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SUMMARY. Glass-ceramics based on the mineral cordierite have good mechanical and thermal properties. They can be easily prepared from glasses whose compositions lie in the primary phase field of cordierite on the 5 % CaO plane of the CaO-MgO-SiO2-A1208 system. For uniform crystallization in the bulk, addition of a suitable nucleating agent is essential. The addition of to-~2 wt % of TiO~ is considered to be suitable for both ternary and quaternary glasses. During controlled heat treatment of these glasses extensive phase separation occurs. The nucleation and crystallization processes that follow have been studied by differential thermal analysis, X-ray diffraction, and light and electron microscopy. Some of the physical and mechanical properties of these glass-ceramics have been measured. THE high melting point and the very low thermal expansion coefficient of the mineral cordierite are two important properties that have been utilized in the development of industrial ceramic materials in which this phase is a major constituent. Although cordierite ceramics are commonly manufactured by a sintering process, the possibility of producing glass-ceramic materials based on cordierite has already been explored in several laboratories (e.g. British Patent 829,447). The thermal, electrical, and mechanical properties of such a material are considered to be satisfactory, but there are production limitations due to the high melting points and the high viscosity of ternary MgO-A1203-SiO~ glasses even after the addition of suitable nucleating agents. The present work, of which this report is a preliminary account, is aimed at examining several quaternary glasses with compositions in the primary phase field of cordierite in the CaO-MgO-AI.~Oa-SiO2 system with a view to studying their crystallization behaviour. Previous work.Cordierite is an incongruently melting compound having a stable primary phase field in the MgO-A120~-SiO2 system (see Osborn and Muan, I96o). Its mineralogy and polymorphism have been extensively studied (Deer, Howie, and Zussman, I962). The form stable at all temperatures and called low-cordierite is orthorhombic. High-(or ~-) cordierite is hexagonal and is considered to be metastable at all temperatures. Cordierite crystal structures provide typical examples of orderdisorder in aluminosilicates.Crystallization of cordierite from glasses of widely different compositions in the system MgO-A120~-SiO2 has been recently studied in great detail by Schreyer and Schairer (I96I a and b). One important discovery in these studies was the observation that quartz solid solutions previously described as t~-cordierite by Karkhanavala and Hummel (I953) and silica O by Roy (I959) form metastably on devitrification of these glasses at relatively low sub-solidus temperatures and are usually the first crystalline 9 Copyright Building Research Station, Ministry of Public Building and Works.

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