A class of feldspar with formula KAlSi_{3[/sbu]O[sub]8} which, unlike sanidine, exhibits complete Al and Si order, with Al occupying the T1 site. When microcline exhibits irregular discontinuous bands that result from the exsolution of albite, it is known as perthite. Microcline forms as the result of radiation damage, naturally arising from the decay of 40K. Its blue-green color results from the presence of Pb and H₂O, although it is colorless if no H₂O is present. The following is observed experimentally, where indicated heating and indicates irradiation,




Apparently, water is mediating a transition from colorless Pb[sup
]+2[/sup] to blue-green Pb⁺³ is some poorly understood manner. Therefore, irradiating a species containing water changes its color. Mild heating a blue sample causes electrons to return to their equilibrium position, resulting in a transition to colorless. Applying a great deal of heat actually drives off water, so a sample so treated remains colorless even after irradiation.