adjective Geology.
of or pertaining to rocks rich in dark, ferromagnesian minerals. Compare basic ( def 4 ), ultramafic.

1910–15; ma(gnesium) + Latin f(errum) iron + -ic Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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American Heritage
Science Dictionary
mafic   (māf'ĭk)  Pronunciation Key 
Relating to an igneous rock that contains a group of dark-colored minerals, composed chiefly of magnesium and iron. Compare felsic.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Mafic rocks are darker colored and are mostly made up of magnesium and iron.
The dark, or mafic, minerals are richer in iron and magnesium.
Deposits related to subaerial mafic extrusive rocks.
Permeability is slow in mafic rocks because the dark minerals commonly weather
  to clay.
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