Nuclear magnetic resonance spectroscopy was used to study polycrystalline -spodumene ͑-LiAlSi 2 O 6 ͒ as well as glassy specimens with the same chemical composition.7 Li spin-lattice relaxation measurements were carried out in a broad temperature range and for several Larmor frequencies. In addition to a pronounced rate maximum at high temperatures, stemming from the long-range Li motion in these aluminosilicates, we found a weak maximum in the crystalline modification near 120 K. The latter result confirms the existence of a local double-well structure in which the Li ions reside. The ionic motion was also monitored by solid-and stimulated-echo spectra as well as by the decay of the Jeener-Broekaert echo. Under conditions which are discussed in detail, the latter is a direct measure of the hopping correlation function. For the glass this function was found to decay faster and more stretched than that of the crystal at a given temperature. Furthermore, the relevant barriers against the high-temperature long-range Li motion are larger in the crystal as compared to the glass.
ABSTRACT. Dissolution rates of 39 rock-forming minerals were determined at 20°C and 100°C and different pH values, using experimental set-ups which achieve near-zero cation concentrations. Weathering rates in nature for rock above surface were estimated from data in this study and data in the literature. Extraction in a Soxhlet extractor with boiling 2.5 m acetic acid causes an acceleration of such estimated natural weathering rates by a factor of 0.25 ؋ 10 3 to 7 ؋ 10 4 for nearly all important rock-forming minerals, including the carbonates. The applicability of such a comparative test for rock resistance to chemical weathering was corroborated by testing 32 different rocks and building stones. Run times of only one to four days were needed for a reliable result.
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