Feldspar
As feldspar a large group very frequently occurring silicate minerals of the general chemical composition (Ba, Ca, Na, K, NH4) (Al, B, Si) 4O8 is called. The elements in brackets can be represented each other, but are always in the same proportions to the other components of the mineral (substitution). Feldspars crystallize in either the monoclinic or triclinic crystal system in, have an average Mohs hardness of 6 to 6 ½ and a highly variable color, from colorless through white, pink, green, blue to brown enough. The stroke color is white.
Feldspars are considered the main rock-forming minerals of the earth's crust. The melting temperature range is 1150-1250 ° C.
Etymology
Feldspar is derived from the German words field and Spath. Spath is " a stone that does not contain ore ". As a plural form of both feldspars and feldspar in the literature can be found.
Classification
Starting from the end members orthoclase, albite and anorthite can the feldspars into three different groups:
Alkali feldspars
Alkali feldspars are mixed crystals of the Ab -Or - solid solution series with end members albite ( NaAlSi3O8 ) and potassium feldspar ( KAlSi3O8 ) and orthoclase / microcline. They form the mixed crystals of anorthoclase, Na - sanidine and sanidine and have a high content of potassium and sodium. However, they are stable only miscible at high temperatures. On cooling it comes to separation, which are manifested in sodium-rich lamellae in K-feldspar ( Perthit ) or in potassium-rich lamellae in albite ( antiperthite ). The procedure itself is called perthitische segregation.
Plagioclase
Plagioclase (also Kalknatronfeldspate ), crystals of the Ab -An- solid solution series with end members albite ( NaAlSi3O8 ) and anorthite ( CaAl2Si2O8 ), on the other hand are characterized by a high content of calcium and sodium. As can be seen macroscopically no differences, the mixed crystals are divided according to the proportion of anorthite: albite ( 0-10%), oligoclase (10-30%), andesine ( 30-50%), labradorite (50-70 %), Bytownite (70-90 %), and anorthite (90-100 %). This series of mixed crystals of plagioclase also has temperature-dependent segregation, but these are less pronounced than in the alkali feldspars and show themselves only in microscopically thin lamellae in the crystal. It has three miscibility gaps, which again divided according to the proportion of anorthite and as Peristerit segregation ( 2-16 %), Bøggild segregation ( 47-58 %) and Huttenlocher segregation ( 67-90 % ), respectively.
Ternary feldspars
Ternary feldspars lie inside the triangle from K-feldspar- albite - anorthite with a but with decreasing temperatures increasingly large miscibility gap.
Individual minerals, varieties and modifications
The designation of a single mineral can also be done by percentages: For example, does one a on-off - mixed crystal of 60 % albite and 40% anorthite with Ab60An40 or only briefly AB60 or AN40 and is referred to him because of this composition as andesine.
- Albite NaAlSi3O8 - triclinic Pericline, Cleavelandit
- Maskelynit
- Peristerit, Aventurine Feldspar ( Sunstone )
- Adularia, moonstone
Education and Locations
Feldspars occur most commonly in the form of columnar or tafeliger, often twinned crystals and are found in both igneous and metamorphic and sedimentary rocks in.
Which Feldspattyp be found in a given igneous rock depends on the chemical composition and temperature of the initial melt: Kalifeldspate form at relatively low temperatures in siliziumdioxidreichen magmas that form the rocks of granite and rhyolite during cooling. If the melt, however, rather poor in silica and crystallized at relatively high temperature, so arise plagioclase, which then can be found in rocks such as gabbro or basalt.
By natural weathering results from feldspar clay ( soil ):
The resultant is a clay mineral.
Use
Some varieties of feldspar like Labradorite or orthoclase find suitable quality as a gemstone use.
Feldspar is next to kaolin and quartz is an important ingredient in the manufacture of porcelain.
Especially pure feldspar is used as a starting material for dental restorations (inlays, veneers ). The mineral rock is crushed, cleaned and fired. Chemical processes do not play a role. The pottery from industrially produced is very similar in physical properties of the natural tooth structure, making them fits perfectly into the tooth.
In the manufacture of stoneware tiles feldspar is used to sinter the glaze in one operation. Even before the burning of the sherds the glaze is applied and oven heated to about 1350 ° C. The feldspar contained in shards melts and provides not only strength and integrity of the work and to the integration of the glaze on the finished tile.