Perlite is an amorphous volcanic glass (SiO2) that has relatively high water content, typically formed by the hydration of obsidian. Perlite has the unusual characteristic of expanding and becoming porous when it is heated. It can expand to as much as twenty times its original volume. Expansion occurs when the glassy lava rock is heated to 1600 degrees F (871 degrees C) and the water molecules trapped in the rock turn into vapor causing the rock to expand. (This is the same principle as the water in popcorn that causes the kernel to pop when it is heated.) Before it is expanded, perlite is commonly gray, but can also be green, brown, blue or red. After it has been heated, perlite is typically light gray to white.
Because of susceptibility to devitrification and alteration, commercial deposits of extrusive volcanic rocks like perlite and pumice are generally confined to younger geological terrains. Perlite deposits, which are rarely older than Oligocene, are exploited in the western United States, Mexico, Greece, Turkey, Italy, western Russia, Hungary, the Czech Republic, and Japan. Among them, the United States (particularly Arizona, New Mexico, and Nevada) and Greece account for more than 60% of world production and, in fact, Greece exports product to eastern U.S. markets.
Perlite is mined using open pit methods such as ripping or blasting, or both. If the perlite is soft and friable, brecciated, or extensively jointed, ripping is employed with significant cost savings. Blasting is required where perlite cannot be readily broken using rippers, but care must be taken to achieve fragmentation without production of excessive fines or oversized material. Once broken and sometimes crushed, the perlite is loaded on trucks or conveyor belts by front-end loaders, excavators, or scrapers for transport to the processing plant. Selective mining is used to minimize associated rhyolite or obsidian. If perlite textures vary, it is often blended to produce consistent milling characteristics and to meet market specifications.
The United States is one of the world’s largest producers and consumers of crude perlite and expanded perlite. A number of western states including Utah and Oregon produce perlite, with New Mexico being the most important perlite-producing state. Other countries that produce large amounts of crude and expanded perlite include China, Greece, Italy, Philippines, Mexico and Turkey.
Pumice is common on volcanic islands such as Lipari near Sicily, Yali and Nisisros in Greece, and the Canary Islands of Spain. Italy, Greece, and the United States control more than 50% of the world’s pumice supply. Smaller tonnages come from Chile, Germany, Spain, Turkey, Italy, New Zealand, France, Ecuador, Ethiopia, Guadeloupe, Iran, Martinique, Argentina, and the Dominican
Republic (production from some areas may include volcanic tuff, ash, pumiceous lapilli, or scoria).