A spectroscopic study of irradiation coloring of amazonite: structurally hydrous, Pb-bearing feldspar

Irradiation-induced color in amazonite can develop only in potassium feldspar having both structurally bound H_2O and Pb impurities. Amazonite color is controlled by either (1) an absorption minimum in the β spectrum between three overlapping bands in the ultraviolet and a broad band at 625 to 643 nm, resulting in a blue color, (2) a combination in β of one UV band and a broad band at 720 nm, resulting in a green color, or (3) both of the above superimposed, resulting in a blue-green color. All optical variations correlated with an EPR pattern indicative of Pb^(3+) or Pb^(1+). The different types of color are associated with a limited range in Pb content and structural state. For constant Pb content, the intensity of color is linearly related to the amount of structurally bound H_2O, up to a limiting value. Dependence of color intensity on both Pb and H_2O concentration strongly suggests that lead and water occur in a 1 :1 ratio in the color centers. The first order reaction kinetics of amazonite color formation by irradiation and the observation that water is not consumed in the process suggests that water plays a catalytic role in the irradiative transformation of Pb^(2+) to the amazonite chromophore.