| Etymology | | | | Anorthite endmember CaAl2Si2O8 |
| Feldspar is derived from the German Feld, "field", and | | | | Solid solutions between K-feldspar and albite are called |
| Spath, "a rock that does not contain ore". "Feldspathic" | | | | alkali feldspar. Solid solutions between albite and |
| refers to materials that contain feldspar. The | | | | anorthite are called plagioclase, or more properly |
| alternative spelling, felspar, has now largely fallen out of | | | | plagioclase feldspar. Only limited solid solution occurs |
| use. | | | | between K-feldspar and anorthite, and in the two other |
| Compositions | | | | solid solutions, immiscibility occurs at temperatures |
| This group of minerals consists of framework or | | | | common in the crust of the earth. Albite is considered |
| tectosilicates. Compositions of major elements in | | | | both a plagioclase and alkali feldspar. In addition to |
| common feldspars can be expressed in terms of | | | | albite, barium feldspars are also considered both alkali |
| three endmembers: | | | | and plagioclase feldspars. Barium feldspars form as |
| Potassium-Feldspar (K-spar) endmember KAlSi3O8 | | | | the result of the replacement of potassium feldspar. |
| Albite endmember NaAlSi3O8 | | | | |