Steiningerite, Ba ₂ Zr ₂ (Si ₄ O ₁₂ )O ₂ , a new cyclosilicate from the Löhley quarry, Germany

Abstract The new mineral steiningerite, ideally Ba 2 Zr 2 (Si 4 O 12 )O 2 , was discovered along fissures and in cavities in melilite nephelinite samples retrieved from the currently active Löhley quarry, Eifel Volcanic Fields, Germany. Steiningerite is associated with minerals of the pyroxene group (augite–diopside), leucite, perovskite, titanite and accessory fluorapatite, fresnoite, wöhlerite, götzenite, fersmanite, magnetite and minerals of the pyrochlore group. It usually forms colourless or creamy white, euhedral, short prismatic to thick tabular, partly pseudocubic crystals up to 100 μm in size but also occurs rarely as individuals reaching 0.5 mm in size. The mineral is transparent to translucent, exhibits a vitreous lustre and no visible cleavage. The calculated density of steiningerite is 3.78 g/cm 3 . Optically, steiningerite is non-pleochroic and uniaxial (+), with ω = 1.711(3) and ε = 1.750(3) (λ = 589 nm). The empirical formula of holotype steiningerite, calculated on 14 anions, is (Ba 1.36 K 0.56 Na 0.09 Sr 0.05 Ca 0.02 ) Σ2.08 (Zr 1.52 Ti 0.25 Nb 0.13 U 0.05 Fe 0.02 Hf 0.01 ) Σ1.98 (Si 4.00 Al 0.03 ) Σ4.03 O 12 (O 1.59 F 0.41 ) Σ2.00 . Steiningerite crystallises in space group P 4 /mbm , with refined unit-cell parameters a = 8.894(2) Å, c = 8.051(2) Å, V = 636.9(3) Å 3 and Z = 2. The crystal structure, determined from single-crystal intensity data, was refined to R = 0.0310 for 444 unique reflections with I > 3σ( I ). The mineral is isotypic with the synthetic KTaSi 2 O 7 and structurally similar to the mineral rippite, K 2 (Nb,Ti) 2 (Si 4 O 12 )(O,F) 2 . The hetero-polyhedral framework is formed by the chains of (Zr,Ti)O 6 octahedra, running parallel to the four-fold axis, which are combined via Si 4 O 12 rings. Each (Zr,Ti)O 6 octahedron shares four vertices with four SiO 4 tetrahedra, belonging to four different Si 4 O 12 units. This arrangement of atoms creates channels along the c axis, with a pentagonal cross-section, in which charge-balancing Ba 2+ and K + ions are located. Extra-framework alkaline and alkaline-earth cations have twelve-fold coordination. The occurrence of the new mineral in a melilite nephelinite, along with its high-temperature mineral association and the absence of H 2 O and OH groups, confirmed by Raman and FTIR spectroscopies, indicate high-temperature conditions of formation and suggests a pneumatolytic origin of steiningerite.