Rocks and Minerals STUDY GUIDE MINERALS There are four basic characteristics of minerals (SNIC). All minerals are… Solid Natural (not man made) Inorganic (not alive and never was) Crystalline structure- the atoms in minerals are arranged in a geometric pattern that is repeated. A natural mineral deposit that is large enough and pure enough to be mined for profit is called an ore. MINERAL IDENTIFICATION Mineral identification refers to the way to tell one mineral from another. Color is the most obvious, but the least reliable methods of identifying minerals because some minerals are the same color and color can be changed by the environment. Luster describes how light is reflected from a mineral’s surface. Luster can be described as metallic or non-metallic. Streak is the color of the mineral when it is broken up into powder. The mineral is rubbed on a "streak plate", which is a piece of porcelain. When the mineral is rubbed across the streak plate some of the mineral is broken off and ground into a powder. Streak is more accurate than color because it shows the true color of the mineral. Hardness is a measure of how easily a mineral can be scratched. The Mohs Hardness Scale lists hardness of ten minerals with (talc)1 being softest and (diamond)10 the hardest. Cleavage & Fracture refer to the way the mineral breaks. Cleavage refers to the tendency to split or crack along parallel or flat planes. Fracture occurs when a mineral breaks at curved or irregular surfaces. Volume is the amount of matter in a given space. Mass divided by volume is density. Specific gravity is another name for density. ROCKS The three main rock classifications are Igneous, Sedimentary, and Metamorphic. Rocks can change from one type to another through various processes over millions of years. Rocks are classified into these groups by the way they were formed. In order to identify a rock, you must determine its mineral content. 90% of all rocks contain silicon and oxygen and are called silicates. Rocks that do not contain both oxygen and silicon are called non-silicates. IGNEOUS Igneous rock is formed from molten material from a volcano (lava) or from deep inside the earth (magma) which cools and hardens. “Igni” is Latin for fire. Extrusive Igneous Rock is formed when lava cools quickly at the surface to form small crystals. Rocks formed in this way have a fine – grained texture. Examples of extrusive igneous rocks are Pumice, Basalt, and Obsidian. When lava erupts and cools in the ocean, it forms new land. Intrusive Igneous Rock is formed when magma cools inside the earth. Rocks formed this way have large crystals. They have a coarse – grained texture. Granite is an example of an intrusive igneous rock. SEDIMENTARY Sedimentary rock form when sediment is compacted and cemented. They form in in flat layers or strata. Fossils are found in these layers. Compaction occurs when small sediments are pressed together. Cementation occurs when the sediments are glued together by minerals deposited between the sediments. Examples: Shale, Sandstone, limestone METAMORPHIC Metamorphic rocks form when any rock is pushed down in the Earth and exposed to heat, pressure, and chemicals. Examples: shale becomes slate; limestone becomes marble IGNEOUS Igneous rocks start as magma. Igneous rocks forms when magma (molten rock under the surface) and lava (molten rock on the surface) cools and hardens into igneous rock.