Mendipite

Mendipite
Mendipite from the Torr Works Quarry (Merehead Quarry), Cranmore, Somerset, England. Specimen size 2 cm
General
Category	Halide mineral
Formula (repeating unit)	Pb3Cl2O2
Strunz classification	3.DC.70 Oxyhalide
Dana classification	10.3.1.1
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pnma or P212121,
Unit cell	a = 9.52 Å, b = 11.87 Å, c = 5.87 Å; Z = 4
Identification
Formula mass	724.50 g/mol
Color	Colorless to white, brownish cream, grey, yellowish, pink, red, or blue; nearly colorless in transmitted light.
Crystal habit	Columnar or fibrous aggregates, often radiated, and cleavable masses.
Cleavage	Perfect on {110}, fair on {100} and {010}
Fracture	Conchoidal to uneven
Mohs scale hardness	2½ to 3
Luster	Pearly to silky on cleavages; resinous to adamantine on fractures.
Streak	White
Diaphaneity	Translucent, rarely transparent
Specific gravity	7.24
Optical properties	Biaxial (+)
Refractive index	nα = 2.240, nβ = 2.270, nγ = 2.310
Birefringence	δ = 0.070
2V angle	Measured: 90°, calculated: 84°
Solubility	Soluble in dilute nitric acid, HNO3
References

Mendipite is a rare mineral that was named in 1939 for the locality where it is found, the Mendip Hills in Somerset, England. It is an oxyhalide of lead with formula Pb₃Cl₂O₂.

Most references assert that mendipite crystallises in the orthorhombic crystal system, disphenoidal class 2 2 2, meaning that it has three mutually perpendicular axes of twofold symmetry, with space group P2₁2₁2₁, meaning that each of these axes is a screw axis. One reference, however, gives the crystal class as orthorhombic m m m with space group Pnma, which has a higher symmetry. In each case the "P" means that the mineral has a primitive unit cell.

Unit cell parameters:

Mendipite is colorless to white, brownish cream, grey, yellowish, pink, red, or blue. It is nearly colorless in transmitted light. It has a white streak and its luster is pearly to silky on cleavages, and resinous to adamantine on fractures. The mineral is translucent, and rarely transparent. It is biaxial (+), with refractive indices N_x = 2.24, N_y = 2.27, N_z = 2.31. These values are quite high, compared with ordinary glass at 1.5. This is typical of lead minerals.

Mendipite is found in columnar or fibrous aggregates, often radiated but more rarely straight long fibers, and in cleavable masses. The cleavage is perfect on {110} and fair on {100} and {010}. Fracture is conchoidal (shell-like) to uneven and the mineral is soft, with hardness only 2½ to 3, a bit less than that of calcite. Because of the lead content the specific gravity is high, at 7.24, or 7 to 7.2, just a little less than that of mimetite, another lead mineral. Mendipite is soluble in dilute nitric acid, HNO₃. It is not radioactive.