PREMELTING EFFECTS IN MINERALS - AN EXPERIMENTAL-STUDY

High-temperature calorimetry experiments show that the isobaric heat capacity (C(p)) of minerals increases in an anomalously rapid way when the melting point is approached. These premelting effects can begin a few hundred degrees below the melting point, and they should also be observed for other physical properties such as the thermal expansion, compressibility and sound velocities. Premelting could thus affect the properties of partially molten rocks. In this study, new evidence for diopside (CaMgSi2O6), pseudowollastonite (CaSiO3), anorthite (CaAl2Si2O8), akermanite (Ca2MgSi2O7), Na2SiO3, L2SiO3 and CaMgGeO4 olivine obtained by means of calorimetry, electron microscopy and vibrational spectroscopy, demonstrates that premelting represents unquenchable configurational changes within phases remaining crystalline up to the reported melting points. The enthalpy and entropy effects of premelting are consistent with cation disordering over the various crystallographic sites of the structures. More generally, such temperature-induced cation disordering could be at the roots of intrinsic anharmonicity in oxide and silicate compounds.