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| Extrusive Ore Deposits |
Perlite
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.
Sources:
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
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).
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