Fish and Wildlife Service - Salt Plains National Wildlife Refuge

Selenite Crystals

The salt plains are a unique geological area; the 11,000-acre barren area is near perfectly flat with a wafer thin salt crust. It is classified as the "largest such saline flat in the central lowlands of North America". The salt was formed by repeated flooding by sea water millions of years ago. The sea water was cut off from the sea and evaporated, depositing thick layers of salt. The area was subsequently covered by erosion from mountain ranges. Below the plains, ground water travels through the salt-saturated sand and comes to the surface where it evaporates, leaving the crust of salt. The concentrated
saline solution combines with gypsum to promote selenite crystal growth in a portion of the salt flats.

What is Selenite, and How are Selenite Crystals Formed?
Selenite is a crystallized form of gypsum. Chemically, it is a hydrous calcium sulfate. Gypsum is a common mineral that takes on a great variety of crystal forms and shapes. On the Salt Plains, the crystals are formed just below the salt encrusted surface. They are seldom found deeper than 2 feet below the surface.
Crystals take on the characteristics of their environment; the finer the soil, the more clear the crystals. Iron oxide in the soil gives the crystals their chocolate brown color. What is So Special About These Crystals? Because these crystals form in wet soil, sand and clay particles are included within the crystal. These particles often form an "hourglass" shape inside the crystal. This hourglass shape cannot be found in selenite crystals in other places of the world; it is only found here at the Salt Plains of NW Oklahoma. Because this is the only place in the world to find these crystals, the selenite crystal with the hourglass-shaped
sand inclusion was designated as the State Crystal of Oklahoma on April 4, 2005. Learn More.
	Single crystals, penetration twins, and clusters are the typical crystal shapes most frequently encountered on the refuge. Exceptional individual crystals measuring up to 7 inches long have been found, along with complex combinations weighing as much as 38 pounds. In certain places on the Salt Plains, gypsum and saline solutions in the soil are sufficiently concentrated to promote crystal growth. When temperature and brine conditions are ideal, the crystals may form very rapidly.
When heavy rains or floods bring great quantities of freshwater to the plains, some of the selenite crystals may go back into solution until conditions are right for recrystallization.
CRYSTAL DIGGING WILDLIFE ON THE SALT FLATS

Last updated: July 30, 2007