Silicates with incorporation of NH₄⁺, Rb⁺, or Cs⁺

Abstract

This thesis presents results of an experimental study on the incorporation of NH₄⁺, Rb⁺, and Cs⁺ in micas and feldspars. The studied micas are dioctahedral. The feldspars are sanidine-like. In chapter II it is shown that the ammonium analogue of muscovite, called tobelite, with the formula NH₄Al₂Si₃AlO₁₀(OH)₂ can be synthesized using NH₃ in water, sal volatile or urea as donors for NH₄⁺. The synthesis conditions may be quite high, even 500⁰C at a pressure of 4 to 5 kbar. These synthesis pressures are higher than ones reported up to now in the literature, and they demonstrate that tobelite is not necessarily a mineral that is restricted to low pressure and temperature environments. All tobelite found so far seem to have formed at low temperature and pressure. In the literature, tobelite is mentioned to occur in relationship to fossil fuel deposits, and in chapter II it is elucidated why such a relationship may occur. The synthesis of NH₄AlSi₃0₈, its characterization with XRD, SEM and IRspectroscopy, its study by thermal analyses, and its morphological description are reported in Chapter III. NH₄AlSi₃0₈ is very close in structure to sanidine (KAlSi₃0₈). The similarities of NH₄AlSi₃0₈ with the mmeral buddingtonite (NH₄AlSi₃0₈.1\2H₂O) are so striking, that it is likely that buddingtonite and the ammonium analogue of sanidme are the same. The first description of buddingtonite reports that it has zeolitic properties. The half mole of lattice water in NH₄AlSi₃0₈.1\2H₂O is, according to descriptions in the literature, not present in the structure in an ordered way. A detailed study (Chapter IV) of the original samples of buddingtonite, kindly made available by the principal author of the article in which buddingtonite was described firstly, shows that the zeolitic properties, as well as the lattice water can be ascribed to an admixture of montmorillonite, a clay mineral, in the original samples. Buddingtonite is not redefined in chapter IV, but it is strongly suggested to consider buddingtonite and NH₄AlSi₃0₈ as the same silicates and to disregard the zeolitic half mole of water of buddingtonite. Chapter V describes the synthesis and characterization of the Rb-analogue of 2M!muscovite (RbAl₂Si₃AlO₁₀(OH)₂) and comparison of this silicate with muscovite and other dioctahedral micas. Chapter VI describes the synthesis, characterization, morphology and structure of the Rb-analogue of sanidine (RbAlSi₃0₈). To get an indication of the influence of the mcorporation of Rb in muscovite and sanidine on the reaction of muscovite to sanidine, corundum and water, the location of the equilibrium of the reaction of (RbAl₂Si₃AlO₁₀(OH)₂) to RbAlSi₃0₈, corundum and water was studied at 2 kbar. The results are also given in chapter VI, and indicate that the temperature at which the equilibrium is probably situated is only slightly higher (probably maximal 20 DC) than for the reaction of the K-silicates. If complete replacement of K by Rb does not have much effect, incorporation of Rb in muscovite and sanidine on trace element level will probably not influence the position of the equilibrium at all. In chapter VII, the incorporation of Rb in muscovite by ion-exchange is studied. Muscovite as well as its Rb-analogue are brought in contact with alkali cWoride solutions to establish ion exchange of K and Rb. All exchange experiments are carried out at a pressure of 2 kbar. The most important result is that there exists a solvus between the endmembers RbAl₂Si₃AlO₁₀(OH)₂ and KAl₂Si₃AlO₁₀(OH)₂ (muscovite). The exact location of the solvus remams somewhat unclear, which is probably caused by the ability of micas to form intergrowths. Chapter VIII gives an account of the synthesis and characterization of a Cs-containing mica with a structure resembling that of muscovite. On basis of the data it can not be proven that the mica is the Cs-analogue of muscovite, because no accurate chemical analyses could be gathered. The mica is formed at a temperature of 400 DC at 2 kbar and at 400 DC and 500 DC at pressures of 2 and 5 kbar. Investigated conditions are 300, 400, 500 and 600 ⁰C at pressures of 0.5,2, and 5 kbar. Pollucite (CsAlSi₂0₆) always occurs together with the mica, as well as another Al-containing phase (boehmite, or diaspore or corundum, depending on the conditions)