Raman Microscopy of Selected Chromate Minerals

A series of related chromate bearing minerals including crocoite, phoenicocroite, hemihedrite, iranite, macquartite, fornacite and vauquelinite have been analysed by Raman microscopy. These minerals are closely related and often have related paragenesis. Raman microscopy enables the selection of individual crystals of these minerals for Raman spectroscopic analysis. This is of importance as often the crystals are found adjacent to each other in the same matrix through paragenetic relationships. The Raman spectrum of crocoite shows three bands in the CrO4 stretching region at 856, 841 and 825 cm-1, phoenicocroite at 856, 848, 839 and 826 cm-1, hemihedrite at 847, 837 and 824 cm-1, iranite bands at 865, 846 and 818 cm-1, macquartite at 857, 840 and 814 cm-1. The Raman spectra of fornacite and vauquelinite are complicated by the presence or AsO4 and PO4 units in the structure and often the spectra are broad and vary with chemical composition. A comparison of the spectra of these minerals is made with that of the hexavalent chromate mineral edoylerite. Raman microscopy is a powerful and underutilized technique in terms of mineralogy for the study of closely related minerals such as these chromate minerals.