Hydrous minerals are crucial because their occurrence is associated with seismic activity through the dehydration process that occurs in the earth's crust and/or mantle. We have developed a technique to observe the dehydration reaction of brucite using electrical conductivity variation under sealed conditions. The electrical conductivity of brucite was measured as a function of temperature. The confining pressure for the measurements was 1 GPa, which represents that of the lower crust. Two types of remarkable electrical conductivity variation were observed. During the first heating, the conductivity of the sample showed a linear variation below 700 K, as was expected from the Arrhenius equation. Once the temperature was increased to near the dehydration boundary, the sample showed a high conductivity. Even though only a small amount of H2O was formed after dehydration, bulk conductivity of the sample varied greatly, presumably caused by a combination of the presence of coexisting solid and fluid phases and a mixed electronic and ionic conduction mechanism operating in the sample.
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