Mafic & ultramafic rocks

Most geology students know what a basalt or gabbro is, but occasionally even introductory texts will refer to different mafic rocks and ultramafic rocks. This list very briefly describes a few of these, just enough to differentiate between them based on composition and formation.

Mafic (45–52% silica)

Intrusive

  • Troctolite: mostly calcium-rich plagioclase; think of it as olivine-rich anorthosite or a pyroxene-depleted gabbro
  • Anorthosite: over 90% plagioclase
  • Norite: calcium-rich plagioclase (labradorite), orthopyroxene, and olivine

Subvolcanic

  • Diabase (dolerite): mafic subvolcanic rock (formed at shallow depth within the crust) with composition of basalt/gabbro (i.e. mostly pyroxenes); fine but visible texture of lath-shaped plagioclase crystals in a matrix of clinopyroxene

Extrusive

  • Basalt: mostly plagioclase and pyroxenes; magnesium, iron, and calcium oxide each constitute 5-15% by weight
    • Picrite: high-magnesium, olivine basalt with 20-50% olivine; used by itself, picrite refers to the basalt variety, but olivine-rich gabbro can be called picrite gabbro
    • Eucrite: basically, basalts found on asteroids and meteors; have reduced, metallic nickel-iron (Ni-Fe) instead of iron oxide minerals
  • Boninite: high Mg; formed in fore-arc of subduction zones; depleted in incompatible trace elements

Ultramafic (less than 45% silica)

PeridotiteIntrusive

  • Peridotite: ultramafic intrusive rock with mostly olivine and pyroxene
    • Lherzolite: 40-90% olivine
    • Dunite: over 90% olivine
    • Harzburgite: olivine and orthopyroxene with no more than 5% clinopyroxene
    • Websterite: over 50% pyroxene

Subvolcanic

  • Kimberlite: sometimes contains diamonds

Extrusive

  • Komatiite: ultramafic extrusive rock with mostly magnesium-rich olivine, calcium-rich plagioclase, and pyroxene; common during the Archean, when the earth was much hotter
Cortney

About Cortney

Geology lover. Proud owner of a 2014 Honda CRF250L. Grew up on NES, N64, & Gameboy. Collects maneki neko (lucky cats). Married to a gearhead. Email cortney@luckysci.com. Wishlist.

Leave a Reply