The illite spectral maturity (ISM) method uses short-wavelength infrared reflectance spectroscopy (SWIR) to measure K-white mica (KWM) physicochemistry within very low-grade metamorphic pelites. The three ISM measures used in this study parameterize KWM absorption features at 1900 nm and 2200 nm in terms of area, depth, and asymmetry. Through comparison with the powder X-ray diffraction (XRD)−derived Kübler index, we demonstrate that ISM differentiates anchizonal and epizonal from diagenetic domains in very low-grade pelites. The wavelength (wvl) of the 2200 nm absorption feature (2200wvl) provides a measure of the celadonite substitution in KWM. It shows a linear correlation (R2 = 0.85) with the KWM b cell dimension (as determined by powder XRD), and can be used to differentiate the metamorphic pressure facies and related metamorphic thermal gradients in pelites of greenschist facies and anchizonal metamorphic grade. The boundaries between low/medium-pressure facies and medium/high-pressure facies series can be defined at 2204 and 2220 nm, respectively. In addition to their use as laboratory-based techniques, both ISM and 2200wvl show potential for remote sensing studies.