Infrared and Raman spectroscopic characterization of the borate mineral colemanite CaB3O4(OH)3 H2O - implications for the molecular structure.

Abstract

Colemanite CaB3O4(OH)3_H2O is a secondary borate mineral formed from borax and ulexite in evaporate deposits of alkaline lacustrine sediments. The basic structure of colemanite contains endless chains of interlocking BO2(OH) triangles and BO3(OH) tetrahedrons with the calcium, water and extra hydroxide units interspersed between these chains. The Raman spectra of colemanite is characterized by an intense band at 3605 cm_1 assigned to the stretching vibration of OH units and a series of bands at 3182, 3300, 3389 and 3534 cm_1 assigned to water stretching vibrations. Infrared bands are observed in similar positions. The BO stretching vibrations of the trigonal and tetrahedral boron are characterized by Raman bands at 876, 1065 and 1084 cm_1. The OBO bending mode is defined by the Raman band at 611 cm_1. It is important to characterize the very wide range of borate minerals including colemanite because of the very wide range of applications of boron containing minerals.