Orpiment (#81)

Lustrous translucent complex crystals to 3/8" maximum dimension in a calcite etched vug.
North America Getchell, Humboldt County, Nevada 3" x 2 ¾" x 2"

 

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Orpiment Crystal Structure

Orpiment is an arsenic sulfide mineral, As2S3. The color varies from yellow to deep yellow-orange, so all of the yellow to orangish material on this specimen is orpiment. This mineral gives us the opportunity to become acquainted with the only type of chemical bonding in minerals we have not encountered in specimens of lower number in this display: van der Waals bonding. This is the crystal structure of orpiment:

Green spheres represent arsenic, yellow ones sulfur. It is obvious that the structure consists of layers of linked As and S atoms (the layers are seen edge-on in this view). It also seems apparent that noth ing connects one layer to another, but in fact, something does.

The electrons that "surround" an atom are in constant motion, and it may be that at one instant, the electrons "around" a particular atom happen to be more on one side of it than the other. It may also be that, at the same instant, the electrons of another atom (nearby, but not chemically bonded to the first) are distributed similarly, so that the slightly negative side of one is toward the slightly positive side of the other, resulting in a very weak attraction. With a large number of atoms involved (true of any specimen large enough to see), this fleeting interaction occurs between enough atoms at any instant that a macroscopic attraction occurs - in this case, the layers are held together. Because the interaction is very weak, the mineral is very soft, and the layers come apart (cleave) very easily.