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Feldspar: Fältspat, Feldespato, Feldspat, Feldspath, Feldspato, Feltspat, Földpátok, Lauko špatas, Laukšpats, Maasälpä, Päevakivi, Skaleń, Skupina živca, Veldspaat, Živec, مجموعة الفلدسبار, فلدسپات, 長石, 장석, 长石, פצלת השדה, Полевые шпаты, Фелдспат.
The feldspars are a group of minerals (aluminium silicate) with a variable proportion of potassium, sodium and calcium. Certain rarer members of this series contain barium, strontium, ammonia, etc...
The feldspar group constitues almost half of the composition of the earth’s crust.
Aside from Sanidine and Celsian, which are generally only of interest to scientists and collectors, feldspar has an important industrial role. Orthose and the Plagioclases feldspars, mixed with quartz and kaolin, are used in the fabrication of ceramics, porceline and refractory products.
Most of the feldspars crystallize in monoclinic and triclinic forms.
Feldspar is divided into two groups which are distinguished by their structure and chemistry :

-  Potassic Feldspar: Orthose, Sanidine, Microcline. These feldspars have the same chemical composition, but different structures and thus are considered different minerals.

-  Plagioclase Feldspar: Albite, Oligoclase, Andesine:, Labradorite, Bytownite, Anorthite. This series is classified by the chemical composition of the mineral based on the sodium and calcium content. The two extremes are Albite (NaAlSi3O8) which is sodium rich and Anorthite (CaAl2Si2O8) which is calcium rich:

Nom % de sodium and calcium % d’ (Ab) et d’ Anorthite (An)
Albite(Na 100%, Ca 0%) AlSi3O890-100% Ab; 0-10% An
Oligoclase(Na 90%,Ca 10%) Al1-2Si3-2O870-90% Ab; 10-30% An
Andesine(Na 70%, Ca 30%) Al1-2Si3-2O850-70% Ab; 30-50% An
Labradorite(Na 30%, Ca 70%) Al1-2Si3-2O830-50% Ab; 70-50% An
Bytownite(Na 10%, Ca 90%) Al1-2Si3-2O810-30% Ab; 70-90% An
Anorthite(Na 0%, Ca 100%) Al2Si2O80-10% Ab; 90-100% An

Other, more rare feldspars that we haven’t covered here do exist.

Ferro-axinite:axinite ©gemfrance.com
Mineral species belonging to the Axinite group. Discovered by Schaller in 1909 at St Christophe in Oisans - Isere - France.
See Gallery

Fire Opale: SiO2,nbH2O Opale de feu ©gemfrance.com
From ICA website: Fire opals are unique in the lush world of the opals. They were already admired as symbols of the most fervent love in ancient times, in India and in the ancient Persian kingdom, and among the people of Central America and the Amerindians. It was believed that a gem, that bubbled over with vivacity to such an extent as the fire opal, could only have been created in the waters of paradise. The Mayas and Aztecs loved this gemstone and liked to use it in mosaics and for ritualistic purposes. They called it quetzalitzlipyollitli, the ’stone of the bird of paradise’. Yet, one day, the gemstone knowledge of Mexico’s natives, which had been handed down from generation to generation, somehow sank into oblivion for a long time ... ... until, in or around the year 1835, the fiery treasures hidden in the Mexican highlands were remembered, and work was gradually begun on the systematic mining of the places where they had been found. Today, the fire opal is regarded as the national gemstone of that country.

It is in Mexico that the most significant fire opal deposits in the world lie. Rock strata containing opals run through the Mexican highlands, with their many extinct volcanoes. With few exceptions, the gemstone, which lies hidden in cavities and crevices, is extracted in open-cast mines, the work giving rise to impressive canyons with walls up to 60 metres high and labyrinthine passages which wind their way through the mining areas. _ To read more->
See the gallery->

Fluorite: CaF2 fluorite ©gemfrance.com
From the Latin « fluere » = to flow, because of its relatively low melting temperature. It was used as a flux in the smelting of ore.
Fluorite or "Fluorine" in French, is the most common natural fluoride. fluorite ©gemfrance.com
This mineral species has long been regarded with wonderment due to its wide variety of colors. It had several names (Spatum vitreum , Calx fluorata...) before being called Fluorine, or Fluorite. (Following today’s common usage, all gemstone names, with few exceptions, have replaced the suffix « ine » with the suffix « ite »).
The commercial and industrial term for Fluorite is Fluorspar...
Its simple crystal structure was determined using X-ray diffraction by William H. Bragg.
Fluorite is an attractive gemstone for collectors with a large range of colors such as red, pink, yellow, green, blue, violet...

Forsterite フォルステライト Mg2 SiO4
The Forsterite is the magnesium rich end-member of the olivine solid solution series.
The date of the discovery is 1824.
See the gallery->

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