The third section focuses on crystals. A crystal is a solid that is made up of atoms arranged in a regular orderly pattern. The edges of perfect crystals are straight, and the angles between edges remain the same size for like crystals. Crystals can be described by their shapes. The shapes of crystals are determined by the arrangement of atoms inside them. There are seven crystal systems or shapes. They include cubic, tetragonal, hexagonal, orthorhombic, monoclinic, triclinic, and trigonal. When identifying minerals that are crystalline, it is useful to know which crystal system the mineral represents. Since many samples do not have perfectly formed crystals, it is often necessary to make an intelligent guess. Although different crystal shapes help distinguish minerals from one another, crystal shape in itself is not always a valid test. Various crystals of the same mineral may develop the same external surfaces or faces even though they may not be the same size or shape. On the other hand, the crystals of two different minerals sometimes have the same shape. No two crystals are exactly alike since the conditions in which they develop vary. They need enough space to grow and if the space is restricted, unusual features or distortions may result. Crystals may range in size from microscopic to several yards long. Habit refers to the shape of a crystal.
When studying crystals it is essential to recognize that all matter is made up of atoms and that atoms contain smaller particles called protons, neutrons, and electrons. A proton has a positive charge, while an electron has a negative charge and a neutron has neither a positive or negative charge. If an atom has the same number of electrons and protons the atom is neutral. However, sometimes an atom gains or loses electrons. When the number of electrons and protons in an atom differ the atom has an electrical charge. An atom with an electrical charge is referred to as an ion. Particles of matter are held together by atomic bonds. An ionic bond refers to the bond formed when two atoms trade electrons. Since the force of attraction in an ionic bond is very strong, many compounds that contain ionic bonds are solids. An example of a solid that contains ions arranged in a regular pattern is a crystal. A crystal lattice is formed by the pattern of positive and negative ions. The shape of a crystal is determined by its crystal lattice.