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GEOLOGY ARTICLE

Selenite: the mysterious crystal of Crowley's Ridge
by KELLY BALDWIN

Science Project Report by Kelly B (5th Grade)

03.09.01: I was interested in finding out more about selenite and how it is formed. That is why I came up with this year's science project. On September 9, 2000, I collected the selenite crystals used in my project while I was on a Dixie Mineral Council field trip hosted by the Memphis Archaeological and Geological Society.

I collected these crystals from a bluff above Copperas Creek, which is named after a mineral called copperas. Copperas Creek is located east of Wynne, Arkansas, along Highway 64-B. This area is part of Crowley's Ridge. Crowley's Ridge is an area about 50 miles west of the Mississippi River.

Parts of Arkansas, Tennessee, and Mississippi were once covered by a shallow ocean (part of the Gulf of Mexico). This shallow ocean was home to many sea creatures. The brown clay and fine-grained sand of the Copperas Creek bluff is from the Eocene era, 37.5 to 54 million ago. During the Eocene era dinosaurs had already disappeared. Reptiles, marine animals and plants flourished in the Crowley's Ridge area. The horse first appeared during the Eocene period.

On various tributaries of Copperas Creek, Eocene fossils can be collected. These fossils include sharks' teeth, fish vertebrae, carbonized wood, and ray teeth. I have collected different types of ocean shells and sharks' teeth on Crow Creek, just a few miles south of Copperas Creek. Crow Creek is also part of Crowley's Ridge.

In addition to the Eocene fossils, selenite can also be found along Copperas Creek. Selenite is a very soft mineral with a hardness of 2 on the Mohs Hardness Scale. On the Mohs Scale talc is the softest with a hardness of 1 and diamond is the hardest with a hardness of 10. Selenite can be scratched with your fingernail. The specific gravity of selenite is 2.24 to 2.3. Specific gravity means that a certain amount of selenite weighs 2.24 times as much as the same amount of water. When you scratch a harder or darker substance with selenite, the selenite leaves a white streak.

The chemical composition of selenite is CaSO4 2H20, Hydrous Calcium Sulfate. One of the most common sulfate minerals in the EarthÕs rocks is calcium sulfate, also known as gypsum. One form of gypsum is alabaster which, because of its softness, can be made into ornaments such as candlesticks. Selenite is a naturally occurring crystalline form of high purity gypsum and is used in the ceramics industries and to manufacture surgical grade cast. Crushed gypsum is used in wallboard, some cements, fertilizer, paint filler, cast (or surgical plaster used to set broken limbs) and Plaster of Paris. Plaster of Paris got its name from the first gypsum mines near Paris, France. The word selenite comes from the Greek word for moon, and means "moon rock".

Selenite forms in crystals which can be tabular (many-sided), bladed (fanlike), or blocky (cubic). Most of the selenite crystals in the CrowleyÕs Ridge area radiate outward from their center. Selenite has a pearl-like luster, especially on cleavage surfaces. The Copperas Creek selenite deposits range from transparent to dark gray or brown. The non-transparent crystals have microscopic particles of clay, carbon and secondary minerals trapped inside. Some of the crystals also have a dull green mineral, called glauconite, trapped inside. This mineral sometimes causes the selenite to have a slight green tint. The Copperas Creek selenite fluoresces green under shortwave ultraviolet light, and duller green under longwave ultraviolet light.

Selenite is an evaporate. An evaporate is a sedimentary rock made up of minerals that precipitated minerals from the ocean sediment after the evaporation of the liquid in which they dissolved. The chemical process in which a dissolved material is changed into a solid and separated from the original liquid is called precipitation.

Marine evaporites are formed by the evaporation of seawater. Gypsum is one of the most important salts precipitated from seawater. The presence of gypsum in the CrowleyÕs Ridge area of Arkansas is an indication that the area was once an ocean. In North America, marine evaporites from sediments lie underneath as much as thirty percent of the entire land area. This is because much of the land of North America was once covered by a shallow ocean.

The most abundant evaporites are gypsum and rock salt. Selenite is a form of gypsum. It was formed when the salinity of the sea water dissolved in the sedimentary layer of clay, sand, and other sedimentary rocks. Since selenite is an evaporite formed in the sedimentary layers of an ancient ocean, the different forms of selenite may be found at different layers on the Copperas Creek bluff.

Selenite may not be found in all areas that were once covered by ocean. How the sedimentary layers were formed, as well as the minerals that make up the sediments can vary the results from one situation to another. In the oceans there is sedimentation from tidal lagoons, surfs, river deposits, and ocean shelves.

RESOURCES:
(1) "Gypsum"; www.rsmm.com/gypsum.html; January 27, 2001.
(2) "Selenite"; www.neatstuff.net/avalon/info/info-s.html; January 27, 2001.
(3) Leet, Don and Judson, Sheldon; Physical Geology; Prentice-Hall, Inc.; Englewood Cliffs, NJ; 1971.
(4) Sides, Paul; "Crowley's Ridge Field Study"; MAGS Newletter; Memphis Archaeological and Geological Society; Memphis, TN; October, 2000.
(5) Skinner, Brian J. and Porter, Stephen C.; The Dynamic Earth; An Introduction to Physical Geology; John Wiley & Sons; New York, NY; 1989.

EDITOR'S NOTE: Kelly won "First Place" in 5th Grade at the Collierville Elementary Science Fair in February, 2001, and "Outstanding Achievement in Earth Sciences" at the Memphis Shelby County Science Fair in March, 2001.