Crystal Form and Structure
CRYSTAL FORM. Well-developed crystals are bounded by a number of faces, which are usually flat. Any two adjacent faces meet at a straight edge, and three or more edges meet at a point, which is known as a solid angle. If all the faces of a crystal are alike, as in a cube (Plate 4) and an octahedron (Plate 78), the crystal is termed a simple form. If a crystal contains elements of two or more simple forms, it is called a combination (Plate 33). If in the combination the faces of the constituent simple forms are developed to the same extent, the combination is said to be in equilibrium, but if one form is dominant it may be called the primitive figure, whose edges and corners have been modified by the smaller faces of the other figure.
CONSTANCY OF THE INTERFACIAL ANGLE. In a growing crystal there is a continual change in the relative size of adjacent faces, but the angle between these faces, known as the interfacial angle, remains constant. Crystals of different sizes belonging to the same mineral and having the same crystal habit are therefore always found to have a similar shape. In every mineral species there are certain characteristic angles formed by corresponding faces of all its crystals. These angles occur not only in natural crystals but also in artificially produced crystals of die same substance. Constant angles are not only present between faces of the actual crystal but also between its internal cleavage surfaces. The angle between cleavage faces of the rhombohedron of calcite, for instance, is always 1050 5 regardless of where the crystal was found or how it was formed.