or Pollucite, a rare mineral, consisting of hydrous caesium and aluminium silicate, H 2 Cs 4 A1 4 (SiO 3) 9. Caesium oxide (Cs 2 0) 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 62 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.