Hydroxyl speciation in felsic magmas

The hydroxyl speciation of hydrous, metaluminous potassium and calcium aluminosilicate glasses was investigated by ²⁷Al-¹H cross polarization and quantitative ¹H MAS NMR spectroscopy. Al-OH is present in both the potassium and the calcium aluminosilicate glasses and its ¹H NMR partial spectrum was derived from the ²⁷Al-¹H cross polarization data. For the calcium aluminosilicate glasses, the abundance of Al-OH could not be determined because of the low spectral resolution. For the potassium aluminosilicate glasses, the fraction of Al-OH was quantified by fitting its partial spectrum to the quantitative ¹H NMR spectra. The degree of aluminum avoidance and the relative tendency for Si-O-Si, Si-O-Al and Al-O-Al bonds to hydrolyze were derived from the measured species abundances. Compared to the sodium, lithium and calcium systems, potassium aluminosilicate glasses display a much stronger degree of aluminum avoidance and a stronger tendency for the Al-O-Al linkages to hydrolyze. Combining our results with those for sodium aluminosilicate glasses (Malfait and Xue, 2010a), we predict that the hydroxyl groups in rhyolitic and phonolitic magmas are predominantly present as Si-OH (84-89% and 68-78%, respectively), but with a significant fraction of Al-OH (11-16% and 22-32%, respectively). For both rhyolitic and phonolitic melts, the AlOH/(AlOH+SiOH) ratio is likely smaller than the Al/(Al+Si) ratio for the lower end of the natural temperature range but may approach the Al/(Al+Si) ratio at higher temperatures.