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The occurrence, detection and significance of moganite (SiO2) among some silica sinters

Published online by Cambridge University Press:  05 July 2018

K. A. Rodgers  and
G. Cressey
K. A. Rodgers*
Affiliation:
Department of Geology, University of Auckland, Private Bag 92019 Auckland, New Zealand
G. Cressey
Affiliation:
Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK
*
*E-mail: [email protected]
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Abstract

Moganite, monoclinic SiO2, is a component of microcrystalline, quartz-bearing, sinters of New Zealand derived from crystallization of non-crystalline and paracrystalline opaline silicas. It occurs at levels of <13 vol.% of the SiO2 phases present in sinters between 20,000 and 200,000 y old but is generally either absent or below the level of detection in Tertiary sinters. Unambiguous identification of moganite is most readily accomplished by laser Raman spectroscopy; the technique allows individual microtextural elements of a sinter's fabric to be analysed. Conventional scanning X-ray powder diffraction procedures are limited in their ability to discern the characteristic moganite diffraction lines from the very similar quartz pattern, especially in those samples where moganite is at low concentration and/or unanticipated. However, powder diffraction, using a position-sensitive detector system, allows not only the identification of the moganite pattern in the presence of a large proportion of quartz, but also semiquantitative estimates of the different silica phases present in bulk sinter samples of ~450 mg. Moganite is part of the sinter maturation sequence. It occurs as a metastable phase that will ultimately transform to quartz, given sufficient time or a change in ambient conditions.

Keywords

moganitequartzsilicasinterRamanposition-sensitive detector
Type
Research Article
Information
Mineralogical Magazine , Volume 65 , Issue 2 , April 2001 , pp. 157 - 167
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2001

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