Wyoming State Geological Survey

Wyoming Trona

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Wyoming hosts the world's largest trona deposits, an estimated resource of 127 billion tons. Of this, 40 billion tons are mineable using conventional “hard rock” methods.

Trona, or natural soda ash, is a sodium sesquicarbonate compound that occurs as an evaporite mineral in the Wilkins Peak Member of the Eocene Green River Formation in southwest Wyoming. Trona, which is 70 percent sodium carbonate, is refined to produce sodium compounds, particularly soda ash. Baking soda is pure soda ash. Soda ash is the second ingredient by weight in glass after silica, where it lowers the melting temperature of silica sand. It is also used in soaps, detergents, inorganic chemicals, water purification, flue gas desulfurization, and a number of other products and processes. The manufacture of compounds such as sodium phosphate, caustic soda, and sodium cyanide (used in gold refining), also depends on trona.

Trona is white, gray, amber, or yellow with vitreous, or glass-like, luster. Trona is soluble in a dilute (10 percent) hydrochloric acid and water solution. It has a monoclinic crystal structure and is soft, about 2.5 to 3 on the 10-point Mohs hardness scale. For a better understanding of crystal structure, hardness, and other characteristics of trona and other minerals, WSGS Bulletin No.72, Minerals and Rocks of Wyoming, or a mineralogy text is recommended.

Trona deposits were first discovered in Wyoming during October 1937 when Mountain Fuel Supply Company took core samples from the John Hay #1 well while drilling for oil and gas. Core from this well contained thick zones of trona and other evaporite minerals, some of which (such as shortite) were identified for the first time. In 1939, the U.S. Geological Survey began a strategic minerals study and in 1940, published a reference to the minerals found in this core. Westvaco Chemical Corporation produced the first trona from trona Bed 17 at a depth of 1,500 feet in 1947.

Trona currently forms as an evaporite mineral in places such as Africa and California. Wyoming’s trona is believed to have formed in the same manner. During the Eocene (48 million to 53 million years ago), an immense lake known as Lake Gosiute occupied the Green River Basin. Over millions of years, Lake Gosiute expanded and contracted due to regional climatic cycles. During intervals of dryness and evaporation, Wyoming’s rich trona deposits were deposited in parts of the Wilkins Peak Member of the Green River Formation. The Wilkins Peak mMmber consists of trona beds up to 40 feet thick separated by lake-bottom shaly sediments, sandstone, and carbonate rocks. Trona beds in the Wilkins Peak member lie at depths of 600 to 2,000 feet beneath the Green River Basin. Other members of the Green River Formation are sediments deposited in a freshwater lake environment. For detailed geologic information on the various theories of trona formation, see WSGS Public Information Circular No. 40.

Aerial view of the Ciner Wyoming trona mine and plant. [Credit: R.E. Harris]

Flattened massive fibrous trona.

Map of trona deposits and mines in Green River Basin, Wyoming.

Download the 2014 Wyoming Trona Summary Report.

References

Dyni, J.R., 1997, Sodium carbonate resources of the Green River Formation, in Jones, R.W., and Harris, R.E., eds., Proceedings of the 32nd Annual Forum of Industrial Minerals—Laramie, Wyoming, May 19–21, 1996: Wyoming State Geological Survey Public Information Circular 38, p.123–143.

Garrett, D.E., 1998, Origin of the Green River trona deposit (from a chemical and other saline mineral perspective, in Dyni, J.R., and Jones, R.W., eds., Proceedings of the first international soda ash conference volume II Wyoming State Geological Survey Public Information Circular 40, p. 15–20.:

Hausel, W.D., 2005, Minerals and rocks of Wyoming: Wyoming State Geological Survey Bulletin 72, 159 p.

Kostick, D.S., 1994, Soda ash in Carr, D.D., ed., Industrial minerals and rocks, 6th edition: Society for Mining, Metallurgy and Exploration, Inc., p.929–958.

Kostick, D.S., 1998, The origin of the U.S. natural and synthetic soda ash industries, in Dyni, J.R., and Jones, R.W., eds.,Proceedings of the first international soda ash conference volume I: Wyoming State Geological Survey Public Information Circular 39, p.11–33.

Kostick, D.S., 2007, Soda ash, in U.S. Geological Survey Mineral Commodity Summaries, 2006, p.150–151, accessed March 2007, at http://minerals.usgs.gov/minerals/pubs/commodity/soda_ash/sodaamcs07.pdf.

Leigh, R.T., 1997, Wyoming Trona–An overview of the geology, in Jones, R.W., and Harris, R.E., eds., Proceedings of the 32nd Annual Forum of Industrial Minerals–Laramie, Wyoming, May 19–21, 1996: Wyoming State Geological Survey Public Information Circular 38, p. 319.

Leigh, R.T., 1998, Wyoming trona: An overview of the geology, in Dyni, J.R., and Jones, R.W., eds., Proceedings of the first international soda ash conference volume II: Wyoming State Geological Survey Public Information Circular 40, p. 1–14.

Mendenhall, W.C., 1940, Occurrence of a deposit of trona: Science, v.91, no. 239, p.11–12.