The presence of K-feldspar with an "adularia habit" in limestones and low-grade marbles is a feature well known to mineralogists (Baskin, 1956; Kastner, 1971; Smith, 1974, p. 258-261; Hearn et al., 1986), but has been little studied by sedimentary petrologists. The term "adularia habit" (Smith, 1974; p. 260) indicates that the crystals of K-feldspar exhibit a rhombohedral habit, yielding diamond- and hexagonal-shaped cross sections, and does not imply a specific structural state. These crystals are generally extremely small and rarely exceed 1 mm (Reynolds, 1929; Baskin, 1956; Glover and Hoseman, 1970; Kastner, 1971; Hearn et al., 1986). Mallick (1965), however, described porphyroblasts of K-feldspar up to 10 mm long from marbles in the Late Precambrian Katanga Sequence in Zambia. The origin of these crystals is generally ascribed either to authigenesis (Kastner, 1971; Kastner and Siever, 1979) or to circulating high saline brines (Hearn et al., 1986). The term "authigenic" is here taken to mean in situ formation between deposition of the sediment and a subsequent episode of low-grade metamorphism (Kastner, 1971). Generally, authigenic albite is more common than K-feldspar in carbonate rocks (Kastner, 1971; Kastner and Siever, 1979). This note describes the occurrence and chemical composition of porphyroblastic K-feldspar from a new locality in low-grade marbles of the Pan-African Damara Sequence in the Outjo District, Namibia.