Minerals. Atoms, Elements, and Chemical Bonding. Definition of a Mineral 2-1

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1 Minerals In order to define a what we mean by a mineral we must first make some definitions: 2-1 Most of the Earth s surface is composed of rocky material. An element is a substance which cannot be broken down into other substances by chemical means. What is a rock? Rock is naturally formed, consolidated material composed of grains of one or more minerals. Poor medieval alchemists! An atom is the smallest possible particle which retains the properties of an element. Unconsolidated individual mineral grains can also occur. For example, in soils. What is a mineral? Definition of a Mineral Atoms, Elements, and Chemical Bonding What do we mean by a crystalline solid or chemical composition? How do atoms bond together? Why do atoms bond together? Why do some atoms bond together while others do not? To answer these questions we need to delve into some chemistry.

2 Chemical Bonding Atoms can join together in specific proportions to form molecules. Examples? Why do atoms like to get together and form bonds? According to quantum mechanics, electrons can only occupy certain orbits. 2-2 A few elements already have complete outer orbital levels (Noble gases). Are they likely to react? Other elements will attempt to fill their outer levels by reacting with other atoms. How? Ionic Bonding In some cases one atom really wants to get rid of electron(s) while another atom really wants electron(s). In this case the first gives electron(s) to the second. Only a certain number of electrons are allowed to occupy each orbit (usually 8). The two atoms are now electrically charged (the first positive, the second negative). Atoms are most stable when they have a full outer orbital level. ==> they electrically attract each other forming an ionic bond. Covalent Bonds When the outer orbital level isn t either nearly empty or nearly full atoms don t have a strong tendency to gain or lose electrons. Metallic Bonds In metallic bonds, rather than share electrons with a single neighbor, atoms form a lattice of positively charged nuclei and allow their outer electrons to roam free. In these cases atoms tend to complete their outer orbitals by sharing electrons, rather than completely transferring electrons. This type of bonding is called a covalent bond. This is why metals are typically such good conductors of electricity. Intermolecular Bonding In general, covalent bonds are quite strong. A final type of bonding is one that can occur between molecules. In some molecules which are covalently bonded, one atom has a greater affinity for the electrons.

3 Thus the electrons spend most of their time near one atom. Properties of the Most Common Minerals 2-3 In some sense, the bond is partially ionic. More than 3000 minerals have been identified. This leads to an uneven distribution of charges which can lead to attraction between molecules. Even when the atoms have the same affinity for electrons, random fluctuations can lead to brief fluctuations in the distribution of charges van der Waals bond. More are being identified every day (~40/year). Fortunately for budding geologists, most are extremely rare. Only about 250 are present in significant amounts. Of these only about 50 are common in the crust. Much weaker than other types of bonds. Today we will examine some of the most common minerals and their properties. Which are the most common? Most Abundant Elements in the Entire Earth: Crust composition differs from the Earth as a whole and from the Mantle. Element Proportion by Weight (%) Iron (Fe) 34.8 Oxygen (O) 29.3 Silicon (Si) 14.7 Magnesium (Mg) 11.3 Sulfur (S) 3.3 Nickel (Ni) 2.4 Calcium (Ca) 1.4 Aluminum (Al) 1.2 Others 1.6 Silicon, oxygen more abundant in the crust than mantle. Al, Ca, Na, K, U are also enhanced in the crust. Iron and magnesium more abundant in the mantle.

4 Oxygen and silicon are by far the most abundant elements in the crust. Isolated silicate structure: 2-4 Abundant in the mantle too (though less dominant). Because of this, by far the most abundant minerals are those containing oxygen and silicon (90%+). These minerals are called silicates. In this structure, none of the tetrahedra share oxygen atoms. The tetrahedra are bonded together by positively charged ions. Note: silica is a term referring to oxygen + silicon. All silicates have a base unit consisting of one silicon atom surrounded by 4 oxygen atoms called the silica tetrahedron: (SiO4) 4 The main categories of silicates are based on how these tetrahedra are connected. Example: Olivine in which the tetrahedra are held together by 2 ions of Fe or Mg per tetrahedra: (Fe,Mg)2SiO4 Bonds tend to be quite strong ==> No preferred bond directions ==> Single Chains In this structure each tetrahedra shares two oxygen atoms with neighbors forming long chains. These chains are held together by positive ions between the chains. The bonds between chains are relatively weak. Would you expect these types of minerals to exhibit cleavage? Double Chains These are similar to single chains except two chains are attached to each other by shared oxygen atoms in every other tetrahedra. This group is called the amphiboles. Double chains held to each other by positive ions. As with the pyroxenes, bonds between double chains relatively weak. Would you expect them to be hard or soft? Would you expect these minerals to exhibit cleavage? Minerals of this structure form the pyroxene group. What about their hardness?

5 Sheets Framework Structures 2-5 In this structure each tetrahedra shares 3 oxygen atoms with its neighbors forming a sheet. In this type of structure each tetrahedra shares all four of its oxygen atoms with neighbors. Examples include the mica and clay mineral groups. Most abundant group of minerals in the crust. Sheets held together very weakly by positive ions. Expected cleavage? Quartz (SiO2) Any planes of weakness? Hardness? Bonds are quite strong in quartz: Feldspars: Most abundant minerals in the crust. Al ions replace some of the Si in the tetrahedra. Nonsilicate Minerals While the silicates are the most abundant minerals, there are other classes of minerals which are important. Other ions (Na, Ca, K) interspersed within the framework as well. Most of our economically valuable minerals (ores) are nonsilicates. Because of the inclusion of these other ions, Feldspars often do exhibit some cleavage. Carbonates (contain the (CO3) 2 group): Calcite (CaCO3) most abundant nonsilicate. Found in limestone and marble.

6 Sulfides (contain S 2 ): 2-6 Halides (contain Cl, F, or Br ions): Halite, NaCl common salt. Flourite, CaF2 Major source of many metals. e.g. Galena (PbS) major ore source for Pb. Pyrite (FeS2) fools gold Oxides (metals combined with O but not containing Si, S, or C): Hematite, Fe2O3 major ore of iron. Sulfates (contain (SO4) 2 ): Gypsum (CaSO4 2H2O) Bauxite, Al2O3 major ore of aluminum Native Elements: Gold (Au) Silver (Ag) Diamond (C) Introduction to the Rock Cycle Igneous Rocks Rocks generally consist of consolidated mineral grains. Different minerals are found in different types of rocks. Rocks fall into three broad categories: Igneous Rocks Sedimentary Rocks Metamorphic Rocks Igneous rocks form when a melt cools down and solidifies. Igneous rocks are in some sense pristine rock. Can be formed either underground or on the surface If the rock is exposed at the surface the environment will start to work on it... Each of these types of rocks are formed in different ways and tend to contain different minerals.

7 Sedimentary Rocks Metamorphic Rock 2-7 Eventually a rock exposed at the Earth s surface will be broken apart, either mechanically or chemically. If a rock is buried to substantial depth it will be subjected to substantial pressure and temperature. Often this material will be transported and eventually deposited. If high enough, the pressure and/or temperature may lead to a transformation of the rock and the minerals it contains. If it is then buried it may become cemented together forming a sedimentary rock. ==> Metamorphic rock. Thus igneous rocks can be transformed into sedimentary rocks. Sedimentary rocks into metamorphic rocks. The concept that different types of rocks can be transformed by various processes into the other classes is known as the rock cycle. Identifying Minerals 1. Crystal Form if a crystal is large enough, its form can be distinctive and help identify a mineral. 2. Luster the way a mineral reflects light. 3. Color obvious. Unfortunately, the color of a mineral is often not very diagnostic. 4. Streak Color of a mineral in powdered form. 5. Hardness How resistant a mineral is to being scratched. Note that the sequence does not have to go igneous - > sedimentary -> metamorphic -> igneous 6. Cleavage/fracture Does a mineral break along preferred planes or not? 7. Specific gravity How dense a mineral is. While difficult to pin down in the field, one can often tell a particularly heavy or light mineral. 8. Other properties Magnetic? Taste, smell...