| Crystallization of Feldspar Minerals |
The Feldspar group contains two parallel but interchangable solid solution series, and is unique in that the series end members can crystallize in different crystal systems.
Some feldspars crystallize in the triclinic crystal system, and others in the the monoclinic crystal system.
The difference between the triclinic crystal system and the monoclinic crystal system is only one angle.
Monoclinic minerals have two asymmetrical angles and one symmetrical angle, whereas triclinic minerals have three asymmetrical angles and no symmetrical angles.
Albite, which is triclinic, has its third angle slightly asymmetrical, and it is therefore classified as a triclinic mineral, but the angle of asymmetry (i.e. off of 90º) is so minute it can hardly be detected, and it is therefore very similar to orthorhombic minerals.
The same is true for all Feldspar minerals that crystallize in the triclinic system - their third angle of symmetry is asymmetrical by only a fractional amount.
This is the reason all Feldspar crystals, even those crystallizing in different systems, have crystals that can be very similar in appearance.
This is also the reason why Orthoclase and Microcline are often regarded as a single mineral, since the only difference between them is one minute asymmetrical angle.
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