Hydrogen in Nominally Anhydrous Crustal Minerals

Systematic infrared and nuclear magnetic resonance investigations of common crustal minerals were undertaken to better understand the geologic significance of minor components of structural hydrous species within these nominally anhydrous minerals. The absolute hydrogen concentration in three alkali feldspars and eight plagioclase samples was measured with ¹H nuclear magnetic resonance spectroscopy. The mid-infrared integral absorption coefficient was determined to be 15.3 ± 0.7 ppm⁻¹cm⁻², allowing quantitative analysis of OH and H₂O in feldspars with infrared spectroscopy. A survey of hydrous species in igneous feldspars found that feldspars contain structural OH (0-512 ppm H₂O), H₂O (0-1350 ppm H₂O), and NH₄⁺ (0-1500 ppm NH₄⁺) groups as well as fluid inclusions and alteration products. Composition and crystal structure influence the type of hydrous species that can be incorporated into feldspars, but the concentration and speciation of structural hydrogen is at least partially determined by the geologic environment. The diffusivity of H in OH-bearing plagioclase was determined at 800-1000°C (D0=5.7±2.5x10⁻⁴ m²/sec and Q=224±33 kJ/mol). A millimeter-sized volcanic feldspar phenocryst would be expected to lose a significant proportion of its OH concentration on the timescale of a typical eruption (hours to weeks). The structures and compositions of low albite and ussingite, Na₂AlSi₃O₈(OH), are similar. The strong hydrogen bonding in ussingite is found to be fundamentally different from the hydrogen bonding environment of OH in feldspars. Comparison of the infrared spectra of structural isomorphs reedmergnerite, NaBSi₃O₈, and low albite suggest that OH is incorporated in both structures through protonation of the most underbonded oxygen site. The concentration of structural OH in diopside was determined for four granulite facies siliceous marble samples from the Adirondacks, New York. Diopside OH concentration increases monotonically with incr