Eclogite tumbled stones from Greenland.

The rock eclogite is mafic (basaltic) in chemical composition. It is sometimes classified as peridotites, but is not actually an ultramafic rock. Eclogite is also sometimes classified as mantle rocks, which is in fact incorrect because it is not originally mantle material. Because the eclogite facies occurs at higher than normal pressures in the crust, eclogite is rare and indicative of special tectonic conditions. Unweathered, eclogite is a striking rock, with minerals consisting of red to purple garnets (almandine to pyrope) in a matrix of poisonous green clinopyroxene (omphacite). Although these two minerals are characteristic of eclogite, kyanite, rutile, quartz (sometimes as its high-pressure polymorph coesite), retrograde amphiboles such as glaucophane, retrograde titanite, phengite and paragonite (white micas), zoisite, dolomite and diamond may also occur. If many retrograde minerals are present, it is also referred to as "retroeclogite". Eclogite is the result of high-pressure metamorphism of rocks with a mafic composition (e.g. gabbro or basalt). The depth required to form eclogite in the Earth is normally only reached in the mantle. However, eclogite has a different chemical composition from ultramafic mantle rocks such as peridotite. Since the 1980s, eclogites have therefore usually been explained as pieces of crust that were subducted into the mantle and then returned to the surface. Exactly how this uplift can take place is still being investigated. Sometimes xenoliths of eclogite are found in kimberlite pipes. Lenses of eclogite occur in granulite or amphibolite terrains throughout the world. Examples include the Musgrave block in Australia (formed during the Petermann orogeny), where eclogite lenses occur in granulite terrains, the Caledonides in Norway and Greenland, where eclogite lenses occur together with peridotite lenses in felsic gneisses and amphibolites, and Alpine mountains such as in Greece or the Alps . In those mountain ranges, the eclogite lenses usually occur in ophiolites. Eclogites with glaucophane occur in the Italian Alps. The occurrence of eclogite always indicates a rare tectonic situation.