A room-temperature phase transition in maximum microcline

A type of hysteresis, similar to that observed in the heat capacity measurements (Openshaw et al., 1979), has been found in the room temperature unit cell parameters of a microcline sample 71104 and likewise indicates the existence of two forms of this microcline. The A-form (obtained on cooling the sample to approximately 80 K) has significantly different values of b, β and V to the B-form which is the more stable form above 300±10 K. The transition from the A- to the B-form occurs over a period of months and has an associated ΔV of ?0.0011 nm³. The cell parameters of the B-form have been measured up to 1278 K and show significant changes: a and V increase, b constracts, and c is unchanged on increasing temperature. The calculated thermal expansion ellipsoid is nearly uniaxial and similar in shape to that for sanidine. Below room temperature the isobaric thermal expansion coefficent α_p, for a natural microcline is nearly four times as large as that for sanidine. Above room temperature α_p for 71104 microcline decreases markedly with increasing temperature. This implies a rapid change in the thermal expansion behavior of microcline which has been correlated with the proposed phase transition.