Forsterite
Forsterite is a commonly occurring mineral from the mineral class of " silicate and germanate ". It crystallizes in the orthorhombic crystal system with the chemical composition of Mg 2 [ SiO 4 ] and developed to mainly tabular prismatic crystals, as well as granular aggregates.
Forsterite forms with fayalite and with tephroite a complete mixed batch, the intermediate members are called olivine.
Special Features
Pure forsterite is grayish white or colorless by lattice defects or impurities. In nature, forsterite is only rarely found in pure form, but almost always with fluctuating levels of fayalite and / or tephroite. So color-giving are in the fayalite ( brown to black) predominant iron ions or in tephroite (gray, red) predominant manganese ions, which the forsterite with an increasing percentage of its light green to yellow green and give brown -green to black -green color.
Etymology and history
Was first found in 1824 on the forsterite Monte Somma in Italy and described by Armand Lévy, who named the mineral after Adolarius Jacob Forster ( 1739-1806 ), an English mineral collector and dealer.
Classification
In the old ( 8th edition ) and new classification of minerals according to Strunz ( 9th Edition ) belongs to the forsterite to the Department of " island silicates ( nesosilicates ) " and there to the olivine group, formed from the minerals fayalite, forsterite, Laihunit, Liebenbergit and tephroite.
The revised 9th edition of the Strunz'schen Mineral classification divides this department, however, precisely according to the presence or absence of other anions and the coordination of the cations involved. The forsterite is in accordance with the division of the " island silicates without additional anions; with cations in oktahedraler coordination " and there is still a member of the olivine group, but it has been expanded to include the minerals Glaukochroit and Kirschsteinite.
The commonly used in English-speaking classification of minerals according to Dana assigns the forsterite similar to the new Strunz'sche Mineral classification in the department of " island silicates: SiO4 groups with all cations only in oktahedraler coordination ". The there also encountered olivine is like in the old Strunz'schen systematics of the members fayalite, forsterite, Laihunit, Liebenbergit and tephroite, but extended to the olivine, for the recognition by the IMA / CNMNC still missing.
Modifications and varieties
The connection Mg2 [ SiO4 ] is trimorph, so come next to the orthorhombic forsterite nor as also orthorhombic, albeit with a different space group and other cell parameters, crystallizing ringwoodite ( γ - (Mg, Fe ) 2 [ SiO4 ] ) and as a cubic crystallizing wadsleyite ( β - (Mg, Fe ) 2 [ SiO4 ] ) before.
Education and Locations
Forsterite is formed in mafic -ultramafic igneous and metamorphic converted into, dolomitic limestone. In paragenesis, where he appeared with a number of minerals such as, among others, calcite, chromite, dolomite, enstatite, corundum, magnetite, phlogopite, plagioclase and spinel.
So far forsterite at more than 570 localities (as of 2009) are detected. Apart from its type locality Monte Somma was forsterite in Italy nor in the Vulture area, as well as in Apulia, Lazio, Liguria, Lombardy, Piedmont, found in Sicily, Trentino Alto Adige, Tuscany and Umbria.
Other localities are Algeria, Angola, Antarctica, Ethiopia, Australia, Bolivia, Botswana, Brazil, Bulgaria, China, Germany, Finland, France, Greece, Greenland, India, Indonesia, Iraq, Japan, Kazakhstan, Canada, Colombia, Madagascar, Mexico, Myanmar, Namibia, New Caledonia, New Zealand, North Korea, Norway, Austria, Poland, Russia, Scotland ( UK ), Sweden, Switzerland, Slovakia, Spain, South Africa, Tajikistan, Tanzania, Turkey, Hungary, Ukraine and the USA.
In April 2011, a U.S. team of researchers reported the discovery of Forsteritkristallen in the proto -stellar cloud of the protostar HOPS -68 using the Spitzer Space Telescope. Scientists believe that the first amorphous material is annealed close to the protostar and thereby crystallized before it is transported by transport processes in the cooler outer region of the dust shell. In other cosmic environments forsterite was detected by infrared spectroscopy - for example, in several comets (including Halley's Comet, Comet Hale- Bopp ), in the dust shells pulsating red giant, in planetary nebulae and in protoplanetary disks (ie resulting planetary systems ).
Crystal structure
Forsterite crystallizes in the orthorhombic space group Pbnm with the values determined from the year 2007 in several measurements lattice parameters a = 4.80 Å; b = 10.35 Å and c = 6.06 Å and four formula units per unit cell.
Use as a gemstone
The minerals of the olivine group are mainly processed to semi-precious stones with good quality. Clear varieties usually get this one facet cut in different shape, cloudy varieties rather a cabochon cut. Commercially they are under the name " Peridot " or " chrysolite " available.
Likelihood of confusion because of the color especially with beryl, chrysoberyl, demantoid, diopside, prasiolite, prehnite, Sinhalit, emerald, tourmaline and vesuvianite.
Manipulation and imitation
To enhance weakly colored stones are occasionally placed them in a ring or pendant frames a green film. Also imitations of colored glass or synthetic corundum and spinel are issued by rogue traders as peridot. In contrast to these, the forsterite or its solid solutions in the high birefringence can be seen, which is seen in the view by thicker, faceted stones in the doubling of the bottom edges of the facets without motion.