Phylloid algae in the Pennsylvanian Providence Limestone (Desmoinesian) in western Kentucky are typical in appearance: thin, arcuate, leaflike forms with a calcite mosaic interior. Cathodoluminescence and backscattered electron imaging reveal well-preserved skeletal structure and their identity as Archaeolithophyllum sp. Johnson, 1956 and A. missouriensum Johnson, 1956. Thin skeletal walls are composed of nonluminescent neomorphic calcite. Polygonal hypothallial and perithallial cells in the skeletons are filled by luminescent calcite cement. Chemical staining also distinguishes cell morphology when ferroan calcite cement is present in skeletal pores, in contrast to the nonferroan calcite forming skeletal walls. Hypothallial cells in Archaeolithophyllum are 25 mu m x 50 mu m and are commonly arranged in arcuate layers. Skeletal walls (5 mu m thick) are composed of calcite and lack distinctive relict features, although some are microporous and contain fine, disseminated pyrite. Some thalli contain large irregularly shaped hypothallial cells (50-100 mu m) partitioned by skeletal walls. These are interpreted as poorly calcified thallus fragments. Neomorphic calcite of the skeletons contains high concentrations of Sr (average 3702 ppm, range 339-10,549 ppm), strongly suggesting that the original Archaeolithophyllum skeleton was aragonitic. Archaeolithophyllum shares morphological, structural, and compositional characteristics with Holocene peyssonnelid algae. This evidence substantiates earlier hypotheses that Archaeolithophyllum may be an ancestral peyssonnelid alga. Calcite cements filling skeletal cells include a bright-yellow-CL phase and two subsequent phases of dull-red and dull-yellow luminescence. Mg, Mn, and Fe are enriched in these calcites, reflecting reducing diagenetic fluids particularly enriched in Fe. Cementation of skeletons by ferroan calcite during early diagenesis appears to have protected skeletons from dissolution and promoted slow rates of neomorphism. Conditions of a nearly closed system are constrained by the high retention of Sr in the skeletal calcite.