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The chemical composition of a mineral can vary, but only within fixed limits.  This means that a mineral always has the same basic "recipe", at least as far as major components or, in baking terminology, ingredients, are concerned.  The principle is much the same as making a cake:  The proportions of basic ingredients (such as flour, sugar, butter, and baking powder) can only be varied slightly without making it inedible, while changing the special ingredients that are typical of one type of cake will turn it into a different type.

The structure of a mineral determines how much variation is possible.  For instance, the mineral quartz (SiO₂) has very little compositional variation; it is essentially composed of one atom of silicon (Si) for every two oxygen (O) atoms, and its structure makes it difficult for other sorts of atoms to become included.  Other minerals are much more variable, and some materials, such as limonite, are so extremely variable that they are considered to be "mineraloids", rather than true minerals.

The chemical formula of minerals with variable composition tends to be very complex.  One example is apophyllite, whose formula is: (K,Na)Ca₄Si₈O₂₀(F,OH) 8H₂O. Variations in mineral composition can produce differences in structure, and are reflected in differences in the color, density, and other physical and optical properties.  These differences are used to define different "varieties" of a particular mineral.  For example, red or pink tourmaline is called rubellite, blue tourmaline is indicolite and iron-rich and black tourmaline is schorl.

Ordered Atomic Structure:

This means that the atoms (or ions) in a mineral are arranged in a regular, repeated, three-dimensional array (this is what defines a crystalline solid). Solids such as common opal and chrysocolla that do not have an ordered atomic arrangement (referred to as amorphous) thus are not minerals. In between crystalline and amorphous solids lie the "glasses" (including man-made glass and naturally-occurring obsidian). The atoms of these are partly ordered, usually due to very rapid cooling from a molten state, creating what are called "supercooled liquids". Supercooled liquids tend to flow under the effects of gravity, but far too slowly to be visible to the human eye.

Atoms are the smallest subdivision of matter that retain the characteristics of the elements. They consist of a very small, massive nucleus composed of protons and neutrons surrounded by a much larger region of circling electrons.

Atoms resemble a miniature solar system. At the center, corresponding to the sun, is the nucleus which is made up of protons and neutrons. Proton have a positive charge; the neutron, as the name implies, is electrically neutral. Each electron, which, like a planet of the solar system, moves in an orbit around the nucleus, carries a negative charge. Since the atom as a whole is electrically neutral, there must be as many electrons as protons.

The basic difference between atoms of different elements is due to the electrical charge of the nucleus, which in turn, is related to the number of protons. This number (which is equal to the number of electrons) is called the atomic number. The elements in the periodic table are arranged according to increasing atomic number.

The chemical properties of the elements depend on how the electrons are arranged around the nucleus. Electrons are considered to be arranged about the nucleus in energy levels, or shells. The number of shells for each atom range from 1 to 7. In the periodic table the elements are arranged in groups where the number of electrons in the outermost shell is the same for the atoms of each element in a group and is equal to the group number. It is the number of these outer shell (valence) electrons that determines the chemical properties of an element.