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accessory mineral A mineral phase within a rock whose presence does not affect the root name of the rock. For instance, the root name ‘granite’ is defined by the presence of quartz, alkali feldspar, and mica. These are the ‘essential minerals’. The presence of the mineral sphene does not affect the root name and hence would be an example of an accessory mineral. Apatite and zircon are also common accessory minerals.