Thermal phase transition of potassium sulfate, K₂SO₄ - A high temperature polarizing light microscopic study

The thermal phase transition of K₂SO₄ has been investigated by high temperature polarized light microscopy. K₂SO₄ undergoes a first-order transition at 587° C where the orthorhombic low temperature form (Pmcn) transforms into a hexagonal high temperature modification (P6₃/mmc). Prior to the beginning of the phase transition, K₂SO₄ shows an anomalous optical behavior. The crystal apparently becomes optically uniaxial twice at 338° and 425° C, respectively, and truly optically uniaxial at 587° C. The phase transition propagates through an intermediate temperature form, which is sandwiched between the low and the high temperature forms and moves in a definite direction, 〈130〉 (orthorhombic indices), in the vicinity of the phase transition. Passing through the phase transition point on cooling, dark belts crossing each other are observed which are a result of the transformation twins parallel to {110} and {130}.