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CYANITE, a native aluminium silicate, Al2SiO5, crystallizing in the anorthic system.

In anorthic crystals there is no relation between the optical and crystallographic directions, and the exact determination of the optical orientation is often a matter of considerable difficulty.

Although the crystals of all felspars present a general resemblance in habit, they are usually regarded as belonging to two systems, some felspars being monoclinic and others anorthic.

It is a calcium and aluminium silicate, CaAl2Si2O3, and crystallizes in the anorthic system.

In the sixth and last system, the triclinic—or anorthic, or asymmetric—the axes are again three, but in this case, none of them are equal and none at right angles.