ultramafic

/AH1 L T R AH0 M AA0 F IH0 K/
noun
  1. 1

    A rock with such properties.

adjective
  1. 1

    Describing igneous rocks that contain magnesium and iron and only a very small amount of silica, such as are found in the Earthโ€™s mantle.

Synonyms

Source: https://en.wiktionary.org/wiki/ultramafic