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Structure of Minerals All minerals are crystalline The atoms are arranged in a regular pattern A crystal is a regular geometric solid with smooth surfaces called crystal faces The orderly arrangement of ions determines the shape of the crystal. Crystal Structure Each kind of mineral has its own crystal form. Halite (NaCl) typically forms a crystal in the shape of a cube Quartz (SiO2) may form long, regular six sided crystals. The angles at which crystal faces meet is always the same for each kind of mineral. To describe these shapes crystallographic axes are used. They are drawn perpendicular to crystal faces. Six Crystal Systems Silica Tetrahedron More than 90% of the minerals in Earth’s crust are members of a mineral family called silicates. They are compounds of silicon and oxygen plus one or more metallic element such as aluminum or iron In all silicates the basic building block is four O atoms packed closely around a Si atom, held together by covalent bonds. The basic unit forms a silica tetrahedron Silicates are classified according to the way the tetrahedrons are linked. Fig 3.16 p. 40 Crystals and Physical Properties The orderly arrangement of the atoms or ions helps explain the other properties of the mineral. A mineral is a solid because of the close packing of its ions or atoms and the strong forces of attraction between them The hardness of a mineral depends on the arrangement of its ions or atoms Carbon forms diamond, the hardest natural mineral. Carbon also forms graphite, a very soft mineral that flakes easily Carbon atoms are arranged in layers or sheets in the graphite structure. Diamond and Graphite Diamond structure
