POLLUX, or POLLUCITE  , a rare mineral, consisting of hydrous caesium and aluminium silicate, HZCs4A14(SiO3)9 . Caesium oxide (CssO) is present to the extent of 30-36 %, the amount varying somewhat owing to partial replacement by other alkalis, chiefly sodium . The mineral crystallizes in the cubic system . It is colourless and transparent, and has a vitreous lustre . There is no distinct cleavage and the fracture is conchoidal . The hardness is 61 and the specific gravity 2.90 . It occurs sparingly, together with the mineral " castor " (see PETALITE), in cavities in the granite of the island of Elba, and with beryl in pegmatite veins at Rumford and Hebron in Maine .