Crystal structure and spectroscopic behavior of synthetic novgorodovaite Ca-2(C2O4)Cl-2 center dot 2H(2)O and its twinned triclinic heptahydrate analog

摘要

Synthetic novgorodovaite analog Ca-2(C2O4)Cl-2 center dot 2H(2)O is identical to its natural counterpart. It crystallizes in the monoclinic I2/m space group with a = 6.9352(3), b = 7.3800(4), c = 7.4426(3) , beta = 94.303(4)A degrees, V = 379.85(3) (3) and Z = 2. The heptahydrate analog, Ca-2(C2O4)Cl-2 center dot 7H(2)O, crystallizes as triclinic twins in the P space group with a = 7.3928(8), b = 8.9925(4), c = 10.484(2) , alpha = 84.070(7), beta = 70.95(1), gamma = 88.545(7)A degrees, V = 655.3(1) (3) and Z = 2. The crystal packing of both calcium oxalate-chloride double salts favors the directional bonding of oxalate, C2O4 (2-), ligands to calcium ions as do other related calcium oxalate minerals. The pi-bonding between C and O atoms of the C2O4 (2-) oxalate group leaves sp (2)-hydridised orbitals of the oxygen atoms available for bonding to Ca. Thus, the Ca-O bonds in both calcium oxalate-chloride double salts are directed so as to lie in the plane of the oxalate group. This behavior is reinforced by the short O center dot center dot center dot O distances between the oxygens attached to a given carbon atom, which favors them bonding to a shared Ca atom in bidentate fashion. Strong bonding in the plane of the oxalate anion and wide spacing perpendicular to that plane due to repulsion between oxalate pi-electron clouds gives rise to a polymerized structural units which are common to both hydrates, explaining the nearly equal cell constants similar to 7.4 which are defined by the periodicity of Ca-oxalate chains in the framework (monoclinic b ae triclinic a). When compared with novgorodovaite, the higher water content of Ca-2(C2O4)Cl-2 center dot 7H(2)O leads to some major differences in their structures and ensuing physical properties. While novgorodovaite has a three-dimensional framework structure, in the higher hydrate, the highly polar water molecules displace chloride ions from the calcium coordination sphere and surround them through OwH center dot center do