ultrabasic

adjective

1. (of rocks) containing iron and magnesium, with little or no silica.

adjective

1. (of such igneous rocks as peridotite) containing less than 45 per cent silica

1. Containing magnesium and iron and only a very small amount of silica. Used of igneous rock, and often used interchangeably with ultramafic. Peridotite is an ultrabasic rock.