麻豆传媒

A comparison was undertaken of the thermal behaviour of the zeolites analcite and stilbite with that of the associated hydrated silicates apophyllite and prehnite, and with hemimorphite, an unrelated hydrated silicate.

Preliminary D.T.A. and T.G.A. investigations to 1400掳C were followed by high temperature diffractometry using a furnace of my own design and construction. Debye-Scherrer photographs were used for the determination of minor phases. Diffractometer and single crystal data were processed with a computer to calculate cell parameters and thermal expansions.

Analcite is orthorhombic with space group D⁴₂. Before-destruction at 865掳C, the lattice suffers two changes. After several intermediate phases, high temperatures yield corundum and kyanite.

Apophyllite suffers a gradual destruction to an amorphous phase, which is complete at 370掳C. Wollastonite and pseudowollastonite are formed at 740掳C. Wollastonite is unstable; on cooling it forms hydrates, and converts to pseudowollastonite at 1130掳C. A new polymorph of the latter is formed at l300C.

Hemimorphite shows a change at 300掳C. At 570掳C it converts to 螔-Zn₂SiO₄. At 830掳C willemite is produced, which modifies reversibly at 1150掳C. Fluorescence colours are susceptible to the smallest lattice transformations.

The controversial space group C⁴_2v for prehnite has been confirmed by electron microscopy. At 760掳C the lattice of prehnite is destroyed; it is immediately replaced by wollastonite and anorthite. At 1130掳C , wollastonite is destroyed and replaced by diopside at 1300掳C.

Stilbite changes to epistilbite at 145掳C. At 240掳C and 470掳C epistilbite shows two changes followed by gradual destruction. Semi-amorphous silica formed in this process yielded 螔-cristobalite and bytownite. Bytownite is replaced, at l340掳C, by an unidentified cubic phase. 螔-cristobalite was found to be metastable down to room temperature.

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Pages: 284
Supervisor: C.G.I. Fried1aender