Structural equilibria of alkali silicate glasses and melts are investigated based on the Q(n) fractions (n = number of bridging oxygen in SiO4 tetrahedron) obtained from high-temperature Raman spectra. A mathematical treatment of the entropy of mixing, Delta S-mix, of structural groups of Q(n) is conducted and applied to the investigation of phase transformation and phase separation. Characteristics of Q(n) traces and the counter-map of Delta S-mix on Q(n) diagram are found to possess useful information to understand their mechanisms. An extended Q(n) diagram is also proposed to understand the melt structure and properties for the wide composition range.