Sources and Origins of the Metals

Abstract

The natural sources of metals in the environment lie with the rocks and processes by which they formed and which affected them after lithification. The primary rocks are called igneous and have a wide range of mineral and chemical composition. Minerals are inorganic and each of the more than 3000 minerals known is unique in its chemical composition and its orderly internal crystalline structure. Igneous rock minerals form at different cooling stages during the crystallization of a magma (molten rock material within the earth) containing all chemical elements. The union of different minerals or mineral groups and the proportions in which they are present at a given stage of crystallization, together with crystal size (texture) are the basis for classifying the principal igneous rocks. Igneous rocks that form (lithify) as magma cools within the earth slowly are comprised of minerals that solidify into large crystals that can be seen with the naked eye. The rocks are subsequently exposed at the earth’s surface by erosion after being uplifted during mountain-forming processes. Igneous rocks that cool rapidly when magma is extruded onto the earth’s surface by volcanic activity (e.g., lava) have very fine crystals that can be seen only when such rock is cut into “thin sections” and viewed through a high-power petrographic microscope.