| Orthoclase | 90° |
| Microcline | 89° 30′ |
| Oligoclase | 86° 32′ |
| Andesite | 86° 14′ |
| Labradorite | 86° 14′ |
| Bytownite | 86° 14′ |
| Anorthite | 86° 50′ |
[Orthoclase]
KAlSi₃O₈
Occurs in granites, syenites, gneisses and light-colored lavas; hardness, 6; specific gravity, 2.57; color white to gray or pink; cleavage in two directions perfect and at 90°, in the third direction imperfect; luster vitreous; translucent on thin edges.
Orthoclase is monoclinic, and when formed in cavities develops as crystals, but it is usually a constituent of igneous rocks, in which case the crystals have not had the opportunity to develop the crystal faces, and the orthoclase is in grains or irregular masses; and the best way of determining the mineral is the cleavage, the two perfect cleavage planes intersecting at right angles. Twinning is frequent but of the simple type, only two crystals being united, similar to either B or C on [plate 34].
It is found in granites, gneisses or lavas, wherever they occur, being especially characteristic of the granites of the Rocky Mountains.
[Microcline]
KAlSi₃O₈
[Pl. 35]
Occurs in granites and gneisses as crystals or irregular masses; hardness, 6; specific gravity, 2.56; color white to gray, pink, or greenish; luster vitreous; translucent on thin edges.
Microcline has the same composition as orthoclase, but is in the triclinic system, the c axis being inclined a half degree away from a right angle with the b axis. This is best seen in the cleavage pieces, the two perfect cleavage planes meeting at 89° 30′, and this is the only test for determining this mineral by the unaided eye. Pike’s Peak is the best known locality for microcline, and there it occurs in fine large crystals of greenish color, which are known as Amazon stone.
[Albite]
NaAlSi₃O₈
Occurs in small crystals, or more often in lamellar masses in granites or in seams in metamorphic rocks; hardness, 6; specific gravity, 2.62; color white to gray; luster vitreous.