Highly hydratable rhyolitic glass

Category Petrology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author de Pablo, Liberto۱; Doval, Mercedes۲; de La Iglesia, Angel۳
Holding Date 06 October 2008

Highly hydratable rhyolitic tephra from explosive volcanism associated to stratovolcanoes in the Transmexican Volcanic Belt, consisting of ~98 % aluminosilicate glass free of pumice, absorbs spontaneously and reversibly an unusually high 12.6 wt % H2O which is lost at 72o . Three glasses are integrated in the rhyolitic glass. Type I, which contains 74.84-75.94 wt % SiO2, 14.44-10.26 wt % Al2O3, 9.58-12.18 wt % MgO+FeO+CaO, 1.14-1.62 wt % K2O+Na2O, Al2O3/SiO2 0.16, and the highest average cation field strength. Type II, which devitrifies to microlitic feldspar, contains 71.80-77.77 wt % SiO2, 12.34-17.19 wt % Al2O3, 3.17-7.94 wt % MgO+FeO+CaO, 2.75-7.36 wt % K2O+Na2O, Al2O3/SiO2 0.20, and the lowest average field strength. Type III, more viscous, which devitrifies to cryptocrystalline aggregates of pyroxene plus amphibole, contains 80.45-83.88 wt % SiO2, 7.46-10.32 wt % Al2O3, 5.70-9.62 wt %, MgO+FeO+CaO, 0.50-3.40 wt % K2O+Na2O, Al2O3/SiO2 0.11, and an average cation field strength intermediate between Types I and II glasses. Compositional variations lead to assume fractionation to low-SiO2 glass that went through an early crystallization of feldspar and to high-SiO2 more evolved glass that crystallized augite and hornblende. There is no molecular water dissolved in the glass and the only high-temperature water is the 0.57 wt % that separates at 454o from amphibole. Absorbed molecular water is manifested by intense OH-stretching of a 848 cm-1 wide-at-the-base strong absorption from H-bonded OH-groups of increasing bond strength and decreasing frequency from the polar OH bond with H+ attached without bonding to oxygen atoms from the glass. 27Al is essentially as high shielded IVAl of chemical shift 56.241 ppm, in (Si,Al)O4 tetrahedra coordinated by bridging oxygens that could predominate in Type II glass. An intense shift of 108.732 ppm is attributed to deshielded IVAl in unpolymerized (Si,Al)O4 tetrahedrons terminated in non-bridging oxygens possibly common to Types I and III glasses. Weak shifts of -0.937 and 17.6 ppm arise from VIAl and of 33.3 and 38.0 ppm from VAl point to melts of high average cationic field strength or from high pressure high temperature reactions. The substantial hydration that characterizes the glass appears to derive from Al/Si replacement in a structure consisting of significant uncoordinated tetrahedra.