Pyromorphite

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.^[5] 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.^[6] Crystals with a barrel-like curvature are not uncommon. Globular and reniform masses are also found.^[7]

Pyromorphite
General
Category	Phosphate mineral Apatite group
Formula	Pb5(PO4)3Cl
IMA symbol	Pym[1]
Strunz classification	8.BN.05
Crystal system	Hexagonal
Crystal class	Dipyramidal (6/m) (same H-M symbol)
Space group	P63/m
Identification
Color	Dark green to grass-green or green, yellow, yellow-orange, reddish orange, yellow-brown, greenish-yellow or yellowish-green, shades of brown, tan, grayish, white and may be colorless; colourless or faintly tinted in transmitted light.
Crystal habit	Prismatic to acicular crystals, 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	[2][3][4]

Pyromorphite is part of the apatite group of minerals and bears a close resemblance physically and chemically with two other minerals: mimetite (Pb₅(AsO₄)₃Cl) and vanadinite (Pb₅(VO₄)₃Cl).^[8] The resemblance in external characters is so close that, as a rule, it is only possible to distinguish between them by chemical tests. They were formerly confused under the names green lead ore and brown lead ore (German: Grünbleierz and Braunbleierz).^{[citation needed]}

History

The mineral was first distinguished chemically by M. H. Klaproth in 1784,^{[9][10][11][12]} and it was named pyromorphite by J. F. L. Hausmann in 1813.^[13][14] The name is derived from the Greek for pyr (fire) and morfe (form) due to its crystallization behavior after being melted.^[3]

Properties

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 between 6.5 and 7.1.^[7]

Isomorphism

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 Stříbro (pronounced Mies in German) in Bohemia, nussierite from Nuizière, Chénelette, near Beaujeu, Rhône, France, and cherokine from Cherokee County in Georgia.^[15]

Biology

Paecilomyces javanicus is a fungus collected from a lead-polluted soil that is able to form biominerals of pyromorphite.^[16]

Gallery

• 
  A pocket of crystals of pyromorphite from China
• 
  Miniature of sharp, lustrous, apple green color crystal "sprays" on matrix
• 
  Fine crystals of pyromorphite from Daoping Mine, Guangxi Zhuang Autonomous Region, China
• 
  Green pyromorphite crystals densely carpet the display side of the large matrix
• 
  Green pyromorphite microcrystals cover the vuggy, quartz-rich matrix. Seams of tiny cerussite crystals and crusts of contrasting, powder-blue caledonite round out this very rich lead ore specimen from an old Leadhills mine.

See also

• Lead apatite
• List of minerals