Strengite is a relatively rare iron phosphate mineral with the formula: FePO₄ · 2H₂O. The mineral is named after the German mineralogist Johann August Streng (1830–1897). Lavender, pink or purple in hue, it is similar to variscite

Variscite

Variscite is a hydrated aluminium phosphate mineral |H2O]]). It is a relatively rare phosphate mineral. It is sometimes confused with turquoise; however, variscite is usually greener in color....

and is partially soluble, particularly in conditions where there is a low pH and low oxidation-reduction potential.