Lecture 3 Minerals Homework today, due next Th Field trip Sat Nov 9. --Start on the atomic level to see how minerals and rocks are put together. --Learn why minerals look the way they do from on atomic perspective Mineral = a naturally occuring inorganic substance witha fixed atomic structure that has a fixed or slightly variable composition and physical properties. Rocks are a conglomeration of minerals. Minerals are composed of atoms. Elements and Atoms write as table Elements are the most fundamental substances into which matter can be separated by chemical means. An Atom is the most fundamental individual partical that retains the chemical properties of a given element. Atoms are the fundamental particals that comprise a given element. A Compound is a substance comprised of more than one element. A Molecule is the most fundamental particlal that retains the chemical properties of compunds. It is a combination of atoms. Atomic Structure Atoms contain a nucleus and one or more energy shells of electrons. The nucleus contains positively charged protons and neutral neutrons. The number of protons determins the atomic number, which define the different elements. The number of protons + neutrons determins the atomic weight The atomic weight is the mass in grams of a mole (6X1023 atoms) of atoms. Idea is that both protons and neutrons have mass. Each element has a different atomic number. Electrons are negatively charged and their number matches the number of protons. Therefore atoms are normally electrically neutral (1+=1-). [show periodic table] Electrons are confined to specific energy-levels called valence-shells. The number of electrons in each valence-shell is fixed: #1=2; #2=8, #3=8 (for the purposes of this class! actually it is 18, but a stable state is reached at 8) ect.... Atoms like it (they are most stable) when their outermost shells are filled. Atoms accomplish this by either accepting or donating electrons. By accepting or donating electrons, atoms become electrically charged ions. Positive (donated electron) = cation Negative (accepted electron) = anion. Chemical formulas are always written cations first, anions second. Elements whose atoms' outermost shells are almost filled accept electrons and become anions (-); while those elements whose atoms' outermost shells contain just one electron readily donate electrons and become cations (+). Bonds If two atoms need an electron to stabalize their shells, they can share an electron and form a covalent bond.
H2O is an example This is a directional bond and two atoms are definitely paired together. It is a monogamous relationship. If electons are transferred, electrically charged ions are formed. Opposites attract. Cations attracted to anions. -No such thing as a homo-anion. This attraction forms an electrostatic ionic bond . Bond can be divided between several different ions (i.e., a fraction of the attraction strength goes to each neighboring ion). It is a polygamous relationship. NaCl is an example. If a covalent bonds occur within inner energy-level electron shells, it is called a metallic bond. Electons in the outer shells are free to roam and cause high electrical and thermal conductivity. Very weak bonds between already-formed compounds are called Van der Waals Bonds. The Sheet minerals often have Van der Waals bonds. Check out talc in lab. Strongly bonded ions can create new electrically charged ions (usually anions) called complex ions. Examples include carbonate (CO32-), and silicate (SiO44-). The silicate anion is extremely important. Oxygen and Silica make up ~70% of the mantle and crust by mass. --The Si tetrahedra is the building block of the common rock forming minerals. [Draw picture of Si tethahedra and demonstrate how charge is 4-]. Crystal Structure --Minerals have definite crystal structure. Cations stay in the same place with respect to the anions consistently. Physical Properties These are important because they form the basis for the first lab. This is how we identify minerals in hand sample. All properties relate to underlying atomic structure. Crystal form directly reflects the underlying atomic structure. It is the angle between crystal faces that forms the diagnotic feature. Habit refers to a characteristic way a mineral grows, but does not necessarily conform to crystal faces or forms. Cleavage reflects the tendancy of a mineral to break along preferred plains of weaknesses. Sometimes, not always, conforms to crystal faces. --Plains of weakness may reflect weaker bonds Luster refers to how a mineral shines in the light. Color often reflects composition. Some ions, especially the transition metals, preferentially absorb various portions of the light spectra. Specific Gravity reflects the composition and the density of the packing of the ions. The heavier the constituents and the more densly packed, the more dense the mineral. Hardness directly reflects the stregth of the bonds. Diamond has strong covalent bonds. Talc has weak Van Der Waals bonds. Mineral Groups Organized by anions. Carbonates CO32- Calcite/aragonite = Ca2+CO32- or CaCO3 limestone/marble This is where a portion of Earth's carbon is stored. Silicates --Buildng block is the silica tetrahedron [fig 2.19] --linking tetrahedra is called polymerization. Clays, Micas, Chlorites sheet silicates one obvious direction of cleavage. Micas impt in ig & mm rocks clays in regolith chlorites in mm rocks. Quartz three dimensional network of tetrahedra no cleavage Feldspars also three dimensional Most abundant minerals on Earth Found in all rock-types. Al, K, Ca, Na, and Si important. Solid solution and ionic substitution. ROCKS combination of minerals Texture + mineral assemblage (composition) Textures -Igneous interlocking grains -- size gives indication of cooling rate mineral assemblage -- gives indication of temperature -Metamorphic interlocking grains (recrystalized), oftern with foliation. Mineral assemblage -- gives indication of T and P. -Sedimentary accumulation of individual grains held together by deposition of cement or recrystallization. Texture -- gives indication of sedimentary environment/process Mineral assemblage -- gives indication of source region.