Trona

Trona, or natural soda ash, is a sodium sesquicarbonate compound (Na2CO3.NaHCO3.2H2O) that occurs as an evaporite mineral in the Wilkins Peak member of the Eocene Green River Formation in southwest Wyoming. Trona, which is 70% sodium carbonate, is refined to produce sodium compounds, particularly soda ash (Na2CO3). 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 (NaOH), and sodium cyanide (used in gold refining) also depends on trona.

Massive fibrous trona from the OCI mine in the Green River Basin.

Trona is white, gray, amber, or yellow with vitreous, or glass-like, luster. Trona is soluble in a dilute (10%) 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 of 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 United States Geological Survey (USGS) began a strategic minerals study and later published a reference to the minerals found in this core in 1940. Westvaco Chemical Corporation produced the first trona from trona Bed 17 at a depth of 1,500 feet in 1947 (Kostick, 1998).

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 (40 to 50 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 member consists of trona beds up to 40 feet thick separated by lake-bottom shaley 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 Garrett (1998), and Leigh (1998).

Green River Basin trona deposits
Green River Basin trona deposits

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 (Leigh, 1997). The soda ash industry in Wyoming’s Green River Basin produces approximately 11 million tons annually. This accounted for more than 95% of U.S. production, and about 38% of the world’s production in 2006 (Kostick, 2007).

Aerial view of the OCI trona mine and plant. Photo by R.E. Harris.
Aerial view of the OCI trona mine and plant. Photo by R.E. Harris.

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 & 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, http://minerals.usgs.gov/minerals/pubs/commodity/soda_ash/sodaamcs07.pdf, website accessed 3/30/2007.

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.

For detailed information about trona, visit the following company sites:

FMC Corporation 

General Chemical 

OCI Chemical Corporation

Solvay Chemicals

Recommended Reference Material

Further information on Wyoming trona can be found in the following WSGS publications:

WSGS Public Information Circular 39, Proceedings of the first international soda ash conference, vol. I, edited by J.R. Dyni, and R.W. Jones, 1998, 95 p.

WSGS Public Information Circular 40, Proceedings of the first international soda ash conference, vol. II, edited by J.R. Dyni, and R.W. Jones, 1998, 215 p.

WSGS Bulletin 72, Minerals and Rocks of Wyoming, by W.D. Hausel, 2005, 159 p.