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Laser Raman identification of silica phases comprising microtextural components of sinters

¹ Research Associate, Australian Museum, College Street, Sydney, NSW, Australia
² Department of Geology, University of Auckland, Private Bag 92019, Auckland, New Zealand

^* E-mail: wendy12{at}clear.net.nz

Laser Raman spectroscopy is particularly well suited for routine characterization of the crystalline and the better ordered paracrystalline silica phases occurring in silica sinters: opal-C, quartz and moganite. The non-crystalline and poorly ordered paracrystalline silica phases (opal-CT and opal-A) are identified with difficulty, partly as a result of the high level of fluorescence exhibited by the younger sinters that are dominated by these phases, and partly as a result of the ill-defined nature of the broad phonon scattering bands produced by them. Nonetheless, given the limited number of phases present and the high level of phonon scattering from the better ordered phases, judicious use of a Raman microprobe enables the character of most siliceous microtextural sinter components to be determined readily. Microprobe examination of a series of New Zealand sinters, ranging from Late Quaternary to Pliocene in age shows that a silica phase inhomogeneity that exists in some outcrops reflects an underlying phase heterogeneity obtaining in the microstructure of the sinter; it is a consequence of the phase transformation process. In the older sinters, quartz, moganite and opaline cristobalite may be present in a single fabric element but in quite different proportions to their abundance in adjacent elements. Identification of opaline lepispheres partially pseudomorphed by quartz+moganite and the association of quartz+moganite in microcrystals exhibiting common quartz forms, cautions against the use of morphology as a means of identifying silica phases at the microstructural level.

KEYWORDS: silica, sinter, laser Raman spectroscopy, opal, New Zealand

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