Upper-shoreface sand samples were collected from 15 sites around the island. In addition, a nearly 2 m (78 in) sediment core was collected from offshore of Cat Island at a subsurface depth of approximately 3 m using a Vibracore sampler (Fig. 1). The mineralogy of the samples was determined using standard petrographic microscope techniques and X-ray diffraction (Lynch, 1997). High-energy upper- and middle-shoreface, surface, and subsurface sands are composed of quartz and 0-5% potassium feldspar and plagioclase feldspar regardless of sample depth or location on the main shore-parallel island body or on the longshore reworked "T." However, the lower 1.5 m (60 in) of the offshore core consists of clay-rich sand with an average mineralogy of 34% clay, 52% quartz, and 14% feldspar. These offshore sands coarsen upwards into middle-shoreface sands with little clay at the top of the core.

Carbonate grains were removed from the samples by washing in 10% acetic acid, and the samples were split into < 60 mesh and > 60 mesh size fractions. Heavy minerals in the <60 mesh fractions were concentrated by differential settling in a 2.85 g/cc sodium polytungstate solution, and 300 translucent grains were identified in each sample. The translucent heavy mineral suite from the shore-parallel island body averages 22% kyanite, 26% staurolite, and lesser amounts of apatite, zircon, tourmaline, and other minerals (Fig. 2). Samples from the longshore reworked "T" average 25% kyanite, 27% staurolite, and 48% other minerals. Three samples from the lower portion of the core average 28% kyanite, 23% staurolite, and 49% other minerals, and two samples from the upper portion of the core average 25% kyanite, 21% staurolite, and 54% other minerals. This heavy mineral suite is very similar to that found on other barrier islands and beaches in the Northern Gulf of Mexico. Abundant epidote and amphibole, with smaller amounts of garnet, zircon, titanite (sphene), staurolite, and other minerals characterizes the heavy mineral suite of Mississippi River sediment (Fig. 2). The heavy mineral suite from Cat Island implies that the most probable provenance of the sediment is the crystalline metamorphic region of the south-ern Appalachians.

The mineralogy of the core samples changed as the result of westward longshore currents reworking the original shore-parallel island into the unusual "T" shape. The samples became coarser grained and clay-poor as a more high-energy environment migrated over the originally low-energy offshore setting. Concurrently, the clay mineral composition changed from a suite with subequal amounts of illite/smectite and illite (~40% each) and lesser amounts of kaolinite and chlorite (~10% each), to a suite with abundant illite (~50%) and subequal amounts (~10-20% each) of kaolinite, chlorite, and illite/smectite (Fig. 3). The likeness of the heavy minerals present in both the low-energy, clay-rich distal core samples, and the high-energy, clay-poor nearshore core samples indicates that the clay mineral variation was not due to a change in provenance.