Application of Raman spectroscopy to distinguish adularia and sanidine in drill cuttings from the Ngatamariki Geothermal Field, New Zealand

Abstract

Adularia and sanidine are polymorphs of potassium feldspar commonly present in felsic, hydrothermally altered volcanic deposits. Sanidine is a high-temperature volcanic mineral, whereas adularia forms post deposition by hydrothermal processes. Petrographically differentiating between these polymorphs in hydrothermally altered volcanic rocks may be utilised to distinguish geological units as well as provide insights into fluid-rock interactions. However, petrographic identification may be difficult or not possible in fine-grained drill cuttings. Here, polymorphic-sensitive, Raman spectroscopy and electron microprobe analyses are utilised to characterise adularia and potential sanidine in drill cuttings from the Ngatamariki Geothermal Field, Taupo Volcanic Zone, New Zealand. Differences in Raman spectra are capable of distinguishing between adularia and sanidine whether using peak positions or principal component analysis. All the Ngatamariki Geothermal Field potassium feldspars analysed by Raman spectroscopy were found to be adularia, as expected, with typical high K, low Ca compositions between Or 94 and Or 99 confirmed with electron microprobe analyses. This applied approach demonstrates that Raman spectroscopy is a fast and effective method for lending confidence to adularia and sanidine identification, which can be utilised in geothermal fields worldwide.

Concentric Structures and Hydrothermal Venting in the Western Desert, Egypt

“…Its euhedral habitus reminds of adular, but the X-ray analysis identified it as K-feldspar phase. Sanidine may be rimmed by K-feldspar generation GII and was detected by XRD and confirmed by EDS analysis with up to 5% Na 2 O (Eastwood et al, 2015). In the proposed paragenetic diagram this mineral crystallizes before K-feldspar generation GII, but the context of its crystallization could not be well constrained.…”

The Whakamaru magmatic system (Taupō Volcanic Zone, New Zealand), part 1: Evidence from tephra deposits for the eruption of multiple magma types through time

“…The TVZ is a northeast-southwest rifted arc in the North Island of New Zealand (Figure 1) (Wilson et al, 1995). The central TVZ is one of the most active silicic volcanic systems in the world (Houghton et al, 1995;Wilson et al, 1995), having produced at least 6000 km 3 of silicic magma over the last ~1.6 Ma , with silicic activity starting at ~1.9 Ma (Eastwood et al, 2013;Chambefort et al, 2014). Over this time, there have been three ignimbrite flare-up periods in the TVZ, which were especially intense periods of ignimbrite-forming volcanism (Houghton et al, 1995).…”

A Mineralogical, Geochemical, and Geochronological Study of ‘Valencianite’ from La Valenciana Mine, Guanajuato, Mexico

“…For practical reasons, we do not adhere in this work to the IMA rule stating that non-official mineral names or varieties must be written in italics or between quotation marks; the terms adularia and valencianite are written in plain text from now on. Adularia is found in many geological settings, but it is commonly unnoticed in optical microscopy and X-ray diffraction analyses due to their similarity with the more common orthoclase and sanidine (e.g., Eastwood et al [16]). Nevertheless, many plutonic, metamorphic, and sedimentary rocks contain adularia crystals or overgrowths formed during cooling or diagenesis in the presence of fluids.…”