Volatiles in phyllosilicate minerals

Abstract

Volatiles other than water are known to occur in hydrous ring-structure silicates like beryl and cordierite^1,2. However, there has been limited study of volatiles in phyllosilicates. Ammonia has been reported in micas^3,4; Banks⁵ measured sulphur in biotite, sericite and chlorite by electron microprobe; and Norman⁶ reported evolution of H₂S and CO₂ when chlorite, sericite and kaolinite were heated to 500 °C. We have studied the thermal evolution of volatiles from phyllosilicates by mass spectrometry. The gases observed were, in approximate order of their abundance, CO₂, H₂S, H₂, SO₂, NH₃, CO, CH₄, N₂, Ar and He. These gases represent 0.35–9 wt% of the volatiles observed; the remainder is water. Step-heating studies indicate H₂S and CO₂ are not generated by the thermal breakdown of sulphide and carbonate minerals. Rather, they are bound within the phyllosilicate structures in a complex way. We show here that gases in chlorite, muscovite and biotite are related to their mode of origin. Phyllosilicates from hydrothermal ore deposits evolve several times more volatiles, principally CO₂ and H_2/S, than phyllosilicates not from ore deposits.