Feldspar mineralogy, reported as concentrations of KAlSi 3 O 8 , NaAlSi 3 O 8 , and CaAl 2 Si 2 O 8 , can be determined from FTIR spectroscopy. In this study end-member FTIR spectra were constructed from sample spectra and their associated K-, Na- and Ca- feldspar concentrations using a least-squares full spectrum program. This was necessary because none of the samples have single-phase, pure end-member composition. The K-, Na-, and Ca-feldspar concentrations are computed by fitting the sample FTIR spectrum to the end-member FTIR spectra using a nonnegative least-squares program. For the feldspar samples examined we found that when the compositions determined from infrared spectroscopy are compared to the compositions derived from chemistry, the standard error of estimate for K-, Na-, and Ca-feldspar is 1.0, 0.9, and 0.7 absolute mole %, respectively.