Crystal structure and spectroscopic behavior of synthetic novgorodovaite Ca2(C2O4)Cl2·2H2O and its twinned triclinic heptahydrate analog.

Synthetic novgorodovaite analog Ca₂( C₂O₄) Cl₄·2H₄O 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) Å, β = 94.303(4)°, V = 379.85(3) ų and Z = 2. The heptahydrate analog, Ca₂( C₂O₄)Cl₄·7H₄O, crystallizes as triclinic twins in the P1 space group with a = 7.3928(8), b = 8.9925(4), c = 10.484(2) Å, α = 84.070(7), β = 70.95(1), γ = 88.545(7)°, V = 655.3(1) ų 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 π-bonding between C and O atoms of the C₂O₄ ^2- oxalate group leaves sp2-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...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 π-electron clouds gives rise to a polymerized structural units which are common to both hydrates, explaining the nearly equal cell constants ~7.4 Å which are defined by the periodicity of Ca-oxalate chains in the framework (monoclinic b ≈ triclinic a). When compared with novgorodovaite, the higher water content of Ca₂( C₂O₄) Cl₄·7H₄O 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···Cl hydrogen bonds. As a result, polymerization in Ca₂( C₂O₄)Cl₄·7H₄O solid is limited to the formation of two-dimensional Ca₂( C₂O₄)(H₄O)₅ slabs parallel to (001), inter-layered with hydrated chloride anions. This layered structure accounts for (001) being both a perfect cleavage and a twin interface plane. The infrared and Raman spectra of both salts are also briefly discussed. [ABSTRACT FROM AUTHOR]