Raman spectra of several compositions of (Mg, Fe, Ca)SiO 3 pyroxenes were collected at ambient conditions. More than 10 Raman vibrational modes were observed for these pyroxenes in the wavenumber range between 200 and 1200 cm -1 . In general, these pyroxenes are characterized by (1) the Si-O stretching modes above 800 cm -1 ; (2) the Si-O bending modes between 500 and 760 cm -1 ;(3) SiO 4 rotation and metal-oxygen translation modes below 500 cm -1 . For a constant Ca content, frequencies of the Raman modes in the enstatite-ferrosilite (opx) and diopside-hedenbergite (cpx) series generally decrease with an increase in Fe content. This phenomenon is attributed to an increase in both the bonding lengths and the reduced mass as Fe 2+ is substituted for Mg. However, two modes at ~900 cm -1 in the enstatite-ferrosilite series increase in frequencies as Fe content increases. A possible explanation is to the shortening in the Si-O-Si bridging bonding bonds when the M2 sites are preferentially occupied by the iron cation. The effect of Fe substituting for Mg on the frequency shift in cpx is less profound than 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)-pyroxenes, especially the orthopyroxene series, can be semi-quantitatively determined on the basis of the peak positions of their characteristic Raman modes. EXPERIMENTAL METHODSChemical compositions of all natural single-crystals of pyroxene (Table 1) were analyzed by the electron microprobe. In each sample, several different spots were analyzed to check the homogeneity of the sample. We also examined series of synthetic orthopyroxene with compositions (En 97.5 , En 80 , En 75 , En 70 , En 60 , En 50 , En 40 , En 35 , En 30 , En 25 , En 17 , and En 10 ) that were produced by H. Yang. See Yang and Ghose (1994) for details. The chemical composition of all the specimens in this study is plotted in Ca-Mg-Fe pyroxene diagram (Fig. 1). Strictly speaking, wollastonite is not a pyroxene. However, it is also included in this study because it is one of the end-members in the MgSiO 3 -FeSiO 3 -CaSiO 3 composition diagram.Raman spectra were excited using the 514 nm line of an Argon-Ion laser. We used a Raman spectrometer of the Renishaw Company, which contains a micro-objective that focuses the size of the laser beam to about 5 µm at the surface of the sample. A charge-coupled device is used as a detector to collect the signal in an 180° geometry. The position of the Raman peaks was determined by the PeakFit program. Typical acquisition time was about 10 minutes. Normally, the intensity of the modes varies with the orientation of the single crystal under investigation whereas the wavenumber of the modes remains the same. Therefore, in each sample, several orientations are attempted and an average in the peak position was taken for each vibrational mode. The spectral resolution for each mode is on the order of ±1 cm -1

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