Schorl tourmaline,  Norway.

Schorl tourmaline, Norway.

The general formula for tourmaline, which is a group mineral is NaFe2+3Al6(BO3)3Si6O18(OH)4  It is a Boron silicon mineral, and the black variety is called schorl, which is the sodium iron end member of the group. This Norwegian specimen clearly shows at least three elongated prismatic crystals.