| SHELL MATERIAL: Recent shell material, Belize. Two SEM images showing the structure of the nacreous layer of a bivalve shell at different magnifications. Nacre represents the pearly material found in some mollusks (pearl oyster shells and pearls, pearly nautiloids, and a variety of aragonitic bivalves including Nucula, Nuculana, Pinctada, and Pteria). As can be seen here, nacre consists of stacked overlapping, vertical columns of tabular aragonite crystals that are separated by extremely thin sheaths of organic material. Although this structure is lost during diagenesis, it is useful in identification of modern shell fragments. Chapter 9: GRAINS: Skeletal Fragments Mollusks; p. 166. Memoir 77: A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis by Peter A. Scholle and Dana S. Ulmer-Scholle |