Silica and quartz content in an imaginary rock
To get a feeling for the numbers used in rock classification, I have calculated the silica content of an imaginary rock made of 20% quartz, 60% orthoclase feldspar and 20% muscovite mica, and presented the results in Table 2.1 below. The composition of this rock is a bit simple, but it would qualify as a so called "alkali-feldspar granite".