X-ray analyses have established that melanothallite breaks down at 400 + or - 10oC with the formation of tenorite CuO, which is preserved during later cooling down to room T. This indicates the possibility of producing tenorite from melanothallite under the variable T conditions of fumarole activity. The thermal expansion of melanothallite is markedly anisotropic with negative expansion along the a axis. The orthorhombic cell of melanothallite, becomes more nearly tetragonal, as T falls, and from extrapolation, becomes tetragonal (pseudotetragonal) at a T of -250oC with a = b = 0.968, and c 0.737 nm.-D.A.B.