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The chemical formula of mineral Nepheline is indicated by (Na, K)AlSiO4 or Sodium Potassium Aluminum Silicate. This Nepheline is actually a Silicate mineral. It is also a member of the Feldspathoid Group of minerals. Nepheline is most commonly used as mineral specimen and it is usually found exhibiting nice and interesting microscope appearance when viewed with the aid of petrographic polarizing light microscope used in optical mineralogy. Nepheline is also used as a raw material for special kinds of glass and ceramics. Nepheline is also known as a major rock forming mineral that is not often sold in rock shops due to a lack of good crystals or attractive specimens. In optical mineralogy, Nepheline is known as a major component of several igneous rocks called Nepheline syenite, Nepheline monzonite, and nephellinite. The basic difference between these is in the amount and types of feldspars present. In Nepheline syenite potassium feldspars or K-spars are the predominant feldspar. In the Nepheline monzonite rocks both k-spars and plagioclase feldspars are present in near equal proportions. And finally in the nephellinites there is little of any of the feldspars present and the rock is mostly Nepheline. There are actually a very few natural nephelines that have pure chemistry indicated by NaAlSiO4 although it produces a stable structure and it is manufactured for use in ceramics and glass production. Nepheline is also a known member of the feldspathoid group of minerals. In optical mineralogy, feldspathoids are minerals whose chemistries are close to that of the alkali feldspars but are poor in silica content. And as a function of the fact, they can be found in silica poor rocks containing silica poor minerals and no quartz. It has been found in optical mineralogy that if quartz were present when the melt was crystallizing, it would react with any feldspathoids and form a feldspar. There are few localities with feldspathoids.

 

            Nepheline is known to be very reactive to acids although it does not bubble like many of the carbonates. Is Nepheline is powdered it will dissolve in hydrochloric acid and if clear specimens are dipped in acid they will become cloudy or frosted. This acidity of mineral could be helpful in distinguishing Nepheline from similar looking feldspars, cryolite, and scapolite. The greasy luster of mineral Nepheline is also diagnostic. Massive Nepheline with a greasy luster is given the variety name called eleolite. Eleolite derived its name from the Greek word for oil. Nepheline derived its name from the Greek word for cloud. This is actually an allusion to its cloudy and translucent crystals and masses when viewed with the aid of polarized microscopes used in optical mineralogy.

 

 

            Mineral Nepheline is most commonly found off white to gray or brown and occasionally other tints that could appear more splendidly exhibited when viewed with the aid of petrographic polarizing light microscopes. Nepheline is mostly found greasy to dull in weathered specimens when viewed in reflected light of polarizing microscopes used in the field of optical mineralogy. Nepheline crystals are most commonly found translucent to more rarely transparent in appearance. Nepheline is also known to crystallize in the hexagonal system of crystal formation. In optical mineralogy, the hexagonal system of crystallization comprises crystals having four axes. Three of which are positioned in a single plane with equal length and are symmetrically spaced. The fourth axis is found to be perpendicular to the other three axes. The crystal habit of mineral Nepheline as described in the field of optical mineralogy is usually massive or granular which can be seen more clearly visible when specimen is closely evaluated with the aid of petrographic polarizing microscopes. It can be also found forming some prismatic to columnar crystals that are found with simple hexagonal cross section when examined under geological polarizing microscope.

 

            Mineral Nepheline is rarely seen showing poor prismatic cleavage in three directions when viewed under polarized microscope. The fracture that is most commonly found exhibited by the mineral under polarizing microscope is usually conchoidal to uneven. The hardness measure of mineral Nepheline when it is evaluated using the Mohs scale method is usually ranging from 5.5 to 6. Nepheline is most commonly found leaving a white streak when rubbed on a white porcelain streak plate. The specific gravity measure of mineral Nepheline is 2.6+ grams per cubic centimeters, which is considered average value. The best field indicators of mineral Nepheline include luster, associations, hardness, reaction to acids, and locality. Nepheline is most commonly found associated with other interesting minerals such as albite, biotite, sodalite, apatite, cancrinite, calcite, and some other feldspathoids. Nepheline notably occurs at several localities including Kola Peninsula in Russia, Mount Vesuvius in Italy, as well as Canada and USA.



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Friday, May 15th, 2009 at 3:17 am
Category:
The Silicates Mineral Class
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