Class : Halides
    Subclass : Oxychlorides
    Crystal system : Cubic
    Chemistry : Hg₆Cl₃O(OH)
    Rarity : Rare

Eglestonite is a rare oxychloride found in the surface oxidation zone of mercury deposits, particularly in desert regions. It results from an alteration of cinnabar in an environment rich in chlorides : these can be brought by outcropping evaporites or by saline air if the deposit is close to the sea. It was named in honor of Thomas Egleston, founder of the Columbia School of Mines and who was also a professor of mineralogy and metallurgy. Eglestonite occurs in dodecahedral crystals, sometimes cubic, octahedral, or even acicular, often streaked parallel to the edges. Eglestonite is also often massive. It is an orange-brown to yellowish-brown mineral, turning black on exposure to light, with a strong adamantine luster. It has a low hardness (2.5) but is very dense (more than 8). In addition to cinnabar, it is almost always accompanied by calomel, another mercury chloride, terlinguaite and native mercury.

Main photo : Eglestonite from Backofen Mine, Landsberg, Obermoschel, Nordpfälzer Land, Donnersbergkreis, Germany © Stephan Wolfsried

No twin known for this mineral species.

No fake listed for this mineral species.