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

Published online by Cambridge University Press:  05 July 2018

R. Martin ,
K. A. Rodgers  and
P. R. L. Browne
R. Martin
Affiliation:
Department of Geology and University of Auckland, Private Bag 92019, Auckland, New Zealand
K. A. Rodgers
Affiliation:
Department of Geology and University of Auckland, Private Bag 92019, Auckland, New Zealand
P. R. L. Browne
Affiliation:
Department of Geology and University of Auckland, Private Bag 92019, Auckland, New Zealand Geothermal Institute, University of Auckland, Private Bag 92019, Auckland, New Zealand
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Abstract

Alunogen and meta-alunogen are the dominant phases present in transient sulphate efflorescences that are the latest products of the alteration of ignimbrite country rocks in the long-lived Te Kopia geothermal field. Meta-alunogen pseudomorphs alunogen and both species occur as white, fibrous, tangled masses, as prismatic, parallel growths, and as thin, platy, crystals, 8–15 μm across, that coalesce in an open cellular network. Small (<2 mm diam.) spherical aggregates of radiating, acicular halotrichite (Fe0.51Mg0.49Al2(SO4)4.22H2O), 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 assemblage reflects the prevailing conditions and ionic activities of a local micro-environment. Kaolinite formed by acid sulphate alteration is now being altered by steam to yield alunogen. In turn, alunogen can react with silica, or co-dissociate with silicic acid, to form kaolinite. The alternating dissolution and reprecipitation of kaolinite and alunogen moves aluminium in and through the surficial environment at Te Kopia.

Keywords

alunogenhalotrichitekaolinitealuminiumgeothermalefflorescencealteration
Type
Research Article
Information
Mineralogical Magazine , Volume 63 , Issue 3 , June 1999 , pp. 413 - 419
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1999

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Footnotes

*

Research Associate, Australian Museum

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