The feldspars are the most common mineral group
in the crust of the earth. Feldspars are tectosilicates.
They have the same basic structure as quartz, except
that one in every four Si is replaced by Al. Therefore,
instead of Si4O8, feldspars contain AlSi3O8. As Al has a
3+ charge, whereas Si has a 4+ charge, the resulting charge
difference is made up by K or Na for the alkali feldspars,
and by Na and Ca for plagioclase feldspars. There is an
additional complication, however, in that Ca has a 2+
charge. For Ca feldspars, therefore, there needs to be a
replacement of two in every four Si are replaced by Al.
The end members of the feldspar series are Anorthite -
CaAl2Si2O8, Albite - NaAlSi3O8 and Sanidine/Micro-
cline/Orthoclase - KAlSi3O8.