyrope, Schorlomit, Spessartine, Skiagit, Uvarovite and wadalite in the subsection " island silicates without additional anions; Cations can be found in oktahedraler and usually greater coordination ".

The mainly common in English-speaking classification of minerals according to Dana assigns the garnet group in the department of " island silicate minerals ." Here it is, however, divided into the subgroups " Pyralspit Series" ( System-Nr. 51.04.03a ), " Ugrandit Series" ( System-Nr. 51.04.03b ), " Schorlomit - Kimzeyit Series" ( System-Nr. 51.04.03c ), "Hydro grenade " ( System-Nr. 51.04.03d ) and " Tetragonal Hydro grenade " ( System-Nr. 51.04.04 ) within the subdivision " island silicates: SiO4 groups with cations and > coordination " to find.

Subgroups, individual minerals and varieties

Traditionally, the garnets are divided into the following subgroups. The compositions listed below refer to the so-called end-members of the garnet group, that is in them, the indefinite in the general formula positions E and G are fully occupied with atoms of an element:

• Pyrope (magnesium - aluminum - garnet): Mg3Al2 [ SiO4 ] 3
• Almandine ( iron-aluminum - garnet): Fe3Al2 [ SiO4 ] 3
• Spessartite ( manganese - aluminum - garnet): Mn3Al2 [ SiO4 ] 3

Also included in this group

• Majorite (magnesium - iron - garnet): Mg 3 (Fe, Al, Si ) 2 [ SiO4 ] 3
• Knorringit (magnesium -chromium garnet): Mg3Cr2 [ SiO4 ] 3
• Calderit ( manganese-iron - garnet): Mn3Fe23 [ SiO4 ] 3
• Momoiit (IMA 2009-026 ): (Mn 2 , Ca ) 3 ( V3 , Al ) 2 ( SiO4) 3

• Uvarovite (Calcium -chromium garnet): Ca3Cr2 [ SiO4 ] 3
• Grossular (calcium - aluminum - garnet): Ca3Al2 [ SiO4 ] 3 Grandit: Intermediate member of mixed batch grossular - andradite
• Plazolith: Intermediate member of mixed batch grossularite katoite

Also included in this group

• Goldmanit (calcium vanadium garnet): Ca3V2 [ SiO4 ] 3
• YAG ( yttrium -aluminum-garnet synthetic ): Y3Al2 [ AlO 4 ] 3

• Schorlomit (calcium - iron -titanium garnet): Ca3Ti24 [ Fe23 SiO12 ]
• Morimotoit (calcium iron2 titanium garnet): Ca3Ti4 Fe2 [ SiO4 ] 3
• Kimzeyit ( calcium-zirconium - aluminum-titanium - garnet): Ca3Zr2 [ Al2SiO12 ]

• Hibschite: Ca3Al2 [( SiO4) > 1.5 ( (OH ) 4) <1.5 ], does not count as a separate mineral but as an intermediate solid solution series grossularite katoite.
• Katoite: Ca3Al2 [( SiO4) > 1.5 ( (OH ) 4) <1.5 ]
• Hydroandradit: Ca3Fe23 [( SiO4) > 1.5 ( (OH ) 4) <1.5 ], does not count as a separate mineral but as a variety of andradite.
• Henritermierit: Ca3 ( Mn3 , Al) 2 [( SiO 4 ) 2 ( OH ) 4]

Is also included in this group of wadalite: Ca6Al5 [ O8 | ( SiO4) 2Cl3 ]

• Blythit (manganese garnet): MN32 MN23 [ SiO4 ] 3, not approved mineral, as purely hypothetical end member of the garnet group as Mn3 analogue of spessartine.
• Khoharit (magnesium - iron - garnet): Fe3 - analogue of pyrope and purely hypothetical end member, therefore not a recognized mineral.
• Skiagit: Fe2 analogue of andradite or Fe3 - analogue of almandine and purely hypothetical end member kbar crystallized only at pressures in excess of 100, so not a recognized mineral.

Modifications and varieties

• Achtaragdit (also Achtarandit, English Akhtaragdit. ): Pseudomorphs of grossularite katoite mixed crystals ( Hydrogrossular ) to mayenite and possibly also of Hibschite after wadalite from Wiljui in Russia. Achtaragdit found mostly in the form of tetrahedral or triakistetraedischer crystals of whitish-gray to gray- brown in color.
• Bredbergit (after James Dwight Dana, 1900): Obsolete and no longer common name for a magnesium-rich andradite - variety
• Demantoid ( by Nils Nordenskiöld, 1870 ): Due to foreign admixtures green colored andradite - variety
• Melanite ( by Abraham Gottlob Werner, 1799): Expected to be titanium rich variety of andradite and μέλας was named after the Greek word for black, since it occurs predominantly in gray-black to jet-black crystals or coarse aggregates.
• Topazolith ( according to PC Bonvoisin, 1806): Light yellow, " topas -like" andradite variety, which was first discovered in the Valle di Lanza, in the Italian region Piedmont
• Xalostocit: Identification of a dense intergrowth of translucent pink Grossularen with white marble, which was named in the Mexican state of Morelos after the site Xalostoc.

Education and Locations

Garnets are found in massive form or grainy, but also often as macroscopic crystals, which can be up to 700 kg.

Particularly frequently finds grenade in metamorphic rocks such as gneiss, mica schist or eclogite; next they occur ( beach sediments, river sediments ) in igneous rocks and sedimentary in heavy mineral soaps. Most naturally found gem garnets today come from the U.S., from South Africa and Sri Lanka.

The exact chemical composition is always with that of the surrounding rock in the context: for example, comes the magnesium-rich pyrope peridotites and serpentinites often before, during green Uvarovite occurs mainly in chromium-containing Serpentinite.

Metapelites ( mica schist, gneiss )

In the metamorphosis of siliceous pelites to almandinreiche Garnets are from about 450 ° C during the reaction chloritoid biotite H2O of garnet to chlorite . At low temperatures, the garnet solid solutions are rich in spessartine and are increasingly almandinhaltiger with increasing temperatures. From about 600 ° C. Garnet is formed in the breakdown of staurolite. With rising temperatures, the garnets are increasingly richer in pyrope and even at the onset of molten rock shell can be re-formed, for example, in the reaction of biotite sillimanite plagioclase quartz K-feldspar garnet to melt. It was not until temperatures of 900 ° C, garnet breaks down to spinel quartz or at high pressures to orthopyroxene sillimanite.

Metabasites ( granulite, eclogite )

In the Suite of the metabasites (eg metamorphic basalts ) delivers rock-forming garnet in eclogites and garnet on mixed crystals are rich in pyrope and grossular.

With increasing pressure, garnet forms the transition from granulite facies to eclogite facies from ca 10kBar, 900 ° C in the reaction of orthopyroxene and plagioclase to garnet, clinopyroxene and quartz. In blueschists form first Fe - rich garnets are pyrope - and grossular - rich on the way to eclogite - facies increasingly.

Decomposition of garnet ( Retrograde conversions)

Grenade suffer under certain circumstances lithofaziellen within metamorphic rocks a transformation or decomposition. The result of these processes is called Kelyphit. This creates a number of new minerals.

Crystal structure

Garnets crystallize in general with cubic symmetry in the space group Ia3d. The unit cell contains eight formula units and has, depending on the composition of an edge length of 1.146 nm ( pyrope ) to 1,256 nm ( katoite ).

Structure of the garnet structure

O2 - anion

The oxygen anions occupy the general grid position 96h with point symmetry 1 Each O2 - anion is surrounded by four cations there:

• A T- cation, associated with a strong, predominantly covalent bond with about 1 valence bond
• A G- cation to which there is a predominantly ionic bond with approximately 0.5 vu ( Valenzeinheiten )
• Two e- cations, to which only weak ionic bonds with about 0.25 vu exist.

The oxygens form not, as in other oxide structures with a high density, a closest packing of spheres. Large 8-fold coordinated ions would find no place in a cubic or hexagonal close -packed oxygen. Due to the complex combination of all coordination polyhedra shared corners and especially many common edges, the garnet structure yet reached a high density.

The cations occupied depending on the size and charge of three different, specific grid positions where they are surrounded by 4, 6 or 8 oxygens.

TO4 tetrahedra

The T- cations ( Si4 ) sit on the lattice position 24d with the point symmetry 4, where they are surrounded by four oxygen ions at the corners of a tetrahedron.

The calculated TO bond lengths are between 0.163 nm ( pyrope, almandine ) and 0.165 nm ( Goldmanit ). The TO4 tetrahedral coordination have two pairs of different length edges:

• Two opposite edges that are adjacent to any occupied lattice position with lengths between 0.274 nm ( almandine, andradite ) and 0.276 nm ( Goldmanit )
• Two likewise opposing Did bordering EO8 dodecahedron with lengths between 0.250 nm ( pyrope, almandine ) and 0.258 nm ( Uvarovite, Goldmanit ).

GO6 octahedra

The G- cations sit on the lattice position 16a with the point symmetry 3, where they are surrounded by six oxygen ions at the corners of an octahedron. The determined GO bond lengths are between 0.19 nm ( pyrope ) and 0.20 nm ( andradite ). The GO6 - coordination polyhedron has two different edges:

• 6 edges that are adjacent to any occupied lattice position with lengths between 0,262 nm ( pyrope ) and 0.289 nm ( Andratit )
• 6 edges that are adjacent to the e- position with lengths between 0,269 nm ( grossular ) and 0,283 nm ( andradite )

EO8 dodecahedron

The e- cations sit on the lattice position 24c with the point symmetry 222, where they are surrounded by 8 Sauerstoffern, at the corners of a dodecahedron ( trigondodecahedron ) lie. The measured EO bond lengths are between 0.22 nm ( pyrope ) and 0.25 nm ( andradite, Glodmanit ). The EO8 - coordination polyhedron has 4 different edges:

• 2 edges that border on neighboring TO4 tetrahedron with lengths between 0.250 nm ( pyrope, almandine ) and 0.258 nm ( Uvarovite, Goldmanit ).
• 4 edges that border on neighboring GO6 octahedra with lengths between 0,269 nm ( grossular ) and 0,283 nm ( andradite )
• 4 edges that border on neighboring EO8 dodecahedron with lengths between 0.27 nm ( pyrope ) and 0.297 nm ( grossular )
• 8 edges that are adjacent to any occupied lattice position with lengths between 0.278 nm ( pyrope ) and 0.287 nm ( Grossular, Goldamnit )

Linking of coordination polyhedra

The TO4 tetrahedra and octahedra GO6 are linked via common oxygen atoms at their corners to form a framework of alternating tetrahedra and octahedra. Garnets are island silicates and their TO4 tetrahedra are not directly interconnected.

The EO8 dodecahedron are linked via common edges to 3R whose plane is perpendicular to the body diagonal of the unit cell. This EO8 - Dodekaederringe are mutually linked to form a skeleton so that each dodecahedron is one of two such rings. About more Did the dodecahedron with the tetrahedra and octahedra of TO4 GO6 skeleton are connected, the interstices of which it fills.

Lowering of the symmetry

In particular, grossular andradite - mixed crystals are weakly birefringent and optically biaxial. Even with almandine optical anisotropy was boabachtet. The optical properties are very sensitive indicators of deviations from the ideal cubic structure. In X-ray studies of grenades they could, however, are seldom found. Some works result for garnets triclinic (I -1) or ortorhombische ( Fddd ), but also tetragonal ( I41/acd ) or monoclinic ( C2 / m) symmetry. The causes of this symmetry lowering numerous causes are mentioned:

• Plastic deformation
• Lowering of the symmetry by lattice strain
• Magneto-optical effects by incorporation of rare earth elements
• Different distribution (order) of cations on octahedral then different
• Lowering of the symmetry by parent incorporation of OH groups

Use

As abrasives

Garnet is used for its hardness as abrasive in sandblasting and waterjet cutting.

In technology

In particular, artificially produced crystals with garnet structure are used in precision and optical instruments. In contrast to the natural minerals other elements are built here in the tetrahedral space instead of silicon often. Yttrium-aluminum- garnet (YAG, Y3Al2 [ Al O4 ] 3 ), in which about one percent of Yttrium3 ion is replaced by Neodym3 ions, is a frequently used laser crystal (Nd: YAG ) laser. The yellow luminescence converter of white LEDs has been a cerium - doped YAG at the beginning of development. Yttrium iron garnet ( YIG) and relatives are used as microwave ferrite, resonator or filter in the high frequency technology.

As a gemstone

Grenade found in different variations as gemstones use. One distinction the dark red pyrope, which is also called Kaprubin, the red and black almandine, the emerald Uvarovite, yellow-green andradite, the black Schorlomit and Melanite, the transparent -green demantoid and the orange-red spessartite. There are also grossular. There are also some years a new variant, the orange mandarin garnet. Garnets are also called gems of the little man.

"Star of Idaho ", the largest six -armed star garnet

Pink almandine - pyrope solid solution

Brooch with green demantoid

Pendant Uvarovite, about 2 cm long