Rockbridgeite
Rockbridgeite, (Fe2+0.5Fe3+0.5)2Fe3+3(PO4)3(OH)5: orthorhombic, fibrous, dark green to black, hardness 3.5 to 4.5, density 3.45. Type locality Rockbridge Co.
Rockbridgeite is a basic iron phosphate, (Fe2+0.5Fe3+0.5)2Fe3+3(PO4)3(OH)5 on the IMA list (G, 1949, USA): orthorhombic, fibrous and radiating to 3 cm, dark green to black and often colour-banded, hardness 3.5 to 4.5, density 3.45. It is named for Rockbridge Co., Virginia, its type locality, and forms a series with frondelite.
In short
- The species. The Handbook of Mineralogy sheet gives an orthorhombic mineral, typically fibrous, compact radiating to 3 cm, in crusts, botryoidal and drusy masses; cleavage {100} excellent; hardness 3.5 to 4.5; density 3.45; dark green, olive-green, greenish black or black, commonly colour-banded, reddish brown when oxidized. The September 2026 IMA-CNMNC Master List writes (Fe2+0.5Fe3+0.5)2Fe3+3(PO4)3(OH)5, status G, 1949, USA.
- The 3.60 check. Worked on standard atomic weights the IMA formula weighs 649.17: 43.01% iron, 14.31% phosphorus. With the sheet’s cell (a 13.783, b 16.805, c 5.172 Å, Z 4) that gives a density of 3.599, the sheet’s calculated 3.60; the measured figure is 3.45. A 20.00 g piece reads 14.20 g in water.
- The Fe2+ ladder. On the IMA list, rockbridgeite, frondelite, ferrorockbridgeite, ferrirockbridgeite, manganrockbridgeite and plimerite all carry Fe3+3(PO4)3 with two more metal sites. Counted per 3 P, ferrous iron runs from 0 in ferrirockbridgeite to 1 in rockbridgeite and up to 2 in ferrorockbridgeite. The sheet’s Rockbridge Co. analysis works out at 0.57 Fe2+ and 4.27 Fe3+.
- Documented failure: the 1:1 column. The sheet’s second chemistry column is its formula with Fe2+:Mn2+ = 1:1, and it reproduces our arithmetic to the hundredth. On the IMA formulas, that composition sits exactly halfway between rockbridgeite (Fe2+ on the two sites) and frondelite (Mn2+ there). The sheet prints no manganese-free ideal.
- Where, and with what. In “limonite” iron ores, and as an alteration product of primary phosphates in complex granite pegmatites. Associates on its sheet: triphylite, huréaulite, barbosalite, roscherite. strengite and cacoxenite list it in turn; its numbers on those pages agree with this one.
| Row | Formula or source | IMA status, year, country | Fe2+ per 3 P | Fe3+ per 3 P | Other metal per 3 P | Anion side |
|---|---|---|---|---|---|---|
| Ferrirockbridgeite | (Fe3+0.67☐0.33)2(Fe3+)3(PO4)3(OH)4(H2O) | A, 2018-065, USA | 0 | 4.34 | 0.66 vacant | (OH)4(H2O) |
| Rockbridgeite | (Fe2+0.5Fe3+0.5)2Fe3+3(PO4)3(OH)5 | G, 1949, USA | 1 | 4 | none | (OH)5 |
| Frondelite | (Mn2+0.5Fe3+0.5)2Fe3+3(PO4)3(OH)5 | G, 1949, Brazil | 0 | 4 | Mn2+ 1 | (OH)5 |
| Ferrorockbridgeite | (Fe2+,Mn2+)2Fe3+3(PO4)3(OH)4(H2O) | A, 2018-004, Germany | up to 2 | 3 | Mn2+ on the same 2 sites | (OH)4(H2O) |
| Manganrockbridgeite | Mn2+2Fe3+3(PO4)3(OH)4(H2O) | A, 2022-122, Germany | 0 | 3 | Mn2+ 2 | (OH)4(H2O) |
| Plimerite | Zn2Fe3+3(PO4)3(OH)4(H2O) | A, 2008-013, Australia | 0 | 3 | Zn 2 | (OH)4(H2O) |
| Sheet column (1), Rockbridge Co., Virginia | Analysis: FeO 6.14, Fe2O3 50.84, P2O5 31.76, MnO 0.40, MgO 0.76, CaO 1.12, H2O 8.53 | — | 0.57 | 4.27 | Mg 0.13, Ca 0.13, Mn 0.04, Al 0.03 | H2O 8.53 wt% |
| Sheet column (2), the sheet’s ideal | Handbook formula with Fe2+:Mn2+ = 1:1 | — | 0.50 | 4.00 | Mn2+ 0.50 | (OH)5 |
What is rockbridgeite? A fibrous green iron phosphate
Rockbridgeite is a mixed-valence iron hydroxyl phosphate, orthorhombic, usually fibrous or radiating; euhedral crystals are rare. The Handbook of Mineralogy rockbridgeite sheet gives point group 2/m 2/m 2/m, space group Bbmm, a 13.783, b 16.805, c 5.172 Å, Z 4. Euhedral crystals are rare; the usual habit is fibrous, elongated along [001], compact radiating to 3 cm, in crusts, botryoidal and drusy masses; see what botryoidal means.
Physical and optical properties. Cleavage {100} excellent, {010} good, {001} fair; fracture uneven; brittle; hardness 3.5 to 4.5; measured density 3.45, calculated 3.60. Dark green, olive-green, greenish black or black, commonly colour-banded; bronze-brown or reddish brown when oxidized; vitreous to dull; semitransparent. Biaxial (+), α 1.873 to 1.875, β 1.880, γ 1.895 to 1.897, 2V moderate, dispersion r < v, rarely r > v, strong; pleochroic, X pale brown to pale yellow-brown, Y bluish green, Z dark bluish green. The sheet prints no streak and this page supplies none. How to read a break is on cleavage or fracture.
Name and type locality. The sheet names it for the occurrence in Rockbridge Co., Virginia, USA, “taken as the type locality”, and puts type material at Harvard University (80600); its powder pattern is from Irish Creek, Rockbridge Co. The IMA list gives 1949, USA, first reference American Mineralogist 34 (1949), 513. What the term means on a label is on what is a type locality.
Rockbridgeite formula: the 3.60 check and two ways of writing it
The IMA and Handbook formulas carry the same atoms for manganese-free material. The IMA list writes (Fe2+0.5Fe3+0.5)2Fe3+3(PO4)3(OH)5: one ferrous and one ferric iron share two sites, three more ferric irons fill the rest. The Handbook writes (Fe2+,Mn2+)Fe3+4(PO4)3(OH)5. Either way the count is one Fe2+, four Fe3+, three phosphate groups and five hydroxyls; the charges balance at 14 each way.
Worked. On standard atomic weights the formula weighs 649.17: as oxides, P2O5 32.80, Fe2O3 49.20, FeO 11.07 and H2O 6.94 wt%; by element, 43.01% iron and 14.31% phosphorus, an Fe:P of 1.667, the figure the strengite page gives. The sheet’s cell has a volume of 1198.0 Å3; with Z 4, the density comes out at 3.599, the sheet’s calculated 3.60. The measured 3.45 is lower. A 20.00 g piece would read 14.20 g in water at 3.45, 14.44 g at 3.60; the method is on how to measure specific gravity at home.
Documented failure: the sheet’s ideal is a 1:1 midpoint. The sheet’s column (2) is labelled as its formula with Fe2+:Mn2+ = 1:1. Worked, it gives P2O5 32.82, Fe2O3 49.23, FeO 5.54, MnO 5.47 and H2O 6.94, the printed figures to the hundredth. On the two IMA formulas that composition, half Fe2+ and half Mn2+ on the divalent share of the two mixed sites, sits exactly between rockbridgeite and frondelite, the series partner the sheet names. The sheet prints no manganese-free ideal; the IMA column above is ours, worked from the IMA formula.
Is it rockbridgeite or one of its newer relatives? The Fe2+ ladder
Six IMA rows share the M2Fe3+3(PO4)3 pattern, and they differ mainly in what sits on the two mixed sites. Method: a regex over the September 2026 list for “Fe3+3(PO4)3(OH)” found seven rows; one, wilhelmgümbelite, carries extra water and loses superscripts in the text extraction, and is left out. The six are rockbridgeite and frondelite (both G, 1949), plimerite (2008), ferrorockbridgeite and ferrirockbridgeite (2018) and manganrockbridgeite (2022). Four of them write (OH)4(H2O) where rockbridgeite and frondelite write (OH)5.
The ladder. Per three phosphorus, ferrous iron is 0 in ferrirockbridgeite, 1 in rockbridgeite, and up to 2 in ferrorockbridgeite, whose two sites hold Fe2+ and Mn2+. Frondelite and manganrockbridgeite put manganese there, plimerite zinc. The Rockbridge Co. analysis on the sheet, converted to cations per 3 P by script, gives Fe2+ 0.57 and Fe3+ 4.27, with Mg 0.13, Ca 0.13, Mn 0.04 and Al 0.03: Fe:P 1.614, and 88% of the iron ferric against 80% in the formula.
What that does and does not mean. On ferrous iron alone, the type-county analysis falls between the ferrirockbridgeite and rockbridgeite rows. The sheet calls it rockbridgeite and so does this page; a valence count from one old wet analysis does not rename a specimen, and the sheet itself says the colour turns reddish brown when oxidized. A label reading rockbridgeite on material from a German pegmatite is a fair starting point; which of the newer rows a given piece belongs to is a laboratory question.
Where is rockbridgeite found? The sheet’s localities
Where it forms. In “limonite” iron ore deposits, and as an alteration product of primary phosphates in complex granite pegmatites. The sheet calls it widespread and locally abundant. Associates: triphylite, huréaulite, barbosalite, roscherite and “limonite”. The order in which such pegmatite phosphates form, as the sheets word it, is worked through on what is mineral paragenesis.
Where the sheet places it. In the USA: South Mountain, southeast of Midvale, Rockbridge Co., Virginia; the Palermo #1 and Fletcher mines, North Groton, New Hampshire; the Tip Top, Bull Moose and other mines near Custer, and the Big Chief, Etta and other mines near Keystone, South Dakota. The Énio pegmatite mine near Galiléia and near São Jose da Safira, Minas Gerais, Brazil; the El Criollo pegmatite, Córdoba, Argentina; Sabugal, Mangualde, Bendada and Moreira, Portugal; the La Vilate quarry, Haute-Vienne, France; Hagendorf and the Kreuzberg, Pleystein, Bavaria, and Ullersreuth, Saxony; the Moculta phosphate quarry, South Australia; the Sandamap pegmatite, Namibia; the Pearl mine, Zimbabwe; Tazenakht, Morocco. The sheet names no British locality.
Rockbridgeite and its neighbours. The strengite page lists the places the two sheets share; dufrénite, the other dark green fibrous iron phosphate, is separated from rockbridgeite on its own page, by optics, weight and X-ray. No fetched source gives rockbridgeite 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.