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- What are the coordination numbers for Si, Mg, and Ca?
- Rotate the structure so you are looking down the a axis. Then rotate it so you are looking down the b axis. Then the c axis. Which view gives you an end-on view (so you are looking parallel to the chains of Si tetrahedra? Which view reveals a layered structure showing Si tetrahedral chains (in layers) bonded together by Ca and Mg cations?
- Tremolite is termed a double chain silicate because pairs of chains of silica tetrahedra linked together extend infinitely in one direction within the crystal. Locate the two linked chains of Si tetrahedra. Describe how the tetrahedra are linked to form the chain. Find the "bridging oxygens" which connect the two chains. If the model were extended in space, predict the location of the next bridging oxygen for additional unit cells on either side of the model.
- Look down the c-axis and notice that some of the double chains are oriented in the opposite direction as the other chains (i.e., their silica tetrahedra "point" in different directions"). The oxygen atoms on the "points" of silica tetrahedra are termed "apical oxygens". What is their coordination number?
- This structure of alternating tetrahedral-octahedral-tetrahedral (T-O-T) sites forms the basic architecture of the amphibole group. Look at the model so that you can see the third dimension of these T-O-T structures. These structures are known as "I-beams" because of their shape in cross-section. What is the spatial relationship between the I-beams in the structure? Are they connected somehow?
- Using Fig. 13.66 in Klein and Hurlbut (21st edition, revised) as a guide, locate the M1, M2, and M3 sites in the structure. "M" stands for metal. What cation(s) is(are) housed in these sites? Why do you think Ca occupies a unique crystallographic site, M4, instead of going into M1, M2, or M3?
- Where is hydrogen located in the amphibole structure? Which atom is it bonded to?
- Note that there are some very large holes in the structure, most easily seen if you look down the c-axis. These are known as the A-sites. Compare the computer model with Fig. 13.66 if you have trouble locating them. What is the coordination of these sites? These sites are empty for tremolite, but are commonly filled, or partially filled, for other amphioboles. What elements would be expected to occur in these sites? If the A-site is partially filled, what also must happen elsewhere in the structure? (Hint: look up the mechanism for coupled solid solution in your textbook).
- Compare the tremolite (amphibole) structure with the diopside (pyroxene) structure. What aspects of these crystal structures are similar, and how are they different?
- This isn't a question for you to answer, but look at Figures 13.69, 13.70, 13.71 in Klein and Hurlbut (21st edition, revised). A class of related minerals is called "biopyriboles". Note that it is possible to have not only single and double chains, but also triple chains, and these structures can alternate within a single crystal.