a nonmetalic element analogous to carbon. it always occurs combined in nature, and is artificially obtained in the free state, usually as a dark brown amorphous powder, or as a dark crystalline substance with a meetallic luster. its oxide is silica, or common quartz, and in this form, or as silicates, it is, next to oxygen, the most abundant element of the earth's crust. silicon is characteristically the element of the mineral kingdom, as carbon is of the organic world. symbol si. atomic weight 28. called also silicium.