Turquoise in the oxidized zone of Yuertan pyrite mine in Baihe County of Shaanxi Province has experienced severe weathering and retrograding. Its color has changed from sky-blue through yellowish green to white, with its texture from massive and smooth to loose and porous. Recent studies of typical turquoise samples by such means as thin section observation, chemical analysis and X-ray powder diffraction and infrared absorption spectroscopic analysis indicate that the factor responsible for weathering and retrograding of turquoise is the replacement of phosphate ions by sulfate ions. And the last important retrograding replacement product is woodhouseite.