Rockbridgeite

Rockbridgeite is an anhydrous phosphate mineral in the "Rockbridgeite" supergroup with the chemical formula Fe²⁺Fe³⁺₄(PO₄)₃(OH)₅. It was discovered at the since-shut-down Midvale Mine in Rockbridge County, Virginia, United States. The researcher who first identified it, Clifford Frondel, named it in 1949 for its region of discovery, Rockbridge County.

Rockbridgeite
General
Category	Minerals
Formula	Fe2+Fe3+4(PO4)3(OH)5
IMA symbol	Rkb[1]
Strunz classification	8.BC.10
Dana classification	41.9.2.1
Crystal system	Orthorhombic
Crystal class	Dipyramidal (2/m2/m2/m) (same H-M symbol)
Space group	Bbmm
Identification
Formula mass	648.96 g/mol
Color	Varies from green, to black, to brownish green, to reddish brown
Crystal habit	Euhedral crystals rare; typically fibrous in crusts, botryoidal and drusy masses
Twinning	Cruciform twins possible
Cleavage	Perfect on {100}, Distinct on {010}, Distinct on {001}
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	4.5
Luster	Vitreous to dull
Streak	Greenish gray
Diaphaneity	Subtranslucent
Specific gravity	3.40
Optical properties	Biaxial (+)
Refractive index	Nx = 1.875, Ny = 1.880, Nz = 1.897
Birefringence	0.0220
Pleochroism	(x): light brown to light yellow-brown (y): bluish green. (z): dark bluish green.
Solubility	Soluble in HCl, but not in HNO3 or H2SO4[2]
References	[2][3][4][5]

Structure

The stable form of rockbridgeite is orthorhombic 2/m2/m2/m, space group Bbmm. Face-sharing trimers in the structure share octahedron corners, forming chains of octahedra extending in the direction of the b-axis. Adjacent chains formed from two trimers and (PO₄) groups extending along the direction of the a-axis, resulting in sheets parallel to (001). Each chain of octahedra is connected by a layer of tetrahedra extending along the direction of the a-axis.^[6]

Unit cell

The unit cell parameters are a = 14 Å, b = 17 Å, and c = 5.2 Å, with 4 formula units per unit cell (Z = 4).
More accurate values are given by various sources as follows.

• a = 13.783 Å, b = 16.805 Å, c = 5.172 Å, β = 100.313(1)°^[2][4]

Optical properties

Rockbridgeite varies from green, to black, to brownish green, to reddish brown.^[5] It is subtranslucent, with a greenish gray streak and vitreous to dull luster.^[5] The refractive indices are N_x = 1.875, N_y = 1.880, N_z = 1.897.^[4]

Physical properties

Rockbridgeite shows perfect cleavage on {100}, distinct cleavage on {010}, and distinct cleavage on {001}. Rockbridgeite is a brittle mineral, with an uneven fracture, hardness 4.5 and specific gravity 3.40. It is soluble in hydrochloric acid HCl, but not in nitric acid HNO₃ or sulfuric acid H₂SO₄.

Environment

Rockbridgeite primarily forms in igneous rocks when the primary iron and manganese phosphates in granite pegmatites undergo mineral alteration through redox reactions, resulting in these phosphates becoming oxidized and causing rockbridgeite to form. More rarely, rockbridgeite can also form in sedimentary rocks in association with limonite from iron ore deposits. Associated minerals include triphylite, hureaulite, barbosalite, roscherite, and limonite.^[5]

Localities

The type locality is the since-shut-down Midvale Mine in Rockbridge County, Virginia, United States. Other localities for rockbridgeite include sites in Brazil, Portugal, Bavaria, Saxony, South Australia, Namibia, Zimbabwe, and Morocco. Type material is conserved for reference in Harvard University in Cambridge, Massachusetts, United States.^[5]