Silvialite, ideally Ca₄Al₆Si₆O₂₄SO₄, is tetragonal, I4/m, Z = 2, with a = 12.160(3), c = 7.560(1) Å, V = 1117.9(8) ų c:a = 0.6217:1, ω = 1.583, ε = 1.558 (uniaxial negative), D_m − 2.75 g/cm³, D_cale 2.769 g/cm³ and H (Mohs) = 5.5. It is transparent and slightly yellow, has a good {100} cleavage, chonchoidal fracture, white streak and a vitreous lustre. It occurs in upper-mantle garnet-granulite xenoliths hosted by olivine nephelinite, from McBride Province, North Queensland, Australia. The empirical formula, derived from electron-microprobe analysis, is (Na_1.06Ca_2.86)(Al_4.87Si_7.13)O₂₄ [(SO₄)_0.57(CO₃)_0.41]. Crystal-structure refinement shows disordered carbonate and sulfate groups along the fourfold axis. Silvialite is a primary cumulate phase precipitated from alkali basalt at 900–1000°C and 8–12 kbar under high f_SO2 andf_O2. The name silvialite, currently used in literature to describe the sulfate analogue of meionite, was suggested by Brauns (1914).