Eclogite is a coarse-grained, mafic-to-ultramafic grouping of metamorphic rocks of special interest on account of the variety of minerals they contain and their microscopic structures and geological relationships. The fresh rock can be very striking in appearance, with red to pink garnets, almandine-pyrope in a green matrix of sodium-rich pyroxene, and pale green or nearly colorless augite (omphacite). Accessory minerals are kyanite, rutile, occasional quartz, hornblende, phengite , paragonite , zoisite, dolomite, corundum, and, rarely, diamonds. Feldspar is rare; if present, belongs to basic varieties rich in lime. Other minerals which have been found in eclogites are bronzite, olivine and glaucophane. The last mentioned is a bright blue variety of hornblende with striking pleochroism.
Eclogite typically results from low-to-medium temperature and high-pressure metamorphism of mafic igneous rocks (typically basalt) as it plunges into the mantle in a subduction zone. Under mantle conditions from 30 to 100 km depth at 500–1000 °C and > 2 GPa (20 kbar), the basic mineralogy is transformed into the eclogite assemblage. If the eclogite melts, it can then be treated as an igneous rock on solidification.
Some consider eclogite to be representative of the upper mantle and see tholeiitic basalt volcanism typical of eclogite melts. Eclogite that is brought to shallow conditions is quite unstable, and retrograde metamorphism often occurs with secondary amphibole and plagioclase forming reaction rims on the primary pyroxene minerals. Xenoliths of eclogite reportedly occur in the kimberlite pipes of the diamond mines of South Africa. Eclogites are also common in alpine garnet peridotites in Greenland.
The eclogites in their chemical composition show close affinities to gabbros. Examples are known in Saxony, Bavaria, Carinthia, Austria and Norway. A few eclogites also occur in the north-west highlands of Scotland. Glaucophane-eciogites have been met with in Italy and the Pennine Alps.