Mottramite

Mottramite is an orthorhombic anhydrous vanadate hydroxide mineral, PbCu(VO4)(OH), at the copper end of the descloizite subgroup. It was formerly called cuprodescloizite or psittacinite (this mineral characterized in 1868 by Frederick Augustus Genth). Duhamelite is a calcium- and bismuth-bearing variety of mottramite, typically with acicular habit.
Mottramite is a member of the adelite-descloizite group. 
Mottramite, which is a copper rich member, forms a series with descloizite, which is a zinc rich member. These two minerals usually contain significant percentages of both copper and zinc and are seldom pure. Mottramite also forms a series with duftite.
It was discovered in 1876 and named for the locality, Mottram St Andrew, Cheshire, England, where ore was stockpiled, although it was probably mined from Pim Hill Mine, Shrewsbury, Shropshire, England.
Crystallography.
Mottramite is an orthorhombic mineral belonging to the crystal class 2/m 2/m 2/m, with space group Pnma. The unit cell has sides of lengths a = 7.6 to 7.7 Å, b = 9.2 to 9.5 Å and c = 6.0 to 6.1 Å. There are four formula units per unit cell (Z = 4), the molar mass is 402.69 g and the calculated density is 6.19 g/cm3. The structure is composed of chains of edge-sharing CuO6 octahedra and very distorted Pb(O,OH)8 polyhedra linked through VO4 groups into a tight three-dimensional network.
Appearance.
Drusy crusts of tiny intergrown crystals are common, also encrustations and mammillary or botryoidal surfaces. The crystals are equant dipyramids or prisms parallel to the c crystal axis, but always microscopic. The colour is various shades of green, yellow-green, blackish brown or nearly black. Crystals often grow step by step, with the different steps or zones having different colours. The streak is yellowish green, or yellow, and the crystals are transparent to opaque, with a greasy lustre.
Physical properties.
No cleavage has been observed. The mineral is brittle and breaks with a subconchoidal to uneven fracture. It is quite soft, with Mohs hardness 3 to , just a little harder than calcite. The hardness is slightly greater on crystal surfaces. It is a heavy mineral, with specific gravity 5.9, because of the lead content. It is readily soluble in acids.
Optical properties.
Orthorhombic crystals (and triclinic and monoclinic crystals) have two directions in which light travels with zero birefringence; these directions are called the optic axes, and the crystal is said to be biaxial. The speed of a ray of light travelling through the crystal differs with direction. The direction of the fastest ray is called the X direction and the direction of the slowest ray is called the Z direction. X and Z are perpendicular to each other, and a third direction Y is defined as perpendicular to both X and Z; light travelling along Y has an intermediate speed. Refractive index is inversely proportional to speed, so the refractive indices for the X, Y and Z directions increase from X to Z. 
For mottramite the orientation with respect to the crystal axes a, b and c is X = c, Y = b and Z = a. The refractive indices are nα = 2.170(2), nβ = 2.260(2) and nγ = 2.320(2). The maximum birefringence δ is the difference between the highest and lowest refractive index; for mottramite δ = 0.150. 
The angle between the two optic axes is called the optic angle, 2V, and it is always acute, and bisected either by X or by Z. If Z is the bisector then the crystal is said to be positive, and if X is the bisector it is said to be negative. Mottramite is usually biaxial (–), and rarely biaxial (+). The measured value of 2V is 73°. Also 2V can be calculated from the values of the refractive indices, giving a value of 46°, which differs considerably from the measured value. 2V depends on the refractive indices, but refractive index varies with wavelength, and hence with colour. So 2V also depends on the colour, and is different for red and for violet light. This effect is called dispersion of the optic axes, or just dispersion (not to be confused with chromatic dispersion). If 2V is greater for red light than for violet light the dispersion is designated r > v, and vice versa. For mottramite dispersion is strong, usually with r > v, and rarely with r < v. The mineral is pleochroic; when viewed along the X or Y direction it appears canary yellow to greenish yellow and when viewed along the Z direction it appears brownish yellow.
Occurrence.
The type locality is Mottram St Andrew, Cheshire, England, UK and type material is conserved at the Natural History Museum, London 52314-52315. Mottramite is a secondary, supergene mineral found principally in the oxidized zones of vanadium bearing base metal deposits, especially sandstones. Associated minerals are descloizite, duftite, mimetite, wulfenite, cerussite, azurite and dioptase.