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The feldspar group is tectosilicates of the aluminosilicate type containing alkali and alkaline earth elements (some also ammonia cations as pseudo-alkali cation) in various proportions. The pure single-alkali/alkaline earth cation forms are microcline and orthoclase (KAlSi3O8, both with the same chemical formula but different elementary cell (crystal structure) microcline triclinic pinacoidal and orthoclase monoclinic prismatic), albite (NaAlSi2O8), and anorthite (CaAl2Si2O8).
The theoretical end-member composition of the feldspar groups is shown in Table 1.
Due to the fact that the species are miscible (according to the phase diagram), commercial feldspars are mixtures of the pure mineralogical species, and the composition depends on the genetic conditions of the deposit they are sourced from. Beneficiation can be as easy as mining and crushing up to complex processes like flotation or electrostatic separation.
Table 1 Theoretical end-member composition of the feldspar groups and nepheline (Carr 1994)
Table 2 Typical data of the pure mineral species (Modified from Robinson (2003))
Nepheline is also a mineral of the tectosilicates group with the stoichiometric formula (Na,K)AlSiO4. Nepheline syenite is a rock formed from nepheline and feldspars. Other than typical feldspars, it is formed with silica deficit and so no free crystalline silica occurs in the rock. Typically, the beneficiation consists of crushing and density and magnetic separation.
A few typical data of the pure mineral species are listed in Table 2.
Natural feldspars and nepheline syenites are highly variable in color. The purest ones typically are white and highly transparent and their streak color is white. Most common impurities deteriorating the color are iron oxides tinting them grayish to brownish.
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