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Alkali feldspar series

 

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Alkali Feldspar Series Members (KAlSi3O8- NaAlSi3O8)

Anorthoclase, sanidine, orthoclase, and microcline all have the same composition (K,Na)AlSi3O8.  Except for anorthoclase, potassium (K) typically exceeds the amount of sodium (Na) in each of these minerals.  Anorthoclase, sanidine, orthoclase,and microcline are polymorphs.  Polymorphs have the same chemical composition but different crystal structures.  The difference in crystal structure is reflected in crystal symmetry.  The shifts in crystal symmetry are mainly due to the degree of ordering of aluminum (Al) and silicaon (Si) on two tetrahedral sites in the crystal structure. The degree of ordering of Al and Si, and the extent to which exsolution of K-rich and Na-rich lamellae have occurred are a reflection of the temperature at which the feldspar crystallized and/or cooling history.  Perfect disorder of Al and Si distribution occurs at high temperature and is correlated with low symmetry.  Perfect ordering of Al and Si on the two tetrahedra sites occurs as a result of low temperature crystallization or very slow cooling and is correlated with higher symmetry.

Sanadine and anorthoclase are high temperature alkali feldspars.  Both of these minerals crystallize in the monoclinic crystal system and the  2/m crystal class.  Al and Si are randomly within the crystal structure. Sanidine contains more potassium than anorthoclase, which is sodium-rich.  Sanidine and anorthoclase are found exclusively in volcanic rocks that cooled extremely rapidly.  This rapid cooling, almost quenching, did not allow enough time for diffusion of Al or Si and  K+ and Na+ ions or exsolution to occur.  The monoclinic symmetry stable at high temperature was "locked in" by the quick cooling.

Orthoclase is the alkali feldspar that typically crystallizes at intermediate temperature (lower than sanidine and anorthoclase, and higher than microcline).  Orthoclase crystallizes in the same crystal system and class as sanidine and anorthoclase (monoclinic 2/m) but the distribution of Al and Si in orthoclase is partially ordered in orthoclase due either to the lower temperature or crystallization or to a slower rate of cooling.  Orthoclase typically forms in intrusive rocks that cooled fairly slowly. The slow cooling rate allowed both the ordering of Si and Al on the tetrahedral sites and the exsolution of microscopically visible lamellae to occur.

MicroclineMicrocline is the alkali feldspar that typically crystallizes at low temperature (lower than orthoclase) or as a result of a very slow cooling rate.  Microcline is triclinic and in crystal class 1, indicating that the only symmetry that it has is a center of symmetry).  The distribution of Al and Si in microcline is completely ordered on the two tetrahedral sites and the exsolution lamellae are often large enough to see without a microscope.  Microcline typically forms in intrusive rocks, especially pegmatites, that cooled extremely slowly. 

Alkali Feldspar Crystal symmetry and class Crystallization temperature Cooling rate Al/Si distribution Exsolution lamellae
Sanidine Monoclinic 2/m High Extremely rapid Perfectly disordered Require electron microprobe
Anorthoclase Monoclinic 2/m High Extremely rapid Perfectly disordered Require electron microprobe
Orthoclase Monoclinic 2/m Intermediate Slow cooling Partially ordered Microscopically visible
Microcline Triclinic 1 Low Extremely slow cooling Perfectly ordered Macroscopically visible

Albite is the sodium-rich alkali feldspar with the composition NaAlSi3O8.  Albite is also the Na-rich end member of the plagioclase feldspars, but ignore that for the moment.  At high temperature, Al and Si are highly disordered within the albite structure and so monalbite crystallizes in the monoclinic system (crystal class 2/m).  As temperature falls, the highly disordered distribution of Al and Si becomes increasingly more ordered resulting in a lessening of the symmetry from high albite, to intermediate albite until the completely ordered low albite crystallizes at low temperature.  Al and Si are perfectly ordered on the two tetrahedral sites in low albite.  Low albite is triclinic, in the same crystal class, 1, as microcline.

Alkali Feldspar Crystal symmetry and class Crystallization temperature Al/Si distribution
Monalbite Monoclinic 2/m Very high Perfectly disordered
High albite Triclinic 1 High Partially ordered
Intermediate Albite Triclinic 1 Intermediate Fairly-well ordered
Low albite Triclinic 1 Low Perfectly ordered

 

 

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