GEO-113: ENVIRONMENTAL GEOLOGY
MINERALS AND ROCKS
In order to fully appreciate the hazards and resources presented by the various geologic processes you must have a basic knowledge of minerals and rocks - make up all geologic materials. Can't understand rocks (or any other geologic material) without knowing about minerals.
I. Mineral: A basic component of all geologic materials. It is a naturally occurring, usually inorganic, homogeneous solid element or compound with a definite but not fixed composition and a regular ordered internal (crystalline) structure. What does this all mean?
1) Naturally occurring: Can not be strictly human-made or the result of human activity. Must be found somewhere in nature.
2) Inorganic: Typically not just made by living organisms and not made of organic compounds (combinations of N, O, H, and C).
2) Homogeneous solid: No liquids or gasses. Cannot be subdivided into simpler minerals. Not a mixture of other minerals. At some small scale this often is the case.
3) Definite, but not fixed composition: Made up of one or more of the naturally occurring elements. Composition is not random. It can be defined by a chemical formula - expresses the identity and ratio of the atoms present.
Ex. NaCl, C, SiO2, KalSiO3O8
Composition may vary within limits - atomic substitution (K->Na, Fe->Mg).
Atom is the smallest unit of an element that retains the chemical characteristics of that element. Made of even smaller particles.
Nucleus: Contains protons and neutrons. Contain most of the mass of the atom but little of its volume.
Protons are positively charged. Number of protons in nucleus gives atomic number (Z). This determines what element the atom is. Ex., C always has 6, O always has 8, Fe always has 26, and U always has 92.
Neutrons have no charge. Their number (N) in nucleus determines what isotope of the element the atom is. Z + N = A (atomic mass number)
18O has 8 protons and 10 neutrons. 16O has 8 protons and 8 neutrons. Same element, but different isotopes. 235U has 92 protons and 143 neutrons.
Electrons: "Orbit" around the nucleus (not in predetermined orbits like planets, but in probability clouds). The are negatively charged and have almost no mass. Number and distribution determines the size and charge of the atom and its chemical properties.
Gain electrons - atom becomes an anion (negatively charged)
Lose electrons - atom becomes a cation (positively charged)
Opposites attract - form bonds that hold structure together.
4) Crystalline structure: Atoms are internally arranged in a regular or specific pattern that repeats in three dimensions. Structure is not a random (glass). Crystal may be very small or very large, may be well-formed (smooth surfaces) or not well-formed (irregularly shaped). Depends on growth conditions.
Composition and/or structure are unique for each mineral. If they aren't then not a different mineral. One or both must be different.
Diamond and graphite are both made of pure carbon (C). However, their structures are very different. Gives them very different physical properties. They are polymorphs (same composition, different structure) of C.
Salt (NaCl) and galena (PbS) share the same structure, but have very different compositions. They are isostructural.
5) Physical properties: Can be used to easily identify one mineral from another.
1) Color: Can be very diagnostic (pyrite) or of little use (quartz).
2) Hardness: resistance to being scratched. Use Moh's Scale of Hardness (1-10). A relative scale, not absolute. Can compare an unknown mineral to substances of known hardness
3) Density: Weight per unit volume (gm/cc) Water has a value of one. Minerals range from 1.5 to 10+. Depends on structure and composition.
4) Cleavage: Mineral breaks along one or more planar surfaces. Number and angle between them can be diagnostic.
5) Form: External shape of the mineral
6) Reactivity with acids: Carbonates react with HCl.
7) Important mineral groups
1) Silicates: Si + O. Further classified according to structure - linkage of Si and O to form sheets, chains, or 3-D networks. Most important mineral group for the crust and upper mantle of the earth. Includes quartz, feldspar (most abundant mineral in crust; made of Si, O, Na, Al, K, Ca), ferromagnesian silicates (Si, O, Fe, Mg: olivine, pyroxene, garnet; these are the most abundant in mantle and are dark colored), micas, and clays (sheet structure; contain water; very abundant in soil).
2) Non-silicates: Include the following
3) Carbonates (contain CO3): include calcite found in marine rocks and shells.
4) Sulfates (contain SO4): include gypsum used in ceramics and dry wall.
5) Sulfides (contain S): include galena and many ores of Cu, Au, Ag, and Ni.
6) Oxides (contain O): Magnetite, hematite (rust) and many ores of Fe, Ti, and Cr.
7) Native elements: C, S, Pt, Au, and Ag
Approximately 3500 uniquely defined different minerals. List grows all the time.
II. Rocks: Solid, cohesive aggregate of one or more minerals. Three major types: Igneous, Metamorphic, and Sedimentary.
Every rock tells a story. They are each like a photograph. They tell you what was going on in some place at some time. Look at enough rocks and you get a movie of earth history.
1) Igneous: crystallize from a melt of molten rock (magma)
Volcanic - crystallize at or near the surface. Quickly cooled, therefore fine grained or aphanitic texture (typically <1 mm). Form from lava (magma at the surface). Sometimes produce volcanic glass (obsidian), a quenched lava.
Plutonic - crystallize at depth. Slowly cooled, therefore coarse grained or phaneritic texture (typically >1 mm).
Igneous rocks classified on the basis of texture and composition (as seen in their mineralogy). Giving a rock a name is important. It communicates a lot of information about a rock. It's more than just a name.
HI-T; OL, PYX
LO-T; QTZ, K-FELD
2) Sedimentary: Form at or near the earth's surface (low temperature and pressure). Made up of particles (sediment) or dissolved compounds derived from pre-existing rocks cemented together. Form by the weathering and erosion of older rocks.
Clastic - sediment comes from breakup of pre-existing rocks. Pieces created by erosion. Name them on the basis of particle size.
From smallest to largest: Shale - siltstone - sandstone - conglomerate
Chemical - sediment crystallizes from compounds in solution (seawater). Compounds created by chemical weathering. Name them on the basis of mineralogy.
Calcite - limestone Halite - rock salt
Both types my have fossils and much empty pore space. Sediments settle in water or air due to gravity and form layers.
3) Metamorphic: Produced by a change in a pre-existing rock. Usually due to an increase in pressure and/or temperature often caused by burial or position near a hot magma. Changes occur entirely in solid state. Rocks recrystallize as old minerals often react away and new minerals form that are stable at the new conditions.
Foliated - minerals oriented in a planar fashion. Grain size becomes large as P, T increase and rock becomes more recrystallized. With increasing level of metamorphism, rock textures and names change. Composition is not fixed.
Slate -> Phyllite -> Schist -> Gneiss ->melting (igneous)
Low P, T High P, T
Unfoliated - minerals have no real preferred orientation. Names based on composition of original rock (protolith). Level of metamorphism.
Marble - originally a limestone
Quartzite - originally a sandstone
Amphibolite - originally a basalt
Presence or absences of particular minerals can give very specific P, T information. This can tell you where in the crust the rock formed.
III. Physical Properties of Rocks
Porosity - amount of empty void or pore space in the rock. Placed where water, oil, or gas can be stored.
Permeability - rate of flow through pores. Measure of how connected are the pores. Higher permeability allows fluids to be extracted and replenished more easily.
Igneous and metamorphic rocks have interlocking crystals - usually low porosity and permeability.
Clastic sedimentary rocks (depends on grain size, shape, and sorting) usually have high porosity and permeability. Chemical sedimentary rocks usually low.
Fractures increase both in rocks of all types.
IV. Rock Deformation
Rocks deform (strain) when placed under stress. Stress can be caused by compression, tension, and shear.
Rocks may strain elastically - stretch. They return to original shape when stress is stopped. Like a rubber band.
Rocks may strain plastically - flow. They do not return to original shape when stress is stopped. Like a piece of silly putty or metal.
Rocks may strain brittlely - crack and break like glass.
Temperature, amount and rate of strain may effect which type of strain occurs. Brittle behavior favored by cold temperatures and rapid rates. Plastic behavior favored by high temperatures. Elastic behavior favored by low amounts of strain.
V. Rock Cycle
Expresses relationship between the three rock types. How one can change to another by way of different geologic processes. Does not specify why these changes occur - need a mechanism that controls geologic processes. Talk about that next.