Strengite: Properties, Information

Strengite is a relatively rare iron phosphate mineral with the chemical 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. 

Strengite is often found as a secondary mineral in granite pegmatites, iron ores, and gossans. It typically forms as an alteration product of primary phosphate minerals like triphylite. The formation of strengite requires sufficient oxidation to convert the ferrous iron (Fe²⁺) in triphylite to ferric iron (Fe³⁺).

This oxidation process can occur in various environments, including areas with high levels of oxygen or exposure to oxidizing agents. The presence of strengite can thus serve as an indicator of the degree of oxidation that a rock has experienced.

Strengite, MINERAL
Beautiful Violet Strengite Crystals. From: Folgosinho, Gouveia, Guarda District, Centro Region, Portugal. Photo Credit: Christian Rewitzer
 
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 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

Chemical Properties:

  • Composition: Iron(III) phosphate dihydrate. FePO₄ · 2H₂O
  • Color: Typically ranges from pink to red or purple, sometimes colorless or with a peach-blossom pink hue.

Physical Properties:

  • Crystal System: Orthorhombic.
  • Crystal Habits: Often occurs as small, globular, or botryoidal aggregates, sometimes as tabular or prismatic crystals.
  • Cleavage: Good in one direction (basal cleavage).
  • Fracture: Conchoidal to uneven.
  • Hardness: 2 to 2.5 on the Mohs scale, which makes it quite soft.
  • Density: Specific gravity is about 2.87.

Optical Properties:

  • Luster: Vitreous to pearly on cleavage surfaces.
  • Transparency: Transparent to translucent.
  • Streak: White.

Other Characteristics:

  • Magnetism: Not typically magnetic, but presence of iron might show weak magnetic properties when influenced by strong magnets due to impurities or the iron content.
  • Fluorescence: Strengite can sometimes exhibit fluorescence under UV light, although this is not a defining characteristic for all specimens.
  • Solubility: Soluble in acids.
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