The Biga Peninsula of NW Turkey is host to many kaolin and halloysite deposits with mineralization occurring at the intersections of fault zones in contact with Late Eocene-Miocene calc-alkaline volcanic rocks. Distinguishing between the relative overprinting of hypogene by supergene processes in these deposits is a challenge and important because they affect the physical-chemical properties of minerals and their potential for industrial applications. This study examines the Saribeyli-Sigirli and Bodurlar kaolin deposits in NW Turkey, which were formed from similar volcanics as evidenced by Ar-40/Ar-39. Late Eocene (34.2 +/- 0.20 Ma) to Early Oligocene (32.7 +/- 0.17 Ma) ages for both primary volcanic rocks and alunites are consistent with surrounding rocks in the C, anakkale region. Criteria used to distinguish hypogene alteration from supergene alteration processes come from X-ray diffraction (XRD), Fourier-transform infrared (FTIR) and Raman spectroscopies, thermal gravimetric analysis (TGA), scanning and transmission electron microscopy (SEM, TEM), and elemental analyses. Isotopic delta O-18 depletion and delta D enrichment of the Saribeyli-Sigirli deposit suggests that it was more influenced by magmatic waters than was the Bodurlar deposit. The Bodurlar deposit contains a paucity of dickite compared to the Saribeyli-Sig. irli deposit, which is evidenced by lower TGA endotherms, higher ratios of XRD intensities for reflections at 1.316 angstrom and 1.307 angstrom, distinctive FTIR absorbance bands at 3620 cm(-1) and 3652 cm (-1), and relative Raman intensities of the v(1) and v(5) vibrational modes.