Physical Geology

Slide1 : GEOL 2101 - Winter 2003 Physical Geology Office: NSci 353 (Northeast side by door) Phone: (510) 885-3083 Email: lstrayer@csuhayward.edu Office Hours: M & T 11:30 – 1:00 p.m. (& arranged) Dr. Luther M. Strayer Structural Geology & Tectonics

Slide2 : Chapter 2 Matter & Minerals Common uses of minerals: gem stones (diamond, ruby, emerald, amethyst, etc.) graphite in pencil lead talc for talcum powder quartz for silicon chips halite for table salt copper for electronics and wiring Many thousands of uses! If we can't grow it, then we have to mine it!

Slide3 : Matter & Minerals Geologic processes depend upon geologic materials (rocks, which are composed of minerals) volcanic eruptions earthquakes landslides erosion mountain building ground water A basic knowledge of Earth materials is essential to understanding geologic processes.

Slide4 : Mineral: any naturally occurring inorganic solid that possess an orderly internal structure and a definite chemical composition. 1. occur naturally (concrete, synthetic diamonds, etc. are excluded. 2. inorganic (teeth, seashells, trees, etc. are excluded) 3. solid (gases and liquids are excluded) 4. orderly internal structure (amorphous solids like glass are excluded) 5. definite chemical composition example C Diamond NaCl Halite (Na,Ca)Al(Si,Al)Si2O8 Plagioclase

Slide5 : Mineral: any naturally occurring inorganic solid that possess an orderly internal structure and a definite chemical composition. There are nearly 4000 minerals and each is uniquely defined by its chemical composition and internal structure.

Slide6 : Rock: any solid mass of mineral, or mineral-like, matter that occurs naturally as part of our planet. A rock may be monomineralic, that is, contain only one mineral (ex. limestone) or polymineralic, with many minerals (ex. granite). A few rocks are composed of nonmineral materials (mineraloids) such as obsidian, pumice and coal.

Slide7 : A few minerals, such as diamond, graphite, gold and sulfur are made up entirely of one element; most minerals are a combination of two or more elements that are chemically bonded. Two simplified models showing atomic structure: The nucleus is the central region containing dense protons (+) and neutrons (0). Electrons (-) surround the nucleus and travel at high speeds. (A) is the more common model of atoms - but electrons do not travel in orbital planes. (B) is the more realistic model of the atom where electrons are found in shells. The number of protons found in an atom's nucleus determines the atomic number and name of the element. All atoms with 6 protons are carbon, however, the number of neutrons in the nucleus of the carbon atom can vary.

Slide8 : Periodic Chart of the Elements

Slide9 : Bonding in Minerals Elements combine with each other to form a wide variety of more complex substances or compounds. A chemical bond is the strong attractive force that links individual atoms together. The forces that hold atoms are electrical in nature - that is, they involve the electrons of the atoms. Most atoms are chemically reactive and bond in order to achieve a noble gas configuration (8 electrons in the outer shell) while keeping overall electrical neutrality in the new compound. There are several different types of bonds that are common in minerals: a. ionic b. covalent c. hybrid d. metallic

Slide10 : Ionic Bonding An electron is transferred from one element (metal such as Na) to another (non-metal such as Cl) and the bond is maintained by the electrostatic atttraction of the unlike charged atoms Na+ = +1 Cl- = -1 Ionicic bonding occurs in the mineral halite - common salt.

Slide11 : Ionic Bonds In ionic bonds, one or more electrons are transferred from one atom to another. Thus, one atom as an excess of electrons and has a negative charge and the other has a deficit of electrons and has a positive charge. Atoms which have and electrical charge (because of an unequal number of electrons and protons) are called ions. cation - positively charged ion anion - negatively charged ion In this example, sodium (Na) donates one electron to chlorine (Cl). Thus Na is positively charged (cation) and Cl is negatively charged (anion). Since each ion is oppositely charged, they are attracted to one another (opposite charges attract and like charges repel each other). It is this electrostatic attraction between the cations and anions that bonds the mineral together.

Slide12 : Properties of chemical compounds are dramatically different from the properties of the pure elements. Pure chlorine is a poisonous gas and pure sodium is a highly reactive metal. However, bonded together, they form common table salt. This figure illustrates the arrangement of Na and Cl atoms in halite. The crystal consists of alternating Na and Cl atoms, positioned so that each positive ion (cation) is surrounded by negative ions (anions). Since each ion is surrounded by other ions of opposite charge, there is attraction that holds the structure together. If ions of like charge where packed next to one another, they would repel one another and the structure would not be stable. (A) The structure has been "opened up" to show the arrangement/structure of ions. (B) This represents how the actual ions are configured.

Slide13 : Covalent Bonding Atoms share electrons in outer shells. The shared electrons give the atoms (in this example, carbon C) a neutral charge. Covalent bonds are very strong - diamond consists of carbon atoms covalently bonded together.

Slide14 : Covalent Bonds In covalent bonds, atoms share one or more outer electrons from their outer shell. In the illustration, two chlorine atoms share a pair of electrons. Note that in the previous example, Cl formed an ionic bond with Na. However, in this example, a covalent bond is formed. Thus the type of bonding not only depends on the element but what it is bonding with. The most common group of minerals are the silicates. Silicate minerals consist of silicon covalently bonded with oxygen. These are minerals such as quartz, feldspar, amphibole, pyroxene and olivine and micas.

Slide15 : Other Bonds Covalent and Ionic bonds are only ideal or theoretical models. In the real world, most bonds are hybrids - there is some degree of electron donation (ionic) and sharing (covalent). An additional complication is that ionic and covalent bonds may coexist between different atoms in a complex mineral structure. For example, in mica minerals, the silicon and oxygen are covalently bonded together. However, these silicon and oxygen units are then bonded to various metal ions by ionic bonds. Metallic bonds In a small number of minerals, metallic bonds are important. These are chemical bonds where electrons are allowed to migrate from atom to another and is common in metal minerals such as gold, copper and silver. This ease of electron migration is the reason why metals conduct electricity so well.

Slide16 : The Structure of Minerals What determines the particular crystalline structure of a mineral? 1. Size of the ions (most important) 2. Charges of the ions This figure shows the ideal geometric packing for ions of different sizes. The relative sizes of the ions determines how many ions of the opposite charge that you can pack. We have seen that Na and Cl ions are packed together to form a cubic structure. This orderly arrangement of ions is reflected in the physical properties of the mineral Halite: 1. it forms cube-shaped crystals 2. it has cubic cleavage ( breaks in 3 perpendicular directions)

Slide17 : Polymorphism Every specimen of a mineral must have the same internal structure. If two minerals have the same chemical formula but have different structures they are polymorphs. Diamond and graphite have the same chemical formula (C) but have different structures. The structure of each mineral determines the physical properties. Diamond has very strong covalent bonds and thus is very hard and resistant. Graphite has weaker electrical bonds which accounts for its softness and excellent cleavage.

Slide18 : The (microscopic) structure of a mineral is expressed in the macroscopic physical properties of the mineral: 1. Crystal Form - symmetry and shape of crystals. 2. Luster - how light is reflected on the surface. 3. Color - not diagnostic for many minerals. 4. Streak - color when mineral is powdered on unglazed tile. 5. Hardness - resistance of mineral to abrasion or scratching. 6. Cleavage - tendency to break along planes of weak bonds. 7. Fracture - minerals that do not exhibit cleavage. 8. Specific Gravity - ratio of the weight of a mineral to the weight of an equal volume of water. 9. Other Properties - these properties are important for a small number of minerals. a. magnetism b. double refraction c. reaction to acid d. taste Physical Properties of Minerals

Slide19 : Mineral Groups There are nearly 4000 minerals that have been identified so far. Approximately 40 - 50 new ones every year! Luckily, only a few dozen are abundant and are "rock-forming" minerals. In addition, only 8 elements make up the bulk of these mineral and represent over 98% of the Earth's crust. Minerals are classified by their chemistry and put into chemical groups. The two most abundant elements are oxygen and silicon which combine to form the framework for the most common mineral group - the silicates (SiO4 4-). Other mineral groups include the carbonates (CO3 2-), sulfides (S2-), oxides (O2-) and halides (column VIIA - periodic table).

Slide20 : Silicates All silicate minerals have the same fundamental building block - the silica tetrahedron. It consists of 4 oxygens ions surrounding the smaller silicon ion. The silica tetrahedron is a complex ion (SiO44-) with a charge of -4. The simplest way for silicate minerals to achieve electrical neutrality is by bonding with positively charged ions (cations) such as Fe2+ , Ca2+ , Mg2+ , Na+ and K+. Silica tetrahedra may link together to form single-chains, double-chains, and sheets by sharing oxygen ions between them.

Slide21 : Silicates Complex silicate minerals are neutralized by the inclusion of metallic cations that bond them together in complex crystal configurations. The cations that most commonly link silicate minerals together are the same elements that are the most abundant in the Earth's crust. Note that each of these cations has a particular size and charge. Mg2+ and Fe2+ have the same charge and are nearly the same size. Thus they may substitute for one another in minerals. Example: Olivine Also note that Ca2+ and Na+ are approximately the same size. However, they have different charges and the crystal lattice must substitute something else in addition to maintain electrical neutrality. Example: Plagioclase (Na,Ca)Al(Si,Al)Si2O8

Slide22 : Common Silicate Minerals Feldspars are the most abundant minerals - more than 50% of the Earth's crust. Quartz is the second most abundant (only common mineral made of only SiO2).

Slide23 : Ferromagnesian (Dark) Silicates Nonferromagnesian (Light) Silicates Tend to be dark colored due to the presence of iron (Fe) and magnesium (Mg), and a high specific gravity. Olivine Pyroxenes Amphiboles Biotite Garnet Tend to be light colored and have lower specific gravity than the Mafic minerals. Muscovite Plagioclase Feldspars Orthoclase Feldspars Quartz

Slide24 : Important Nonsilicate Minerals Other mineral groups are based on ions other than silica tetrahedron. They are less common than the silicate minerals but are important components of the Earth and have economic uses.