Strengite Mineral Information

Strengite is a relatively rare iron phosphate mineral. Strengite is the end member of a series with Variscite, with Strengite being the iron-dominant member and Variscite being the aluminum-dominant member.

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.

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 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.
Strengite Mineral Information
Strengite Mineral Information
Photo Copyright: Dieter Strangfeld

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.
Strengite Mineral Information
Strengite specimen from the Leveäniemi Mine, Svappavaara from North Sweden.
Photo: Joy Desor Mineralanalytik

The mineral is named after the German mineralogist Johann August Streng (1830–1897). 

The specimen in the photo is From: Hagendorf South Pegmatite, Hagendorf, Waidhaus, Neustadt an der Waldnaab District, Upper Palatinate, Bavaria, Germany

Properties of Strengite

Composition: Chemical Formula FePO₄ · 2H₂O (Iron(II) phosphate dihydrate)

Color: Primarily purple, violet, pink, peach-blossom red, carmine red, greenish white; colorless to pale pink in transmitted light.

Luster: Vitreous to sub-vitreous, meaning glassy with a slightly waxy or greasy appearance.

Streak: White.

Hardness: 3 1/2 to 4 on the Mohs scale, making it relatively soft and vulnerable to scratches.

Cleavage: Good in one direction (010), poor in another (001). This means it tends to break along specific planes.

Crystal Form: Orthorhombic, usually found in radial fibrous, botryoidal, or spherical aggregates and crusts. Less commonly, it can form prismatic or lath-shaped crystals.

Density: 2.84 - 2.87 g/cm³, relatively heavy for its size due to the presence of iron.

Transparency: Can be transparent, translucent, or opaque. Gem-quality strengite is usually translucent to transparent.

Fracture: Conchoidal, meaning it breaks with smooth, curved surfaces.

Solubility: Insoluble in common acids and water.

Magnetism: Non-magnetic.

Fluorescence: Inert, meaning it does not glow under ultraviolet light.

Pleochroism: Slight pleochroism can be observed, where the color appears slightly different depending on the viewing angle.

Refractive Index: 1.697 - 1.708 (α), 1.708 - 1.719 (β), 1.741 - 1.745 (γ). This refers to the way light bends as it enters the mineral, influencing its brilliance and dispersion.

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