Prof. C.ValentiMinerals and Rocks 1

INTRODUCTION TO EARTH MATERIALS Knowing the names and important characteristics of the most common elements

and minerals is essential to understanding how they combine to form the rocks of the earth. Knowing some basic principles of chemistry is also important for identifying the various types of rocks, understanding the processes of weathering, and the composition of the earth’s interior and crust.

Minerals are essential natural resources for humans: metals are used for jewelry, aluminum cans, buildings, wiring, pencils, baby powder, diamonds are used for drill bits, silicon for computer chips, nutrients.

Earth materials are produced by volcanic eruptions, weathering and erosion events. Types of minerals and rocks serve as indicators of the chemical and physical events which formed. So to understand earth processes and how to interpret the geologic past, we need to have a foundation of earth materials.

Our Objective is to:(1) Define what a mineral is; (2) How each mineral is composed of certain chemical elements in a orderly arrangement; (3) how the arrangement and characteristics of atoms control the physical properties of minerals; (4) How to determine physical properties and use them to identify common minerals. Mineralogy, the study of minerals. Minerals are the building blocks of rocks,

where a rock is any solid mass of a mineral or conglomeration of more than one mineral.

Definition of a mineral. (1) Naturally occurring (2) inorganic (3) solid (4) crystal structure with an orderly internal structure where atoms are arranged in a definite and repeating pattern (5) definite chemical composition that can be expressed by a chemical formula representing the kinds of chemical elements present and their proportions..

Rocks. Rock is naturally formed, consolidated material composed of grains of one or more minerals. Assemblage of one or more minerals occurring in the solid state. There are three groups of rocks based on how they form.

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Prof. C.ValentiMinerals and Rocks 2ATOMIC STRUCTURE The earth is composed of about 4000 different minerals, which are defined by their

chemical composition and internal structure as dictated by the definition of a mineral.

An element is a substance that cannot be broken down into other substances by ordinary chemical methods. The atom is the smallest possible particle of an element that retains properties of that element. The atom contains three types of subatomic particle-protons, neutrons, and electrons. Need overhead of atom and periodic table.

Protons. A proton is a subatomic particle that contributes mass and a single positive electrical charge to an atom. The atomic number of an element is the number of protons in each atom. EACH ATOM OF AN ELEMENT HAS THE SAME NUMBER OF PROTONS. (i.e. oxygen has an atomic number of 8, or 8 protons). A neutron is a subatomic particle that contributes mass but no charge to an atom (a neutron is actually a proton and an electron). Protons and neutrons form the nucleus of an atom. Therefore, even though it the nucleus occupies a tiny volume of the atom, it contains virtually all the atoms mass. The atomic mass number of the atom is the total number of neutrons and protons in an atom. (i.e. oxygen 16).

Neutrons. The number of neutrons (and subsequently the mass of the element) can vary. Atoms containing different numbers of neutrons but the same number of protons (remember, each atom of an element has the same number of protons - pretty much defines the element) are called isotopes of that element. (i.e. isotopes of oxygen containing 10 instead of 8 neutrons have been found. The atomic weight of an element is the weight of an AVERAGE atom of an element taking into account the abundance of each isotope as found in nature (given in atomic mass units). (e.g. Sodium has 1 isotope, therefore its atomic mass number and atomic weight are the same-23. Chlorine has two isotopes with mass numbers of 35 and 37. The atomic weight is 35.5 taking into account the abundance of the two isotopes as found in nature).

Electrons. An electron is a single negative electric charge that contributes virtually no mass to an atom. Electrons move rapidly within specific energy levels (called shells) around the nucleus and therefore the space they occupy takes up virtually the entire volume of the atom. The number of electrons in an atom is generally equal to the number of protons in the nucleus. Electrons are negatively charged, and protons are positive, therefore the charges balance each other and the atom is electrically neutral. Atoms are happiest when the electron shells around the nucleus are full. The innermost shell is full when it possesses 2 electrons, outer shells generally require 8 electrons to be full. Besides the first shell which houses 2 electrons, a stable configuration occurs when the valence shell contains eight electrons. (Noble gasses).

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Prof. C.ValentiMinerals and Rocks 3BONDING Bonding. Because atoms may have varying numbers of valence electrons in their

outer shells, they want to have them ‘full’ so they tend to bond with other elements to become stable, and subsequently form compounds. Most minerals are chemical compounds. Forces holding atoms together are electrical. To be stable, atoms will complete the outermost energy level by combining with other elements to obtain the stable electron configuration of a noble gas. To do this, atoms may have to gain, lose or share electrons with other atoms. If they gain or lose electrons, this will result in atoms to become charged, (+ or -) so that they act like magnets and stick to each other.

Ions and Ionic Bonding. Because atoms have different numbers of electrons in their outer shells they may gain or lose electrons so that the atom is no longer electrically neutral. Atoms that gain or lose electrons are called ions. I.e. Sodium, with an atomic number of 11 (meaning 11 protons, and therefore 11 electrons occupying 3 shells - 2,8,1) loses its outermost shell’s electron so that it has only two FULL shells, the innermost containing two electrons and the outermost containing 8. The sodium ion formed is electrically charged - the loss of one negatively charged electron gives the Na atom a charge of +1. Chlorine, with an atomic number of 17 (17 protons, therefore 17 electrons in three shells - 2,8,7) has only 7 electrons in its outermost shell, but wants 8, and therefore captures and incorporates another electron adding an additional negative charge and giving the chlorine ion a charge of -1. Because opposite charges attract, the positively charged Na will bond to the negatively charged Cl. The mineral halite (table salt) is composed of equal numbers of Na and Cl atoms arranged in a simple, orderly crystalline pattern due to this “electron exchange” between ions. Ionic compounds consist of an orderly arrangement of oppositely charged ions assembled in a definite ratio that provides overall electrical neutrality.

Covalent Bonding. Not all atoms combine by transferring electrons to form ions. Other atoms share electrons to become stable. Diatomic oxygen, hydrogen and chlorine, will bond to itself (2 atoms) to remain stable if it has nothing else to bond to. Here, outer shells overlap and each atom shares an electron, so both atoms are stable (but together). Covalent bonds are much stronger than ionic bonds (silicate) minerals, so they are much more stable minerals in the earth. Diamond, C is a mineral with carbon covalently bound to all other carbon. Hardest most stable mineral on earth.

Isotopes. The mass number of an atom is the number of neutrons and protons in he nucleus. Atoms of the same element always have the same number of protons (atomic number), but may have varying numbers of neutrons (mass number). Variations of these same elements are called isotopes. Carbon 12, carbon 13, carbon 14 all have the same number of protons (atomic number) but different numbers of neutrons, 6, 7, 8 respectively. Since neutrons are not charged, changes in numbers of neutrons will not change the charge of the atom despite changing the isotope.

MATERIALS OF THE EARTH’S CRUST

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Prof. C.ValentiMinerals and Rocks 4 Elements of the earth’s crust. Abundance of elements in the earth’s crust by

weight - Oxygen, Silicon, Aluminum, Iron, Calcium, Sodium, Potassium and Magnesium. (In order of abundance by weight, O and Si are the most abundant elements at the earth’s surface, being the building blocks of most minerals, which form rocks.) The eight most abundant elements account for more than 98% of the weight of the crust.

Minerals. A mineral is composed of atoms arranged in a very orderly, three-dimensional structure (crystalline structure). Defined as the primary building block of rocks (rocks are made of one or more minerals), which are naturally occurring, inorganic, crystalline (definite ordered atomic arrangement of atoms), solid having a definite chemical composition expressed as a chemical formula.

By weight, oxygen accounts for half the crust, but occupies 93% of the volume of an average rock (It’s Big). Because silicon is the second most abundant element in the crust, most minerals contain silica - a term for oxygen plus silicon.

SILICA TETRAHEDRON AND SILICATE MINERALS The fundamental building block is the silica tetrahedron, four oxygen atoms

surrounding one small silicon atom (silicates are the most common rocks at the earth’s surface forming ~92% of the earth’s crust/igneous. The term silicates is used for substances that contain silica. Quartz is made up exclusively of oxygen and silicon atoms (SiO2). Most silicate minerals also contain one or more other elements. Can get various substitutions and or chaining of tetrahedron by attaching them to different elements. Common minerals at the earth’s surface include quartz, feldspar, mica. All abundant minerals at the earth’s surface have a lot of silica and oxygen (recall two most common elements).

A single silica tetrahedron has a formula of SiO4-4 because silicon has an ionic charge of +4 and the four oxygen ions each have a -2 charge - therefore -8 altogether. For the tetrahedron to be stable it must (1) balance the negative charge with positively charged ions or (2) share oxygen atoms with adjacent tetrahedron (reduces the number of negative charges).

The structure of silicate minerals can range from an isolated silicate structure which uses positively charged ions to hold the tetrahedron together to framework silicates which uses shared oxygen atoms among adjacent tetrahedra to hold the structure together.

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Isolated silicate structure. The individual silica tetrahedrons are bonded together by positively charged ions. (I.e. olivine - contains two ions of magnesium (+2) or iron (+2) for each tetrahedron (Fe,Mg)2SiO4.) SiO4 (-4). To become neutral, these tetrahedra join together with positively charged ions (iron, magnesium, calcium, potassium, sodium). Silica tetrahedra are linked together by cations. Each isolated silicate minerals require bonding to 4 cations to become neutral.

Tetrahedra may link up with tetrahedra to form a variety of configurations; other silicate structures. Oxygen ions in tetrahedra will share electrons with other oxygen ions of other tetrahedra and link up forming chains. Forms single chains, double chains and sheet structures. Chain silicates. The pyroxene group is a single chain silicate structure formed

by the sharing of two oxygen atoms of each tetrahedron with adjacent tetrahedra. The ratio of silica to oxygen is 1:3 with a formula of Si)3-2. The negative 2 charge is balanced by positive ions that act to hold the parallel chains together.

Double chain silicates. The amphibole group is a double chain silicate structure formed when every other tetrahedra along a chain shares an oxygen ion with adjacent chain. It structure is like two single chain silicates bound together by the sharing of oxygen (rather than having positively charged ions between the single chains). The double chains are connected by positively charged ions to form the crystal structure.

Sheet silicates. Form when each tetrahedron shares three oxygen ions and the positive ions that hold the sheets together are sandwiched between the silicate sheets (mica).

Framework Silicates. Form when all four oxygen ions are shared by adjacent tetrahedron to form a complex three dimensional framework. (quartz, feldspar).

The ratio of oxygen atoms to silicon atoms differs in each of the silicate structures. In the isolated tetrahedron there are 4 oxygen atoms for every silicon atom, in the single chain the oxygen-to-silicon ratio is 3 to 1, and in the three-dimensional framework this ratio is 2 to 1. Consequently, as more of the oxygen atoms are shared, the percentage of silicon in the structure increases. The silicate minerals therefore have a high or low silicon content based on their ratio of oxygen to silicon. In turn, as you increase the silica to oxygen ratio, you decrease the inclusion of positively charged metallic ions like iron and magnesium. Therefore minerals rich in silica are low in iron and magnesium and vice versa.**Felsic minerals contain large quantities of feldspar and silica, are lighter in color and density. Felsic rocks contain lower percentages of iron and magnesium. **Mafic minerals contain more iron and magnesium percentages and are darker in color and denser. Examples include augite, olivine, hornblende, and micas. (Made of heavier elements).

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Prof. C.ValentiMinerals and Rocks 6 Important non-silicate minerals. Other minerals are considered ‘scarce’ as

compared to silicate minerals but are very important. Carbonates calcite, calcium carbonate, salts halite and gypsum.

Minerals1. It must be a crystalline solid2. It must occur naturally3. It must be inorganic4. It must have a definite chemical composition5. It must possess characteristics physical properties

Crystalline solid - Crystallinity is an orderly arrangement of atoms. Atoms are arranged in a regularly repeating, orderly pattern. The type, or ‘architecture’ of a crystal is the relative size of adjacent atoms.Natural and Inorganic Substances - Excludes manmade crystalline compounds and substances that are part of plants and animals.Definite Chemical Composition - due to the orderliness of crystalline substances. Every sample of a given mineral will always produce the same ratios of elements. Therefore, the composition of any mineral can be expressed as a chemical formula. (i.e. quartz is SiO2, always has 2 Oxygen to 1 silica.)

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PHYSICAL PROPERTIES OF MINERALSPhysical Properties - Characteristic physical properties are the bases by which minerals are usually identified, like color, streak, luster, hardness, external crystal form as well as taste, smell, magnetism. To identify an unknown mineral, determine its physical properties then match the properties with the appropriate mineral, using a mineral identification key - Appendix A.Color - The most ambiguous of physical properties (i.e. quartz may be white, pink, black, yellow or purple.) Color is extremely variable. Minute chemical impurities can strongly influence it. Streak - Streak is the color of the pulverized mineral, and is more reliable than the color of the specimen itself. Scraping the edge of a mineral sample across an unglazed porcelain plate leaves a streak that may be diagnostic of the mineral. However, few silicate minerals leave a streak because most are harder than the porcelain streak plate.Luster - The quality and intensity of light that is reflected from the surface of a mineral. Luster is either metallic or nonmetallic. A metallic luster gives a substance the appearance of being make of metal…but may be shiny or dull, like the two sides of a piece of aluminum foil. Nonmetallic luster does not look like metal or rust. Glassy (or vitreous) luster is a type of nonmetallic luster which gives the mineral a glazed appearance, like glass or porcelain. Most silicate minerals have glassy luster. Earth luster resembles unglazed pottery and is characteristic of many clay minerals. Includes resinous luster (resin), silky luster, and pearly luster.Hardness - harder minerals are able to make a scratch on a softer mineral. Minerals can be compared to Moh’s hardness scale on which ten minerals are designated as standards of hardness. Can also compare the hardness of minerals to common objects like a fingernail - 2.5, penny 3.5, knife - about 5.External Crystal Form - a set of faces that have a definite geometric relationship to one another. Most minerals are able to develop their characteristic crystal faces only if they are surrounded by a fluid that can be easily displaced as the crystal grows. In rocks, however, most minerals grow while competing for space with other minerals. Cleavage - Cleavage is the ability of a mineral to break along preferred planes. It may be indicated by the mineral’s tendency to split apart along certain preferred directions. A mineral tends to break along certain planes because the bonding between atoms is weaker there. (quartz is equally strong in all directions and therefore has no cleavage, mica is easily split apart into sheets. Minerals may be characterized by one, two, or more cleavage directions (mica - one plane, feldspars - two directions at 90 degree angles to each other, halite - three directions at 90 degrees to each other, calcite - three cleavages.)Fracture - is breakage not along a cleavage plane. Minerals which have no cleavage have an irregular fracture. Conchoidal fracture.Specific Gravity - The density or ‘heaviness’ of a mineral in comparison to the weight of equal volumes of water. Liquid water has a specific gravity of 1. Most silicate minerals weigh two and a half times as much as equal volumes of water.Other properties - taste, smell, striations - straight, parallel lines on the flat surfaces of one of the two cleavage directions, magnetism, double refraction of calcite, reaction of carbonate minerals with weak acid to form CO2.

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Prof. C.ValentiMinerals and Rocks 8THE ROCK CYCLEThe definition of a rock is any naturally formed, nonliving, firm, and coherent aggregate mass of solid matter that constitutes part of a planet. The word mineral does not appear in the definition because rocks can be made of materials that are not minerals such as natural glass (obsidian) and organic matter.

There are three large families of rock, each defined by the process that form the rocks.1. Igneous rocks. Form by the cooling and consolidation of magma.2. Sedimentary rocks. Form either by chemical precipitation of material carried n

solution (ocean, lake, river) or by deposition of particles of regolith transported in suspension by water, wind, or ice.

3. Metamorphic rock. Forms when a parent rock is changed as a result of high temperatures, high pressures, or both. The parent rock undergoes a series of chemical reactions as a result of the high temperature/pressure conditions resulting in the formation of new compounds.

Rocks do not remain in their original form indefinitely but instead are always in the process of transformation. Explain rock cycle. The rock cycle is the largest of many natural cycles on earth and relates geologic processes to the formation of the three types of rocks. To operate, the rock cycle requires the earth’s other natural cycles. The tectonic cycle is required for energy, the geochemical cycle for materials, and the hydrologic cycle for water.

The general model of the rock cycle is that the three rock families - igneous, metamorphic, and sedimentary - are involved in a worldwide recycling process. The cycle starts through the formation of igneous rocks by crystallization of a magma in the interior. The igneous rock then is uplifted to the surface in the mountain-building process. There the rocks are exposed to erosion and weathering, which produce sediments. The sediment is cycled back to the interior by burial and lithification to sedimentary rock. Further burial leads to metamorphism or melting, starting a new cycle.

Although the term rock cycle implies an orderly progression as above from one type of rock to another, such a regular sequence does not occur. The rock cycle simply expresses the important concept that rocks are not permanent, but change continuously over geologic time. The three groups of rocks are related by the rock cycle in that each is formed from the others. In geologic time it is common for earth processes to convert a sedimentary rock to a metamorphic rock or an igneous rock to a sedimentary rock. For example, igneous rocks form by crystallization from a magma, a mass of melted rock that originates deep in the crust or upper mantle by the melting of preexisting rock of any kind - other igneous rocks, metamorphic rocks, or sedimentary rocks.

The tectonic cycle provides several environments for the rock cycle. Because the plates move in different directions, they bump and grind together at their boundaries

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Prof. C.ValentiMinerals and Rocks 9constructing and modifying the earth’s crust. For instance, most melting and formation of igneous rock take place along convergent plate boundaries. A convergent boundary develops where two plates are moving horizontally toward each other and therefore are colliding. Collisions can occur between an oceanic plate and a continental plate, between two oceanic crust plates, and between two continental plates.

When a continental plate collides with a denser oceanic plate, the oceanic plate sinks beneath the continental plate and dives into the mantle. This process is called subduction. Because oceanic crust is covered by the sea, the upper part of the subducting plate consists of water soaked seafloor rock. As the subducting plate sinks, water from the subducting plate causes the rock to melt forming huge quantities of magma. Some magma solidifies within the crust to form coarse grained igneous rock, while some erupts onto the earth’s surface to form volcanic igneous rock. Many of the world’s mountain chains form at these types of subducting zones through volcanic eruptions and the crumpling and buckling of the earth’s crust.

Subduction also occurs where two oceanic plates collide. When two oceanic plates collide the older, cooler, and denser oceanic plate subducts into the mantle. Great quantities of magma form and rise toward the earth’s surface onto the seafloor forming submarine volcanoes which eventually grow above sea level to form a chain of volcanoes called an island arc.

As two continental plates collide, subduction cannot occur because the continental crust is too light to sink into the mantle. As a result, the two continents crumple against each other forming huge mountain chains in the process of orogeny.

The igneous rocks formed during plate collisions now exposed at the surface far from its birthplace in the earth’s hot interior, begins to weather. The rock debris, consisting of altered and unaltered minerals and the dissolved substances produced by weathering are transported to the ocean, there to be deposited as layers of sediment. As these sediments laid down are buried by successive layers of sediment, they gradually lithify into sedimentary rock. Any type of rock, however, whether metamorphic, sedimentary, or igneous, can be weathered and eroded in this way to form new sedimentary rock.

As the lithified sedimentary rock is buried more deeply under layers of sediment, it gets hotter. As temperatures climb to over 300C the rock minerals start to change into new kinds of minerals that are more stable at the higher temperatures and pressures of the deeper parts of the crust. This is the process of metamorphism, which transforms different types of rocks into metamorphic rocks. Metamorphic rocks are produced by mineralogical and textural transformations of all kinds of rocks - igneous, sedimentary, and metamorphic - under the influence of high temperatures and pressures deep in the earth. With further heating, however, the rocks may melt forming a new magma from which igneous rocks crystallize.

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As is apparent the rock cycle never ends. It is always operating, at different stages in different parts of the world. At any given moment, mountains are forming and eroding in one place and sediments are being laid down and buried in another. The rocks that make up the solid earth are being recycled continuously. No particular rock is permanent over geologic time, all rocks change slowly from one of the three rock types to another.

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WHAT TO KNOW: Minerals and Rocks

Silicate minerals are the most abundant on the earth’ surface where the elements oxygen and silicon dominate (silicon tetrahedron).

Felsic and mafic minerals, their color and density characteristics and how they are relevant to igneous rocks.

Rock cycle and how rocks form and are classified. The atom and its sub atomic particles. Know subatomic particle charges, and

which contributes to mass and atomic number. Also, what an isotope is and how an atom is physically different from its isotope.

Know valence shells, and how many electrons are required to be stable. When would an electron donate/accept an electron.

What are atomic bonds. How do ionic bonds form? What happens to the electrons during ionic bonding?

Covalent bonding. How is it different from ionic? Which type of bonding is more stable and why?

What are the most abundant elements of the earth’s crust? Definition of a mineral. Physical properties of minerals, how to use them, what they indicate. Silicate minerals. The silica tetrahedron, isolated, chain, double chain, sheet and

framework silicates. How are they different? How does the ratio of oxygen to silica change with these structures? What happens to the cation ratio as you change structure?

Principles of Bowen’s Reaction Series

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