A Vibrational Spectroscopic Study of Perhamite, An Unusual Silico-Phosphate

The silico-phosphate mineral perhamite has been studied using a combination of electron and vibrational spectroscopy. SEM photomicrographs reveal that perhamite morphology consists of very thin intergrown platelets that can form a variety of habits. Infrared spectroscopy in the hydroxyl-stretching region shows a number of overlapping bands which are observed in the range 3581–3078 cm-1. These wavenumbers enable an estimation to be made of the hydrogen bond distances in perhamite: 3.176(0), 2.880(5), 2.779(6), 2.749(3), 2.668(1) and 2.599(7) Å. Intense Raman bands are observed in the region 1110–1130 and 966–996 cm-1 and are assigned to the SiO4 and PO4 symmetric stretching modes. Other bands are observed in the range 1005–1096 cm-1 and are attributed to the ν3 antisymmetric bending modes of PO4. Some low intensity bands around 874 cm-1 were discovered and remain unclassified. Bands in the low-wavenumber region are assigned to the ν4 and ν2 out-of-plane bending modes of the OSiO and PO4 units. Raman spectroscopy is a useful tool in determining the vibrational spectroscopy of mixed hydrated multi-anion minerals such as perhamite. Information on such a mineral would be difficult to obtain by other means.