rhyacoliteMin.|raɪˈækəlaɪt| [ad. G. ryakolith, f.Gr. ῥυακο-, ῥύαξ mountain-stream, lava-stream: see -lite.] A variety of orthoclase. c1830in Encycl.Metrop. (1845) VI. 526/2 Ryakolite.1852Brooke & Miller Phillips' Min. 369 Rhyacolite..is found..in volcanic matter at the foot of Vesuvius.