1271 J. Indian Chem. Soc., Vol. 93, November 2016, pp. 1271-1277 Effect of silica on the crystallization behavior and microstructure of mica glass-ceramics in the system SrO·4MgO·Al 2 O 3 ·6SiO 2 ·2MgF 2 Priyabrata Manna, Debdas Mali and P. K. Maiti * a Ceramic Engineering Division, Department of Chemical Technology, University of Calcutta, 92, Acharya Prafulla Chandra Road, Kolkata-700 009, India E-mail : prabir.maiti@gmail.com Manuscript received 28 June 2016, revised 18 August 2016, accepted 26 August 2016 Abstract : The effect of varying the SiO 2 content on the crystallization and microstructural behavior of strontium con- taining glasses based on the system SrO·4MgO·Al 2 O 3 ·6SiO 2 ·2MgF 2 was investigated by differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). DTA analysis revealed that the glass transition temperature ( T g ) and crystallization peak temperature ( T p ) increased by increasing silica content. It was also shown that the higher the silica content, the lower is the aspect ratio of the fluorophlogopite crystals and appearance of pores in the body. The analysis of X-ray diffraction patterns revealed that strontium fluorophlogopite crystals exist as major phase in all heat treatment temperatures. Keywords : Glass-ceramics, DTA, X-ray diffraction, scanning electron microscope. Introduction The glass-ceramics, a polycrystalline material com- posed of at least one crystalline phase dispersed in a vitre- ous matrix, is produced by the controlled crystallization of highly viscous glass-forming melts. The transition oc- curs from ‘vitreous to crystalline’ in glass-ceramics via two kinds of transformation : nucleation and crystal growth. The first one is an endothermic process and the second one is an exothermic process 1 . The two dimensional mica crystals are nucleated internally and crystallized from fluo- rine containing glasses. The most unique feature of mica glass-ceramics is that they can be machined to precise tolerances and surface finish with conventional metal working tools 2–7 . Mica glass-ceramics, particularly phlo- gopite system, have excellent machinability due to the layered lattice structure of the sheet silicates arranged in a ‘house of cards’ microstructures, which causes a perfect basal cleavage along the (001) planes of mica crystals. Besides above, they possess higher strength, fracture tough- ness, thermal shock resistance, and have excellent dielec- tric properties. Beall 7 undertook the first study of the alkaline earth fluormica glass-ceramics, later on Hoda and Beall 2,3 in- vestigated glasses of different compositions containing barium, calcium and strontium based on fluorophlogopite stoichiometry. Most of the compositions were susceptible to crystallization during casting. They also observed that these alkaline earth containing materials have the proper- ties of high mechanical strengths, refractoriness, and in case of strontium containing glass-ceramics, a unique phe- nomenon of water swelling and self disintegration to clay like particles. Greene et al. 8 investigated on the alkaline earth con- taining glass-ceramic system based on(1-Z) BaO : Z K 2 O : (6-X) MgO : X MgF 2 : (3-Q) Al 2 O 3 : Q B 2 O 3 : 8SiO 2 system (where Z=0, 0.25, 0.5, 0.75 and 1.0, X=2, 2.5 and 3.0 and Q= 0, 0.5 and 1) and observed that barium substitution by potassium results in increase in molar vol- ume and co-efficient of thermal expansion and reduction in fractional glass compactness, microhardness and glass transition temperature. Honda et al. 9 studied on a new type of fluorophlogopite, which was formed in the machinable glass-ceramics system SiO 2 ·Al 2 O 3 ·MgO·Na 2 O/K 2 O·F – . They observed that this type of mica 4–5 times more machinable than house of cards types phlogopite glass-ceramics and also reported that phase separation in the base glass took place during nucle- ation and crystallization through controlled heat treatment.