GEOL 2311 Midquarter Exam I Name Crystal Chemistry Score: / 100 1. Chose a subdiscipline of geology and describe how mineralogy plays a central role in that field. (2 pts) Looking for reasonable answers here. 2. Name two attributes of what defines a mineral (4 pts) Possibilities naturally occurring solid highly ordered atomic arrangement definite (but not fixed) chemical composition usually formed by inorganic processes 3. What are the two most common elements in the earth s crust? (2 pts) O & Si 4. Name four more elements that comprise > 1 wt. % of the earth s crusts (4 pts) Al, Fe, Mg, Ca, Na, K 5. Ti 48 is the most abundant isotope of titanium, which has an atomic number of 22. How many of each atomic particle are in this isotope? (3 pts) protons 22 neutrons (48-22) 26 electrons 22 What is the approximate atomic weight of Ti 48 in atomic mass units? ~48 amu (1pt) 6. Ignoring the transition metals, the columns in the periodic table of elements are organized by increasing numbers of electrons in the outermost shell (3 pts) The rows in the periodic table are organized by number of electron shells (3 pts) 7. Elements with low ionization potential are typically cations or anions?(circle one, 1 pt) 8. Why are the electronegativities of inert (or noble) gasses much lower than other ions? All outer electron sites are filled resulting in most stable configuration (2 pts) 9. Calculate the net + and charges of these minerals to determine if they are electrically neutral? (note: some are fictional minerals; 5 pts)? CaMg 2 Si 2 O 9 14+ vs. 18- (Na,K)AlSiO 4 _8+ vs 8- TiO 3 4+ vs_6- Al 2 SiO 4 (OH) 2 _10+ vs 10- Ca 5 (PO 4 ) 3 (OH) 25+ vs_25-
10. To electrically balance the formula for magnetite, Fe 3 O 4, how many of the iron atoms are Fe 2+ and how many are Fe 3+? Two Fe 3+ and One Fe 2+ (+8 total) (2 pt) 11. Rank the types of bonding in order of strength (1- highest; 5 lowest) (4 pts) 2 Ionic 4 Hydrogen 3 Metallic 5 Van der Waals 1 Covalent 12. What types of bonds are most likely between these elements or molecules (6 pts) K-SiO2 Van der Waals C-C Covalent Na-Cl Ionic Fe-O Ionic Al-Al Metallic Na-OH Hydrogen 13. Many ion pairs show gradations between ionic and covalent bonding; what attribute of the two elements determines whether the bonding will be predominantly one or the other? Difference in electronegativity; greater difference promotes more ionic bonds (2 pts) 14. Are these statements about ionic radii true or false (4 pts) Ionic radii increase with down periodic table columns True Ionic radii of cations increase across periodic table rows False (decrease) Ionic radii of cations decrease with increasing coordination number False (increase) The ionic radius of an element in a metallic bond is greater than that element in an ionic bond True 15. Why are atomic structures dominated by covalent bonds not portrayed as coordination polyhedra? Covalent bonding distorts the shape of atoms such that they do not have spherical forms (2pts) 16. What are the polyhedral shapes of anions surrounding cations with the following coordination numbers CN=4 tetrahedron CN=6 octahedron CN=8 cubic (3 pts) 17. What ratio determines the coordination number of anions surrounding a cation? Ratio of cation radius to anion radius (2 pts)
18. What is the largest number of closest neighbors a cation may have 12? and under what conditions will that likely occur? When cation has approximately the same radius as the surrounding anions; i.e. radius ratio ~ 1 (2pts) 19. What is the electrostatic valence of these bonds for the coordination indicated? Si-O (CN-4) 4/4 Na-Cl (CN-6) 1/6 Ca-F (CN-4) 2/4 (3pts) Which is the strongest bond? Si-O (1 pt) 20. Below is an illustration of an SO 4 anionic complex. What is the net valency charge of this anionic complex? -8 +6 = -2 (1 pt) How much charge is available for each oxygen ion to contribute to another bond? -1/2 (1pt) 21. Below is an illustration of the core silicate structure for ring silicate minerals. What is the formula for this structure? Si 6 O 18 (2 pt) What is the net valency charge of this molecular structure? -36 +24 = -12 (1 pt) How much charge is available for each unbound oxygen to contribute to another bond? -1 (1 pt) 22. Connect with a line, the element pairs that are likely to substitute for each other in solid solution sites (4 pts). Na Fe 3+ Al Mn Ca K Fe Sr
23. Ca-Na and Mg-Fe are common solid solution pairings; which elements are the high temperature components? Ca and Mg (2 pts) 24. If Na substitutes for Ca in plagioclase, what other substitution must occur? Si for Al Why to maintain charge balance Ca+2 Na+1 // Al+3 Si+4 (3 pts) 25. In the discontinuous mineral series of Bowen s Reaction Series, decreasing temperature promotes the successive formation of olivine, pyroxene, amphibole, and biotite. What general changes occur in the structural types of silicates represented by these minerals? Silica tetrahedral become more clustered (2 pts) 26. What intensive variables are typically portrayed in phase diagrams for igneous rocks? temp and compositions (2 pts) for metamorphic rocks? Temp and pressure (2 pts) 27. What is the stability state of most igneous and metamorphic minerals exposed at the earth s surface. metastable (2 pts) 28. Complete this statement: A homogeneous mineral phase is said to be in equilibrium with its environment when its free energy is as low as possible under the particular conditions of pressure and temperature (2 pts) 29. Indicate whether these statements about phase stability are true or false (4 pts). Increasing temperature, decreases the entropy of a mineral False, increasing T increases the disorder (Entropy) in a mineral Increasing pressure, decreases the volume of a mineral True, compresses the mineral In the figure to the left, which mineral (A or B) is more stable at low pressure and temperature? A If these two phases are different compositions, they would be considered polymorphs. False, polymorphs are minerals with the same composition but different crystal structure
30. Answer the following questions about the phase diagram below. Al2SiO5 A B C This is an example of phase diagram of how many components? One (1 pt) Which phase(s) should be present at P-T conditions defined by point A? Kyanite (1 pt) How many degrees of freedom will maintain equilibrium at conditions represented by point A? Two (1 pt) Which phase(s) should be present at P-T conditions defined by point B? Kyanite and Sillimanite (1 pt) If pressure increases from B, which phase has the greater molar volume? Sillimanite (the phase with the lower molar volume will be most stable, and that phase then would be kyanite) (1 pt) If temperature increases from B, which phase has the greater entropy? Sillimanite (phases with the greater entropy will be he most stable) (1 pt) At what pressure and temperature will all three phases be in equilibrium? T - ~500ºC P ~ 350 Kb (2 pts) A sample at conditions represented by C is found to have sillimanite rimmed by andalusite. How do you explain this relationship? Suggests that the sample was at a higher P & T forming sillimanite at one time, but has now dropped in P&T such that andalusite is now stable, but the reaction has not gone to completion. (2 pts)