Pyromorphite

phosphate

cerussite

Formula: Pb5(PO4)3Cl phosphate
Isostructural with mimetite and vanadinite
Specific gravity: 7.04
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; colourless or faintly tinted in transmitted light. colourless when pure
Solubility: Slightly soluble in hydrochloric acid and sulphuric acid; moderately soluble in nitric acid and KOH; slightly soluble in carbonated water (Mindat)
Environments:

Hydrothermal 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.

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 sandstones 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 Ingray Gill, Cumbria, England, 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 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 Burgham mine, Shelve, Shropshire, England, UK, pyromorphite is associated with baryte and quartz (RES p283).

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)

Common impurities: F,Ra,Ca,Cr,V,As

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