Mullite and cristobalite formation in fired products starting from halloysitic clay

Ceramic raw materials from High Latium (Central Italy) consisting of halloysite-10 Å, amorphous silica, sanidine and other accessory phases were subjected to heat treatment by rapid- and slow-firing processes. The products were studied by X-ray diffraction and optical microscopy. The main newly-formed high-temperature phases were mullite and cristobalite. The discussion centres around the formation and features of these newly-formed crystalline phases. Particular attention is paid to any mineralogical differences between the products from the two modes of firing. © 1992.