Hydroxylgugiaite: A new beryllium silicate mineral from the larvik plutonic complex, Southern Norway and the ilímaussaq alkaline complex, South Greenland: the first member of the melilite group to incorporate a hydrogen atom

Hydroxylgugiaite, ideally (Ca₃A₁)R₄(Si_3.5Be_2.5)R₆O₁₁(OH)₃, is a new mineral species from two localities in the Larvik plutonic complex in Porsgrunn, Telemark, Norway, and one locality in Iĺimaussaq, Greenland. Hydroxylgugiaite crystals occur as squat dipyramids {111} (30 × 50 μm) or as elongate tetragonal prisms. The crystals are translucent, white to pale grey in color, with a white streak and vitreous luster. It is brittle, with no apparent cleavage. Hydroxylgugiaite is uniaxial positive with ω = 1.622 ± 0.002 and ϵ = 1.632 ± 0.002. There is no pleochroism and birefringence is low. The average of eight analyses of a single grain of type material (oxide wt.%) gave Na2O 2.04, CaO 32.90, FeO 0.22, MnO 0.74, BeO 13.47 (LA-ICP-MS), Al₂O₃ 0.74, SiO₂ 44.06, F 1.74, H2O (assuming 3 OH + F) 4.93, Total (-0.73 O = F) 100.10. Potassium, strontium, and magnesium were measured but not detected. The calculated density is 2.79 g cm-3. The empirical formula on the basis of 14 anions including 3 OH- + F- is: (Ca_2.76Na_0.31Mn_0.05Fe_0.01)R_3.13(Si_3.45Be_2.53A_l0.07)R_6.05O₁₁[(OH)_2.57F_0.43]R₃. The formula from crystal-structure analysis of the Saga specimen is: (Ca_3.02A_0.98)R₄(Si_1.79Be_0.21)R₂(Be_2.29Si_1.71)R₄O₁₁(OH)₃. Combined structural and chemical data gives the fol lowing formula for the Nakkaalaaq specimen: (Ca_2.88A_0.98Na_0.12Mn_0.02)R₄(Si_1.80Be_0.17Al_0.03)R₂ (Be_2.32Si_1.68)R₄O₁₁[(OH)_2.70F_0.30]R₃; with simplified formula (Ca,A)4(Si,Be)2(Be,Si)4O11(OH)3. The crystal structure of hydroxylgugiaite is tetragonal in acentric space group P421/m, with a 7.4151(2), b 7.4151, c 4.9652(1) Å, V 272.9(1) Å3, and Z=1. It has been refined to an R index of 0.028 on the basis of 342 observed reflections and a correction for the {110} twin law. It is an H-bearing member of the melilite group. The structure has two distinct layers. The one crystallographically distinct Ca site with eight-fold coordination is a square antiprism polyhedron. The Ca polyhedra are in a layer with the H atoms. A second layer consists of corner-sharing Si/Be atoms in tetrahedral coordination with O. One H atom is bonded to an apical O atom that is not shared by two tetrahedra. This H atom is present only when there is a Ca-site vacancy. The other H atom is loosely bonded to the same O atom but at a different site. The IR spectrum supports this H-bonding scheme. Additional hydroxylgugiaite data is given for the other localities.