10:00 AM - 10:15 AM
[SMP23-04] Redetermination of the crystal structure of gowerite, Ca[B5O8(OH)][B(OH)3]·3H2O, and possible formation environment
Keywords:Gowerite, Borate minerals, Hydrogen bonding, Single-crystal XRD, Ab initio quantum chemical calculation
The crystal structure of gowerite, Ca[B5O8(OH)][B(OH)3]·3H2O, monoclinic, a = 12.872(4), b = 16.326(4), c = 6.5634(18) Å, β = 121.319(3)°, V = 1178.3(6) Å3, Z = 4, space group P21/a, has been refined by full-matrix least-squares on F2 to an R1 index of 4.13 % for 1730 unique observed [Fo ≧ 4σ(Fo)] reflections measured with MoKα radiation. The H positions were located on difference Fourier maps and refined using the constraints of O-H distances and interatomic distances between H atoms. The crystal structure is characterized by a curved corrugated two-dimensional sheet in the ac plane. The sheet is composed of the double three-membered borate rings and Ca atoms coordinated to six O atoms from the double three-membered borate rings. The Ca atom is further coordinated to a water molecule and an isolated B(OH)3 triangle oriented approximately perpendicular to the sheets. The linkage between the sheets is provided only by the hydrogen-bonding interactions. Three water molecules are accommodated in the space between the sheets. The important feature in gowerite is that in the triangle geometry the B-O bond lengths between B atom and hydroxyl group cannot be clearly distinguished from the B-O bond length between B and O atoms. Ab initio quantum chemical calculation revealed that molecular orbitals are spatially well spread over the whole three-membered borate rings, indicating the existence of the covalent bonding character between B and O atoms. Structural units of the fundamental building blocks (FBBs) in the parent borate minerals are released by mineral weathering and inherited to the FBBs in gowerite during the secondary mineralization process.