Rockbridgeite
Rockbridgeite is named after the type locality at the Midvale Mine, South Mountain, Midvale, Rockbridge County, Virginia, USA. It is a common mineral but rarely available as a faceted gem. It’s not that it couldn’t be faceted but that it is mostly unremarkable as a gem. It usually has a dark green color but that changes with oxidation to a reddish brown, dark brown or black. Rockbridgeite is a ferrous iron rich mineral but its chemistry is a bit odd in that it includes two different iron atoms. One is ferric (with a plus 3 charge) and the other is ferrous (with a plus 2 charge). Rockbridgeite forms from the alteration (oxidation) of primary iron and manganese phosphates.
Distribution: Widespread; may be locally abundant. In the USA, in Virginia, on South Mountain, southeast of Midvale, Rockbridge County; at the Palermo #1 and Fletcher mines, North Groton, Grafton County, New Hampshire; in the Tip Top, Bull Moose, and other mines, near Custer, Custer County, and at the Big Chief, Etta, and other mines, near Keystone, Pennington County, South Dakota. From the Énio pegmatite mine, northeast of Galiléia, and near São Jose da Safira, Agua Boa, Minas Gerais, Brazil. From the El Criollo pegmatite, Cerro Blanco, Tanti district, 45 km west of Córdoba, Córdoba Province, Argentina. At Sabugal, Mangualde, Bendada, and Moreira, Portugal. From the La Vilate quarry, near Chanteloube, Haute-Vienne, France. In Germany, at Hagendorf, and on the Kreuzberg, Pleystein, Bavaria; and from Ullersreuth, Saxony. In the Moculta phosphate quarry, northeast of Angaston, South Australia. From the Sandamap pegmatite, west of Usakos, Namibia. In the Pearl mine, Miami, Zimbabwe. At Tazenakht, Morocco.
| Chemical Formula: | (Fe2+,Mn2+)Fe3+4(PO4)3(OH)5 |
| Iron Manganese Phosphate Hydroxide | |
| Molecular Weight: | 648.96 gm |
| Composition: | Manganese | 2.12 % | Mn | 2.73 % | MnO |
| Iron | 40.88 % | Fe | 8.30 % | FeO / 49.21 % Fe2O3 | |
| Phosphorus | 14.32 % | P | 32.81 % | P2O5 | |
| Hydrogen | 0.78 % | H | 6.94 % | H2O | |
| Oxygen | 41.91 % | O | |||
| 100.00 % | 100.00 % | = TOTAL OXIDE |
| Crystallography: | Orthorhombic – Dipyramidal |
| Crystal Habit: | Euhedral crystals rare; typically fibrous, elongated along [001], compact radiating, to 3 cm, in crusts, botryoidal and drusy masses. |
| Twinning: | None |
| Cleavage: | Excellent on {100}, Good on {010}, Fair on {001} |
| Fracture: | Irregular/Uneven |
| Tenacity: | Brittle |
| Moh’s Hardness: | 3.5 – 4.5 |
| Density: | 3.45 (g/cm3) |
| Luminescence: | Not Fluorescent |
| Radioactivity: | Not Radioactive |
| Other: | Soluble in HCl, but not in HNO3 or H2SO4 |
| Color: | Dark green, olive-green, greenish black, black, commonly color-banded; may be bronze-brown, reddish brown when oxidized. |
| Transparency: | Transparent to Translucent, Opaque |
| Luster: | Vitreous to Dull |
| Refractive Index: | 1.875 – 1.897 Biaxial ( + ) |
| Birefringence: | 0.0220 |
| Dispersion: | Strong; r < v, rarely r > v |
| Pleochroism: | Visible; X = pale brown to pale yellow-brown; Y = bluish green; Z = dark bluish green |
