Strengite

What is strengite? The iron analogue of variscite

Strengite is FePO4·2H2O, a purple to pink orthorhombic phosphate in which ferric iron takes the place variscite gives to aluminium. The Handbook of Mineralogy strengite sheet gives point group 2/m 2/m 2/m and space group Pcab, a 10.122, b 9.886, c 8.7233 Å, Z 8. Crystals are variable in habit, may be dominated by {111}, lathlike along [001], or elongated along [100] or [010], to 5 cm, with many forms; generally radial fibrous, as botryoidal or spherical aggregates and crusts. Twinning rarely on {201}.

Physical and optical properties. Cleavage good on {010}, poor on {001}; hardness 3.5; measured density 2.84 to 2.87, calculated 2.84. Purple, violet, pink, peach-blossom-red, carmine, greenish white, or nearly colourless; streak white; lustre vitreous; transparent to translucent. Biaxial (+), α 1.697 to 1.708, β 1.708 to 1.719, γ 1.741 to 1.745, 2V moderate to small, dispersion r < v, strong. A 20.00 g piece reads 12.96 to 13.03 g in water, as on variscite. How to read a break is on cleavage or fracture.

What the lists record. The September 2026 IMA list writes Fe3+(PO4)(H2O)2, status G, 1877, Germany, first reference Neues Jahrbuch für Mineralogie, Geologie und Paläontologie (1877), 8. The sheet puts it in the Variscite group, says it forms a series with variscite and is dimorphous with phosphosiderite, and names it for Johan August Streng (1830–1897), German mineralogist at the University of Giessen. Type material is at Wroclaw University, Poland.

Strengite formula and composition: the Pleystein check

On standard atomic weights Fe3+(PO4)(H2O)2 weighs 186.84 and is 29.89% iron, 16.58% phosphorus and 19.28% water. As oxides that is Fe2O3 42.73, P2O5 37.98 and H2O 19.28, the ideal column the sheet prints (37.99 for P2O5, a rounding apart). The phosphosiderite sheet prints the same ideal column for its dimorph.

The Pleystein analysis. The sheet’s analysis from Pleystein, Bavaria, reads P2O5 38.24, Fe2O3 43.40, H2O 18.89, total 100.53. Converted to moles, iron against phosphorus is 1.009, within 1% of the formula’s 1.000; the water is 0.39 below the ideal. The sheet prints no aluminium in it, so this is material at the iron end of the series.

From the cell. V = a × b × c = 872.9 Å3; with Z 8 and the formula weight, the density works out at 2.843, the sheet’s calculated 2.84 and at the bottom of its measured 2.84 to 2.87. Weighing is on how to measure specific gravity at home; hardness testing on how to test mineral hardness properly.

Strengite and rockbridgeite: what the two sheets share

Rockbridgeite is a mixed-valence iron phosphate that the strengite sheet names as an associate. The IMA list writes (Fe2+0.5Fe3+0.5)2Fe3+3(PO4)3(OH)5, status G, 1949, USA. The Handbook of Mineralogy rockbridgeite sheet gives orthorhombic, typically fibrous, compact radiating to 3 cm, in crusts, botryoidal and drusy masses, hardness 3.5 to 4.5, density 3.45, dark green, olive-green, greenish black or black, commonly colour-banded; named for Rockbridge Co., Virginia, taken as the type locality. A 20.00 g piece of it would read 14.20 g in water, against strengite’s 12.96 to 13.03 g. On the two sheets the colours do not overlap: purple to pink against dark green to black.

The Rockbridge ledger. Method: 22 place names were taken by hand from the strengite sheet’s Distribution field and matched against rockbridgeite’s by script, accent-folded and on word boundaries. 8 are on both: Hagendorf and the Kreuzberg, Pleystein, Bavaria; the Mangualde pegmatite, Portugal; the Palermo #1 and Fletcher mines, New Hampshire; Midvale, Rockbridge Co., Virginia; the Bull Moose mine, South Dakota; and the El Criollo pegmatite, Argentina. For comparison, 6 of the 22 are on the cacoxenite sheet and 2 on the variscite sheet.

Documented failure: association is not reciprocal. Strengite’s Association line names rockbridgeite. Rockbridgeite’s names triphylite, hureaulite, barbosalite, roscherite and “limonite”, not strengite; neither do the beraunite and dufrénite sheets. A shared place shows both minerals are recorded there, not that one sheet’s author found them on the same specimen.

Where is strengite found? The sheet’s localities

Where it forms. A late secondary mineral in complex granite pegmatites; in “limonite” iron ores and gossans; with magnetite iron ores; rarely a cave mineral. Associates: beraunite, hureaulite, dufrénite, bermanite, stewartite, cacoxenite, rockbridgeite, vivianite, apatite, leucophosphite and phosphosiderite.

Where the sheet places it. In Germany, the Eleonore and Rotläufchen iron mines near Giessen, Hesse, and Hagendorf and the Kreuzberg, Pleystein, Bavaria; Cyrilov, Czech Republic; the Leveäniemi mine, Svappavaara, and the Kiruna iron mines, Sweden; Iglesias, Sardinia; the Mangualde pegmatite, Portugal; the Bomi Hill Caves, Liberia. In the USA, the Palermo #1 and Fletcher mines, New Hampshire; Midvale, Virginia; large crystals from Indian Mountain, Alabama; Three Oak Gap and the Coon Creek mine, Arkansas; the Wood mine, Tennessee; the Bull Moose mine, South Dakota; Pala, California. Also the Sapucaia pegmatite mine in Minas Gerais; the El Criollo pegmatite, Córdoba, Argentina; and the Iron Monarch quarry, Iron Knob, South Australia.

Nothing more is claimed. No fetched source gives strengite a use, and this page states none. The reference section continues at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.

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