Tripoli
Tripoli is a microcrystalline form of quartz (SiO2) which is derived by the alteration of chert, chalcedony, or novaculite, or leaching of highly siliceous limestones. Tripoli has numerous applications, mainly as an abrasive in polishing, buffing, and burnishing compounds, in scouring soaps and powders, and recently, as a filler or extender in plastics, rubber, in sealants and epoxy resins, and as a pigment in paints. Firing tests on tripoli blocks have shown its potential as a high-quality lightweight aggregate.
Tripoli is present in 3 general areas of Arkansas: northwestern Arkansas near Rogers in Benton County; in the Ouachita Mountains near Hot Springs in Garland County; and near Athens in Howard County. The deposits of northwestern Arkansas were formed by the weathering of cherty limestones of the Boone Formation (Mississippian), while the Ouachita Mountains deposits were formed by the leaching of a limy phase within the Upper Division of the Arkansas Novaculite (Mississippian). Originally, portions of the Upper Division contained as much as 30 percent carbonate. Tripolitic zones in the Bigfork Chert (Ordovician) in western Saline County were investigated for commercial potential and are now being utilized.
Analyses of Arkansas tripoli reveal that silica content is greater than 99 percent. A typical analysis of processed tripoli from Arkansas novaculite is 99.49 % SiO2, 0.0l5 % TiO2, 0.102 % Al2O3, 0.039 % Fe2O3, 0.021 % MgO, and 0.014 % CaO, for a total of 99.68 %. Although all 3 areas have been mined, there only one mine and processing facility is presently active. Malvern Minerals Company of Hot Springs in Garland County markets their products under registered trade names. Tripoli has been mined by both underground (Ozark region) and open-pit (Ouachita region) methods. The mined material is dried, crushed, pulverized, disaggregated, and sized by screening or air-flotation. The range of particle size of individual quartz grains composing tripoli is from 0.5 to 10 microns and equidimensional. The color of the tripoli varies within the same deposit. Colors include white, cream, tan, and brown, with white being the least prevalent, but most marketable.
The mined output of tripoli usually amounts to about 15,000 short tons per year. In 2005, Arkansas ranked 3rd in the nation out of 4 producing states. Reserves of higher-grade white tripoli are limited, but other color grades are several million tons.
References
Griswold, L. S., 1892, Whetstones and the novaculites of Arkansas: Arkansas Geological Commission Survey Annual Report for 1890, v. III, 443 p.
Holbrook, D. F., and Stone C. G., 1978, Arkansas Novaculite – A silica resource, in Johnson, K. S. and Russell, J. A., eds., Thirteenth Annual Forum on the Geology of Industrial Minerals: Oklahoma Geological Survey Circular 79, p. 51-58.
Keller, W. D., Stone, C. G., and Hoersch, A. L., 1985, Textures of Paleozoic chert and novaculite in the Ouachita Mountains of Arkansas and Oklahoma and their geological significance: Geological Society of America Bulletin, v. 96, p. 1353-1363.
Steuart, C. T., Holbrook, D. F., and Stone, C. G., 1984, Arkansas Novaculite: Indians, whetstones, plastics, and beyond, in McFarland, J. D., III, and Bush, W. V., eds., Contributions to the geology of Arkansas, v. II: Arkansas Geological Commission Miscellaneous Publication 18-B, p. 119-134.