Chapter 5 MINERALS EARTH S CRUST AND MANTLE COMPOSED PRIMARILY OF ROCK WHAT IS ROCK MADE OF Minerals are the building blocks of the planet WHAT IS A MINERAL Geologic de nition of a mineral is specialized Naturally occurring Solid Ordered atomic arrangement Inorganic Formed geologically De nite chemical composition ICE Is this properly a crystal or not CRYSTALLINE STRUCTURE Atoms in a mineral are speci cally ordered bottom gure A solid with disordered atoms is called a glass top gure Crystalline structure based on atomic patterns CRYSTALS Crystals form when mineral grains can grow unobstructed bound by planar surfaces crystal faces relative sizes of crystal faces are not constant for a given mineral for given mineral the angles between a certain pair of crystal faces is constant ELEMENTS MINERALS IN THE CRUST Most common minerals Silicate minerals contain silicate anion SiO4 4 Feldspars Quartz Oxide minerals contain oxygen anion O2 SILICATES Most important rock building material Tetrahedon structure four large oxygen atoms at the corners with a small silicon at the center Tetrahedrons can link together to form rings chains double chains or sheets MOLECULAR STRUCTURE Silica impacts the properties of the minerals and the rocks which are composed of these minerals Rocks with more silica tend to be more viscous MINERAL PHYSICAL PROPERTIES Common properties Color Streak Luster Hardness Speci c gravity Crystal habit Crystal form Fracture Cleavage HARDNESS Scratching resistance of a mineral Hardness compared to the Mohs Hardness Scale Fingernail 2 5 Copper penny 3 5 Glass Steel 5 5 Steel File 6 5 CLEAVAGE FRACTURE HABIT Cleavage Tendency to break along planes of weakness Produces at shiny surfaces Fracture Occurs for minerals that lack planes of weakness equal molecular bonds in all directions Cleavage is sometimes mistaken for crystal habit Cleavage is through going often forms parallel steps Crystal habit is only based on external surfaces WHAT ARE ROCKS Coherent aggregates cemented together Naturally occurring concrete and brick are not rocks Aggregate consists of mineral crystals or grains with other materials CLASTIC ROCKS from water and lls the space between grains CRYSTALLINE ROCKS Grains interlock with one another like pieces of a jigsaw puzzle THE THREE ROCK GROUPS Clastic rocks grains are bonded by natural cement mineral material that precipitates Igneous rocks formed by cooling and consolidation of magma Sedimentary rocks formed by chemical precipitation of material from water at the Earth s surface or by deposition and cementation of particles and debris transported by water wind or ice Metamorphic rocks igneous or sedimentary rocks that have been changed under high temperatures and or high pressures