Roscoelite

Roscoelite
Roscoelite and gold
General
Category	Phyllosilicate
Formula (repeating unit)	K(V3+,Al,Mg)2AlSi3O10(OH)2
Strunz classification	9.EC.15
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/m
Identification
Formula mass	426.53 g/mol
Colour	olive-green to green-brown
Crystal habit	scales, fans, druses, rosettes, fibrous or felted aggregate.
Cleavage	Perfect plane {0,0,1}
Fracture	platy
Mohs scale hardness	1
Lustre	pearly
Diaphaneity	semi-transparent to translucent
Specific gravity	2.92 - 2.96
Optical properties	Biaxial (-)
Refractive index	nα=1.60 nβ=1.66 nγ=1.67
Birefringence	δ =
Pleochroism	olive green to green-brown
Fusibility	loses water

Roscoelite is a green mineral from the mica group that contains vanadium.

The chemical formula is K(V³⁺, Al, Mg)₂AlSi₃O₁₀(OH)₂. Crystals of roscoelite take on the monoclinic form, and are from the 2/m point group. The appearance is semi transparent to translucent coloured olive brown to green brown. The lustre is pearly. The mineral shows pleochroism with X showing green-brown, and Y and Z axes showing olive-green colour. The mineral was named after Henry Enfield Roscoe who first produced vanadium metal.

Roscoelite is a muscovite with aluminium substituted with vanadium. Vanadium can also be substituted by magnesium, iron, or manganese.

It is soft and the density is 2.93±0.01. The unit cell has dimensions

with an angle between axes of β=101.0°. The tetrahedral cation-oxygen atom distance is 164.1 pm, The distance from the cation to the oxygen in the octahedral plan is 202 pm.

Two kinds of mineral deposits contain roscoelite, either gold-silver-tellurium low temperature epithermal deposits where it occurs along with quartz, fluorite, pyrite and carbonates, or oxidized low temperature uranium-vanadium ores in sedimentary rocks, where it occurs with corvusite, hewettite, carnotite and tyuyamunite. Roecoelite is considered a gangue mineral of no value when found with gold. However it has also been used as a vanadium ore.