mineral in low- to medium-grade metamorphic rock formed by metamorphism of mafic or pelitic rock. It is also common in igneous rocks, usually as a secondary mineral, formed by alteration of mafic minerals such as biotite, hornblende, pyroxene, or garnet. The glassy rims of pillow basalt on the ocean floor is often altered to pure chlorite, in part by exchange of chemicals with seawater. The green color of many igneous rocks, slates, and schists is due to fine particles of chlorite disseminated throughout the rock. Chlorite is a common weathering product and is widespread in clay and in sedimentary rock containing clay minerals. Chlorite is found in pelites along with quartz, albite, sericite, and garnet, and is also found in associate with actinolite and epidote. In his pioneering work on metamorphic facies in the Scottish Highlands, G.M. Barrow identified the chlorite zone as the zone of mildest metamorphism. In modern petrology, chlorite is the diagnostic mineral of the greenschist facies. This facies is characterized by temperatures near and pressures near 5 kbar. At higher temperatures, much of the chlorite is destroyed by reactions with either potassium feldspar or phengite mica which produce biotite, muscovite, and quartz. At still higher temperatures, other reactions destroy the remaining chlorite, often with release of water vapor. Chlorite is one of the most common minerals produced by propylitic alteration by hydrothermal systems, where it occurs in the "green rock" environment with epidote, actinolite, albite, hematite, and calcite. Experiments indicate that chlorite can be stable in peridotite of the Earth's mantle above the ocean lithosphere carried down by subduction, and chlorite may even be present in the mantle volume from which island arc magmas are generated. Members of the chlorite group {| class="wikitable" |- ! Baileychlore ! IMA1986-056 | |- ! Borocookeite ! IMA2000-013 | |- ! Chamosite ! year: 1820 | |- ! Clinochlore ! year: 1851 | |- ! Cookeite ! year: 1866 | |- ! Donbassite ! year: 1940 | |- ! Gonyerite ! year: 1955 | |- ! Nimite ! year: 1968 | |- ! Pennantite ! year: 1946 | |- ! Ripidolite ! chlinochlore var. | |- ! Sudoite ! IMA1966-027 | |} Clinochlore, pennantite, and chamosite are the most common varieties. Several other sub-varieties have been described. A massive compact variety of clinochlore used as a decorative carving stone is referred to by the trade name seraphinite. It occurs in