Pyromorphite

Pyromorphite is a lead phosphate ((Pb5(PO4)3Cl ), which occurs in small, hexagonal barrels, intense green to brown.

This mineral was named by Johann Friedrich Ludwig Hausmann in 1813 from the Greek "pyros", "fire", and "morphos" "form", because, after being melted into a globule at high temperature, the globule begins to take on a crystalline form during cooling.

Pyromorphite has made Chaillac's reputation among mineralogists, along with goethite, honey fluorite and alternating deposits of more or less oxidised barite and fluorite.

Pyromorphite is a mineral typical of oxidation zones in sulfide deposits. It formed well after the Rossignol quartz-galena vein, following an episode of exhumation that brought the deposit close to the surface and allowed the denudation of Mesozoic sediment layers.

Oxidising groundwater then penetrated the vein, oxidising the galena and releasing sulfate and lead, which re-precipitated, probably during evaporation cycles linked to the beat of the water table, in the form of lead phosphate, and more rarely wulfenite (a Pb molybdate, PbMoO4), and lead carbonate, cerussite (PbCO3).

 Pyromorphite was mainly encountered in the open zones of the Rossignol vein and, more rarely, in the barite geodes of the stratified part.

This oxidation episode is probably linked to the hot, humid climatic phases of the late Cretaceous and early Tertiary (Eocene), which are synchronous with iron mineralisation.

 ("siderolitic" iron mineralisation (goethite-hematite)).

Two other French deposits have produced remarkable pyromorphites:

They were probably formed under similar conditions and ages by supergene alteration of galena: the fluorite vein (and accessory galena) at Les Farges in Corrèze, with numerous forms and colours (orange to brown, green, skeletal and hollow crystals, etc.) of pyromorphite, and the sphalerite vein (and accessory galena) at Saint-Salvy (Tarn) where pyromorphite is mainly in acicular and green form. Other occurrences that have produced remarkable specimens, albeit in small quantities, include the Vézis mine, fluorite veins in the Vosges, Brittany and Auvergne.

Pyromorphite has been found worldwide in numerous deposits, with China producing some of the finest specimens.