Penfieldite
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| Penfieldite | |
|---|---|
 | |
| General | |
| Category | Minerals |
| Formula | Pb2Cl3(OH) |
| IMA symbol | Pfd |
| Strunz classification | 03.DC.15 |
| Dana classification | 10.04.01.01 |
| Crystal system | Hexagonal |
| Crystal class | Trigonal Dipyramidal H-M symbol: 6 |
| Space group | P6 |
| Unit cell | 5,360.83 |
| Identification | |
| Formula mass | 537.77 |
| Color | Colorless, white, yellowish, bluish |
| Cleavage | Distinct/good on {0001} |
| Mohs scale hardness | 3–4 |
| Luster | Adamantine, greasy |
| Streak | White |
| Diaphaneity | Transparent |
| Specific gravity | 6.00 |
| Density | Measured: 5.82 – 6.61 Calculated: 6.00 |
| Optical properties | Uniaxial (+) |
| Refractive index | nω = 2.130(1) nε = 2.210(1) |
| Birefringence | 0.080 |
| Solubility | Soluble in water |
Penfieldite is a rare lead hydroxychloride mineral[1] from the class of halides. It was named after Samuel Lewis Penfield. It has been a valid species before the founding of IMA, and was first published in 1892. It had been grandfathered, meaning the name penfieldite is still believed to refer to a valid species.[2][3] When it was first described by Genth in 1892 from Laurion, Greece, the mineral had the formula of Pb3Cl4O.[1]
Penfieldite grows tabular pyramidal crystals, meaning it is longer instead of being wide, and grows in the shape of a pyramid. It is a secondary mineral, meaning that rock minerals went under transformation due to changes in pressure and temperature, and penfieldite is the newly formed stable mineral after this transformation.[2] Singular crystals are usually striped, and can grow up to 3 cm. Pure penfieldite is colorless, however, due to lattice defects or foreign admixtures, it can be white, yellow or even blue. It mainly consists of lead (77.06%) and chlorine (19.78%), and has a negligible amount of oxygen (2.98%) and hydrogen (0.19%) in it. It does not show any radioactive properties whatsoever.[2] After it is dissolved in water, penfieldite leaves a lead oxychloride residue behind that is yellowish white in color.[3] When heated to 180 °C, a 9c periodicity can be observed, meaning the crystal repeats itself every nine layers in the c crystallographic direction. When further heated to 200 °C and above, the crystals are quickly destroyed.[1]