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

Abstract Hydroxylgugiaite, ideally (Ca 3 □ 1 ) Σ4 (Si 3.5 Be 2.5 ) Σ6 O 11 (OH) 3 , is a new mineral species from two localities in the Larvik plutonic complex in Porsgrunn, Telemark, Norway, and one locality in Ilimaussaq, 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 Na 2 O 2.04, CaO 32.90, FeO 0.22, MnO 0.74, BeO 13.47 (LA-ICP-MS), Al 2 O 3 0.74, SiO 2 44.06, F 1.74, H 2 O (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 2.76 Na 0.31 Mn 0.05 Fe 0.01 ) Σ3.13 (Si 3.45 Be 2.53 Al 0.07 ) Σ6.05 O 11 [(OH) 2.57 F 0.43 ] Σ3 . The formula from crystal-structure analysis of the Saga specimen is: (Ca 3.02 □ 0.98 ) Σ4 (Si 1.79 Be 0.21 ) Σ2 (Be 2.29 Si 1.71 ) Σ4 O 11 (OH) 3 . Combined structural and chemical data gives the following formula for the Nakkaalaaq specimen: (Ca 2.88 □ 0.98 Na 0.12 Mn 0.02 ) Σ4 (Si 1.80 Be 0.17 Al 0.03 ) Σ2 (Be 2.32 Si 1.68 ) Σ4 O 11 [(OH) 2.70 F 0.30 ] Σ3 ; with simplified formula (Ca,□) 4 (Si,Be) 2 (Be,Si) 4 O 11 (OH) 3 . The crystal structure of hydroxylgugiaite is tetragonal in acentric space group P 2 1 / m , with a 7.4151(2), b 7.4151, c 4.9652(1) A, V 272.9(1) A 3 , 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.