Mendipite is a rare mineral that was named for the locality where it is found, the Mendip Hills in Somerset, England. It is an oxyhalide of lead with formula Pb3Cl2O2.[
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Crystal structure
Most references assert that mendipite crystallises in the orthorhombic crystal system, 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.
Unit cell parameters:
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a = 9.52 Å, b = 11.87 Å, c = 5.87 Å, Z = 4
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a = 11.87 Å, b = 5.806 Å, c = 9.48 Å, Z = 4
Optical properties
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 Birefringence, with Refractive index Nx = 2.24, Ny = 2.27, Nz = 2.31. These values are quite high, compared with ordinary glass at 1.5. This is typical of lead minerals.
Physical properties
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 Mohs hardness only to 3, a bit less than that of calcite. Because of the lead content the specific gravity is high, at 7.24,
Environment
At the Eleura Mine near Cobar, New South Wales, Australia, oxygenated groundwater reacted with during the Cenozoic, forming supergene sulfides, as well as the beudantite, anglesite and baryte, together with some mimetite and native silver. Cerussite crystallised later, and later still chloride-rich groundwaters reacted with many of these earlier minerals to form more mimetite, as well as blixite, laurionite, mendipite and chlorargyrite.[Australian Journal of Mineralogy (2005) 11-2:89-90]
At the type locality, galena was deposited in Carboniferous Limestone throughout the Mendip Hills during the late Permian or Triassic Period. In the Jurassic Period that followed, these deposits were exposed to the action of seawater, which deposited Manganese that reacted with the galena and adsorbed heavy metals both from the seawater and surroundings. A later event heated the deposits creating the conditions which led to the formation of the suite of unusual secondary minerals, including a number of rare oxohalide minerals, now found at Merehead.
Type locality
The type locality is Churchill, Mendip Hills, Somerset, England, and type material is conserved at the Royal Swedish Academy of Sciences, Stockholm, Sweden. Other localities include Australia, Germany, Greece, Sweden, the UK and the US.
External links
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JMol: http://rruff.geo.arizona.edu/AMS/viewJmol.php?id=06898
