New data suggest the following chemical peculiarities of lazurite: 1. 1. The cell (cubic, a=9 A ̊) formula can be expressed as (R′,R′′)7,5-8(Si,Al)12(O,S)24(SO4,H2O,Cl)a·nH2O; R′ signifies monovalent ions, chiefly Na; R′′ signifies divalent ions, essentially Ca. 2. 2. Si exceeds Al by 0.1 to 0.6 atoms per 9Å cell. Non-cubic (anisotropic and pleochroic) lazurite is common and may be caused by S- 3 ions in parallel orientation. A 'perthitic' composite from Baffin Island has more Cl in the dark isotropic phase, less in the light anisotropic phase. 'Anomalous dispersion' results from absorption at 600 nm. Dark varieties contain more S2- than pale varieties.