VAPOUR DEPOSITION OF ARSENIC-BEARING MINERALS ORIGINATING FROM A BURNING CULM BANK: ST. ETIENNE, THE LOIRE REGION, FRANCE

As-bearing minerals including realgar (As₄S₄), pararealgar (As₄S₄), orpiment (As₂S₃), arsenolite (As₂O₃), native arsenic, and an unidentified Sb-Bi-As-sulphur compound associated with salammoniac (NH₄C_l), mascagnite (SO₄)₂NH₄, native sulphur, and a fluorine-sulphate compound have been identified in a burning culm bank in the Loire region in St. Etienne, France. Field observations and mineral compositions suggest that vapour-solid deposition is the principal mechanism for the formation of these minerals. The minerals crystallized around the outside of gas vents and inside culm-bank fissures at a depth of 0 - 40 cm below the surface. Nucleation temperatures from a cooling gas were 94.5 - 572.70 °C (near the upper limit of mineral stability).

Ion-coupled plasma mass spectrometry and energy dispersive spectrometry-scanning electron microscopy analyses of unbaked shales, the main component of the culm bank, revealed that they are naturally rich in arsenic and sulphur (pyrite and/or arsenopyrite), the primary source of arsenic and sulphur in the coal-fire minerals, whereas porcellainites (baked shales) contain higher concentrations of iron-oxides (mainly hematite). The oxidation of sulphide minerals in the shales during baking possibly mobilized arsenic and sulphur into a gas phase, concentrating iron oxides in the porcellanites. Fluorine originated from muscovite and/or fluorapatite in the shales and antimony is thought to have originally been in solid solution with the Fe-sulphides. The source of bismuth is unknown.

The minerals described herein were formed from harmful coal-fire gas components (NOx, SOx, AsOx, HCl, etc.), harmful to human health. The minerals are soluble in water and a potential environmental hazard because of the slow but prolonged release of their toxins into the hydrologic system. Consequently, the study of minerals that form from burning culm banks is worthy of intense research.