Feldspar

The name feldspar derives from the German words Feld, "field", and Spath, "a rock that does not contain ore." Feldspars are the most abundant mineral in the Earth's crust, making up more than 60% of both the continental crust and the oceanic crust. The feldspars are the most common minerals in the Earth's crust. They consist of three endmembers:

KAlSi3O8 - Orthoclase (or), NaAlSi3O8 - Albite (ab), and CaAl2Si2O8 - Anorthite (an)

KAlSi3O8 and NaAlSi3O8 form a complete solid solution series, known as the alkali feldspars and NaAlSi3O8 and CaAl2Si2O8 form a complete solid solution series known as the plagioclase feldspars.

The general formula for the feldspars is XAl(Al,Si)Si2O2, where X is potassium, sodium, calcium, or barium. The Al occupies some of the tetrahedra. All the X ions are big, much larger than iron or magnesium, which are too small to occupy the voids in the structure fully.
The feldspars have a framework structure, consisting of SiO4 tetrahedra sharing all of the corner oxygens. However, in the alkali feldspars 1/4 of the Si4+ ions are replaced by Al3+ and in the plagioclase feldspars 1/4 to 1/2 of the Si4+ ions are replaced by Al3+.

This allows for the cations K+, Na+, and Ca2+ to be substituted into void spaces to maintain charge balance. Compositions of natural feldspars are shown in the diagram below based on the 3 components -NaAlSi3O8, - Albite (ab), KAlSi3O8 - Orthoclase (or) and CaAl2Si2O8.

feldspar.jpg

Compositions of natural feldspars. From Stephen A. Nelson



The plagioclase feldspars are a complete solid solution series between Ab and An, and can contain small amounts of the or component. Names are given to the various ranges of composition:

Albite - ab90 to ab100
Oligoclase - ab70 to ab90
Andesine - ab50 to ab70
Labradorite - ab30 to ab50
Bytownite - ab10 - ab30
Anorthite - ab0 to an10

In thin section, plagioclase commonly shows the characteristic albite polysynthetic twinning. This twinning is the most characteristic identifying feature of plagioclase, and makes its identification easy when present.
Although some cross-hatched twinning may also occur in plagioclase, it is always very simple with only one or two cross twins per grain. Thus, be careful not to identify plagioclase as microcline. The cross-hatched twinning in microcline is always much more complex.

The Alkali Feldspars form a complete solid solution between ab and or, with up to 5% of the an component. The high temperature more K-rich variety is called Sanidine and the more Na-rich variety is called anorthoclase. The potassium feldspars (KAlSi3O8) differ in the ordering of Al-Si tetrahedra. At high temperatures the tetrahedra are randomly mixed (although Al-O-Al links never occur) and the lattices are therefore of high symmetry. They have long range order even if on a local scale the lattice is disordered.

Sanidine is the highest temperature variety and is completely disordered, orthoclase forms below about 800°C and is partially ordered, and microcline forms below about 600°C and is highly ordered. The complete ordering of Al in microcline breaks down the mirror plane symmetry and microcline becomes triclinic.

Bibliography



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc
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