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Formula: Pb5(PO4)3Cl
Phosphate, apatite group,
apatite supergroup
Isostructural with mimetite and
vanadinite
Crystal System: Hexagonal
Specific gravity: 7.04 measured, 7.109 calculated
Hardness: 3½ to 4
Streak: White
Colour: Green to dark green, yellow, greenish-yellow or yellowish-green, orangish-yellow, shades of brown;
white and
colourless when pure; colourless or faintly tinted in transmitted light
Solubility: Slightly soluble in hydrochloric acid and sulphuric acid; moderately soluble in nitric acid
and KOH; slightly soluble in carbonated water
(Mindat)
Common impurities: F,Ra,Ca,Cr,V,As
Environments:
Pyromorphite is usually a secondary
lead mineral found
in the
oxidation zones of high temperature hydrothermal lead deposits associated with
other oxidised
lead and zinc minerals.
Lead will generally precipitate as
primary
galena from ore fluids rich in sulphur
and lead. Removal of sulphur by precipitation of sulphides, however, may lead
ultimately to an ore fluid
from which
galena cannot be precipitated, even with a high concentration of
lead
in solution.
In these circumstances, pyromorphite, as well as cerussite and
anglesite, could be precipitated as
primary minerals
(Strens (1963), MM 33:722-3).
Pyromorphite forms a complete series with mimetite
(lead chloride arsenate), and many
specimens are
intermediates between the two end-members.
Pyromorphite forms pseudomorphs after
galena and
cerussite
(common), and galena forms
pseudomorphs
after pyromorphite
(Dana, JRS 12.38).
Other pseudomorphs include
apatite
after
pyromorphite and plumbogummite encrusted on and
replacing pyromorphite
(Mindat).
galena is sometimes epitaxial on pyromorphite.
Localities
At Yangshuo, Guangxi, China, plumbogummite
pseudomorphs after pyromorphite have been found
(KL p204).
At Poullaouen, France, galena
pseudomorphs
after pyromorphite have been found
(KL p210).
Morocco
Pyromorphite forms in the oxidised areas of lead ores. The most beautiful Moroccan pyromorphite
crystals are extracted from the old lead mine of Bou Iboulkhir, in the Western Meseta Moulay Bouazza region. The
ore deposits form pyromorphite-rich veins hosted mainly by
schists. Pyromorphite is found as translucent yellow to green,
equant to elongate hexagonal prisms, generally millimeter in size, but rare crystals reach 2 cm in length. The
mineralogy of the pyromorphite-bearing veins includes galena,
iron-manganese oxides,
cerussite and quartz
(R&M 98.2.174-175).
At the Bou Iboulkhir Mine, Moulay Bouazza, Moulay Bouazza Caïdat, Aguelmous Cercle, Khénifra Province, Béni
Mellal-Khénifra Region, Morocco, pyromorphite is typically yellow to yellow-green, but in a very few
specimens a vivid green colour is observed. The colour correlates with the accessory or
trace-element chemistry. Green crystals are richer in calcium, copper and
arsenic, whereas the yellow ones are richer
in chromium
(R&M91-4:321).
At the Berg Aukas mine, Grootfontein, Otjozondjupa Region, Namibia, light green microcrystals of pyromorphite
have been found in
dolomite-lined vugs associated with
willemite and
aragonite
(R&M 96.2.132-133).
Pyromorphite in the Northern Pennines, England, UK, is of
secondary
origin, developed
by alteration of galena in the presence of phosphate-bearing solutions
percolating
through the
upper weathering zone of the deposits
(JRS 1.3.81-82).
At Alderley Edge, Cheshire, England, UK, copper mineralised solutions
percolated through porous
sandstone and deposited
barium, cobalt,
copper,
lead, vanadium and
zinc minerals between the sand grains.
Anhydrite formed as cement in permeable rocks, then
baryte was deposited, followed by
pyrite,
chalcopyrite, sphalerite and
galena. These minerals crystallised from highly saline, sulphate-rich brines,
at a temperature of 50 to 60o C. About 65 million years ago the deposit was uplifted, and oxygenated
ground water
oxidised original sulphide minerals. Galena was oxidised to
cerussite, anglesite and
pyromorphite
(RES pps 49-50). A wide range of compositions in the pyromorphite -
mimetite series occurs, from virtually arsenate-free pyromorphite to
phosphate-rich
mimetite associated with wulfenite
(JRS 5(2).99). A sample of galena with
copper and pyromorphite has been found
(RES p54).
At Closehouse Mine, Lunedale, County Durham, England, UK pyromorphite forms thin encrustations on fracture
surfaces of
baryte, and in places on altered
dolerite
wallrock.
A few specimens have been found which show pyromorphite encrusting
galena
(JRS 1.3.81-82).
At Grasshill Mine, Teesdale, County Durham, England, UK, pyromorphite has been found in cavities, sometimes
surrounding cores of
galena which is in an advanced stage of alteration to
cerussite
(JRS 1.3.81-82).
At Whitfield Brow Mines, Weardale, County Durham, England, UK, pyromorphite occurs as crusts on
shale and quartz, and also on
galena some of which shows advanced alteration to
cerussite
(JRS 1.3.81-82).
At Force Crag Mine, Coledale, Above Derwent, Allerdale, Cumbria, England, UK, a small specimen has been found with an
open face of matrix covered with a combination of matte black
sphalerite, white cerussite and
typical brownish pyromorphite
(AESS).
At Balliway Rigg, Caldbeck, Allerdale, Cumbria, England, UK, vivid greenish yellow botryoidal pyromorphite and
olive coloured hexagonal crystals have been found on a quartz matrix
(AESS).
At the Driggith South Mine, Caldbeck, Allerdale, Cumbria, England, UK, barrel-shaped crystals of moss-green
pyromorphite to 1 mm occur on a quartz boxwork matrix with
limonite
(AESS).
At Dry Gill Mine, Caldbeck, Allerdale, Cumbria, England, UK, pyromorphite is found on a
quartz matrix
(AESS).
At Ingray Gill, Caldbeck, Allerdale, Cumbria, England, UK, pyromorphite crystals encrust
mottramite
on a number of specimens. These crystals, which are late in the
supergene paragenesis, are ideal
endmember pyromorphite
(JRS 12.38).
At the Roughton Gill mines, Caldbeck, Cumbria, England, UK, pyromorphite is the most abundant
supergene mineral.
Pseudomorphs and epimorphs
of pyromorphite after
cerussite and hydrocerussite
occur here,
typically as microcrystalline crusts surrounding and replacing prismatic
cerussite
(JRS 11.21).
At Iron Crag, Roughton Gill, Caldbeck, Allerdale, Cumbria, England, UK, pyromorphite occurs as light yellowish
green vitreous and translucent crystals on a matrix of quartz or of
goethite
(AESS).
Pyromorphite from Iron Crag - Image
At Old Brandley Mine, Catbells, Keswick, Allerdale, Cumbria, England, UK, a specimen has been found of a
quartz matrix with a vug containing iron oxide stained
quartz crystals with a druzy over-growth of green pyromorphite
(AESS).
Pyromorphite from the Old Brandley Mine -
Image
At Wapping mine, Matlock Bath, Derbyshire, England, UK, pyromorphite occurs as alteration crusts on corroded
galena, and also on fluorite and
secondary baryte (which
it seems to replace), associated with cerussite and a little
anglesite
(JRS 4(1).32).
At Whitwell quarry, Derbyshire, England, UK, pyromorphite
pseudomorphs
after cerussite have been found
(KL p204).
At Burgham mine, Shelve, Shropshire, England, UK, pyromorphite is associated with
baryte and
quartz
(RES p283).
At Roar Hill, Ballater, Buchan Grampian, Scotland, UK, lead-bearing vein
mineralisation was exposed during recent work carried out on an unmetalled vehicle track. A small temporary quarry
exposed fluorite-bearing quartz
veins and minor wulfenite in light-coloured granite. At a
second site,
a little further to the west, an oxidised galena-bearing
quartz vein was exposed.
Pyromorphite occurs as lime-green to yellow crusts and thin coatings in
quartz-lined cavities at the track location. Analyses suggest that minor OH
is present in substitution for Cl in some specimens
(JRS 22.33).
At Cystanog Mine, Carmarthen, Carmarthenshire, Wales, UK, pyromorphite has been found as weathered coatings
on vein
quartz
(MW).
At Nant Mine, Llangunnor, Carmarthenshire, Wales, UK, pyromorphite has been found encrusting massive
baryte
(MW).
At Penycefn Mine, Bontgoch, and at Frongoch Mine, Devil's Bridge, both at Ceredigion, Wales, UK, pyromorphite has been found with
cerussite
(MW).
At Llechweddhelyg Mine, Penrhyncoch, Ceredigion, Wales, UK, pyromorphite has been found as crystals in vuggy
goethite gossan
(MW).
At Bwlch-glas Mine, Tal-y-bont, Ceredigion, Wales, UK, pyromorphite has been found with
wulfenite
on surface dumps, and underground coating crystals of quartz
(MW).
At Silver Rake, Halkyn Mountain, Flintshire, Wales, UK, pyromorphite has been found with
cerussite
coating galena
(MW).
At the Kabwe mine, Central Province, Zambia, pyromorphite is commonly associated with
tarbuttite (R&M 94.2.131-133).
Formation
In an acid environment, provided by dissolved carbon dioxide making carbonic acid H2CO3,
galena may be oxidised as follows:
PbS + 2O2 ↔ Pb2+ + (SO4)2-
Although phosphorus is more abundant (0.099%) in the Earth's crust than chlorine (0.017%), chlorine is widely
distributed in the surface environment but phosphorus occurs only as a trace element in most environments. In addition,
only small amounts of chlorine are required to form
pyromorphite but phosphorous is a major constituent. Hence the availability of phosphorus is likely to be an important
factor in the formation of pyromorphite,
especially in an oxidised lead deposit where lead is abundant.
The commonest source of phosphorus is phosphate rock,
which is mostly apatite.
5Pb2+ (from galena) + 3(PO4)3- (from apatite) + Cl- (abundant) ⇌
Pb2+5(PO4)3-3Cl- (pyromorphite)
If phosphorous is not available, galena will weather to
anglesite or cerussite depending
on the acidity
(CG).
Solubility of pyromorphite
Pb5(PO4)3Cl (solid) + 6H+ (aqueous) ⇌ 5Pb2+ (aqueous) +
3H2PO-4 (aqueous) + Cl- (aqueous)
(Min Mag June 1989 Vol 53 pp363-371)
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