Mineralogists group minerals together into six "systems." These systems are based on the form of individual crystals. We will describe the six systems here. We will characterize each system by describing both its faces and its crystallographic axes. An axis is an imaginary straight line that runs through the center of a crystal. Each axis runs from the center of one crystal face to the opposite face. Since crystals are three-dimensional objects, each crystal must have at least three axes.The six systems of minerals are:
- Isometric (Cubic) System with six square faces. All three of the crystallographic axes are of the same length, and they are arranged at right angles to each other. Galena crystals and halite crystals are examples of this system.
- Tetragonal System with two square faces and four lengthened rectangular faces. This is similar to the cubic system except that only two of the axes are of the same length. The third axis is either shorter or longer than the other two. Chalcopyrite crystals are an example of this system.
- Hexagonal System with six rectangular faces of the same size, and two hexagonal faces opposite each other. There are three horizontal axes of the same length. A vertical axis intersects them at right angles. Quartz crystals and calcite crystals are examples of this system.
- Orthorhombic System with three pairs of parallel faces, each pair being of a different size. As with the first two systems, the three axes are at right angles to one another, but of three different lengths. Barite crystals and celestite crystals are examples of this system.
- Monoclinic System with three pairs of faces of different size and shape, only two pairs of which meet at right angles. The three axes are of different length. Two of the axes intersect at right angles, while the third is inclined relative to the other two. Crystals of gypsum are an example of this system.
- Triclinic System with three pairs of faces of different size and shape, none of which meet at right angles. As in the monoclinic system the three axes are of different length. All three of the axes meet at some angle other than a right angle. This system is not represented in the Ohio minerals described in Ohio History Central.