Читать книгу Earth Materials - John O'Brien - Страница 130

1 Discuss the difference between a motif and a node and explain why both are important in illustrating and understanding how long‐range patterns of atoms/ions can be generated by symmetry operations.

2 What are symmetry operations and how are they related to crystal structures? How do simple and compound symmetry operations differ?

3 Carefully define the following symmetry operations and the symbols used to denote each one.translation and unit translation vectorrotation and axis of rotationreflection and mirror planeinversion and centerglide reflectionrotoinversionscrew rotation

4 Discuss the properties and differences between the following types of plane meshes or nets.squarerectanglediamondhexagonal (rhombic)oblique

5 Define the essential properties of each of the seven (if hexagonal and trigonal are separated) crystal systems in terms of the properties of their Bravais lattices and their crystallographic axes (lengths and intersection angles).

6 6 In Weiss parameters and Miller indices, what do the three sequential parameters or indices represent? Convert the following Weiss parameters into the equivalent Miller indices.2, ∞, 1½, 1, 22, ∞, 2, 2, 11/3, , 1

7 Explain the meaning of the following Miller indices in terms of crystallographic axial intercepts with respect to axial ratios(111){111}(1 2){hkl}(hh )

8 What are twinned crystals and what are their chief defining characteristics? What is the essential difference between contact and penetration twins? Give a couple of good examples of each.

9 What are polymorphs? What is the difference between reconstructive and displacive polymorphs? Give an example or two of each. Which is most likely to produce long term metastable polymorphs?