ENERGY ASSOCIATED WITH DISLOCATIONS - A CALORIMETRIC STUDY USING SYNTHETIC QUARTZ

High temperature oxide melt solution calorimetry was used to study the energy associated with dislocations in quartz by comparing undeformed and deformed single crystals of synthetic quartz. Samples were deform ed at 698 K, 1000-1500 MPa at a strain rate of 10(-5) sec(-1). Two set s of calorimetric measurements were made: (i) using a Pt capsule as a container for powdered sample, and (ii) using pellets made from sample powder without any container. For the first set of measurements, the undeformed sample with a dislocation density of < 10(5) cm(-2) yielded an enthalpy of drop-solution (measured enthalpy is sum of heat conten t H-973-H-295 and enthalpy of solution in molten lead berate at 973 K) of 39.22+/-1.00 kJ mol(-1), while the sample deformed in the dislocat ion creep regime with a dislocation density of gave an enthalpy of 6x1 0(10) to 1x10(11) cm(-2) 38.59+/-0.78 kJ mol(-1). For the second set o f measurements the measured enthalpy of the undeformed sample was 38.8 7+/-0.31 kJ mol(-1), and that of a deformed sample with a dislocation density of 3x10(10) to 1x10(11) cm(-2) was 38.24+/-0.58 kJ mol(-1). Th e present study and previous theoretical calculations and estimates ar e consistent and suggest that the energy associated with dislocations in quartz is similar to 0.6+/-0.6kJ mol(-1) for a dislocation density of similar to 10(11) cm(-2); a precise value is difficult to determine because of the overlapping errors. These results indicate that for ge ologically realistic dislocation densities, the maximum excess energy due to dislocations would be similar to 0.5 kJ mol(-1) for most minera ls; the exact value would depend on the Burgers vector as well as the shear modulus.