Building Blocks of Rocks and Economic Resources Minerals Building Blocks of Rocks and Economic Resources GLY 2010 - Summer 2013 Lecture 4

Minerals Minerals are a major building block of most rocks Their properties determine a good part of the physical behavior of the earth

Mineral Definition Naturally occurring Inorganic Crystalline Naturally occurring - not made by man Inorganic - not formed by a living organism - Teeth, kidney stones, etc. are not minerals Crystalline - Composed of atoms arranged in a definite three-dimensional pattern

Crystal Structure Examples Halite (left) is common table salt, or sodium chloride, chemical formula NaCl. The large green spheres represent chloride anions ions, while the yellow spheres represent sodium cations. Fluorite (right) is CaF2 (calcium difluoride). It is Moh’s scale mineral 4, and often occurs in interesting crystals, with either an octahedral or a cubic shape. As the labels indicate, blue spheres are calcium cations, and green spheres are fluoride anions. In order for a substance to be crystalline, the atoms must be arranged in a definite pattern. These figures show two common patterns. There are over 3500 known minerals, with many different possible patterns. Halite Fluorite

Formation of Halite

Atoms Building blocks of all matter - Electrically neutral Originally thought to be tiny, indivisible particles, but now known to split under certain conditions They are also known to be composed of smaller particles Protons (+1) and neutrons (0) in the nucleus, surrounded by cloud of electrons (-1) Nucleus is about 10-15 meters, or 1 fermi unit (Unit was named after the Italian physicist Enrico Fermi, who first split the atom in 1942) The nucleus is surrounded by an electron cloud, whose size is about 10-10 meters Thus the nucleus is about 100,000 times smaller than the entire atom

Atomic Terminology The atomic number equals the number of protons. Thus Hydrogen, with one proton, has atomic number 1, and iron, with twenty-six protons, has atomic number 26. The atomic weight equals the number of protons plus the number of neutrons. If an iron atom has 26 protons, and 30 neutrons, it has an atomic weight of 56.

Chemical Elements An element is composed of atoms with the same atomic number Each element has a unique chemical symbol Hydrogen consists of all those atoms whose atomic number is one, regardless of the atomic weight. For Hydrogen, it is H. For iron, Fe (from the old name, ferium).

Isotopes An isotope of an element is an atom with the correct number of protons for that element, plus a fixed number of neutrons Example: Carbon has three isotopes, each with six protons, and with 6, 7, or 8 neutrons

Stable or Radioactive An isotope may be stable or radioactive Carbon isotopes with 6 or 7 neutrons are stable, while the isotope with 8 neutrons is radioactive

Chemical Symbols Atomic number is shown as a subscript before the element symbol - 1H The atomic weight is shown as a superscript before the symbol - 56Fe The atomic number is actually redundant, since the chemical symbol implies the atomic number

Examples of Chemical Symbols Particular isotopes are shown using a superscript in front of the symbol 1H is normal hydrogen, with one proton and no neutrons 2H is deuterium, with one proton and one neutron 3H is tritium, with one proton and two neutrons - it is radioactive

Ions Ions are charged particles Cations: Atoms that lose one or more electrons become positively charged Anions: Atoms that gain one or more electrons are negatively charges Ionic charge: Shown by a superscript after the chemical symbol, O2-

Use of Isotopes Chemical tracers Study topics such as: Pollution Formation temperature The path of volcanic emissions, etc Radioactive isotopes are used in estimating the age of materials

Compounds Combination of two or more atoms Combination is called a molecule Water H2O Carbon dioxide CO2 Water is a combination of two hydrogen and one oxygen atoms - H2O - here the subscript after the hydrogen tells us there are two hydrogens

Molecules Molecules may consist of just one element Oxygen in the atmosphere is O2 Molecules may consist of several elements, in various amounts Example: Plagioclase feldspar, the most common mineral on earth NaAlSi3O8 - one sodium (Na), one aluminum (Al), three silicons (Si), and eight oxygens (O)

Chemical Bonds The “glue” that holds materials together Responsible for the properties of matter On an atomic scale At the scale of the earth When two atoms combine to form a chemical bond, energy is released

Types of Bonds Ionic Covalent Metallic Hydrogen Van der Waals Each type is discussed separately on following slides

Ionic Bonds Bonds between a cation and an anion They occur when a cation donates one or more electrons to an anion They are strong Dissolve in water Halite, or table salt, for example

Covalent Bonds Equal sharing of electrons by two atoms Very strong bonds Compounds usually not soluble May create molecules that do not readily combine to form larger particles Ex. Carbon dioxide is strongly bonded within the molecule, but weakly bonded between molecules, so it is a gas

Metallic Bonds Outer electrons are loosely held Properties: Opaque, may have a metallic luster Bond strength is moderate Allows them to move between atoms within the solid Produces high thermal and electric conductivity

Hydrogen Bonds Secondary bond between oxygen on one water molecule and hydrogen on another Accounts for high melting and boiling points of water Allows DNA strands to unzip, and later recombine with another strand in sexual reproduction Weak bonds - About 10-15% as strong as an ionic bond Hydrogen bonding in water

Van der Waals Bonds Diamond Graphite Accounts for the formation of liquid Helium at 4 degrees above absolute zero (4K) Occurs in some minerals – Graphite, between layers of carbon atoms Residual, extremely weak bonds form by distortion of electron clouds by the presence of a nearby atom

Molecules - as strong as the weakest bonds within themselves Molecular Properties Molecules - as strong as the weakest bonds within themselves The oxygen-oxygen (in 02) bond is relatively weak, so these bonds break easily and oxygen reacts with other substances The nitrogen-nitrogen (in N2) bond is strong, and nitrogen usually will not react

Mineral Properties Depend on the type and strength of bonds and number of bonds (bond density) within themselves Minerals will be examined in the laboratory, and most properties will be taught there Examples of mineral properties: hardness, cleavage

Hardness A mineral’s hardness is measured by the ability of a surface to resist abrasion Hardness is a directional property, although for most minerals the differences are not observable Fredrik Mohs developed a scale of hardness, based on 10 mostly common minerals

Moh’s Scale of Hardness Any higher number mineral will scratch any lower number. Moh’s scale is not linear – the difference in hardness between low numbers is much less than between high numbers

Cleavage When a mineral always or usually breaks along a particular plane, it is said to have a cleavage plane Minerals may have no cleavage, or up to six planes Cleavage planes occur where there are weak bonds, or low bond density across a plane (Examples on next slides)

Two-directional Cleavage Selenite, a variety of the mineral gypsum, shows cleavage in two directions

Angle Between Cleavage Planes

Three-directional Cleavage Halite, common table salt, shows three directions of cleavage at right angles

Three-directional Cleavage Calcite shows three directions of cleavage, not at right angles This specimen also shows the property of double refraction. The single cross on the paper is transmitted as two crosses within the crystal

Four Directional Cleavage

Crystal and Crystal Faces A mineral sample in which the internal orderly arrangement of atoms is reflected in the symmetrical orderly arrangement of the external surfaces (crystal faces) of a crystal Apatite, showing a hexagonal prism - these are crystal faces, not cleavage planes

Identification of Minerals Minerals are identified based on their physical and chemical properties A combination of properties are needed, just as no single line from a fingerprint can identify a person No single property can identify a mineral. A combination of properties are needed, just as no single line from a fingerprint can identify a person You will learn more about mineral identification in the laboratory

Mineral Classification Classification is based on anion type Minerals with the same anions have similar properties, while those with the same cations often do not Mineralogy, the study of minerals, attempts to classify minerals into groups with similar properties

Anions Anions may be a single ion Ex. Oxygen O2- Anions are often groups of atoms, with the entire group having a negative charge Ex. Carbonates are CO32- , one carbon with three oxygens, and the whole group with a minus 2 charge

Common Anion Groups Silicates, SiO44- Oxides, O2- Sulfides, S2- Carbonates, CO32- Phosphates, PO43-

Occurrence of Minerals Over half of all known minerals are silicates, because oxygen is the most common element on earth, and silicon is the second most common. Silicates are the most important type of rock-forming minerals, those minerals that make up most of the earth’s rocks Most silicate minerals contain other elements in addition to silicon and oxygen

Silicon Tetrahedron The SiO44- tetrahedron is the basic building block of silicate minerals

Silicate Structures

Chemistry of the Continental Crust Eight elements account for almost all of the earth’s crust Chart is based on weight percent Oxygen is the most abundant, and silicon the second, which is why most minerals are silicates

Felsic Minerals Minerals with a lot of aluminum and silicon are light in color, and are called Felsic Many silicate minerals are subdivided based on their chemistry Plagioclase feldspar, the most common mineral in the earth’s crust

Mafic Minerals Minerals with more iron and magnesium, and less silicon, are dark in color and are called Mafic (from the first two letters of magnesium and the first letter of ferium) Augite, a type of pyroxene