Rocks and Minerals: Rocks and Minerals Elements, Compounds and Mixtures: Elements, Compounds and Mixtures Matter is anything that takes up space and has mass.
All matter can be classified into three forms: elements, compounds and mixtures. Element: Element An element is a substance that cannot be separated into simpler substances by ordinary chemical means. Scientists have identified 109 elements.
Each element has a name and a chemical symbol made up of one or two letters.
The smallest part of an element that has all the properties of that element is an atom. Molecules: Molecules Some elements are made up of atoms that are chemically combined to form molecules.
A molecule is two or more atoms held together by chemical forces. Atomic Structure: Atomic Structure Atoms are made up of three main particles: protons, neutrons and electrons. The center of the atom is called the nucleus. Two different kinds of particles are found in the nucleus. One of these is the proton.the proton is a positively charged particle. The other particle that makes up the nucleus is the neutron. A neutron is a neutral particle. Compounds: Compounds A compound is made of atoms of different elements that are bonded together. Water is a compound. Some compounds are made up of aluminum, magnesium, silicon, oxygen and hydrogen. Mixtures: Mixtures Some forms of matter are neither elements nor compounds. Instead they are two or more substances mixed together. Such forms of matter are called mixtures. A mixture is two or more substances physically combined. Most rocks, soil, sea water and air are examples of mixtures. Because the substances that make up a mixture are not chemically combined they can be separated by physical means. Chemical Formulas: Chemical Formulas The combinations of chemical symbols that represent atoms are called chemical formulas.
A chemical formula shows that elements that make up a compound. A chemical formula also shows the number of atoms of each element in a molecule or smallest particle of the compound. Subscripts: Subscripts Subscripts give the number of atoms of the element in the compound. Subscripts are placed to the lower right of the symbols. Minerals: Minerals A mineral is a naturally occurring, inorganic solid that has a definite chemical composition and crystal structure. In order for a substance to be called a mineral, it must have all five of the characteristics described in this definition. Inorganic: Inorganic A mineral must be inorganic, or not formed from living thing or the remains of living things.
Solid: Solid A mineral is always a solid. Like all solids, a mineral has a definite volume and shape. Chemical Composition: Chemical Composition A mineral has a definite chemical composition. A mineral may made of a single pure substance, or element, such as gold, copper or sulfur. Most minerals are made of two or more elements chemically combined to form a compound. Crystal Structure: Crystal Structure A mineral’s atoms are arranged in a definite pattern repeated over and over again. Atoms not confined, the repeating pattern of a mineral;s atoms forms a solid called a crystal. A crystal has flat sides that meet in sharp edges and corners. All minerals have a characteristic crystal structure.
There are 2500 different kinds of minerals. Formation and Composition of Minerals: Formation and Composition of Minerals Many minerals come from magma, the molten rock beneath the Earth’s surface. When magma cools, mineral crystals are formed. How and where magma cools determine the size of the mineral crystals. When magma cools slowly beneath the Earth’s crust, large crystals form. When magma cools rapidly beneath the Earth’s surface, small crystals for Crystal Formation: Crystal Formation Crystals may also form from compounds dissolved in a liquid such as water.When the liquid evaporates, or changes to a gas, it leaves behind the minerals as crystals. Halite, or rock salt, forms in this way. Most Abundant Elements: Most Abundant Elements The eight most abundant elements in the Earth’s crust are oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium. There are about 100 common minerals formed from the eight most abundant elements.Of these 100, fewer than 20 are widely distributed and make up almost all the rocks in the Earth’s crust.
Identifying Minerals: Identifying Minerals Minerals have certain physical properties that can be used to identify them, such as color, luster, hardness, streak, density, crystal shape, and other special properties.
Color: Color The color of a mineral is an easily observed physical property. Color can be used to identify only those few minerals that always have their own characteristic color, such as malachite which is always green. The mineral azurite is always blue.
Many minerals come in a variety of colors. Some are colorless.Colors can also change. Luster: Luster The luster of a mineral describes the way a mineral reflects light from its surface. Certain minerals have a metallic luster, such as silver, copper and gold. Minerals that do not reflect light have a nonmetallic luster, and are described by terms like glassy, pearly, dull and silky. Hardness: Hardness The ability of a mineral to resist being scratched is known as its hardness. Hardness is one of the most useful properties for identifying minerals. Friedrich Mohs, a German mineralogist, worked out a scale of hardness for minerals ranging from 1 to 10. The number one is assigned to the softest mineral, talc and 10 is assigned to the mineral, diamond. Streak: Streak The color of the powder scraped off a mineral when it is rubbed against a hard , rough surface is called its streak. The streak may be different from the color of the mineral. Streak can be observed by rubbing the mineral sample across a piece of unglazed porcelain, which is called the streak plate. A streak plate has a hardness slightly less than 7. Density: Density Density is the amount of matter in a given space. The density of a mineral is always the same, not matter what the size of the mineral sample. Crystal Shape: Crystal Shape Minerals have a characteristic crystal shape that results from the way the atoms or molecules come together as the mineral is forming. There are six basic shapes of crystal structures: cubic, hexagonal, orthorhombic, monoclinic, tetragonal and triclinic. Cleavage and Fracture: Cleavage and Fracture The terms cleavage and fracture are used to describe the way a mineral breaks. Cleavage is the tendency of a mineral to split along smooth, definite surfaces. Some minerals, like halite, break into small cubes. Micas cleave along one surface, making layers of thin sheets. Most minerals do not break along smooth lines. Special Properties: Special Properties Some minerals can be identified by special properties. Magnetite is naturally magnetic. Fluorite glows under ultraviolet light. Halite tastes salty. Sulfur smells like rotten eggs. Calcite fizzes when hydrochloric acid is added to . Uraninite is radioactive. Ores: Ores The term ores is used to describe minerals or combinations of minerals from which metals and nonmetals can be removed in usable amounts. Metals: Metals Metals are elements that have shiny surfaces and are able to conduct electricity and heat. Metals can be pressed or hammered into thin sheets and other shapes without breaking. Metals cans also be pulled into thin strands. Iron, lead, aluminum, copper, silver and gold are examples of metals. Smelting: Smelting Most metals are found combined with other substances in ores. After the ores are removed from the Earth by mining, the metals must be removed from the ores. During a process, called smelting, an ore is heated in such a way that the metal can be separated from it. Metals are useful. Copper is used in pipes and electrical wire. Nonmetals: Nonmetals Nonmetals are elements that have dull surfaces and are poor conductors of electricity and heat. Nonmetals are not easily shaped. Some are removed from the Earth in usable form. Others must be processed. Sulfur,a nonmetal, is used to make matches, fertilizers and medicines. Gemstones: Gemstones Gemstones are minerals that are hard, beautiful and durable and can be cut and polished for jewelry and decoration. Once a gemstone is cut and polished, it is called a gem. The rarest and most valuable gemstone- diamonds, rubies, sapphires and emeralds, are known as precious stones. All other gemstones, amethysts, zircons, garnets, are known as semiprecious stones. Rocks: Rocks A rock is a hard substance composed of one or more minerals. A rock can also be made of or contain naturally occurring substances that do not perfectly fit the definition of a mineral. Rocks can be composed of volcanic glass or of opal. Both of these substances lack a crystalline structure. Types of Rocks: Types of Rocks Geologists place rocks into three groups according to how they form: igneous, sedimentary and metamorphic. Igneous Rocks: Igneous Rocks Igneous rocks were originally hot, fluid magma within the Earth. Igneous get their name from the Latin word, ignis, which means 'fire'. Sedimentary Rock: Sedimentary Rock Most sedimentary rocks are formed from particles that have been carried along and deposited by wind and water. These particles, or sediments, include bits of rock in the form of med, sand or pebbles. Sediments also include shells, bones, leaves, stems and other remains of living things. Over time they are pressed together to form rocks. Metamorphic Rocks: Metamorphic Rocks Metamorphic rocks are formed when chemical reactions, tremendous heat and great pressure change existing rocks into new kinds of rocks. These new rocks have chemical and physical properties usually quite different from the original rocks. Rock Cycle: Rock Cycle The continuous changing of rocks from one kind to another over long periods of time is called the rock cycle. The rock cycle has no definite sequence. It can follow many different pathways. Granite and the Rock Cycle: Granite and the Rock Cycle Because granite is made of hard materials it is resistant to nature’s forces. It can be slowly worn down until bits of granite flake off and fall in streams and are eventually reduced to sand. The sand from granite, along with other sediments is carried to the sea and is deposited on the floor. The weight of layers piling on puts pressure on lower layers and with calcite the granite becomes part of a sedimentary rock. After many years, under great pressure and temperature the sedimentary rock will change to a metamorphic rock, quartzite Fluid and Fire: Igneous Rocks: Fluid and Fire: Igneous Rocks Igneous rocks are classified according to their composition and texture.
Composition refers to the minerals of which rocks are formed.
Texture means the shape, size, arrangement and distribution of the minerals that make up rocks.
Both are evident in a rock’s appearance. Igneous Rock Textures: Igneous Rock Textures Igneous rocks have four basic types of textures: glassy, fine-grained, coarse-grained and porphyritic. Glassy Igneous: Glassy Igneous Glassy igneous rocks are shiny and look like glass. The minerals that make up a glassy igneous rock are not organized into crystals. Obsidian has a glassy texture. Fine-Grained Igneous: Fine-Grained Igneous Fine-grained rocks, unlike glassy rocks, are made of interlocking mineral crystals. These crystals are too small to be seen without the help of a microscope. The dark gray rock known as basalt has a fine-grained texture. Coarse-Grained Rock: Coarse-Grained Rock Coarse-grained rocks, such as granite, consist of interlocking mineral crystals, which are all roughly the same size and visible to the unaided eye. Porphyritic Igneous Rocks: Porphyritic Igneous Rocks Porphyritic rocks consist of large crystals scattered on a background of much smaller crystals. Sometimes these small background crystals are too tiny to be seen with a microscope. Porphyritic rocks have a texture that resembles rocky road ice cream. Igneous Variety: Igneous Variety Where and how magma cools determines the size of mineral crystals. The longer it takes magma to cool, the larger are the crystals that form. Glassy and fine-grained rocks form from lava that erupts from volcanoes and hardens on the Earth’s surface. Coarse-grained rocks form from molten rock that cools and hardens within the Earth. Extrusive Rocks: Extrusive Rocks Rocks formed from lava are called extrusive rocks. Because lava is brought to the surface by volcanoes, extrusive rocks are also known as volcanic rocks. Basalt and obsidian are two kinds of extrusive rocks that are quite solid. Pumice, another extrusive rock, is filled with bubbles. Intrusive Rocks: Intrusive Rocks Igneous rocks formed deep within the Earth are called intrusive. They form when magma forces its way upward into preexisting rocks and then hardens. Intrusive rocks include granite and pegmatite. Intrusive rocks are also known as plutonic rocks. A mass of intrusive rocks are known as a pluton. Plutons may produce landforms by pushing up layers of rock above them, such as domes. Slowly Built Layers: Sedimentary Rocks: Slowly Built Layers: Sedimentary Rocks The most widely used classification system for sedimentary rocks places them into three main categories according to the origin of the materials from which they are made. These three categories are: clastic rocks, organic rocks and chemical rocks. Clastic Rocks: Clastic Rocks Sedimentary rocks that are made of the fragments of previously existing rocks are known as clastic rocks. Clastic rocks are further classified according to the size and shape of the fragments in them. Conglomerates: Conglomerates Some clastic rocks are made of rounded pebbles cemented together by clay, mud or sand. If over a third of the rock is made of pebbles, the rock is called a conglomerate. The pebbles in conglomerates are smooth and rounded because they have been worn down by the action of water. They are also called puddingstones. Sandstones: Sandstones Clastic rocks made of small, and-sized grains are called sandstones. At least half the particles in a clastic rock must be sand sized in order for it to be considered a sandstone. Sandstones are very common rocks. They are formed from the sand on beaches, in riverbeds and in sand dunes. In a sandstone, the grains are cemented together by minerals that harden. Shale: Shale Many geologists use term shale to describe all the clastic rocks that are made of particles smaller than sand. Shale forms from small particles of mud and clay that settle to the bottom of quiet bodies of water such as swamps. Most shale can be split into flat pieces. Organic Rocks: Organic Rocks Organic rocks come from organisms.Limestone are often but not always organic rocks. Deposits of limestone may be formed from the shells of creatures when they die. Creatures may also cement their shells together and over time form reefs. Coal is also made from the remains of living things. It is made from plants that lived millions of years ago. Chemical Rocks: Chemical Rocks Some sedimentary rocks are formed when a sea or lake dries up, leaving large amounts of minerals that were dissolved in water. Examples of chemical rocks formed this way include rock salt and gypsum. Some limestone rocks are formed by inorganic processes in caves. As water evaporates, a thin deposit of limestone is left behind. Changes in Form: Metamorphic Rocks: Changes in Form: Metamorphic Rocks When already existing rocks are buried deep within the Earth, tremendous heat, great pressure and chemical reactions may cause them to change into different rocks with different textures and structures. The changing of one type rock into another as a result of heat, pressure and /or chemical reactions is called metamorphism. Metamorphic Rocks: Metamorphic Rocks Metamorphic rocks may be formed from igneous, sedimentary or metamorphic rocks. Heat and pressure are great enough to make tock undergo change. Temperatures of 100 degrees to 800 degrees cause some minerals to break down, allowing their atoms to form other more heat-tolerant minerals.Texture, mineral and chemical composition may change. Metamorphic Rocks: Metamorphic Rocks The amount of heat, pressure and chemical reactions varies during metamorphism. Thus the degree of metamorphism also varies. The characteristics of the original rock also affect the degree of metamorphism. Many metamorphic rocks can be produced from more than one kind of rock. Metamorphic Classification: Metamorphic Classification Like igneous and sedimentary rocks, metamorphic rocks can be classified according to texture. The classification for metamorphic rocks are based on the arrangement of the grains that make up the rocks. Foliated Rocks: Foliated Rocks In the first group, the mineral crystals are arranged in parallel layers, or bands. The word foliated comes from the Latin word for leaf. It describes the layers in such metamorphic rocks, which are thin and flat. Most metamorphic rocks are foliated, like schist, slate and gneiss.
Unfoliated Rocks: Unfoliated Rocks In the second, smaller group of metamorphic rocks, the rocks are not banded and do not break into layers. These rocks are said to be unfoliated. Marble and quartzite are examples of unfoliated rocks.