The chemical formula of the mineral Fayalite is indicated by (Fe,Mg)2SiO4 or Iron Magnesium Silicate. Fayalite is actually a Silicate mineral. Fayalite, along with forsterite, belongs to the group called olivine. Mineral Fayalite is known as the iron rich end member of the said group while forsterite is known as the magnesium rich end member. These two minerals form a series where magnesium and iron can actually substitute each other in the structure without too much effect on the crystal structure. Mineral Fayalite also forms a series with another interesting mineral called tephroite. This mineral species on the other hand is known as the manganese rich end member of the series. This new tephroite mineral is actually a member of the more inclusive Olivine Group. More often, Olivine Group of minerals is only set limited to just Fayalite and forsterite. But then, most mineralogists include all silicates having similar structure as olivine just like tephroite mineral. The name of the mineral species Fayalite is derived from the type of locality the Faial Island of the Azores in Portugal. The mineral was actually first discovered from this type of locality in 1940. The name of the mineral was actually given by Gmelin in the same year. Mineral Fayalite is known to crystallize in the orthorhombic system of crystal formation. In optical mineralogy field of study, this orthorhombic crystal system comprises crystals having three mutually perpendicular axes, of which all are of different lengths.

 

Mineral Fayalite is used as refractory sand. It can be also used as abrasive. And since it has the capacity to display a wonderful brilliance under gemological microscope, it is also used as a gemstone. But Fayalite is most commonly used as mineral specimens for it has the capability to show off interesting and fascinating microscope images under polarizing light microscopes used in optical mineralogy field of study. Due to the iron content of mineral Fayalite, it is found having higher index of refraction. Fayalite is also found exhibiting darker color when viewed with the aid of petrographic polarizing light microscopes for mineralogists. Fayalite is also found heavier than forsterite. If the two minerals were to be evaluated otherwise, it would be difficult to distinguish one from the other. And virtually, all specimens of Fayalite and forsterite do contain both magnesium and iron. But these two minerals are often treated and termed as one mineral named olivine. This is done for the reason of simplicity as well as general public recognition.

 

            However, Olivine is not really an officially recognized mineral. One gemstone variety of Olivine is known as peridot and this stone usually exhibits a fascinating and interesting appearance when viewed under gemological polarizing microscopes. Most peridot is actually forsterite because they are found as magnesium rich mineral variety. Unfortunately, they are not Fayalite. But then Fayalite mineral, with its high iron content, would always be less attractive when evaluated with the aid of any polarized light microscopes. And because of this, mineral Fayalite is not commonly used for gemstone purposes. Most Fayalite crystals are found transparent to translucent in appearance. The crystal habit of the mineral Fayalite as described in optical mineralogy usually includes flatten tabular to box shaped crystals that can be very interesting and wonderful when viewed with the aid of petrographic polarizing light microscopes. Good Fayalite crystals are usually rare. Fayalite is most commonly found as grains in alluvial gravels and also as granular pockets in volcanic rocks having high magnesium and iron content. Fayalite crystals can be also found massive.

 

            Fayalite minerals are most commonly found in the igneous rocks with high iron and magnesium content. Olivine minerals are commonly found having high melting point. The first Olivine minerals to crystallize from a mafic magma are those having high melting points. And it has been found that forsterite crystallizes first, followed by Fayalite and the rest are just about to begin their formation. The reason why molten lavas are most often found having crystallized grains of olivine is the early crystallization of olivine itself. Sometimes, some ultramafic rocks can be found composed of almost all olivine crystals. And these are termed dunites or peridotites crystals that can be very fascinating when viewed with the aid of petrographic polarizing light microscopes. Olivine can be also found present in some marbles that are formed from the metamorphosed impure limestones.

 

            Mineral Fayalite is commonly found exhibiting yellowish green to greenish brown to black color that could appear more interesting when viewed with the aid of petrographic polarizing microscope used in the field of optical mineralogy. The lighter green color usually indicates high magnesium content and this specimen is commonly considered as olivine or even forsterite that shows interesting appearance under polarized microscopes used in optical mineralogy. Fayalite is most commonly found exhibiting vitreous luster when mineral specimen is viewed in reflected light of geological polarizing microscope for mineralogists. Fayalite is also usually found exhibiting distinct cleavage found in two directions at 90 degrees when mineral specimen is evaluated with the aid of polarized light microscope for mineralogists. Fayalite also exhibits conchoidal fracture when it is examined under some minor adjustments on the aperture diaphragm of the polarizing microscope for mineralogists. The specific gravity measure of the mineral usually gives an approximate value of 4.3 grams per cubic centimeters, which is above average for non-metallic minerals. The hardness measure of mineral Fayalite when it is evaluated using the Mohs scale method is usually ranging from 6.5 to 7. Fayalite is commonly found leaving a white streak when the mineral is rubbed on a white porcelain streak plate.

 

            Mineral Fayalite is most commonly found showing biaxial negative figure when specimen is viewed between crossed nicols of petrographic polarizing microscope used in optical mineralogy. Fayalite actually exhibits a moderate double refraction when viewed between crossed nicols of polarizing light microscope. The maximum birefringence of the mineral is most commonly found ranging from 0.042 to 0.051 when the mineral is viewed plane-polarized light of petrographic polarizing light microscopes for mineralogists. Fayalite is also usually found exhibiting very high surface relief when mineral is evaluated in plane light of polarized microscope for mineralogists. Fayalite also most commonly found showing weak dispersion when viewed with the aid of polarized microscope for geologists. The index of refraction of mineral Fayalite when it is evaluated in plane light of polarized microscope is usually found as 1.87. The crystals of Fayalite are typically striated and it can be seen clearly visible when viewed under petrographic polarizing light microscopes. There is no specific data found on the toxicity and health dangers for this mineral Fayalite. However, the specimens of this mineral should be treated with great care and use of sensible precautions is advised upon handling them. Fayalite is a non-radioactive mineral.

 

            Mineral Fayalite is most commonly found associated with several other interesting minerals like augite, spinel, hornblende, iron-nickel meteorites, diopside, chromite, serpentine, biotite, plagioclase, obsidian, anorthite, cristobalite, and feldspars. The best field indicators of mineral Fayalite usually include color, environment formation, density, crystal habit and hardness. Fayalite is commonly found in ultramafic igneous rocks. The word mafic is used to describe an igneous rock containing high amount of manganese and iron. The MA stands for the magnesium while F stands for ferrum, which is a Latin word for iron. Many iron-nickel meteorites are found containing Fayalite crystals. And they are commonly found in large crystals that almost occupy over 50 percent of the volume of the meteorite. Fayalite mineral notably occurs at several famous mineral localities including Lipari Islands in Sicili and Mt Vesuvius in Italy, the Salt Lake Crater in Oahu, Hawaii, the Sugarloaf Mountain in the Inyo County, California as well as the Peridot, Gila County in Arizona, USA. It can be also found in Sweden, France and in many other countries of types of localities.