Pyromorphite

Pyromorphite

Yellow pyromorphite from Australia
General
Category Phosphate mineral
Apatite group
Formula
(repeating unit)		 Pb5(PO4)3Cl
Strunz classification 8.BN.05
Crystal system Hexagonal
Crystal class Dipyramidal (6/m)
H-M symbol: (6/m)
Space group P63/m
Identification
Color Dark grass-green, green, yellow, yellow-orange, reddish orange, yellow-brown, brown, tan, grayish, may be colorless
Crystal habit Prismatic to acicular crysals, globular to reniform
Twinning Rarely on {1122}
Cleavage Imperfect- [1011]
Fracture Uneven to sub-conchoidal
Tenacity Brittle
Mohs scale hardness 3.5-4
Luster Resinous to subadamantine
Streak White
Diaphaneity Transparent to translucent
Specific gravity 7.04 measured, 7.14 calculated
Optical properties Uniaxial (-) May be anomalously biaxial -
Refractive index nω = 2.058 nε = 2.048
Birefringence δ = 0.010
Pleochroism Weak
Ultraviolet fluorescence May fluoresce yellow to orange under LW and SW UV
Other characteristics Piezoelectric if biaxial
References [1][2][3]

Pyromorphite is a mineral species composed of lead chlorophosphate: Pb5(PO4)3Cl, sometimes occurring in sufficient abundance to be mined as an ore of lead. Crystals are common, and have the form of a hexagonal prism terminated by the basal planes, sometimes combined with narrow faces of a hexagonal pyramid. Crystals with a barrel-like curvature are not uncommon. Globular and reniform masses are also found. It is part of a series with two other minerals: mimetite (Pb5(AsO4)3Cl) and vanadinite (Pb5(VO4)3Cl), the resemblance in external characters is so close that, as a rule, it is only possible to distinguish between them by chemical tests. Paecilomyces javanicus is a mold collected from a lead-polluted soil that is able to form biominerals of pyromorphite [4]

They were formerly confused under the names green lead ore and brown lead ore (German: Grünbleierz and Braunbleierz). The phosphate was first distinguished chemically by M. H. Klaproth in 1784, and it was named pyromorphite by J. F. L. Hausmann in 1813. The name is derived from the Greek for pyr (fire) and morfe (form) due to its crystallization behavior after being melted.[2]

Cylindrical yellow green pyromorphite crystals, Chihuahua, Mexico

Properties and isomorphism

The color of the mineral is usually some bright shade of green, yellow or brown, and the luster is resinous. The hardness is 3.5 to 4, and the specific gravity 6.5 - 7.1. Owing to isomorphous replacement of the phosphorus by arsenic there may be a gradual passage from pyromorphite to mimetite. Varieties containing calcium isomorphously replacing lead are lower in density (specific gravity 5.9 - 6.5) and usually lighter in color; they bear the names polysphaerite (because of the globular form), miesite from Mies in Bohemia, nussierite from Nuizière, Chénelette, near Beaujeu, Rhône, France, and cherokine from Cherokee County in Georgia.

Stereo image
Right frame 
Small specimen of Pyromorphite from China

See also

References

  1. ↑ http://rruff.geo.arizona.edu/doclib/hom/pyromorphite.pdf Handbook of Mineralogy
  2. 1 2 http://webmineral.com/data/Pyromorphite.shtml Webmineral data
  3. ↑ http://www.mindat.org/min-3320.html Mindat.org
  4. ↑ Fungal transformation of metallic lead to pyromorphite in liquid medium. Young Joon Rheea, Stephen Hillierc, d, Helen Pendlowskic, Geoffrey Michael Gadd. Chemosphere. Volume 113, October 2014, Pages 17–21
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