Mineralpedia Details for Strengite

Named for Johan August Streng, a German mineralogist at the University of Gissen in Gissen, Germany. Strengite can be found in over 200 localities as a late-forming secondary mineral in granite pegmatites as well as in limonite iron ores, gossans, and magnetic iron ores, and can occur rarely as a cave mineral. Strengite is partially soluble.

Ref. Handbook of Mineralogy, Anthony et al (1995) and MSA at http://www.handbookofmineralogy.org/pdfs/strengite.pdf

Formula

Fe³⁺PO₄·2H₂O 

Crystal System

Orthorhombic 

Crystal Habit

Spherical, Radial, Botryoidal 

Cleavage

Good, Poor, None 

Luster

Vitreous (Glassy) 

Color

colorless, pale violet, deep violet, red, carmine red 

Streak

white 

Class

Orthorhombic - Dipyramidal 

Fracture

Brittle - Conchoidal 

Hardness

3.5 

WebMineral

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Mindat

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Purple or lilac crystals aggregates as balls to 3mm across.

Bright cherry red micro crystals to 0.5mm on jet black Robertsite.

Stunning purple crystal groups of Strengite. The groups are very large and the crystals sharp and distinct to 8mm.

3mm light to dark purple spherical crystal aggregates. This strengite has color changing ability depending on the light source.

Lilac or purple Strengite crystals to 1mm in aggregate or clusters to 1cm. These crystals have an amazing color changing properties. They will appear a dull grey to a nice lilac depending what light it is under.

Clusters of purple to pink Strengite crystals, less than 1mm.

Strengite and Phosphosiderite are polymorphs- they have the same chemical formula, but belong to different crystal systems. Phosphosiderite is monoclinic, while Strengite is orthorhombic. At the Bull Moose mine, the two are difficult to discern. But careful observation will usually help. The Phosphosiderite are typically more prismatic, showing striation along the c-axis and the terminations will be diamond shaped. Phosphosiderite will also twin (see Jason Smith's photo on Mindat). The best crystals are a raspberry red (see Edgar Müller photo on Mindat). But color is not reliable as Strengite can also be the same color but usually not that red. The Strengite at the Bull Moose are more equant and show some octahedral faces. Saul Krotki's photo on Mindat shows a good distiction between the two. This is not the best specimen, but is a good representative of the Strengite with a little Phosphosiderite. The matrix is dull black Barbosalite with some Pyrite.

Lilac to purple platy crystals in nice aggregates to 2.5mm.

Last three photos are by Henry Barwood, Professor of Earth Science, Troy University, AL:

Tiny strengite rosettes on kidwellite from the old locality north of the extinct town of Bluffton, Indian Mountain, Cherokee County, Alabama. 

1. FOV is about .5  X .4mm. 
2. FOV is about 1 X 1.5 mm.
3. FOV is about .5  X .4mm.

Raspberry radial crystals to 4mm in Hematite matrix.

A beautiful thumbnail specimen packed with numerous purple Strengite clusters to 2mm.

Superb, fine lilac colored aggregates of Strengite crystals to 5mm. Stunning specimen with color changing properties under different lght sources.

Deep purple Strengite crystals as tiny beads lining a large vug with deep red Bermanite. 

This specimen is for those of you who enjoy the petrology and paragenesis of pegmatites, especially phosphate mineral paragenesis. The piece is an advanced altered phosphate nodule with end products of iron phosphates Rockbridgeite and Strengite. The purple areas are Strengite. The divergent, compact fibrous groups are Rockbridgeite with perhaps zones of brownish Frondelite. The green areas are likely Mitridatite. There are areas where the Strengite forms nice purple beads and lilac bladed crystals. There is also minor Hureaulite.

This is a cool study specimen, one of which you get great pleasure viewing.

Purple prismatic Strengite crystals to 2mm with balls of green Kidwellite. 

Purple gum-drop like spheroidal crystals of Strengite in decayed phosphate nodule.