Nekoite Ca 3 Si 6 O 15 ·7H 2 O and okenite Ca 10 Si 18 O 46 ·18H 2 O 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 1092cm −1 with bands of lesser intensity at 974, 994, 1023 and 1132cm −1 . The Raman spectrum of okenite shows an intense single Raman band at 1090cm −1 with a shoulder band at 1075cm −1 . These bands are assigned to the SiO stretching vibrations of Si 2 O 5 units. Raman water stretching bands of nekoite are observed at 3071, 3380, 3502 and 3567cm −1 . Raman spectrum of okenite shows water stretching bands at 3029, 3284, 3417, 3531 and 3607cm −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.