Identification of biogenetic calcite and aragonite using SEM

Abstract

Identification of calcite and aragonite is very important for studying different fossil or recent biomineralized skeletons. A problem occurs when scanning electron microscopy is used for studying calcite and aragonite present in the same part of skeleton. The same chemical composition of these two minerals produces the same contrast on SEM images. There are three possible ways to distinguish calcite and aragonite in such a mixture. (1) It is possible to recognize the crystal habits of these two minerals, if crystal faces are developed. (2) The geochemical difference can also be an important tool for distinguishing aragonite (containing large cations like Sr, Ba, or Pb) from calcite (containing small cations like Mg, Mn, Fe, Zn, or Ni). However, it is also possible that large cations remain in the calcite crystal structure after phase transformation from aragonite to calcite. (3) The third possibility for distinguishing calcite from aragonite is to use staining methods. Different staining methods used for SEM analyses have had varying degrees of success. SUZUKI et al. (1993) successfully used Meigen's staining method, but results provided by Feigl's staining method were unsatisfactory. The failure when using Feigl's staining method was caused by the erroneous application of the solutions.