In granites, the dark (mafic) minerals are typically one or more of the following; biotite, hornblende, or pyroxene. In the granite image at which you were looking, biotite appears to be the primary dark mineral present. Biotite in granites is typically black or brown, and has a flaky texture that is characteristic of micas.

A number of other minerals occur in granites, but in most cases they are too small to be recognizable on an image like the one you were shown.

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