Vibrational spectroscopy of the borate mineral gaudefroyite Ca4Mn3-x3+ (BO3)3 (CO3) (O;OH) from N’Chwaning II mine, Kalahari, Republic of South Africa.

Abstract

Gaudefroyite Ca4Mn3-x^3+ (BO3)3 (CO3) (O;OH) is an unusual mineral containing both borate and carbonate groups and is found in the oxidation zones of manganese minerals, and it is black in color. Vibrational spectroscopy has been used to explore the molecular structure of gaudefroyite. Gaudefroyite crystals are short dipyramidal or prismatic with prominent pyramidal terminations, to 5 cm. Two very sharp Raman bands at 927 and 1076 cm^-1 are assigned to trigonal borate and carbonate respectively. Broad Raman bands at 1194, 1219 and 1281 cm^-1 are attributed to BOH in-plane bending modes. Raman bands at 649 and 670 cm^-1 are assigned to the bending modes of trigonal and tetrahedral boron. Infrared spectroscopy supports these band assignments. Raman bands in the OH stretching region are of a low intensity. The combination of Raman and infrared spectroscopy enables the assessment of the molecular structure of gaudefroyite to be made.