The white micas are a diverse group of common fine-grained phyllosilicate minerals that include the true micas paragonite, muscovite, and phengite, as well as the K-deficient mica, illite. This group often serves as a useful exploration vector as they provide an indicator of geothermal and geochemical conditions at the time of their formation. Using the Al-OH scalar reported by the ASD TerraSpec® Halo mineral identifier it is possible to track geochemical conditions while the Illite Spectral Maturity (ISM) scalar (Doulblier et al. 2010) reported by Halo provides an indicator of thermal maturity.
The white micas contained in the mineral library used by the ASD TerraSpec Halo mineral identifier are organized by composition. The true white micas are represented by the potassium white micas muscovite and phengite, and the sodic white mica paragonite. The muscovite-phengite series is characterized by increasing substitution of Mg+2 or Fe+2 and Si+4 for Al+3. The illites are classified similarly. Thus, 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) is indicative of increasingly sodic conditions. Similarly, a shift in mineralogy to phengite and/or phengitic illite (labeled as Mg-Illite in the Halo mineral library) is usually indicative of increasingly magnesium-rich conditions.
The ASD TerraSpec Halo mineral identifier is able to track geothermal gradients both by detecting mineralogy and through the reporting of the ISM scalar. With increasing metamorphic grade, smectites convert to illites, which then covert to muscovite (and other true micas). Within the illites, subtle geothermal trends are easily detected and mapped using the Halo’s ISM scalar which tracks the illite dehydration process as the illite converts to true mica.