Introduction Dividing the PT space of metamorphism into several subdivisions independent of the composition of the metamorphosed rock would greatly facilitate communication among petrologists regarding the degree of metamorphism. In 1915, Eskola introduced the concept of metamorphic facies to accomplish this classification. Eskola emphasizes mineral assemblages (in mafic rocks) rather than index minerals. A facies can be defined as: “A group of mineral assemblages, each of which develops in the rock a specific chemical composition, and is characteristic of the P and T under which the rocks were metamorphosed.” Therefore, if the overall composition of the rock and the P and T values ​​at which it crystallized is known, a prediction of the mineral assemblage can occur accordingly." What are the key mineral assemblages and reactions in eclogite facies rocks, The Eight minerals the facies originally defined by Eskota were greenschist, pyroxene hornfels, amphibolite, granulite, sanidinite, glaucopheneschist (blueschist), and eclogite facies (Figuire 1, the metabasaltic rocks contain the characteristic omphacitic pyroxene + almandine- assemblage). pyrope-grossular garnet, creating dense, beautiful green and red rocks (Table 1). Plagioclase is not present in rocks of the eclogitic facies due to its stability. The accessory minerals are kyanite, quartz, rutile, orthopyroxene and coesite formed. over a wide temperature range (temperatures above 600 0 C in Figure 1 Figure 1: Pressure-temperature diagram showing the fields of the various metamorphic facies). The transition from the blueschist facies to the eclogite facies involves glaucophane and paragonite reacting to form the garnet -omphacite assemblage:Gln + Pg = Prp + Jd+ Qtz + H2Oat pressures greater than 1.2 GPa. But glaucophene alone can remain stable in the eclogite facies. Which minerals/reactions provide evidence for their distinctive conditions of formation? Eclogites are very dense rocks with an increasingly higher density than some ultramafic mantle rocks and, due to the high density of eclogites, their origin can easily be related to very high pressure conditions during formation. Eclogites are found in very different geodynamic contexts. Low-temperature eclogites (glaucophane + paragonite = garnet + omphacite) can result from subduction of oceanic lithosphere and form from blueschists (high-pressure, low-temperature eclogites), an example being the Alps. Intermediate eclogites can result from stacking of the continental crust by amphibolites (eclogites at medium temperatures). At high temperature, hydrates are absent in eclogite, and kyanite is often a characteristic additional mineral in addition to Grt + Omp. High-temperature eclogites form in collisional or extensional environments where the geotherm is abnormally “hot” due to magmatic heat transfer from the mantle.