 There may be two or more endmembers in a group or series of minerals.For example, the tectosilicate feldspar can be described as a solid solution of the endmembers K-feldspar [KAlSi3O8], albite [NaAlSi3O8] and anorthite [CaAl2Si2O8].A specific feldspar can have varying quantities of potassium (K), sodium (Na) and calcium (Ca).