Orthorhombic lazurite from the Tultui deposit in the Baikal region

A lazurite sample taken from the Tultui deposits in the Baikal region has been studied using petrographic examination, X-ray diffraction, and local X-ray spectroscopy (electron microprobe). It is established that the line of the basic cell in the X-ray diffraction pattern of the orthorhombic lazurite is widened, so the unit-cell parameters should be recalculated from the interlayer distances of the superstructural reflections. The chemical compositions and unit-cell parameters of the lazurites from the Baikal region and the Pamirs are different. Two schemes of the chemical substitutions of atoms in the mineral are assumed: (1) the hauyne cluster Na₃CaSO₄]³⁺ is replaced with a sodalite cluster Na₄Cl]³⁺, and (2) two hauyne clusters 2Na₃CaSO₄]³⁺ are substituted by nosean clusters Na₄SO₄]⁺ and Na₄H₂O]⁴⁺ with retention of the total charge. The increase in the Na and Cl contents in the orthorhombic lazurite is accompanied by a decrease in the unit-cell dimensions. Orthorhombic lazurites from the southern Baikal region and the southwestern Pamirs were formed in apocarbonate (apocalciphyre) metasomatic rocks at a lower temperature than pyroxene, afghanite, and cubic lazurite.