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

Moganite, monoclinic SiO2, is a component of microcrystalline, quartz-beari ng, sinters of New Zealand derived from crystallization of non-crystalline and paracrystalline opaline silicas. It occurs at levels of <13 vol.% of th e SiO2 phases present in sinters between 20,000 and 200,000 y old but is ge nerally either absent or below the level of detection in Tertiary sinters. Unambiguous identification of moganite is most readily accomplished by lase r Raman spectroscopy; the technique allows individual microtextural element s of a sinter's fabric to be analysed. Conventional scanning X-ray powder d iffraction procedures are limited in their ability to discern the character istic moganite diffraction lines from the very similar quartz pattern, espe cially in those samples where moganite is at low concentration and/or unant icipated. However, powder diffraction, using a position-sensitive detector system, allows not only the identification of the moganite pattern in the p resence of a large proportion of quartz, but also semiquantitative estimate s of the different silica phases present in bulk sinter samples of <similar to>450 mg. Moganite is part of the sinter maturation sequence. It occurs a s a metastable phase that will ultimately transform to quartz, given suffic ient time or a change in ambient conditions.