Clay Mineralogy of Weathered Products and of River Sediments, Puerto Rico

Abstract

ABSTRACT The island of Puerto Rico is composed primarily of an east-west mountain range of andesitic to basaltic volcanics with lesser amounts of calcareous sediments along the north and south shores. The weathering of the volcanics in humid areas produces a soil composed of halloysite, gibbsite, and quartz, whereas weathering in more arid areas produces a soil rich in montmorillonite. The northward flowing Rio Manati drains a volcanic area largely covered by lateritic soils containing halloysite, gibbsite, and quartz; the mineralogy of the stream load, however, shows montmorillonite in addition to halloysite and quartz. The load of the south-flowing Inabon-Caomo river group (on the lee side of the island), derived from a petrologic source area similar to that of the Rio Manati, has a high montmorillonite content with lesser amounts of mixed-layer montmorillonite-vermiculite, vermiculite, and halloysite. The Rio Farajardo draining a volcanic area with high rainfall contains halloysite and vermiculite primarily. The Rio Guayanes, draining a generally wet, granodiorite area, contains kaolinite primarily with lesser amounts of vermiculite and montmorillonite. The Rio Guanajibo, draining an arid to wet, volcanic area, contains mixed-layer montmorillonite-vermiculite primarily, with lesser amounts of halloysite, chlorite, and serpentine. The rivers of Puerto Rico reflect the climate conditions in their watershed insofar as the characteristic top-soil components in their loads is concerned. The mineralogy of the stream load from lateritic source areas, however, consists of, in addition to halloysite, clay minerals such as montmorillonite which originate in lowermost soil horizons; these clay mineral do not represent phases at equilibrium with the climate. Because of the small volume of clay relative to the larger volume of mineralogically unstable volcanic sand in the rivers, however, the bulk sediment being deposited off-shore provides only slight indication of climatic conditions on-shore. This observation indicates that ancient source area climatic conditions inferred from clay fractions of sedimentary rocks are always tenuous, particularly in view of the fact that some (if not all) of the clay fraction of a rock may he derived from altered unstable sand grains.