Minerals and Igneous Rocks PDF

Section 2.2 Minerals and Igneous Rocks Geology is not Rocket Science All the solids on the surface of the Earth that are not of biological or human origin are composed of rocks and/or minerals. There are thousands of different described types or species of rocks and minerals all...

Section 2.2 Minerals and Igneous Rocks Geology is not Rocket Science All the solids on the surface of the Earth that are not of biological or human origin are composed of rocks and/or minerals. There are thousands of different described types or species of rocks and minerals all classified by their physical properties and how they formed. What Makes a Mineral a Mineral? Mineral are natural, usually inorganic solids that have a characteristic chemical composition, an orderly internal structure, and a characteristic set of physical properties. To be a mineral, a substance must have these four characteristics: ○ It must be inorganic- it cannot be made of or by living things; ○ It must occur naturally- it cannot be man-made; ○ It must be a crystalline solid; ○ It must have a consistent chemical composition. What makes A Mineral A Mineral? Types of Minerals The 20 most common minerals are called rock-forming minerals because they form the rocks that make up Earth’s crust. Ten minerals are so common that they make up 90% of Earth’s crust. These minerals are quartz, orthoclase, plagioclase, muscovite, biotite, calcite, dolomite, halite, gypsum, and ferromagnesian minerals. All minerals can be classified into two main groups-silicate minerals and nonsilicate minerals-based on their chemical compositions. Silicate Minerals Silicate minerals are minerals that contains a combination of silicon and oxygen and that may also contain one or more metals Common silicate minerals include quartz, feldspars, and ferromagnesian minerals, such as amphiboles, pyroxenes, and olivines. Silicate minerals make up 96% of Earth’s crust. Quartz and feldspars alone make up more than 50% of the crust. Non-Silicate Minerals Non-silicate minerals are minerals that do not contain compounds of silicon and oxygen. Nonsilicate minerals comprise about 4% of Earth’s crust. Non-silicate minerals are organized into six major groups based on their chemical compositions: carbonates, halides, native elements, oxides, sulfates, and sulfides. Mineral Properties All minerals will have properties specific to their species that can help us to identify and classify them. The properties that are most frequently used to identify minerals are: ○ Hardness ○ Crystal Shape ○ Color ○ Streak ○ Luster Hardness Hardness is the measure of the ability of a mineral to resist scratching. Hardness does not mean “resistance to cleavage or fracture”, or its resistance to shattering. Diamond is the hardest mineral, but will shatter like glass if struck (in)correctly. The hardness of a mineral can be determined by comparing the mineral to minerals of Mohs hardness scale, the standard scale against which the hardness of minerals is rated The strength of the bonds between the atoms that make up a mineral’s internal structure determines the hardness of that mineral. The stronger the bond, the harder the mineral. Crystal Shape A certain mineral always has the same general shape because the atoms that form the mineral’s crystals always combine in the same geometric pattern. The reason for a crystal’s shape is due to how the atoms that compose the crystal are arranged Crystal Structure Systems Color While a mineral’s color is easy to observe, it alone is not a reliable clue for identifying a mineral sample. The color of a mineral sample can be affected by the inclusion of impurities or by weathering processes. Some minerals like quartz exhibit many different colors. Purple amethyst, pink rose quartz, and golden tiger’s eye quartz are all different colors that quartz can exhibit. Streak Streak is the color of a mineral in powdered form Streak is more reliable than color for the identification of minerals. Streak is determined by rubbing some of the mineral against an unglazed ceramic tile called a streak plate. This is a more reliable method that color alone, because minerals will exhibit a consistent color in power form. Luster Luster is the way in which a mineral reflects light A mineral is said to have a metallic luster if the mineral reflects light as a polished metal does. All other minerals have nonmetallic luster. There are several types of nonmetallic luster, including glassy, waxy, pearly, brilliant, and dull or earthy. Cleavage and Fracture Cleavage is the tendency of a mineral to split along specific planes of weakness to form smooth, flat surfaces Fracture is the manner in which a mineral breaks along either curved or irregular surfaces Uneven or irregular fractures have rough surfaces. Splintery or fibrous fractures look like a piece of broken wood. Curved surfaces are conchoidal fractures. Igneous Rocks Igneous rocks are rock that forms when magma or lava cools and solidifies The three factors that affect whether rock melts include temperature, pressure, and the presence of fluids in the rock. Rock melts when the temperature of the rock increases to above the melting point of minerals in the rock or when excess pressure is removed from rock that is close to melting. Rock may melt when fluids, such as water, are added. The addition of fluids generally decreases the melting point of certain minerals in the rock. Partial Melting Different minerals have different melting points, and minerals that have lower melting points are the first minerals to melt. As the temperature increases and as other minerals melt, the magma’s composition changes. The process in which different minerals in rock melt at different temperatures is called partial melting. Fractional Crystallization When magma cools, the cooling process is the reverse of the process of partial melting. Chemicals in magma combine to form minerals, and each mineral has a different freezing point. Minerals that have the highest freezing points crystallize first. The crystallization and removal of different minerals from the cooling magma is called fractional crystallization. Two Major Categories Igneous rocks are classified based on where they cool and harden. Intrusive Igneous rocks cool beneath the surface Extrusive Igneous rocks cool above the surface Where the magma or lava cools determines the rate it will cool which in turn determines crystal size and the texture of the rock. Igneous Rock Textures Intrusive igneous rocks cool very slowly, up to millions of years. This slow cool time allows very large crystals to grow and tend to have a rough texture. Rocks like granite and gabbro exhibit these large crystals Extrusive igneous rocks cool rapidly preventing large crystals from forming resulting in smooth rocks. Basalt and obsidian exhibit this smooth texture. Igneous Rock Composition - Felsic Rocks Another way to categorize igneous rocks is by their chemical composition. Felsic igneous rocks form from felsic magma or lava high in feldspars and silica. The two most common minerals to Granite form in felsic rock are potassium feldspar and quartz. Some of the most common igneous rocks are felsic; rocks like granite, rhyolite, obsidian, and pumice. Pumice Igneous Rock Composition - Mafic The other major chemical category are the mafic igneous rocks. These are rocks high in magnesium Basalt and iron. The most common minerals are plagioclase and pyroxenes. The most common rocks in the mafic group are gabbro and basalt. Rock composed of lava or magma with properties of both mafic and Gabbro felsic lava are referred to as intermediate igneous rocks. Other Igneous Rock Properties Igneous rocks can also have properties depending on how they formed. If the magma or lava that formed the rock contained dissolved gases, the gases can form bubbles in the rock as it cools. This can create a vesicular texture, where the rock contains large amounts of pits and holes. Pumice is the most well known rock Pumice with this property. Other Igneous Rock Properties Some igneous rocks are not formed by the cooling of magma or lava, but rather the compacting of volcanic ash. This rock is very soft and will take on the appearance color of the lava that made it. Tuff Igneous Rock Identification Igneous rocks are classified according to their texture and color. Felsic rocks are lighter in appearance while mafic rocks will be darker in appearance due to the minerals that compose them. Intrusive rocks will have large crystals because of how long they were able to cool off while extrusive rocks will have small or no crystals because of their rapid cooling. Igneous Rock Identification Below is a simplified chart showing the different types of igneous rocks and their properties.