Wasserstoff und Wasserstoffbrücken in Mineralen

Autor(en)

Eugen Libowitzky

Abstrakt

Hydrogen, as a major or trace constituent, has a strong inßuence on the physical and chemical properties of minerals, rocks, and melts (rheology, melting points, elastic constants, etc.). Information about the type of H incorporation, quantiÞcation of hydrous species in nominally anhydrous minerals, and characterisation of proton dynamics and phase transitions is gained by IR-spectroscopy and supplementary by X-ray or neutron diffraction techniques. Olivine and Mg-silicate perovskite seem to be the most abundant minerals in the EarthÕs mantle. Though nominally anhydrous, forsterite crystals show abundant IR absorption bands which are attributed to OH defects mainly correlated with Si vacancies in the structure. Absorption bands in synthetic high-pressure MgSiO3 perovskite are also attributed to OH defects correlated to a vacant cation site in the structure. The observed type of H incorporation is conÞrmed by similar results in isotypic compounds (chrysoberyl, CaTiO3 perovskite). A different type of H incorporation is observed in the grossular - hydrogrossular series. The SiO4 tetrahedra are subsequently substituted by an (OH)4 cluster, which is characterised by two strongIR bands around 3650 cm-1. Whereas natural hydrogrossular contains up to several wt.% H2O, pyrope and almandine contain only up to several hundred ppm H2O. Water molecules in the structural channels of beryl and cordierite occupy two different positions with either the H-H vector parallel (H2O type I) or perpendicular (H2O type II) to the c-axis. Recent investigations indicate that type I water molecules rotate around the channel axis. In addition, hydroxyl groups are proposed in samples with large alkali ions (Rb, Cs). Proton dynamics and enhancement of hydrogen bonds seem to be the driving force for phase transitions in the high-pressure mineral lawsonite and in hemimorphite. Whereas the water molecules and the hydroxyl groups occupy dynamically disordered sites in the high- temperature phases, the protons are ordered at low temperatures and space group symmetry is reduced. In addition, a superstructure is built up in the low-temperature hemimorphite phase. Very strong hydrogen bonds are observed in the minerals mozartite, pectolite and serandite, and in members of the natrochalcite series. Whereas the former three comprise a silanol group, the latter show an H3O2-unit. The very broad, low-energetic OÐH stretching band, which is corrupted by transmission windows and sharp modes, is only visible in polarised IR spectra. However, even though OÐH...O distances as short as 2.44 � are encountered, IR spectroscopy and structural reÞnements suggest asymmetric hydrogen bonds. Quantitative information on H bond lengths, OÐH vector orientation, and hydrogen content can be directly obtained from IR absorption spectroscopy. Unlike isotropic materials, most crystals require special measurement conditions like polarised radiation and single-crystals to determine the total absorbance which is related to the hydrogen concentration. Using stoichiometric mineral hydrates and hydroxides, a calibration line for H in minerals (based upon integrated intensities of OÐH stretching bands) was obtained.

Organisation(en)

Institut für Mineralogie und Kristallographie

Journal

Mitteilungen der Österreichischen Mineralogischen Gesellschaft

Band

143

Seiten

41-53

Anzahl der Seiten

13

ISSN

1609-0144

Publikationsdatum

1998

ÖFOS 2012

104026 Spektroskopie, 105116 Mineralogie

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/6d737337-8af5-4404-bf90-ac0ff2af304b