The preparation of libethenite, a double salt of copper(II) phosphate and copper(II) hydroxide presents the opportunity to discuss the prevalence of double salts in the environment, the relationship between solubility and stability in aqueous solution, the origin of the color of transition metal compounds and gravimetric analyses. Typical results for these analyses are 53.1% copper and 37.0% phosphate with standard deviations of 3.6% and 3.8% for copper and phosphate respectively.