This exercise will hopefully give you a better understanding of some common minerals, their physical properties and the relationship of the physical properties to the crystal structure.  In order to answer the questions below, please refer to Chapter 3 and Appendix IV of your textbook, Press and Siever. This will be group-learning exercise. You should form groups of three or four individuals and plan to get together for at least 15 minutes to discuss the questions posed below. One member should be identified to write the group report and submit your response.  Please make sure all group members are identified on the final report
 

1. Give a definition of the term cleavage.  Using your newly acquired knowledge of crystal structures and the descriptions of cleavage found in Appendix IV, explain the observed difference in cleavage between the mineral quartz and biotite.
2. Explain why minerals such as the feldspars, quartz and muscovite are colorless or light colored, whereas minerals such as biotite, pyroxenes and amphiboles are dark colored.
3. Lets see if we can’t deduce the fundamental nature of the crystal structure of the mineral periclase (MgO).  Magnesium (Mg) has an atomic number of 12.  Consequently, it has 2 electrons in the K shell, 8 electrons in the L shell and 2 electrons in the M shell.  It can achieve a completely filled valence-electron (outer) shell by giving up the two M-shell electrons, and forming an Mg2⁺ cation.  Oxygen, is a perfect candidate to bond to Mg, because it needs two electrons to fill its outer shell, thereby forming the O^2- anion.  We saw in lecture that ionic size considerations dictate that the small Si⁴⁺  cation is most energetically stable by being bonded to (surrounded by) 4 O^2- anions. The Mg²⁺ cation is slightly more than 3 times as large as Si⁴⁺, and consequently is too big to fit in the "tetrahedral" void between 4 O^2- anions.  For Mg, the most stable configuration is to be surrounded by 6 O^2- anions.

A. The name for the 3-dimensional geometric form with four corners is the tetrahedron. What is the name of the 3-dimensional geometric form with 6 corners?
 
B. In lecture, I emphasized the fact that the [SiO4]^4- tetrahedron is the fundamental "building block" for all of the silicate minerals.  How would you describe the fundamental "building block" for periclase?
 
C.  Can electrical neutrality be achieved simply by bonding one Mg with 6 Oxygen ions?
 
D. If you answered "yes" to question B, you may stop here (this option is not highly recommended, however).  If you answered "no" to question B, how can both electrical neutrality and the formula for periclase (MgO) be achieved?  [Hint: consider sharing the corner oxygen ions of the "building block" with adjacent building blocks.]
 
E. Based on the above answers, describe the structure of periclase?  For example, the structure of the mineral pyroxene can be described as "chains of [SiO4] tetrahedra linked together by sharing 2 apical oxygens.  The chains are then bonded to each other by divalent cations such as Mg or Fe".
 
F. Based on this "first-principles guess" of the periclase crystal structure, would you predict this mineral to have a strong cleavage such as that found in minerals such as the micas?