Fabrizio Aumento

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°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°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°C. Wollastonite and pseudowollastonite are formed at 740°C. Wollastonite is unstable; on cooling it forms hydrates, and converts to pseudowollastonite at 1130°C. A new polymorph of the latter is formed at l300C.

Hemimorphite shows a change at 300°C. At 570°C it converts to Β-Zn₂SiO₄. At 830°C willemite is produced, which modifies reversibly at 1150°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°C the lattice of prehnite is destroyed; it is immediately replaced by wollastonite and anorthite. At 1130°C , wollastonite is destroyed and replaced by diopside at 1300°C.

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

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