Rocks and Minerals

ROCKS AND MINERALS ● PROPERTIES OF MINERALS ○ Mineral: Naturally occurring, inorganic solid that has a crystal structure and a definite chemical composition ○ Must have the following FIVE characteristics: ■ Must be formed by processes that occur in the natural world ■ Must be inorganic, meaning that it cannot come from materials that were once part of a living thing ■ A solid - with definite shape and definite volume ■ Particles that make it line up in a repeating pattern which form a crystal ■ Has a definite composition or range of compositions ○ Has certain characteristics that help identify it: ■ COLOR: Used to identify a few minerals that have their own characteristic coloring ■ STREAK: Color of a mineral’s powder ■ LUSTER: How light is reflected from a mineral’s surface ■ DENSITY: Each mineral has a characteristic density ■ CLEAVAGE: If a mineral breaks easily along flat surfaces ■ FRACTURE: When a mineral breaks apart in an irregular way ■ HARDNESS: Using the Mohs hardness scale, this scale ranks ten minerals from softest to hardest ■ CRYSTAL SYSTEM: Crystals grow atom by atom to form a particular structure - these structures are grouped into six different categories based on the number and angle of the crystal faces ○ Examples: Quartz, Halite, Mica ■ Source of variety of materials used to make many products, like gemstones, metals, and ore ● MOHS HARDNESS SCALE ○ Convenient way to help identify minerals ■ A mineral’s hardness is a measure of its relative resistance to scratching ■ A one on the scale is the softest, a ten would be the hardest The Mohs Hardness Scale. Image via U.S. National Park Service. ● CLASSIFYING ROCKS ○ Geologists look at the mineral composition, color, and texture of a rock, which can contain a mixture of minerals or only a single mineral. ■ COLOR: Provides clues to the rock’s mineral composition ● Granite: Light-colored rock having high silica content ● Basalt: Dark-colored rock that is low in silica ■ TEXTURE: The look and feel of the rock’s surface which is made up of particles of minerals or other fine rocks called grains ● Grain size, shape, or pattern ○ Types of Rocks: ■ IGNEOUS ROCK: Created when magma or lava cools and crystallizes ● Formation: May form on or beneath Earth’s surface ○ Extrusive Rocks: Igneous rocks formed from lava that erupted onto Earth’s surface ○ Intrusive Rocks: Rock that forms when magma hardens beneath Earth’s surface ● Texture: Geologists determine whether an igneous rocks is extrusive or intrusive based on its texture (Extrusive: Small, hard to see crystals; Intrusive: large and interlocking crystals) ● Composition: Can sometimes be determined by the color of the rock ■ SEDIMENTARY ROCK: Rock material that forms where rocks are broken down into smaller pieces or dissolved in water as rocks erode or the remains of plants and animals are pressed and cemented together ● Sediment: Small, solid pieces of material that come from rocks or remains of living things ● Sedimentary rock formed through a series of processes which include: ○ Erosion: Forms sediment by particles getting carried away from their source by wind and water ○ Deposition: Process by which sediment settles out of the water or wind carrying it ○ Compaction: Process that presses sediment together growing thick layers that build up over millions of years ○ Cementation: Process in which dissolved minerals crystallize and glue particles of sediment together ■ METAMORPHIC ROCK: Forms when existing rock is changed by heat, pressure, or chemical reactions (MORPH) ● Heat and pressure beneath Earth’s surface can change any rock into metamorphic rock ○ Foliated: Metamorphic rocks that have their grains arranged in parallel layers or bands ○ Non-Foliated: Mineral grains are arranged randomly ○ The Rock Cycle ■ Series of processes that change one type of rock into another type of rock The Rock Cycle. Image via cotf.edu.