Raman spectroscopic characteristics of Mg-Fe-Ca pyroxenes

Raman spectra of several compositions of (Mg, Fe, Ca)SiO3 pyroxenes were co llected at ambient conditions. More than 10 Raman vibrational modes were ob served for these pyroxenes in the wavenumber range between 200 and 1200 cm( -1). in general, these pyroxenes are characterized by (1) the Si-O stretchi ng modes above 800 cm(-1); (2) the Si-O bending modes between 500 and 760 c m(-1); (3) SiO4 rotation and metal-oxygen translation modes below 500 cm(-1 ). For a constant Ca content, frequencies of the Raman modes in the enstati te-ferrosilite (opx) and diopside-hedenbergite (cpx) series generally decre ase with an increase in Fe content. This phenomenon is attributed to an inc rease in both the bonding lengths and the reduced mass as Fe2+ is substitut ed for Mg. However, two modes at similar to 900 cm(-1) in the enstatite-fer rosilite series increase in frequencies as Fe content increases. A possible explanation is to the shortening in the Si-O-Si bridging bonding bonds whe n the M2 sites are preferentially occupied by the iron cation. The effect o f Fe substituting for Mg on the frequency shift in cpx is less profound tha n opx because the larger M2 was occupied by calcium and the substitution of iron and magnesium in the M1 site results in a less significant change in the bond length. The major-element composition of the (Mg, Fe, Ca)-pyroxene s, especially the orthopyroxene series, can be semi-quantitatively determin ed on the basis of the peak positions of their characteristic Raman modes.