The High-Low Quartz Transition up to 10 Kilobars Pressure

The high-low quartz transition of a vein quartz and a composite quartz sample were compared at up to 10,000 bars pressure, using a comparative DTA technique. The differences are smaller than 0.2°C, suggesting that such a composite sample may be preferred for use as a DTA thermal standard. The transition temperature ranges from 572.6°C at 1 bar to 825.3°C at 10,000 bars. The best fit of the data is given by two expressions: Tₚ = 572.5 + 1.55 X 10⁻²P + 5.465 X 10⁻⁶P₂ for the pressure range 1-1,000 bars, and Tₚ = 567.3 + 2.645 X 10⁻²P - 6.525 X 10⁻⁸P₂ for pressures between 1,000 and 10,000 bars. The change in the rate and sign of the curvature may be explained by a gradual inhibition of the high rate of thermal expansion of low quartz just below the high-low quartz transition when the pressure increases from 1 to about 1,000 bars.