Svornostite-(NH ₄ ), (NH ₄ ) ₂ Mg(UO ₂ ) ₂ (SO ₄ ) ₄ (H ₂ O) ₈ , a new mineral from the Blue Lizard mine, San Juan County, Utah, USA, and the establish

Abstract The new mineral svornostite-(NH 4 ) (IMA2024-068), (NH 4 ) 2 Mg(UO 2 ) 2 (SO 4 ) 4 (H 2 O) 8 , was found in the Blue Lizard mine, San Juan County, Utah, USA, where it occurs as sprays and subparallel groups of yellow blades in a secondary assemblage with ammoniozippeite, blödite, boussingaultite, gypsum, hexahydrite, kröhnkite, plášilite and quartz. The streak is very pale yellow. Crystals are transparent with vitreous lustre. The tenacity is brittle, the Mohs hardness is ∼2½, the fracture is curved. The mineral is soluble in H 2 O and has a measured density of 3.06(2) g·cm –3 . The mineral is optically biaxial (+) with α = 1.560(2), β = 1.564(2), γ = 1.589(2); 2V = 43(1)°; orientation: X = a , Y = b , Z = c ; pleochroism: X colourless, Y yellow, Z yellow; X < Y ≈ Z . Electron microprobe analyses provided [(NH 4 ) 1.895 Na 0.065 K 0.040 ] Σ2.000 (Mg 0.755 Mn 0.252 ) Σ1.007 (U 0.996 O 2 ) 2 (S 1.002 O 4 ) 4 (H 1.998 O) 8 . Svornostite-(NH 4 ) is orthorhombic, Pmn 2 1 , a = 13.0259(9), b = 8.2909(4), c = 11.2589(4) Å, V = 1215.92(11) Å 3 and Z = 2. The crystal structure ( R 1 = 0.0243 for 2222 I > 2σ I reflections) contains uranyl-sulfate chains that are linked into sheets by MgO 2 (H 2 O) 4 octahedra and 9- and 10-coordinated NH 4 + groups. The sheet has the same topology as the sheets in several synthetic uranyl selenates. Svornostite-(NH 4 ) is a member of the newly established svornostite group, which also includes svornostite-(K), oldsite-(K), rietveldite and zincorietveldite.