It is probable that many of the gross physical properties of melts and partial melts at depth within the earth are substantially modified by pressure-induced cationic coordination transformations. An isochemical extension of the Bottinga-Weill viscosity model is suggested to account for the effects of coordination changes on melt viscosities. Resulting predictions are that tholeiitic and andesitic melts will undergo, with increasing pressure, decreases in viscosity of factors of 14 and 47 respectively on transformation of aluminum from four- to six-fold oxygen coordination. Analogous transformations in solid phases suggest their occurrence in melts at pressures below 35 kilobars for temperatures not exceeding 1400C. Associated changes in liquid specific volume are estimated to be roughly three and four percent respectively for the aforementioned melt compositions. Transformation of aluminum from four- to six-coordination also greatly decreases shear wave attenuation in partial melts.