Vibrational spectroscopic study of the minerals nekoite Ca3Si6O15·7H2O and okenite Ca10Si18O46·18H2O – Implications for the molecular structure

Nekoite Ca3Si6O15·7H2O and okenite Ca10Si18O46·18H2O are both hydrated calcium silicates found respectively in contact metamorphosed limestone and in association with zeolites from the alteration of basalts. The minerals form two-dimensional infinite sheets with other than six-membered rings with 3-, 4-, or 5-membered rings and 8-membered rings. The two minerals have been characterised by Raman, near-infrared and infrared spectroscopy. The Raman spectrum of nekoite is characterised by two sharp peaks at 1061 and 1092 cm−1 with bands of lesser intensity at 974, 994, 1023 and 1132 cm−1. The Raman spectrum of okenite shows an intense single Raman band at 1090 cm−1 with a shoulder band at 1075 cm−1. These bands are assigned to the SiO stretching vibrations of Si2O5 units. Raman water stretching bands of nekoite are observed at 3071, 3380, 3502 and 3567 cm−1. Raman spectrum of okenite shows water stretching bands at 3029, 3284, 3417, 3531 and 3607 cm−1. NIR spectra of the two minerals are subtly different inferring water with different hydrogen bond strengths. By using a Libowitzky empirical formula, hydrogen bond distances based upon these OH stretching vibrations. Two types of hydrogen bonds are distinguished: strong hydrogen bonds associated with structural water and weaker hydrogen bonds assigned to space filling water molecules.