compoundis a substance in which the atoms of two or more elements bond together. The
elements in a chemical compound are in a certain ratio. Solid water (ice) is probably one
of the simplest compounds that you know. As you can see inFigure3.3, a molecule of the
compound water is made of two hydrogen atoms and one oxygen atom. All water molecules
have a ratio of two hydrogen atoms to one oxygen atom. In ice, all the water molecules are
arranged in a definite, orderly pattern.
Figure 3.3: As this model of a water molecule shows, all water molecules have two hydrogen
atoms (shown in gray) bonded to one oxygen molecule (Shown in red). ( 11 )
Minerals that are not pure elements are made of compounds. For example, the mineral
quartz is made of the compound silicon dioxide, or SiO 2. This compound has one atom of
theelementsiliconforeverytwoatomsoftheelementoxygen. Whenamineralhasadifferent
chemical formula, it is a different mineral. For example, the mineralhematitehas two iron
atoms for every three oxygen atoms, while the mineralmagnetitehas three iron atoms for
every four oxygen atoms. Many minerals contain more complex chemical compounds that
are made of several elements. However, even the elements in more complicated compounds
occur in certain ratios.
Structure of Minerals
The crystal structure of a mineral affects the mineral’s physical properties. Imagine you
have three samples of halite. Each sample was found in a different country. They are all
different sizes and shapes. They may have even been formed by different geologic processes.
Will the samples all have the same crystal structure? Yes! All halite has the same chemical
composition and the same crystal structure, despite physical differences.