Dufrénite
Dufrénite, Ca0.5Fe2+Fe3+5(PO4)4(OH)6(H2O)2: monoclinic, radial fibrous, dark green, density 3.1 to 3.34. Told from rockbridgeite by optics, weight and X-ray.
Dufrénite is a calcium-bearing iron phosphate, Ca0.5Fe2+Fe3+5(PO4)4(OH)6(H2O)2 on the IMA list (G, 1833, Germany): monoclinic, usually radial fibrous crusts, dark green to greenish black, hardness 3.5 to 4.5, density 3.1 to 3.34. Rockbridgeite looks similar; its lowest refractive index, 1.873, sits 0.031 above dufrénite’s highest α, 1.842.
In short
- The species. The Handbook of Mineralogy sheet gives a monoclinic mineral, crystals rare and to 1 mm, usually radial fibrous, botryoidal masses or crusts; two cleavages parallel to the fibre; hardness 3.5 to 4.5; density 3.1 to 3.34; greenish black, dark green or olive-green when fresh, olive-brown to reddish brown when oxidized, typically concentrically banded. The September 2026 IMA-CNMNC Master List writes Ca0.5Fe2+Fe3+5(PO4)4(OH)6(H2O)2, status G, 1833, Germany.
- Rockbridgeite or dufrénite. Both are dark green fibrous iron phosphates of hardness 3.5 to 4.5. The sheets separate them by system (monoclinic against orthorhombic), by α (1.810 to 1.842 against 1.873 to 1.875, a 0.031 gap), by weight (a 20.00 g piece reads 13.55 to 14.01 g in water against 14.20 g) and by X-ray: dufrénite’s pattern has a strong line at 12.00 Å; rockbridgeite’s printed lines stop at 6.87 Å.
- Documented failure: Fe:P. On the IMA formulas, Fe:P is 1.500 in dufrénite and 1.667 in rockbridgeite. The sheets’ real analyses do not keep to that: the two Wheal Phoenix, Cornwall, dufrénite analyses work out at 1.560 and 1.634, the Rockbridge Co. rockbridgeite analysis at 1.614. Iron against phosphorus, by itself, does not tell them apart.
- The Z check. Worked on standard atomic weights, the IMA formula weighs 873.06 and reproduces the sheet’s ideal column (3) to within 0.01. With the sheet’s cell, 1701.7 Å3, it gives the bracketed calculated density 3.41 only if Z = 4; at the printed Z = 2 it gives 1.70, below anything measured. The printed Z fits a formula unit twice the IMA one; the sheet does not say which it counts.
- Cornwall. Both analyses and the powder pattern on the sheet are Cornish (Wheal Phoenix; “Cornwall, England; close to natrodufrénite”), and the Distribution field names the West Phoenix United mines, Linkinhorne, and the Gunheath china clay pit, St. Austell. The IMA list gives Germany as the country, and the sheet marks no type locality.
| Criterion | Dufrénite | Rockbridgeite | Separates them? |
|---|---|---|---|
| Formula (IMA) | Ca0.5Fe2+Fe3+5(PO4)4(OH)6(H2O)2 | (Fe2+0.5Fe3+0.5)2Fe3+3(PO4)3(OH)5 | By analysis only: dufrénite has calcium and water |
| Crystal system, space group | Monoclinic, C2/c | Orthorhombic, Bbmm | Yes, by X-ray or optics |
| Refractive index α | 1.810–1.842 | 1.873–1.875 | Yes: 0.031 gap |
| Refractive index β | 1.813–1.850 | 1.880 | Yes: 0.030 gap |
| Refractive index γ | 1.855–1.925 | 1.895–1.897 | No: rockbridgeite’s γ lies inside dufrénite’s range |
| Birefringence, worked | 0.013–0.115 | 0.020–0.024 | Not reliably |
| 20.00 g piece in water | 13.55–14.01 g (D 3.1–3.34) | 14.20 g (D 3.45) | Yes, on clean material: 0.19 g at the nearest |
| Strongest powder lines | 3.151 (10), 12.00 (9), 5.002 (9) Å | 3.196 (10), 4.842 (5), 3.573 (5) Å | Yes: the 12.00 Å line |
| Cleavage | One perfect, another less perfect, both parallel to the fibre; a third, in traces | {100} excellent; {010} good; {001} fair | Hard to use on fibrous crusts |
| Hardness | 3.5–4.5 | 3.5–4.5 | No |
| Colour | Greenish black, dark green, olive-green; olive-brown to reddish brown when oxidized; concentrically banded | Dark green, olive-green, greenish black, black; colour-banded; bronze-brown or reddish brown when oxidized | No |
| Fe:P, IMA formula | 1.500 | 1.667 | On paper |
| Fe:P, the sheets’ analyses | 1.560 and 1.634 (Wheal Phoenix) | 1.614 (Rockbridge Co.) | No: the analyses overlap |
| Calcium | 0.5 Ca per 4 P (formula); 0.27 and 0.25 in the analyses | None in the formula; 0.18 per 4 P in the Rockbridge analysis | Not cleanly |
What is dufrénite? A banded fibrous iron phosphate
Dufrénite is a monoclinic iron phosphate with a little calcium, mostly seen as dark green radial fibrous crusts and botryoidal masses. The Handbook of Mineralogy dufrénite sheet gives point group 2/m, space group C2/c, a 25.84, b 5.126, c 13.78 Å, β 111.20°, Z 2. Crystals are rare, rounded and tabular, to 1 mm, in sheaflike or subparallel aggregates. Cleavage: one perfect and another less perfect, both parallel to the fibre elongation, and a third perpendicular to those two, in traces; brittle; hardness 3.5 to 4.5; measured density 3.1 to 3.34, calculated [3.41].
Colour and optics. Greenish black, dark green or olive-green when fresh, olive-brown to reddish brown when oxidized, typically concentrically banded; bluish green, reddish brown or yellow in transmitted light; vitreous to silky; translucent to almost opaque. Biaxial (+), α 1.810 to 1.842, β 1.813 to 1.850, γ 1.855 to 1.925, 2V small to very small, dispersion extreme. The sheet prints no streak. Banded radial crusts are explained on what botryoidal means.
Name and lists. The sheet names it to honour Ours Pierre Armand Petit-Dufrénoy (1792–1857), French mineralogist at the National School of Mines, Paris, and puts type material at the University of Chicago (1247) and Harvard (97427). It marks no type locality. The IMA list gives status G, 1833, Germany. The site’s URL drops the accent; the IMA name keeps it.
Dufrénite or rockbridgeite? What separates them
A refractometer reading of α, or an X-ray pattern, separates them; colour and hardness do not. Both sheets describe dark green to black fibrous, banded, botryoidal iron phosphates that turn reddish brown when oxidized, both at hardness 3.5 to 4.5. Dufrénite’s α runs 1.810 to 1.842; rockbridgeite’s 1.873 to 1.875, 0.031 higher. β separates them too, 1.813 to 1.850 against 1.880, but their γ ranges overlap, 1.855 to 1.925 against 1.895 to 1.897, so a single highest-index reading does not. On fine fibres a refractometer reading may not be possible at all.
Weight and X-ray. At the sheets’ measured densities, a 20.00 g piece reads 13.55 to 14.01 g in water as dufrénite and 14.20 g as rockbridgeite, 0.19 g apart at the nearest; a crust on matrix will not give a clean reading. The powder patterns differ more: dufrénite’s second-strongest line is at 12.00 Å (intensity 9), while the longest spacing on rockbridgeite’s printed list is 6.87 Å. Their strongest lines, 3.151 and 3.196 Å, are close. Weighing is on how to measure specific gravity at home.
Documented failure: iron against phosphorus. On paper dufrénite is Fe:P 1.500 and rockbridgeite 1.667, and the strengite page uses those figures. Worked from the sheets’ own analyses, the two Wheal Phoenix dufrénites give 1.560 and 1.634 and the Rockbridge Co. rockbridgeite 1.614, so the ranges cross. Calcium does no better: the formula gives dufrénite 0.5 Ca per 4 P, the Wheal Phoenix analyses 0.27 and 0.25, and the rockbridgeite analysis carries 0.18. Neither sheet names the other as an associate.
Dufrénite formula, worked: the Z check
The IMA formula reproduces the sheet’s ideal analysis, but not its printed Z. Ca0.5Fe2+Fe3+5(PO4)4(OH)6(H2O)2 carries 18 positive charges (1 from calcium, 2 from ferrous iron, 15 from ferric) against 12 on the phosphate and 6 on the hydroxyl. On standard atomic weights it weighs 873.06 and comes out at P2O5 32.52, Fe2O3 45.73, FeO 8.23, CaO 3.21 and H2O 10.32 wt%, the sheet’s column (3) to within 0.01; by element, 38.38% iron and 14.19% phosphorus.
The cell. V = abc·sin β = 1701.7 Å3. With the IMA formula weight and Z = 4 that gives 3.408, the sheet’s bracketed 3.41; with the printed Z = 2 it gives 1.704, half of that and well below the measured 3.1 to 3.34. The printed Z fits a formula unit twice the IMA one. The sheet does not say which unit its Z counts, and this page does not choose.
The analyses. Worked to cations per 4 P, the first Wheal Phoenix analysis gives Fe2+ 0.86, Fe3+ 5.38, Ca 0.27 and Al 0.16, with 86% of the iron ferric (83% in the formula). The second reports no FeO at all, so all its iron is ferric. The sheet says only that dufrénite is olive-brown to reddish brown when oxidized; it does not say whether that analysis was of oxidized material.
Where is dufrénite found? Germany, Cornwall and beyond
Where the sheet places it. A secondary mineral formed principally in iron ore deposits; associates goethite, beraunite and quartz. In Germany, Hirschberg, Ullersreuth and Lobenstein, Thuringia; Hauptmannsgrün near Reichenbach, Saxony; the Rotläufchen mine, Waldgirmes, near Giessen, Hesse; Hagendorf, Bavaria. In England, the West Phoenix United mines, Linkinhorne, and the Gunheath china clay pit, St. Austell, Cornwall. The Mangualde pegmatite, Portugal. In the USA, Rock Run, Alabama; the Palermo #1 mine, New Hampshire; the Tip Top mine, South Dakota; the Sequoia Pit and the Gold Quarry mine, Nevada; the Santa Rita mine, New Mexico. Dandarragan, Western Australia; the El Criollo pegmatite, Córdoba, Argentina.
What the sheet adds. “There are probably additional localities, but requiring confirmation.” Its two analyses are from Wheal Phoenix, Cornwall, and its powder pattern from Cornwall, “close to natrodufrénite”, the sodium relative that the September 2026 IMA list records as redefined in 2026. Six of dufrénite’s places are also on the rockbridgeite sheet, by script over the two Distribution fields: Hagendorf, Mangualde, the Palermo #1 mine, the Tip Top mine, El Criollo and Ullersreuth (placed in Thuringia on the dufrénite sheet and in Saxony on the rockbridgeite sheet); a label from one of those places could be either, and the separation above is the place to start.
Nothing more is claimed. No fetched source gives dufrénite a use, and this page states none. The pegmatite phosphates’ order of formation, as the sheets word it, is on what is mineral paragenesis; 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.