Commercial perlite deposits of New Mexico and North America
— George S. Austin and James M. Barker

Perlite is weathered volcanic glass containing from 2 to 5 wt.% water. When heated to 600°-800°C in 870°-1100°C furnaces, the glass softens and its water is rapidly given off as steam. The steam expands or "pops" the perlite into glass foam that is from 10 to 40 times the original volume and 15 lbs/ft³ or less in density. "Onionskin" or classical perlite is dense, gray to bluish black with concentric fractures and a pearly luster. Granular perlite is lighter weight, microvesicular, highly fractured, and white to gray. Pumiceous perlite is extremely lightweight, frothy, and white to light gray. All types are mined commercially, but granular perlite is dominant in high-volume mines. Perlite deposits are associated with rhyolitic volcanic terranes. Most are Tertiary in age because volcanic glass is relatively short lived over geologic time. The glassy, subhorizontal tops of microvesicular (i.e., permeable), high-silica lava flows are the most favorable sites for weathering into commercial perlite. Large, steep-sided, high-silica lava domes are more common than high-silica flows. However, large domes yield relatively little commercial perlite because of their complex cooling histories and the lack of access to the meteoric water that hydrates the glass during weathering. In 1997, commercial perlite was mined at a record pace of 775,000 short tons in Arizona, California, Nevada, and New Mexico, with more than 85% of the total from New Mexico. Construction uses accounted for 71% of expanded perlite domestic sales. Filter aids accounted for 11%, horticultural aggregate 9%, fillers 7%, and others 2%. Other North American perlite production is from, or has recently come from, Colorado, Idaho, and Texas; the Canadian province of British Columbia; and the Mexican states of Durango, Puebla, and Sonora. New operations are online, or will come online soon, in Oregon and Utah and an Idaho operation will re-open, all of which will increase supply, thus eliminating the shortages of recent years.