The earth’s crust consists of a variety of rocks, formed under different circumstances, and with a wide variety of properties. Rocks usually consist of one or more minerals, ranging from single chemical elements to complex compounds. There are known to be more than 3,000 different minerals. Of the 155 known elements, some of which do not occur naturally, oxygen is by far the most common, making up about 50% of the earth’s crust by weight. Silicon forms about 25%, and the other common elements such as aluminium, iron, calcium, sodium, potassium, magnesium and titanium build up the total to 99% of the earth’s crust. Silicon, aluminium and oxygen occur in the commonest minerals such as quartz, feldspar and mica, which form part of a large group known as silicates, being compounds of silicic acid and other elements. Amphiboles and pyroxenes contain aluminium, potassium and iron. Some of the earth’s commonest rocks, granite and gneiss, are composed of silicates.