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alkali rock

American  

noun

Mineralogy.

  1. any igneous rock with a marked preponderance of alkali and a low percentage of silica.

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Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.