National Wildlife Refuge | Oklahoma
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About the Refuge
The refuge's namesake, the salt flats, is a very unique geological formation.
The salt was formed by the repeated flooding of seawater millions of years ago. The seawater was eventually cut off and evaporated, depositing thick layers of salt and subsequently covered by erosion from surrounding mountain ranges. For millions of years, below the plains the groundwater has continued to travel through the salt-saturated sand and rises to the surface and evaporates, leaving a thin crust of salt. It is the concentrated saline solution combining with the gypsum that promotes the selenite crystal growth in a portion of the salt flats.
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 two feet below the surface.
Crystals take on the characteristics of their environment -- the finer the soil, the more clear the crystals. It is the iron oxide in the soil that 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 is not found in selenite crystals anywhere else in the world -- it is only found here on the Salt Plains of northwest Oklahoma.
Single crystal blades, penetration twins, and clusters are the typical crystal shapes most frequently encountered on the refuge. Exceptional individual crystals measuring up to seven 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.
Because it is so unique, the hourglass-shaped selenite crystal was designated as the State Crystal of Oklahoma in 2005.
Page Photo Credits
Selenite crystals / Tim Jones ©
Last Updated: Sep 28, 2013
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