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A blue to dark-gray variety of plagioclase feldspar that contains a higher proportion of calcium than sodium and occurs in alkaline igneous rocks.
[After Bytown, former name of Ottawa, Ontario, near where a sample of the mineral was found.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
a rare plagioclase found in alkaline igneous rocks
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014