Natural diamond is formed approximately 120-200 km below the earth's surface under high pressure and high temperature conditions. Assuming that the average density of the earth is 5500 kg/m3, use this information and the unary phase diagram for C (Figure 10-17) to calculate the range of the earth's geothermal gradient (rate of increase of temperature with depth). Estimate the pressure below the earth's surface as ρgh where ρ is density, g is gravity, and h is depth. 10 kbar = 109 Pa.