Peridotite

The mantle is composed of rocks called peridotite. Peridotite is named for one of the dominant minerals it contains, Peridot (a.k.a. Olivine) Serpentinite is water-damaged peridotite. Peridotite forms near the crust-mantle boundary at mid-ocean ridges where the driving forces of plate tectonic motion create new ocean crust. The layered complexes are believed to have been formed in place by crystal settling from a previously intruded fluid or magma. Other types seem to have ranged from fluid magmas to semisolid crystal mushes at the time of emplacement like kimberlites. Peridotite is the source of all chromium ore and naturally occurring diamonds, and of nearly all chrysotile asbestos. It is one of the main host rocks of talc deposits and platinum metals and formerly was a major source of magnesite. Nearly all peridotite is more or less altered to serpentine.