What is a mineral? Minerals, the building blocks of rocks, are naturally occurring, usually inorganic solids with specific crystal structures and chemical compositions. A mineral is constructed of atoms, the small units of matter that combine in chemical reactions. An atom is composed of a nucleus made up of protons and neutrons and surrounded by electrons. The atomic number of an element is the number of protons in its nucleus, and its atomic mass is the sum of the masses of its protons and neutrons.
How do atoms combine to form the crystal structures of minerals? Chemical elements react with one another to form compounds either by gaining or losing electrons to become ions or by sharing electrons. Ionic bonds, which form by electrostatic attraction between positive ions (cations) and negative ions (anions), are the dominant type of chemical bond in mineral structures. Atoms that form compounds by sharing electrons are held together by covalent bonds. When a mineral crystallizes, atoms or ions come together in the proper proportions to form a crystal structure—an orderly three-dimensional array in which the basic arrangement of the atoms is repeated in all directions.
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What are the major classes of rock-forming minerals? Silicate minerals, the most abundant minerals in Earth’s crust, are built of silicate ions that are linked in various ways. Silicate tetrahedra may be isolated (linked together only by cations) or bonded together in structures such as single chains, double chains, sheets, or frameworks. Carbonate minerals are made up of carbonate ions bonded to calcium, magnesium, or both. Oxide minerals are compounds of oxygen and metallic elements. Sulfide and sulfate minerals are composed of sulfide and sulfate ions, respectively, in combination with metallic elements.
What are the physical properties of minerals? Geologists use the physical properties of minerals to identify them. These physical properties include hardness—the ease with which a mineral’s surface is scratched; cleavage—its tendency to split along planar surfaces; fracture—the way it breaks along irregular surfaces; luster—the way it reflects light; color—imparted by transmitted or reflected light to crystals or irregular masses or visible as streak (the color of a fine powder); density—mass per unit volume; and crystal habit—the shape in which individual crystals or aggregates of crystals grow.
What determines the properties of rocks? Mineralogy (the kinds and proportions of minerals that make up a rock) and texture (the sizes, shapes, and spatial arrangement of its crystals or grains) define a rock. The mineralogy and texture of a rock are determined by the geologic processes by which it formed.
What are the three families of rocks and how do they form? Igneous rocks form by the crystallization of magmas as they cool. Intrusive igneous rocks cool slowly in Earth’s interior and have large crystals. Extrusive igneous rocks, which cool rapidly at Earth’s surface, have a glassy or fine-grained texture. Sedimentary rocks form by the lithification of sediments after burial. Sediments are derived from the weathering and erosion of rocks at Earth’s surface. Metamorphic rocks form when igneous, sedimentary, or other metamorphic rocks are subjected to high temperatures and pressures in Earth’s interior that change their mineralogy, texture, or chemical composition.
How does the rock cycle explain the transformation of rocks from one type into another? The rock cycle relates geologic processes driven by the plate tectonic system and the climate system to the formation of the three families of rocks. We can view these processes by starting at any point in the cycle, such as the creation of new oceanic lithosphere at a spreading center as two continents drift apart. The ocean basin gets wider until at some point the process reverses itself. As the basin closes and igneous rocks and sediments are subducted beneath a continent, they begin to melt to form a new generation of igneous rocks. The heat and pressure associated with subduction and with the intrusion of these igneous rocks transforms surrounding rocks into metamorphic rocks. Ultimately, the two continents collide, and these igneous and metamorphic rocks are uplifted into a high mountain chain. The uplifted rocks slowly weather, and their fragments are deposited as sediments.
How do deposits of economically valuable minerals form? Ores are deposits of minerals from which valuable metals can be recovered profitably. Hydrothermal deposits of ore minerals are formed when groundwater or seawater reacts with a magmatic intrusion to form a hydrothermal solution. The heated water transports soluble minerals to cooler rocks, where they are precipitated in fractures. The resulting ores may be found in veins or in disseminated deposits. Igneous ore deposits typically form when minerals crystallize from cooling magma, settle, and accumulate at the base of the magma body. They are often found as layered accumulations of minerals. Other ore minerals are chemically precipitated in sedimentary environments to which metals are transported in solution.