Largely overlooked in the national debate over fracking is the emerging fight in the U.S. heartland over mining "frac sand," which has grains of ideal size, shape, strength and purity. Mining companies say the work provides good jobs in rural areas, but some residents fear the increase in mining could harm human health and the environment.

"More and more people are waking up to the fact that there are difficulties with this massive explosion," said Pat Popple, a retired school teacher and principal and anti-sand mining activist.

U.S. frac sand producers sold or used more than 6.5 million metric tons of sand worth $319 million in 2009, according to the U.S. Geological Survey. The tonnage likely will have doubled when 2010 data is released, said Thomas Dolley, a USGS mineral commodity specialist who follows the silica mining industry.

"It's huge," Dolley said. "I've never seen anything like it, the growth. It makes my head spin."

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