Gypsum with Halite ( #6)
|South America||Otuma mine, Pisco, Ica Department, Peru||5" x 3 ¾" x 2"|
Many minerals display a property known as cleavage. It is the tendency of a mineral to split or break along specific planes of weakness in the crystal structure. These are typically planes perpendicular to which there are either few atomic bonds or weak ones. Cleavage may be perfect, poor, or anything in between. Some minerals display no cleavage whatever. There may be only one direction of predictable cleavage, or there may be more than one. Cleavage (or lack thereof), however, is one of the defining characteristics of each mineral, and is an aid in identifying minerals. For example, one of the physical properties of quartz is a lack of cleavage, while mica, which you may have seen, has one direction of perfect cleavage.
We have seen gypsum (labeled selenite) before as specimen #1 in this display. It has the composition CaSO4 - 2H20. The ''water molecules" occur in layers in the crystal structure, indicated by green arrows above, with only weak hydrogen bonds between the layers. (Water molecules is in quotation marks above just to emphasize that there is no liquid water here - just oxygen atoms to which two hydrogen atoms are bonded in the crystal structure.) Gypsum exhibits perfect cleavage in this direction. There are two other directions of good cleavage in gypsum that are not readily visible in this orientation of the crystal structure.