Geo-pic of the Week: Accessory Minerals

Modified by CombineZP(FOV approx. 2 mm, photo by Corbin Cannon)

Accessory minerals are minerals found in igneous rocks that are not used for the classification or naming of the rock. These minerals may be commonly present in a type of rock, but the absence of the mineral would not change the general classification geologists give to the rock.

The two accessory minerals in the center of the picture above are greenish-black needles of aegirine (AY-jur-EEN) and orangish-pink analcime (uh-NAL-seem) crystals. These minerals are frequently found together in igneous intrusions of syenite like the one present at Granite Mountain, where this sample was collected.

Accessory minerals give important clues to geologists when trying to determine details about how a rock formed and how it changed over time. They can make up a substantial portion or a fairly insignificant portion of a rock. Some accessory minerals make up a sufficient portion of the rock to be included as a modifier in the name, such as “biotite syenite”. Adding such a modifier gives geologists quick and useful information about how this rock differs from standard syenite.

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