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Definition of basic

1. Said of an igneous rock with a silica content between 45 and 52 weight-%
(IUGS recommendation for the classification of igneous rocks)

2. 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.

Compare with: femic

3. Said loosely of dark-colored minerals.

Compare with: silicic, mafic

Ref: AGI

4. Said of a plagioclase that is calcic.
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