Chlorite and corrensite are very common secondary minerals in very low-grade metamorphic or hydrothermally altered mafic rocks. Corrensite, although defined as a 1:1 regularly mixed-layered chlorite/smectite or chlorite/vermiculite, has been considered as a unique mineral phase and might thus be a potential index mineral for evaluating very low metamorphic grade. Many lenticular basaltic rocks, which are intercalated with unmetamorphosed to low-grade metamorphosed Tertiary sedimentary rocks, occur sporadically in the Western foothills and the Central Range in Taiwan. The lenticular basaltic rocks in the Western foothills (diagenetic zone) and the western flank of the Central Range (anchizone to epizone) were subjected to different degrees of hydrothermal alteration and/or metamorphism. However, only few occurrences of corrensite in these basaltic rocks were reported. In the present study, the occurrences and mixed-layering features of corrensite and coexisting chloritic minerals in the basaltic rocks were characterized by utilizing optical microscopy, X-ray diffraction, scanning electron microscopy with energy dispersive spectrometry, and transmission electron microscopy (TEM). The results showed that most of these basaltic rocks contain abundant chloritic minerals occurring as replacements of mafic minerals or interstitial glass, or as vein- or vesicle-filling materials. The chloritic minerals include smectite, corrensite, chlorite, mixed-layered chlorite/smectite, or mixed-layered chlorite/corrensite. The compositions of chlorite, corrensite, and smectite are distinctive in terms of their Si/Al ratios, interlayer cations, and total cation numbers. Corrensite, chlorite, and mixed-layered chlorite/corrensite are common secondary mineral assemblages in the anchizone basaltic rocks, whereas only discrete chlorite occur in the epizone. The basaltic rocks in the diagenetic zone contain different assemblages, such as smectite only, chlorite + smectite + mixed-layered chlorite/smectite + mixed-layered chlorite/corrensite, or chlorite + corrensite + mixed-layered chlorite/corrensite. These different assemblages of chloritic minerals and other secondary mineral assemblages might reflect different stages of hydrothermal alteration. Corrensite was positively identified with TEM lattice-fringe imaging in the diagenetic rocks collected from Nangang, Poneikeng, Shanzijiao, Nanshihjiao, Hsiungkong, Shihsiouping, Fusing, and Northern Shihmen Reservoir areas, and in the studied anchizone rocks from Junghua, Kaopo and Baolai areas. The disappearance of corrensite clearly marks the boundary between the anchizone and epizone basaltic rocks. Corrensite, as a chemically and structurally unique mineral phase, is therefore a potential index mineral in very low-grade metamorphic rocks.