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 and is partially soluble, particularly in conditions where there is a low pH and low oxidation-reduction potential.
Strengite is the end member of a series with Variscite, with Strengite being the iron-dominant member and Variscite being the aluminum-dominant member. Though similar in composition, Strengite and Variscite strongly differ in their color. While Variscite has greenish hues, Strengite is mostly in violet or reddish. The color caused by iron will dominate, and will cause Variscite rich in iron to have a violet or reddish color similar to Strengite.
Strengite is named in honor of Johann August Streng (1830-1897), a Professor of Mineralogy at the University of Giessen in Germany.
Strengite is a rare collectors mineral. It doesn't form in large crystals, but small crystal aggregates and crystals make beautiful thumbnail and micromount specimens.
Some of the largest single Strengite crystals were found in Pleystein, Bavaria, Germany. This locality is closed, and these specimens have become classics that are very difficult to obtain. Good Strengite crystals were also found nearby in Hagendorf, the next town over from Pleystein. Outstanding and highly aesthetic hemispherical Strengite balls with a deep violet color come from the Leveäniemi Mine, Svappavaara, Sweden. Portugal has several Strengite localities, most notable is the the Cabeço da Mua mine, Felgar, where it occurs as small pink crystals. In Brazil, small pink and purple balls come from the Boa Vista pegmatite, Conselheiro Pena, in the Doce valley, Minas Gerais.
In the U.S., velvety purple Strengite aggregates come from the phosphate deposit at Indian Mountain, Cherokee Co., Alabama. Other localities include the Three Oaks Gap, Polk Co., Arkansas; the Bull Moose Mine, Custer, South Dakota; and the Stewart Mine, Pala, San Diego Co., California.