Summary: The problem of silicate immiscibility, of petrological and technological interest, is discussed in terms of the structures of silicate crystals and certain phase diagrams. Physical chemical principles are employed to indicate the author's basis for suspecting that certain structures in the crystalline solid are carried over into the melt and are a major contributory cause of silicate immiscibility. These structures appear to be limited to the neso (SiO₄)⁴⁻, soro (Si₂O₇)⁶⁻, cyclo (Si₃O₉)₆₋, and ino (SiO₃)²⁻ types of silicate polymers and do not include the more complex phyllo- and tecto-silicate polymers.