The nature and significance of sulphate-rich, aluminous efflorescences from the Te Kopia geothermal field, Taupo Volcanic Zone, New Zealand

Alunogen and meta-alunogen are the dominant phases present in transient sul phate efflorescences that are the latest products of the alteration of igni mbrite country rocks in the long-lived Te Kopia geothermal field. Meta-alun ogen pseudomorphs alunogen and both species occur as white, fibrous, tangle d masses, as prismatic, parallel growths, and as thin, platy, crystals, 8-1 5 mu m across, that coalesce in an open cellular network. Small (<2 mm diam .) spherical aggregates of radiating, acicular halotrichite (Fe0.51Mg0.49Al 2(SO4)(4). 22H(2)O), potash alum, mirabilite, melanterite and tschermigite are present locally. The cations needed to form these minerals derive from the host rocks with the exception of sulphur and ammonia that come from H2S and NH3 gases ascending with steam. The particular efflorescence assemblag e reflects the prevailing conditions and ionic activities of a local micro- environment. Kaolinite formed by acid sulphate alteration is now being alte red by steam to yield alunogen. In turn, alunogen can react with silica, or co-dissociate with silicic acid, to form kaolinite. The alternating dissol ution and reprecipitation of kaolinite and alunogen moves aluminium in and through the surficial environment at Te Kopia.