It’s Elemental, My Dear: A Very Basic Chemistry of Elements and Compounds - Geology - page 55

SiO 2
This formula indicates that one atom of silica (Si) and two atoms of oxygen (O) have
bonded together, forming the compound.
In the case of geology, most chemical formulas describe minerals, which are solid struc-
tures built of molecules (see Chapter 6). In mineral compounds, sometimes multiple ele-
ments can fill the same spot in the mineral structure. For example, the mineral olivine
has this formula:
(Mg, Fe) 2 SiO 4
Two atoms of either magnesium (Mg) or iron (Fe) will combine with one atom of silica
(Si) and four atoms of oxygen (O). Either magnesium or iron can create the mineral oliv-
ine, so when you write the chemical formula, you put a parenthesis around and a
comma separating the possible atoms that can form that particular chemical compound.