ultramafic rock
Technical term in mineralogy for igneous rocks with very low silica content, rich in magnesium and iron.
Ultramafic rocks are known as rocks derived from the mantle.
苦鉄質岩 (mafic rock) has higher silica and lower magnesium/iron than 超苦鉄質岩.
Compound of 超 (ultra-), 苦鉄質 (mafic, from 苦 'bitter' for magnesium and 鉄 'iron') and 岩 (rock). A calque of English 'ultramafic rock'.