Mineralogy and geochemistry of paralavas in Otago and Southland, New Zealand


Rait, Rachel Jane, 1970-


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Geochemical and mineralogical studies of paralavas and baked sediments were carried out on samples from Otago and Southland, New Zealand.

Paralavas in this study contained plagioclase, pyroxenes, quartz, cristobalite, cordierite, mullite, magnetite, hercynite, glass and some unidentified phases. Wide compositional ranges occur for the glasses, pyroxenes and spinels.

Pyroxenes display calcium tschermak and esseneite substitution. All pyroxenes are calcium rich clinopyroxenes. Plagioclases range from An5oAngo. Iron rich cordierites are present in samples from Croydon Coal Mine, Southland. A variety of cordierite morphologies are exhibited including hexagonal, spiral, skeletal, parallel cluster and multiple terminated grains. A complete range of glass compositions occur from 50wt% Si02 to 80wt% Si02 with respective decreases in iron contents.

Quench textures are displayed by the spinels implying rapid cooling. The environment of formation for the paralavas involved disequilibrium and high temperatures. Cause of combustion may be by spontaneous combustion within the coal, bush fires or lightning.

Paralavas have only been studied to a limited extent and more detailed study is needed to gain a complete picture of paralavas formation, chemistry and geological importance.

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vi, 132, (1) leaves (some folded) : ill. (some col.), maps ; 30 cm.


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Rait, Rachel Jane, 1970-, “Mineralogy and geochemistry of paralavas in Otago and Southland, New Zealand,” Otago Geology Theses, accessed March 1, 2021, http://theses.otagogeology.org.nz/items/show/268.

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