Selenite (#71)

A cluster of opaque to translucent lustrous colorless to lightly iron-stained crystals to 10" in length.
North America Sand Wash, Hanksville, Utah 23" x 18" x 12"

 

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In the Learn More for specimen #1 we also considered selenite, the variety of gypsum that consists of bladed crystals (bladed meaning like a knife blade – one long dimension, one very short (thin) dimension, and one in between, but closer to the thin one). This is another specimen of gypsum, CaSO4 · 2H2O.

Gypsum is number 2 on the Mohs scale of hardness (see Learn More under specimen #7), which means it is so soft that your fingernail will scratch it. Why is that? Look at the drawing of the crystal structure of gypsum:

Yellow spheres represent sulfur, blue calcium, red oxygen, and pink the oxygen atoms of the water molecules. The little salmon colored spheres represent hydrogen of H2O. Notice that there are planes through the structure bounded only by the water molecules. The hydrogen atoms (positive charge) are weakly attracted to the oxygen atoms of other water molecules (negative charge) across these planes. These are called hydrogen bonds. Unlike covalent and ionic bonds, these are very weak, and under the pressure of a fingernail, these bonds are broken. This is also the location of the perfect cleavage of gypsum. The hydrogens are not bonded to any particular oxygen atom, just whichever is closest, so thin cleavage plates of gypsum can be bent slightly without breaking.

Gypsum Crystal Structure