R. E. Openshaw | USGS Science

USGS1979

A room-temperature phase transition in maximum microcline - Heat capacity measurements

The thermal hysteresis in heat capacity measurements recently reported (Openshaw et al., 1976) for a maximum microcline prepared from Amelia albite by fused-salt ion-exchange is described in detail. The hysteresis is characterized by two limiting and reproducible curves which differ by 1% of the measured heat capacities. The lower curve, denoted curve B, represents the values obtained before the sample had been cooled below 300 K. Measurements made immediately after cooling the sample below 250 K followed a second parallel curve, curve A, to at least 370 K. Values intermediate to the two limiting curves were also obtained. The transitions from the B to the A curve were rapid and observed to occur three times. The time required to complete the transition from the A to the B curve increased from 39 h to 102 h in the two times it was observed to occur. The hysteresis is interpreted as evidence of a phase change in microcline at 300??10 K The heat effect associated with the phase change has not been evaluated. ?? 1979 Springer-Verlag.

Physics and Chemistry of Minerals↗

USGS1976

The heat capacities at low temperatures and entropies at 298.15 K of low albite, analbite, microline, and high sanidine

The heat capacities of low albite and analbite, NaAlSi 3 O 8 , and of microcline and high sanidine, KAlSi 3 Og, have been measured from 15 to 375 K using an adiabatic calorimeter. Tables of the thermodynamic functions C° p , (H° T –H° 0 )/T, (G° T –H° 0 )/T, and S° T –S° 0 are presented for these four feldspars from 0 to 370 K. At 298.15 K (25.0°C) the values for S° T S° 0 for low albite, analbite, microcline, and high sanidine are 207.4±0.4, 207.7±0.4, 214.2±0.4, and 214.2±0.4 J/(mol . K), respectively. The effect of the state of Al/Si order upon the heat capacity is quite small. The difference, ΔC° p , between albite-analbite and microcline-sanidine never exceeds 0.5 percent at temperatures below 400 K. With the exception of microcline, the heat capacities of these four feldspars follow a smooth S-shaped curve between 15 and 375 K, with no indication of transitions or anomalous behavior. Above 250 K, the heat capacity of microcline shows a form of thermal hysteresis. In the temperature range 250 to 375 K, the heat capacity of microcline is dependent upon its past thermal history. At 375 K, microcline which had been cooled to 230 K before the measurement of C° p has a heat capacity greater by approximately 1.2 percent than microcline which had not been previously cooled below 295 K. After a day of annealing at about 300 K, C° p of the previously cooled microcline decreases to essentially the same value as the microcline which had never been at a temperature below 300 K.

Journal of Research of the U.S. Geological Survey↗