 The geochemist F. W. Clarke calculated that a little more than 47 % of Earth 's crust consists of oxygen . The more common rock constituents of the crust are nearly all oxides : chlorine , sulfur and fluorine are the important exceptions to this and their total amount in any rock is usually much less than 1 % . The principal oxides are silica , alumina , iron oxides , lime , magnesia , potash and soda . The silica functions principally as an acid , forming silicates , and all the most common minerals of igneous rocks are of this nature . From a computation based on 1 @,@ 672 analyses of all kinds of rocks , Clarke deduced that 99 @.@ 22 % was composed of 11 oxides ( see the table at right ) , with the other constituents occurring in minute quantities . 
