Hydrothermal synthesis of tobelite, NH4Al2Si3Al010(0H)2, from various starting materials and implications for its occurrence in nature

Abstract

Ammonium-bearing micas are prominent nitrogen containing minerals in the Earth's crust. Various starting materials and a range of conditions were applied for the hydrothermal synthesis of tobelite, the ammonium analogue of muscovite. The natural occurrence of the mineral is elucidated. The best crystallised tobelite is carefully investigated with XRD, SEM, DTA, TGA, and lR-spectroscopy. Cell parameters of the tobelite are a0: 5.230 ± 0.007; b0: 9.02 ± 0.01Å; c0: 10.55 ± 0.01 Å; β: 101.560 ± 0.01; V: 487.5 ± 0.7, Å3. Tobelite is indexed on the basis of a 1M cell. Tobelite decomposes at temperature above 500°C in TGA/DTA studies, using heating rates of 10° C/min. Ammonia loss and dehydroxylation are separate processes. Ammonia loss proceeds relatively faster and it is a lower temperature process. In an IR spectrum of tobelite the NH4+ vibrations are readily perceivable, and they are determinative for tobelite with respect to other dioctahedral micas. Tobelite is easily identified by X-ray diffraction. The present synthesis results and literature data indicate that tobelite is readily formed in NH3-rich environments. Such environments are likely to exist in fossil fuel deposits, where decomposition of amino-acids under reducing conditions normally yields NH3-rich gas or fluid phases. Ammonium micas may be formed directly from ammonium bearing clay minerals, or by ammonia incorporation in pre-existing micas.