By diagonalizing the complete energy matrices for a d(5) configuration ion in a trigonal ligand-field and considering the second-order, fourth-order EPR parameters D and (a-F) simultaneously, the substitution position of Mn2+ ion in the CaCO3:Mn2+-system has been studied by analyzing the optical absorption and EPR spectra of single crystal and the ostracum layer of operculum of fresh water snail Pila globosa. The results show that in the single crystal there is an obvious compressed distortion Delta R = -0.158 angstrom and Delta theta = 0.70 degrees, and the distortion in P. globosa is Delta R = -0.107 angstrom and Delta theta = -0.34 degrees similar to -0.23 degrees. Meanwhile the EPR parameters 10(4) (a-F) = 7.02-7.29 cm(-1) and 10(4) a = 6.35 cm(-1) are predicted in P. globosa. (c) 2006 Elsevier B.V. All rights reserved.