Gypsum can be cleaved along particular crystallographic planes using a razor blade. The bonding perpendicular to these cleavage planes is weaker than that in other directions, and hence the crystal breaks preferentially along these planes. Quartz and diamond do not have such distinct cleavage planes, and so cleaving these crystals requires much more effort and care.
There are distinct planes in the gypsum structure, with no bonding between them. These are the cleavage planes. It is much more difficult to cleave gypsum along planes other than these. In contrast, all of the planes in the quartz structure are interconnected and the material is much more difficult to cleave in any direction. This is a demonstration of a way in which the crystal structure of a material can influence its mechanical properties.
Glass is impossible to cleave. As an amorphous substance, glass has no crystallographic planes and therefore can have no easy-cleavage directions. Glassy materials are often found to be mechanically harder than their crystalline equivalents. This is an example of how mechanical properties of crystals and amorphous substances differ.
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