- The Nature of Minerals
- The Formation of Crystals
- The External Form of Crystals
- Outward Appearance Of Crystalline Material
- Distorted and Deformed Crystals and Pseudomorphs
- Crystal Systems
For convenience of study and reference, crystals are divided into six great systems, described by the comparative length and angular relation of their crystallographic axes.
Cubic (or Isometric) SystemA mineral is placed in the cubic system if it can be described by three axes of equal length at right angles to one another. In a cube, if axes were passed from the center of each face to the center of the opposite face, the axes would be equal in length and at right angles to one another. Diamond, spinel and garnet crystallize in the cubic system.
Hexagonal System(pronounced hex-AG-uh-nul) Hexagonal crystals have four axes, three of which are equal in length and intersect at 600 angles. The fourth is perpendicular to these and longer or shorter than the three previously described axes, Ruby, sapphire, emerald and aquamarine crystallize in this system.
Tetragonal System(pronounced tech-TRAG-uh-nul) The tetragonal system is one in which there are three axes that intersect at right angles, but only two of which are equal in length, the third being either longer or shorter than the first two. The basic form resembles a cube elongated in one direction. Zircon crystallizes in this system.
Orthorhombic System(pronounced or-tho-RO M-bik) This system is characterized by three mutually perpendicular axes of unequal length. The basic form resembles a box with length, width and depth unequal. Topaz crystallizes in this system.
Monoclinic System(pronounced mon-oh-KLIN-ik) The three axes in this system are unequal, two intersect at an angle other than at right angles, and a third is perpendicular to these two. The basic form can be visualized as a box deformed. So that the top is still rectangular but one side view is a parallelogram. Jadeite and nephrite crystallize in this system.
Triclinic System(pronounced try-KLIN-ik) The triclinic system is the one of the least symmetry. It is described by three axes, all unequal in length, and Inclined to one another at angles other than 900. A basic form would resemble a box deformed so that all sides are parallelograms. Labradorite and microcline feldspars crystallize in this system.
To describe a crystal form it is necessary to visualize the existence of certain fixed lines of reference, similar to the imaginary line, or axis, from the north to the south pole about which the earth rotates. In an ideal crystal form these lines are of definite length in relation to each other, extend in certain definite directions, and intersect in the middle of the crystal at a point called the ORIGIN. Such imaginary lines are called CRYSTAL AXES (the plural of axis). There must be at-least three axes to describe a crystal, and in one case four are necessary. These are indicated in the accompanying plate entitled "Models of the Six Crystal Systems."
Forms Within Crystal Systems
Within each crystal system there are many different shapes that crystals can take; each basic shape is called a CRYSTAL FORM. The form or combination of forms most commonly taken by crystals of a given gem mineral is known as the HABIT of that mineral. For example, the habit of garnet is the DODECAHEDRON (pronounced doe-dek-uh-HEE-drun); of Zircon, a prism modified by a pointed form called a BIPYRAMID, etc. (see accompanying plate entitled "Examples of Crystal Forms"). For reasons related to the conditions during growth, crystals of the same mineral may take different forms or combinations of forms. For example, ruby and sapphire, varieties of the same gem species, usually occur in crystals with different shapes.