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Aerial magnetometer

An instrument aboard, or attached to, an airplane used to detect the strength of a magnetic field

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Aegirine

A sodium-ferric monoclinic iron silicate, NaFe3+Si2O6, which occurs in soda-rich igneous rock

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Adit

A horizontal opening driven into a hill or mountain for providing access to minerals

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Actinolite

A monoclinic mineral with prismatic cleavage, 2[Ca2(Mg,Fe)5Si8O22(OH)2]

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Amphibole

A mineral group including actinolite, anthophyllite, arfvedsonite, cummingtonite, hornblende, richterite, glaucophane, grunerite, anthophyllite, riebeckite, tremolite, and others

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Amortization

The gradual and systematic reduction of a balance in an account over an assigned period