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