Strengite: Information

Strengite is a relatively rare iron phosphate mineral. The mineral is named after the German mineralogist Johann August Streng (1830–1897). Lavender, pink or purple in hue, it is similar to variscite. 
Strengite is isostructrual with the iron arsenate di-hydrate Scorodite. The term ‘isostructural’ means the two mineral shares the same crystal form. Strengite also forms a chemical series with Varicite which is an aluminum phosphate di-hydrate. 
Strengite forms as an alteration product of primary phosphate minerals such as triphylite. Strengite will occur if the oxidation conditions are high enough to oxidize the iron of triphylite from a ferrous (+2) state to the ferric (+3) state as is needed for strengite. The presence of strengite is therefore an indicator of the how much oxidation the rock has under gone.
Violet Strengite Crystals
Beautiful Violet Strengite Crystals. From: Folgosinho, Gouveia, Guarda District, Centro Region, Portugal. Photo Credit: Christian Rewitzer

Strengite forms a series with the more common mineral variscite, AlPO₄·2(H₂O). A series is where different minerals will easily allow certain atoms to freely substitute for each other. In this case iron and aluminum can substitute for each other in variscite and strengite without any disturbance of the crystal structure. Usually the two minerals are fairly pure in nature, but some nearly intermediate specimens have been found.

Strengite is a beautiful mineral and is hard to find on the mineral markets. The best samples are usually small and micromounts are seemingly more available than cabinet samples. Once a specimen is obtained, it will no doubt become a real treasure for the owner.

The most common occurrence of strengite is as spherical aggregates of rheniform masses. However strengite does occur in distinct crystals on the mineral dufrenite in Rockbridge county, Virginia and in distinct crystal groups from Indian Mountain, Cherokee County, Alabama.

Properties of Strengite:

Formula: FePO₄ · 2H₂O
Crystal System: Orthorhombic
Colour: Purple, violet, pink, ...
Hardness: 3½ - 4
Cleavage is good in one direction, poor in another.
Fracture is conchoidal.
Associated Minerals: are limonite, heterosite, rockbridgeite, variscite, phosphosiderite, strunzite, beraunite, triphylite and many other secondary phosphate minerals.
Notable Occurrences: include Rio Grande de Norte, Brazil; Bull Moose Mine, Custer, South Dakota, Indian Mountain, Alabama and San Diego Co., California, USA; Svappavaara, Sweden; Eleonore Mine, Germany and Mangualde, Portugal.
Best Field Indicators are color, habit, associations and luster.

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