Water solubility in nominally anhydrous minerals measured by FTIR and H-1 MAS NMR: the effect of sample preparation

Samples of enstatite and forsterite were crystallized in the presence of a hydrous fluid at 15 kbar and 1100 degrees C. Water contents in quenched sam ples were measured by H-1 MAS NMR and by FTIR. If the samples were prepared in the same way, similar water concentrations were obtained by both method s. There is no evidence that one or the other method would severely over or underestimate water contents in nominally anhydrous minerals. However, mea sured water contents vary by orders of magnitude depending on sample prepar ation. The lowest water contents are measured by polarized FTIR spectroscop y on clear, inclusion-free single crystals. These water contents probably r eflect the real point defect solubility in the crystals. Polycrystalline ma terial shows much higher total water concentrations, presumably due to hydr ous species on grain boundaries, growth defects, and in submicroscopic flui d inclusions. Grinding the sample in air further increases water concentrat ion. This effect is even more pronounced if the sample is ground in water a nd subsequently dried at 150 degrees C. Polarized FTIR measurements on clea r single crystals of enstatite saturated at 15 kbar and 1100 degrees C give 199 +/- 25 ppm by weight of water. The spectra show sharp and strongly pol arized bands. These bands are also present in spectra measured through turb id, polycrystalline aggregates of enstatite. However, in these spectra, the y are superimposed on much broader, nearly isotropic bands resulting from h ydrous species in grain boundaries, growth defects, and submicroscopic flui d or melt inclusions. Total water contents for these polycrystalline aggreg ates are between 2000 and 4000 ppm. Water contents measured by FTIR on enst atite powders are 5300 ppm after grinding in air and 12 600 ppm after grind ing under water und subsequent drying at 150 degrees C.