CRYSTALS Crystals or solid forms of substances will have “faces”. These will have certain geometrical relationships, resulting in symmetry in crystals. Different solid molecules will have different crystalline structures. Sodium chloride is an easy one because if forms a cubic structure that can be seen with a simple magnifying glass. Figure 37 shows what a cubic crystal looks like molecularly:
Figure 37.
Irregularities or differences in the pattern of crystal shapes are referred to as “crystal habits”. With sodium chloride, the general shape of things is that it grows into a completely cubic shape. Partial melting and impurities can create different habits of the cubic shape. While a crystal can become broken or distorted, it will have the same angles between its corresponding faces. This is referred to as the law of constant angles. When a crystal is cleaved, it will cleave along particular cleavage planes according to the law of constant angles. The cleavage angles are determined by the molecular structure of the molecules in solid form. The different possible ordered shapes include the cube, the rectangle, the parallelogram, the rhombus, and the hexagonal shape. The rhomboid shape is similar to the hexagonal shape, with the rhomboid shape being simpler. These are the two-dimensional shapes that crystals can take. As you know, snowflakes take on a hexagonal shape. Why is this the case? It’s because water crystals or ice forms a hexagonal pattern when ice crystalizes. They form unique
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