This study investigates the use of short-wavelength infrared reflectance spectroscopy for measuring chlorite physicochemical information of value for determination of very low metamorphic grades. The method is termed chlorite spectral index (CSI) and assessed through quantitative comparison with XRD-derived Árkai and Kübler indices (ÁI and KI, respectively) in a set of 41 pelitic samples. The four CSI measures investigated measure the depth (D) of the absorption features at 1900, 2250 and/or 2350 nm, and are defined as reflectance 2250D/1900D index (CSI(H2O)), reflectance (2350D + 2250D × 1.7)/1900D index (CSI(H2O)sum), reflectance 2250D index (CSI(2250)) and reflectance 2350D + 2250D × 1.7 index (CSIsum). The results demonstrate the applicability of the method: all CSI measures show a good correlation with the ÁI, and respective correlation factors range between R2 = 0.840 (for the CSI(H2O)) and R2 = 0.616 (for the CSIsum). The CSI measures also show a good correlation with the KI with R2 = 0.704 (for CSIsum) to 0.769 (CSI(2250)). By applying the correlation equations to define the metamorphic zone boundaries for each of the CSI measures, 64.3–78.6 % of the samples match the metamorphic grade indicated by ÁI for the epizone, 47.1–70.6 % for the anchizone and 70–80 % for the diagenetic zone. All CSI measures show a good reproducibility, with an intra sample variance ranging between 2.31 % CSI(H2O)sum and 3.34 % (CSI(2250)). CSI is not restricted to pelitic rocks and marls, but can also be applied in very low-grade metabasites. In addition to the existing laboratory method, CSI has application potential in remote sensing applications.