Microestructura y polimorfismo en valvas recientes de Diplodon chilensis patagonicus (d'Orbigny 1835)

Abstract

Thanks to its potential as environmental archive, the valves of the freshwater clam Diplodon recently gained the attention of the scientific community. In this work, Scanning Electron Microscopy (SEM) and Raman Spectroscopy were used to investigate the material microstructure and polymorphism of recent Diplodon chilensis patagonicus shells. It was found that the shell is completely built of aragonite. In addition, the external nacreous layer shows to be the most appropriate to do stable isotopes and trace elements measurements. Analyses with confocal microscopy and SEM demonstrated that an annual increment is composed of two bands: one of lighter colors deposited during the cold months and one of darker colors deposited during the summer time. Both bands are nacre, composed of polygonal aragonite platelets of approximately 5-7 μm diameter and 0.6-2 μm thickness. The platelets have its "c"-axis (crystal group: orthorhombic) parallel to the shell surface and are bound together with an inter-crystalline organic material, translucent to the electrons. The nacre deposited in the annual growth mark shows thinner platelets, coinciding with the hypothesis of a decelerated growth associated to the reproduction time. As consequence, the valve presents there higher organic to CaCO3 ratios and explains why the annual mark is dyed more sharply when using Alcian blue.