H2O AND OH IN ECHINOID CALCITE - A SPECTROSCOPIC STUDY

Proton nuclear magnetic resonance (NMR) and visible and near-infrared (VNIR) reflectance spectral data and heating experiments indicate that the Mg-rich calcite (commonly referred to as high-Mg calcite and hereafter as HMC) that is produced by six different species of echinoids contains H2O with mobilities and H bond strengths ranging from those characteristic of liquid H2O to those observed in hydrated magnesium carbonate minerals. Total H2O (inorganic OH- and H2O, i.e., that occurring in inorganic phases rather than organic compounds) contents of echinoid HMC range from 1.97 to 3.38 wt% and differ in amount from one species to another and, within one species, between different skeletal elements. The proportion of total H2O that is rigidly bound is comparatively constant, similar to 0.4 in all samples. Total H2O content shows no correlation with Mg/Ca ratio in these samples. Although echinoid ossicles are generally noted for their chemical homogeneity, the total H2O content of echinoid HMC shows considerable variation within individual ossicles over distances of tens to hundreds of micrometers. Echinoid HMC contains small amounts of OH- (a few tenths to hundredths of a percent by weight). Brucite does not occur, and OH- is present either in solid solution or in a hydrated magnesium carbonate mineral.