Aluminum substitution in quartz and its effect on dissolved H2O

Abstract:

Trivalent aluminum is known to substitute for tetravalent silicon in quartz. However, this charge imbalance requires the presence of an additional +1 ion for charge compensation – a role that can be played by dissolved hydrogen species (“water”) in the form of H⁺ bonded to oxygen. The correlation between the concentrations of aluminum and water in quartz was investigated experimentally and found to be significant. FTIR was used to measure water concentrations in thin (<1mm) doubly-polished samples of synthetic quartz grown over a range of pressures and temperatures. The quartz was grown from a mixture of amorphous silica powder, aluminum hydroxide powder, and distilled water. Two techniques were utilized to obtain accurate measurements representative of the sample. Depending on the size and visible geometry of the crystals, they were measured either with the c-axis parallel to the IR beam or in groups of ten at random orientations. Aluminum was determined via electron microprobe. The molar concentrations of Al and H were found to be positively correlated.