Definition of seybertite
plural -s
: a basic aluminosilicate Ca(Mg,Al)3(Al,Si)4(O,OH)12 of the clintonite group consisting of calcium, magnesium, and aluminum, occurring in monoclinic crystals and foliated masses and having a reddish brown, copper red, or yellowish color and submetallic luster (hardness 4–5, specific gravity 3–3.1)
