Lazurite: Validation as a Mineral Species with the Formula Na7Ca(Al6Si6O24)(SO4)S₃^• -⋅H2O and New Data

The status of lazurite as a valid mineral species has been confirmed. The neotype specimen from the Malaya Bystraya gem lazurite deposit, Baikal Lake area has been studied using electron microprobe, wet chemical analysis, ESR, IR, Raman, X-ray photoelectron spectroscopy, UV-Vis-near IR absorption and luminescence spectroscopy, and powder X-ray diffraction. The empirical formula of the neotype sample is (Na6.97Ca0.88K0.10)7.96[Si6.04Al5.96]12O24(SO4)1.09( S₃^• - )0.55 S_0.05^2 - Cl0.04⋅0.72H2O, where S₃^• - is trisulfide radical anion, which is a blue chromophore. The idealized formula Na7Ca(Al6Si6O24)(SO4) S₃^• - ⋅H2O has been approved by the IMA Commission on New Minerals, Nomenclature and Classification, proposal #20-H. The crystal structure of lazurite is characterized by commensurate and incommensurate modulations; the a parameter of the cubic sub-cell is 9.087(3) Å. The neotype sample is slightly birefringent, with α' = 1.523(2) and γ' = 1.525(2).