Detecting Mineralogy Gradients of White Micas

A diverse group of fine-grained phyllosilicate minerals known as white micas, including the true micas paragonite, muscovite, and phengite, as well as the K-deficient mica, illite, provide an indicator of geothermal and geochemical conditions at the time of their formation, making them a useful exploration vector. Geochemical conditions can be tracked using the Al-OH scalar reported by TerraSpec Halo mineral identifier while the Illite Spectral Maturity (ISM) scalar (Doulblier et al. 2010) reported by Halo can provide an indicator of thermal maturity.

The TerraSpec Halo mineral identifier uses white micas contained in the mineral library organized by composition. The potassium white micas muscovite and phengite, and the sodic white mica paragonite represent the true white micas. Increasing substitution of Mg+2 or Fe+2 and Si+4 for Al+3 characterizes the muscovite— phengite series. The illites are classified similarly. Increasing sodic conditions are, therefore, indicated by a shift in mineralogy from muscovite and/or illite (labeled as K-Illite in the Halo mineral library) to paragonite and/or paragonitic illite (labeled as Na-Illite in the Halo mineral library).

Likewise, increasingly magnesium-rich conditions are indicated by a shift in mineralogy to phengite and/or phengitic illite (labeled as Mg-Illite in the Halo mineral library).

Geothermal gradients can be tracked with the TerraSpec Halo mineral identifier by detecting mineralogy and by reporting the ISM scalar. Smectites convert to illites with increasing metamorphic grade. These are then converted to muscovite (and other true micas). Subtle geothermal trends are easily detected within the illites and mapped through the Halo’s ISM scalar. As the illite converts to true mica, this tracks the illite dehydration process.

This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.

For more information on this source, please visit Malvern Panalytical.

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