This report presents the results of X-ray diffraction (XRD) analyses of samples from nine sites cored during Ocean Drilling Program Leg 204 (Tréhu, Bohrmann, Rack, Torres, et al., 2003). Based on the stratigraphic and structural setting, the sites can be grouped into three different environments. Sites 1248–1250 on the southern summit of Hydrate Ridge represent locations of active seafloor seepage. At Site 1248, high backscatter reflectivity was interpreted to indicate authigenic carbonates at the seafloor (Johnson et al., 2003). Characteristics for Site 1249 are massive gas hydrate accumulations at the seafloor accompanied by carbonate precipitates (Bohrmann et al., 1998; Teichert et al., 2005). Site 1250 is located between the summit and a prominent chemoherm called the Pinnacle. Sites 1244–1247 represent the flanks of southern Hydrate Ridge, and Sites 1251 and 1252 represent a slope basin to the east. Authigenic carbonates have been previously recognized at the Cascadia margin (e.g., Sample and Kopf, 1995) and may serve as a record of the fluid evolution at these sites. Therefore, mineral and chemical composition of authigenic carbonates may help to evaluate the diagenetic environment and the paleofluid composition during carbonate precipitation.