Abstract: The structure of CaF₂-Al₂O₃-SiO₂ system glass was investigated by using the methods of DTA, IR, Raman, XPS, XRD, SEM and EDAX.
It was found that the structure units, [SiO₄] and [AlO₄], were linked to form the basic network by means of angle connection and the ions Ca²⁺
were located as the network modifiers. Some ions F^- entered as the network formers through replacing O^2-, and other ions F^- were located as the
network modifiers. There were also some ions Al³⁺ staying as the network modifiers. Because the F^- entered as the network formers, the structure of
network became more loose and phase separation of glass took place. Two kinds of different phase regions were formed in the glass, and one region contained more
Ca²⁺ and F^-, whereas another region contained more Si⁴⁺ and Al³⁺. When the above glass with phase separation was heat-treated, the crystals of
CaF₂ firstly came out from the regions containing more Ca²⁺ and F^-.

Key words: network, exterior, interior, phase separation