Unloading data for a perthitic feldspar from shock compressed states between 250 and 500 kilobars are reported. Continuous stress wave profiles were measured with manganin transducers and the appropriate Riemann invariant integrated to determine the stress-volume states during unloading. The results are in agreement with data obtained on other silicate minerals. A physical mechanism based on thermal heterogeneity and partial transformation to the high coordination phase (hollandite structure) incurred during dynamic compression provides a consistent explanation for the observed stress-volume behavior.