In its oxide or silicate form, silicon is used to make concrete, bricks, glass, ceramics, and soap. Silicon metal is also the base material for making silicones used in such products as synthetic oils, caulks and sealers, and anti-foaming agents.
Intrusive igneous rocks crystallize below Earth's surface, and the slow cooling that occurs there allows large crystals to form. Examples of intrusive igneous rocks are diorite, gabbro, granite, pegmatite, and peridotite. Extrusive igneous rocks erupt onto the surface, where they cool quickly to form small crystals.
Students need to be aware that igneous rock forms when magma cools and forms crystals. They should know from examining rock samples and / or photographs that igneous rock such as granite contains crystals and that different types of igneous rock contain crystals of different sizes.