Feldspar is a group name. There are two mani minerals subgroups; Plagioclase feldspar and Alkali feldspar. There are many minerals under this name. The feldspars can be looked at in terms of the three components Albite (Ab) - NaAlSi3O8, Orthoclase (Or) - KALSi3O8, and Anorthite (An) - CaAl2Si2O8.  At high temperature, complete solid solution exists between Ab and Or, to form the alkali feldspar solid solution series.  But, as shown in the diagram, the alkali feldspar solid solutions can contain up to 5% of the An component.  Similarly a complete solid solution series exists between Ab and An, to form the plagioclase solid solution series.  Plagioclase can contain up to about 5% of the Or component.