Geol /19/06 Labs 5 & 6 Crystal Chemistry Ionic Coordination and Mineral Structures

Transcription

1 Geol /19/0 Labs 5 & Crystal Chemistry Ionic Coordination and Mineral Structures Handout Oral Mineral Tray Report Samples Ionic Coordination Exercise Investigating Mineral Structures using XtalDraw Oral Mineral Tray Reports Objective: Be able to identify 7 of the 9 unknown minerals in your tray. At least 5 of the 9 samples will be from your list of 59 Minerals. Procedure: May apply any physical properities test to your sample that will aid in identifying the minerals, HOWEVER, when applying a scratch test, please scratch part of the mineral that is not a smooth crystal or cleavage face. When you think you know your minerals, contact me to set up a time outside of class time to present your oral report. These oral reports should only take minutes. During the reports, I will want to how you determined your unknown mineral phase (what were its most diagnostic features) and I will also ask other information about the geologic environment that the mineral forms in, what other minerals it is commonly associated with, and its possible uses. You may use your mineral index file to answer these questions. If you do not correctly identify at least 7 minerals, you can try again some other time. If you do correctly identify at least 7 minerals, you will be given a new tray of 9 unknown minerals within 24 hours. Points: You will receive 2 extra credit points for each mineral you correctly identify. You may do up to 5 total tray reports. You will receive an extra 5 points for successfully identifying the 2 nd tray, 10 points for the 3 rd tray, 15 points for the 4 th tray, and 20 points for the 5 th tray. Ionic Coordination Exercise Objective: To become familiar with the relationship between ionic radius ratios and the packing coordination of ions. Background: Lecture notes and Chapter 3, p Procedure: Do the following: A) Record the data on ionic radii for different cations in their most common coordination with oxygen in rock-forming minerals from Table 3.11 (p. 74). Note that Al, Ca, Na, and K have two common coordination values. Calculate the radius ratios for these cations paired with oxygen in the table below to the nearest 100th. Cation: CN: Ionic Ratio(s) S + 4 C 4+ 4 Ti 4+ Si 4+ 4 Al 3+ 4/ Fe 3+ Fe 2+ Mg 2+ Ca 2+ /8 Na + /8 K + 8/12 / / / / / / / /

2 B) A collection of colored Styrofoam balls are supplied in boxes at the front of the room. The large white balls represent oxygen anions and the other smaller balls represent different cations. Do the following to fill out the table below: 1) determine the radius of the various ball sizes (in cm) (fill in values in Column B); 2) calculate the radius ratios between the cations and the oxygen anions (Column C) 3) figure the predicted coordination number based on the radius ratios calculated (Column D) 4) by experimenting with the spheres, determine how many anionic oxygens you can pack around the six different cations (including oxygen X) while the maintaining contact (bonding) between the anion and the central cation. Some of the oxygens are joined together to aid in packing; toothpicks may also be used to help hold the anions together. Record this observed coordination number in Column E. Does the observed match the predicted? 5) based on the coordination numbers and the radius ratios calculated in Part A above, choose a cation that is the closest match to the cation model spheres (Column F) ) calculate the bond strength (electrostatic valency- e.v.) between each cation and an oxygen anion (Column G; see Pauling s Rules #2, p ). Cations Ion Model White (small) Black (small) Blue Yellow A B C D E F G Sphere Ratio CN (predicted) CN (observed) Possible Cation e.v. of bond Black (large) White (X) White (Oxygen) Investigating Mineral Structures using XtalDraw Objective: To investigate various aspects of different crystal structures using XtalDraw a 3D visualization software. XtalDraw is freeware that can be downloaded from the Dr. Robert Downs (University of Arizona) website: The first part of this investigation will involve translating a simple 3D crystal structure into a 2D map of the ion locations. The second part will involve answering questions about the atomic structures of nine common types of minerals that display a variety of crystal structure types. Background: Lecture notes and Chapter 3, p

3 Procedure: Part A. Mapping out 3D-crystal structure. In the graph below, map out the locations of Ca and F ions in the unit cells of Fluorite and Sr, Ti, and O ions in Perovskite by projecting down the c-axis on to the a-b plane. Use the fractional method shown in Figure 3.45 of Klein. View the ball and spoke crystal models of these minerals given in XtalDraw (note the unit cell boundaries of perovskite are masked by the bonds; turn off bonds to see them).

4 Part B. Examining mineral structures. Examine the ball and spoke and polyhedral models in XtalDraw for the following nine minerals and answer the questions below. Graphite 1. Consider only the bonding within a single horizontal plane. What is the coordination number for each C atom within this plane? 2. What is the bond angle between an C atom and its two nearest neighbors in the horizontal planes? 3. Graphite is renown for its softness and its strong basal cleavage due to weak bonds holding the carbon sheet together. What axes define this weak plane (AB, BC, or AC)? What type of bonds hold carbon sheets together? Diamond 1. What is the coordination number for each C atom? What is the shape of its polyhedra? 2. What makes diamond so hard? 3. Although very hard, diamond is cleavable in many orientations. Cleavability depending on the density bonds in a particular plane and the distance between the planes of ions. Two common cleavage planes in diamond are the (001) plane, which is perpendicular to the C axis, and the (111) plane which cut diagonally across all three axis. Position the unit cell so that the (001), then the (111) planes are horizontal (test the horizontality by spinning the crystal on a vertical axis). Which of these planes is more easily cleaved and why? Quartz 1. What is the coordination number of Si? 2. Compare the structures of diamond and quartz. What are some similarities? What are some differences? 3. Although similarly structured as diamond, why isn t quartz as hard? (see pg. 5)

5 4. Note that XtalDraw shows a polyhedron configuration for quartz, but not for diamond. Why might this be? (clue: answer related to your 2 nd answer for diamond; also see p.9) Halite 1. In halite, which element is the cation? Which element is the anion? 2. What is the coordination number of Na? 3. How many cleavages occur in NaCl? What is the angle between them? Calcite 1. What is the coordination number of C? What is it coordinated with? What does its radius ratio predict should be its CN? 2. What is the coordination number of Ca? What is it coordinated with? What does its radius ratio predict should be its CN? 3. Why might the observed coordination numbers for C and Ca not match the predicted (see p. 77)? Spinel 1. What is the coordination number for Mg? ; for Al? 2. Calculate the electrostatic valency of the Mg-O and Al-O bonds (see p. 75) e.v. (Mg-O) = e.v. (Al-O) = is this mineral isodemic or anisodemic (see p. 7-79)? Olivine (fosterite) 1. What is the coordination number of Mg? ; of Si? 2. Calculate the electrostatic valency of the Mg-O and Si-O bonds (see p. 75) e.v. (Mg-O) = e.v. (Si-O) = Is this mineral isodemic or anisodemic (see p. 7-79)? 3. What is the relationship between the silicon tetrahedra. (i.e., Do they share O 2- to make sheets or chains or networks, or are the related in some other way? 4. What type of silicate structure is this (see Fig. 3.58, p. 90)

6 5. Compare the olivine structure to the spinel structure. Both have model formulas of XY 2 O 4 (i.e., one tetrahedral site and two octahedral sites). Which has the greater density and how would you tell? Pyroxene (Enstatite & Diopside) Biotite 1. In Enstatite, what is the coordination number of Si? ; of Mg? 2. Distinguish the colors of the M1 and M2 octahedral sites in the enstatite structure. Now open the Diopside file. You will see a plot of only the Ca atoms (not sure why the whole structure is not shown here) in a unit cell that is half the size of the enstatite cell in the A direction. According to Table 11.2 (p. 459), these Ca atoms would fill the M2 sites. Calculate the coordination number of Ca for an octahedral coordination. What should be the coordination number of Ca sites? 3. What other difference exists between the enstatite and diopside structure? (clue: enstatite is an orthopyroxene and diopside is a clinopyroxene) 4. Pyroxenes are part of the inosilicate group characterized by single chains of silica tetrahedra bound at their corners. Along what crystallographic direction (A, B, or C) do these chains project? 5. Why is pyroxene said to have a T-O-T-O structure. What might this refer to?. Pyroxene has two strong directions of cleavage that intersect parallel to the c axis. Looking down the C-axis and on to the AB plane, you should be able to recognize where these cleavage planes are: What is their approximate angle? 1. What are the coordination numbers for Si?, Mg?, and K? 2. Calculate the bond strength of the Si 4+, K + and Mg 2+ cations to oxygen. e.v. (Si-O) = e.v. (K-O) = e.v. (Mg-O) = Along what bonds would you expect biotite to most easily cleavable? _ 3. Biotite is a phyllosilicate, which is characterized by a layered sheets of corner bonded silica tetrahedral bound by octahedral sheets hosting Mg (or Fe). Two types of these octahedral sheets are found in phyllosilicates tricoctahedral and dioctahedral (see Mineralogy CD Illustrations of layered silicates). Which of these occurs in biotite? Total Points: 30 Due Date: Tues, Sept. 2.