M > 90% (Ultramafic rocks)
Ultramafic rocks are usually dominated by one or more of the minerals olivine, orthopyroxene and clinopyroxene. Ultramafic rocks with > 40% olivine are peridotites. Various types of peridotite are dunite (> 90% olivine), harzburgite (olivine + orthopyroxene), wehrlite (olivine and clinopyroxene), and lherzolite (olivine + two pyroxenes).
Rocks with > 60% pyroxene are pyroxenites. Pyroxenites are divided into 6 types: orthopyroxenite (> 90% orthopyroxene), clinopyroxenite (> 90% clinopyroxene), websterite (two pyroxenes), olivine orthopyroxenite, olivine clinopyroxenite and olivine websterite.
Common accessory minerals in ultramafic rocks are garnet or spinel.
Hornblende may be important to give various types of hornblendites and e.g. hornblende peridotites.
We will not consider these further.
The upper mantle is composed of peridotite. Mafic minerals usually crystallize before plagioclase from basaltic magma. Ultramafic rocks can therefore form as the early crystallization products. It is important to note that ultramafic magmas are extremely rare so that there is no classification system for volcanic rocks equivalent to the ultramafic classification for plutonic rocks.
The upper mantle is composed of peridotite. Mafic minerals usually crystallize before plagioclase from basaltic magma. Ultramafic rocks can therefore form as the early crystallization products. It is important to note that ultramafic magmas are extremely rare so that there is no classification system for volcanic rocks equivalent to the ultramafic classification for plutonic rocks.
