Unlike rocks which minerals build, a mineral has the same chemical composition throughout. Unlike many of the compounds that we see in the laboratory, the formula of minerals is not quite so predictable. The formula can have a range of values and still be recognizable as a particular mineral. For example, the mineral Olivine, which is a pale green mineral can have a formula ranging from Mg 2 SiO 4 to Fe 2 SiO 4 . Minerals form in and on the Earth in a variety of ways.
Most minerals occur as crystals when conditions are suitable.
In most cases these crystals are too small and too spread out to be of any value in identifying a mineral. In these cases special laboratory tests are necessary. In larger samples, the shape of a crystal can be important in helping to identify the mineral. We will not delve too far into the science of Crystallography here. Instead, we will only refer to the common shapes that help us in the field.