Previous page     Contents     Next page

In previous experiments on water solubility in pyroxenes we showed that the OH substitution is not necessarily coupled to chemical defects. Water solubility in pure MgSiO₃ enstatite reaches 1000 ppm by weight at 1100°C and 100 kbar. In nature, numerous solid solution series and trace element substitutions are possible that could influence the incorporation of OH groups in the pyroxene structure. In order to understand the effect of chemical impurities, two series of experiments were carried out. The first set of experiments involves growing enstatite crystals at 1100°C and 15 kbars and water-saturated conditions from gels of enstatite composition doped with Al, Li and B. The second series consists of similar experiments with gels of diopside composition doped with Al, Li or B as starting material. Water contents in enstatite and diopside were measured with an FTIR-spectrometer. Unpolarised spectra were taken on clear, inclusion-free single crystals several 10 to 100 µm in size.

Figure 3.5-4 compares the water content of pure MgSiO₃ enstatite and enstatite grown from gels doped with Al, B or Li. While boron and lithium have no effect, water solubility greatly increases with aluminum. A nominal substitution of 2 mol% Al for Si increases the water content from 380 ppm to 1100 ppm by weight, probably due to the substitution Al³⁺ + H⁺ for Si⁴⁺ on the tetrahedral site. In the near infrared spectra one can clearly distinguish OH bands arising from the intrinsic solubility from bands due to the coupled substitution of OH and Al. This in turn allows the OH substitution mechanism in natural samples to be identified (Fig. 3.5-5).

In diopside, water solubility increases only slightly with aluminum content. However, all band positions shift to lower frequencies. This suggests that the substitution Al + Al for Mg + Si is predominant, which changes the lattice constants and interatomic distances and, in turn, the position of the OH bands.
 
 

Fig. 3.5-4: Water content of enstatite grown from gels under water-saturated conditions at 1100°C and 15 kbar. Gels were doped with Al or B replacing equal molar fractions of Si or with Li replacing Mg. The horizontal axis gives the composition of the gels in mol% of Al / (Si + Al), B / (Si + B) or Li / (Li + Mg), respectively.

Fig. 3.5-5: Infrared spectra of natural and synthetic enstatite. Unpolarized spectra of clear, inclusion-free single crystals.