matlockite

{{Infobox mineral | name = Matlockite | boxwidth = | boxbgcolor = | image = Matlockite-190524.jpg | imagesize = 260px | alt = | caption = | category = Halide minerals | formula = PbFCl | IMAsymbol = Mtl | molweight = | strunz = 3.DC.25 | dana = 9.2.11.1 | system = Tetragonal | class = Ditetragonal dipyramidal (4/mmm) H-M symbol: (4/m 2/m 2/m) | symmetry = P4/nmm | unit cell = a = 4.11 Å, c = 7.23 Å; Z = 2 | color = | colour = Colourless to yellow and greenish | habit = Flattened, tabular crystals occurring as aggregates, rosettelike, radiating, hemispherical; also massive | twinning = | cleavage = {001}, perfect | fracture = Uneven to subconchoidal | tenacity = Brittle | mohs = 2.5 – 3 | luster = Adamantine, pearly on {001} | streak = | diaphaneity = Transparent | gravity = 7.1 – 7.2 | density = | polish = | opticalprop = Uniaxial (−) | refractive = nω = 2.150 nε = 2.040 | birefringence = | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | prop1 = | prop1text = | 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.

==Description== 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. In 1802, the mineralogist John Mawe described a mineral he called "glass lead" in his book Mineralogy of Derbyshire. 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.