Tephroite

Tephroite, also called manganese peridot, is a rarely occurring mineral from the mineral class of " silicate and germanate ". It crystallizes in the orthorhombic crystal system with the chemical composition Mn2SiO4 and usually develops short prismatic crystals in the centimeter range, but also granular to massive aggregates.

Between the pure Mn ( tephroite ), Mg ( forsterite ) and Fe compounds ( fayalite ) is a complete miscibility, the intermediate members are called Olivine

Special Features

Tephroite gets its reddish brown to flesh- red color by the manganese contained in the compound. In nature, however tephroite is often found in the form of mixed crystals with fluctuating levels of forsterite and / or fayalite. Accordingly, the color varies gray or olive green in direction.

Etymology and history

The forsterite 1823 in the "Sterling Hill Mine " at Ogdensburg in Sussex County (New Jersey) and described was first found by August Breithaupt, of the mineral after the Greek word " tephra " for ash due to its often occurring, ash- gray color.

Classification

In the old ( 8th edition ) and new classification of minerals according to Strunz ( 9th edition ) is one of the tephroite 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 tephroite 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 tephroite 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.

Education and Locations

Tephroite forms in iron-manganese deposits, skarns and related metamorphic, manganese-rich sediments. Accompanying minerals include Alleghanyite, Banalsite, Bustamite, calcite, diopside, Franklinite, Gageit, Glaukochroit, Jakobsit, Manganocalcit, rhodonite, willemite and zincite.

Worldwide, tephroite be proven so far to some 180 localities (as of 2010), as in Antarctica, Australia, Brazil, China, Germany, Finland, France, India, Italy, Japan, Kazakhstan, Kyrgyzstan, Madagascar, Namibia, Norway, Oman, Austria, Russia, Sweden, Switzerland Slovakia, Spain, South Africa, Turkey, the United Arab Emirates, the United Kingdom ( UK ) and the United States (USA).

Crystal structure

Tephroite crystallizes in the orthorhombic space group Pbnm with lattice parameters a = 4.90 Å; b = 10.60 Å and c = 6.26 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.