Strengite is a relatively rare iron phosphate mineral with the formula: FePO<sub>4</sub>÷2H<sub>2</sub>O. The mineral is named after the German mineralogist (1830–1897). Lavender, pink or purple in hue, it is similar to variscite and is partially soluble, particularly in conditions where there is a low pH and low oxidation-reduction potential. The color comes from ferric ion (Fe<sup>3+</sup>).