Fayalite

Fayalite is a rarely occurring mineral from the mineral class of " silicate and germanate ". It crystallizes in the orthorhombic crystal system with the chemical composition FE22 [ SiO 4 ] and developed mostly tabular to prismatic crystals, but also granular to massive aggregates of pale yellow, greenish yellow, yellow-brown and red-brown to black in color.

Fayalite forms with forsterite and with tephroite a complete mixed batch, the intermediate members are called olivine.

Special Features

In nature, fayalite is rarely found in pure form, but almost always with fluctuating levels of forsterite and / or tephroite. Color-giving are in fayalite ( brown to black) predominant iron ions or in tephroite (gray, red) predominant manganese ions. Depending on the percentage proportion of forsterite, which is colorless in the pure state, the colors of fayalite be weakened.

Etymology and history

Was first found the fayalite 1840 on the Ilha do Faial Island ( old spelling Fayal ), one belonging to Portugal Azores island, and described by Christian Gottlob Gmelin, who named the mineral after its type locality.

Classification

In the old ( 8th edition ) and new classification of minerals according to Strunz ( 9th edition ) is one of the fayalite 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 fayalite is in accordance with the division of the " island silicates without additional anions; with cations in octahedral 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 fayalite similar to the new Strunz'sche Mineral classification in the department of " island silicates: SiO4 groups with all cations only in octahedral 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

• Hortonolith and knebelite are names for a manganese-containing fayalite.

Education and Locations

Fayalite formed in ultramafic volcanic and plutonic rocks. In paragenesis, where he appeared with a number of minerals such as, among others, almandine, various minerals of the amphibole, apatite, Arfvedsonite, augite, grunerite, hedenbergite, ilmenite, magnetite, microcline, plagioclase, quartz, sanidine, spinel and tridymite.

So far fayalite was at 250 localities (as of 2010) are detected. Apart from its type locality Ilha do Faial fayalite in Portugal was still found on Pico and the Agua de Pau on São Miguel.

Other localities are Algeria, Egypt, Antarctica, Ethiopia, Australia, Brazil, Chile, China, Germany, Eritrea, Finland, France, French Polynesia, Greenland, India, Indonesia, Ireland, Israel, Italy, Japan, Cameroon, Canada, Madagascar, Morocco, Mexico, Mongolia, Myanmar, Namibia, New Zealand, Netherlands, Nigeria, Norway, Oman, Austria, Romania, Russia, Saudi Arabia, Sweden, Switzerland, Slovakia, Spain, St. Lucia, South Africa, Tajikistan, Czech Republic, Ukraine, Hungary, United Kingdom ( UK) and the United States (USA).

Crystal structure

Fayalite crystallizes in the orthorhombic space group Pbnm with lattice parameters a = 4.82 Å; b = 10.48 Å and c = 6.09 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.

To enhance weakly colored stones, is sometimes placed below 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 fayalite 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.

In high-temperature experiments

Fayalite has the property of reacting reversibly with oxygen at high temperatures:

This reaction can be utilized to adjust at high temperature experiments, a defined partial pressure and a defined fugacity of oxygen. The system is also called fmq buffer ( Fayalit-Magnetit/Quarz-Puffer ).