Feldspars

Feldspars are one of the most common minerals on Earth. There are two solid solution series of feldspar minerals: alkali feldspars and plagioclase feldspars. The end members of these solid solutions are potassium feldspar (orthoclase, microcline) (KAlSi3O8) – albite (NaAlSi3O8) – anorthite (CaAl2Si2O8). So, the alkali felspars range from orthoclase and microcline to albite. Accordingly, chemical formula of these minerals ranges from KAlSi3O8 to NaAlSi3O8 because of K+-Na+ substitution. Ion charge is the same. However, the ion size is not that similar so that K+ and Na+ would change each other very easy, that is why at lower temperatures there is a gap in alkali feldspar sequence. But in higher temperatures this substitution happens and then there is no gap in alkali sequence. This aspect is not illustrated in my picture.

Plagioclase feldspars range from albite to anorthite, so from NaAlSi3O8 to CaAl2Si2O8 because of Na+Si4+- Ca2+Al3+ substitution. Na+ and Ca2+ ions are similar in size, so they tend to substitute each other, and the difference in charge is compensated by Si4+ and Al3+.

The solid solution between K and Ca feldspars doesn’t work because of both ion sizes and charges differences. K+ and Ca2+ differ in ion size and charge and Si with Al cannot compensate those differences.

Intermediate blended minerals are present for both sequences. Barium feldspars (celsian and hyalophane) are not illustrated there.

This illustration is simplified and generalized version of feldspar mineral classification. It is important to know that some feldspars form in high temperature (ex. sanidine) whereas others in low (ex. microcline). Meanwhile this picture generalizes all feldspars and does not tell anything about mineral abundance depending on t-p conditions.

Colors of feldspar minerals have great variety and this illustration can not show all the possible variety. However, few, more common colors are used for each mineral. The ‘colorless’ color is ignored there.

Again, this picture is not for professional mineralogy use but can be used by professionals as fun illustration to put on the wall, show non-geologist friend or explain basics to the beginners.

High resolution poster available there:https://gum.co/RKBcT