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a. Said of an igneous rock having a relatively low silica content,
sometimes delimited arbitrarily as 44% to 51% or 45% to 52% ; e.g.,
gabbro, basalt. Basic rocks are relatively rich in iron, magnesium, and/or
calcium, and thus include most mafic rocks as well as other rocks. Basic
is one of four subdivisions of a widely used system for classifying
igneous rocks based on their silica content: acidic, intermediate, basic,
and ultrabasic. CF: femic
b. Said loosely of dark-colored minerals. CF:
silicic;
mafic.
AGI
c. Said of a plagioclase that is calcic.
Source:
Dictionary of Mining, Mineral, and Related Terms
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