The Detection and Significance of Moganite in Pliocene Sinter at Plumduff, Northland

Abstract

Raman spectral microsampling, from 100-600 cm-', of clean slabs of Pliocene breccias show moganite, monoclinic SiO,, is disseminated through the Plumduff sinter, that otherwise consists predominantly of microcrystalline quartz. Moganite forms as a metastable phase during the recrystallisation of the sinter from opal-CT and opal-C to quartz and these are the oldest sinters in New Zealand in which moganite has been recognised. The distribution of moganite is uneven e.g. it is absent from recrystallised examples of palisade, tufted network, finely laminated, peloidal and plant-rich primary sinter textures but is present in both fenestral network and columnar types. Its distribution reflects heterogeneity of process that has affected the outcrop since development of the initial sinter apron e.g. hydrothermal activity and weathering. The rapidity of Raman analysis and its ability to microsample individual textural elements allow the twin paths of mineralogical and textural maturation of sinters to be mapped together.