Matlockite

Matlockite
Matlockite specimen from Derbyshire, England
General
Category	Halide minerals
Formula (repeating unit)	PbFCl
Strunz classification	3.DC.25
Dana classification	9.2.11.1
Crystal system	Tetragonal
Crystal class	Ditetragonal dipyramidal (4/mmm) H-M symbol: (4/m 2/m 2/m)
Space group	P4/nmm
Unit cell	a = 4.11 Å, c = 7.23 Å; Z = 2
Identification
Colour	Colourless to yellow and greenish
Crystal habit	Flattened, tabular crystals occurring as aggregates, rosettelike, radiating, hemispherical; also massive
Cleavage	{001}, perfect
Fracture	Uneven to subconchoidal
Tenacity	Brittle
Mohs scale hardness	2.5 - 3
Luster	Adamantine, pearly on {001}
Diaphaneity	Transparent
Specific gravity	7.1 - 7.2
Optical properties	Uniaxial (-)
Refractive index	nω = 2.150 nε = 2.040
References

Matlockite is a rare lead halide mineral, named after the town of Matlock in Derbyshire, England, where it was first discovered in a nearby mine. Matlockite (chemical formula: PbFCl) gives its name to the matlockite group which consists of rare minerals of a similar structure.

The mineral, a lead fluorochloride (formula PbFCl), was discovered sometime around the early 1800s at Bage Mine at Bolehill near Matlock, together with specimens of phosgenite and anglesite. Although phosgenite was known at this time, it seems likely that matlockite itself remained unappreciated as a new mineral for some fifty years. It was given the name by Greg in 1851. The first mention of Matlockite may have been in Mawe's Mineralogy of Derbyshire in 1802 in which he gives a detailed description of phosgenite, which is then followed by a mention of a mineral he refers to as "glass lead" - a description which does rather equate to the appearance of matlockite. It is a light, translucent creamy-yellow colour, but heavy in weight having a density that is over 7.1.

A very large specimen 10 cm across, and originating from Derbyshire, exists in the collections of the American Museum of Natural History. A 7 cm specimen can be found in the collection of Derby Museum and Art Gallery.

Matlockite has been reported from a variety of locations since its discovery at the type locality of Derbyshire. The mineral is also found in Tiger, Arizona, Laurium in Greece, a mine near Essen in Germany and near Campiglia in Tuscany. Samples have also been found at locations in South Africa, Peru, Chile, Australia, Austria, France and Italy.