Raman spectroscopic characteristics of Mg-Fe-Ca pyroxenes

Abstract

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

Thermodynamic mixing properties of olivine derived from lattice vibrations

“…13 are not significantly different from linearity. That also holds for enstatiteferrosilite and diopside-hedenbergite solid solution series measured by Huang et al (2000) as exemplified by the vibrational modes in Fig. 1b.…”

“…Figure 1 illustrates that these frequencies depend nearly linear on the composition for a number of solid solution phases. According to Huang et al (2000) and Mernagh and Hoatson (1997) a similar trend is present in pyroxenes and according to Hofmeister and Mao (2001) in silicates with the spinel structure. Guyot et al (1986) have shown that a quantitative theoretical interpretation of the substitutional effect of magnesium by iron is not straightforward.…”

“…Figure 1a indicates that for three internal vibrational modes in olivine these frequencies show a near-linear behavior with composition. According to Huang et al (2000) and Mernagh and Hoatson (1997) such a trend is also present in at least 17 internal-and lattice vibrational modes of pyroxenes and Fig. 1b exemplifies that this is the case for a number of these modes.…”

“…Experimental data for olivine are from Guyot et al (1986), circle, Besson et al (1982), triangle. Data for pyroxene are from Huang et al (2000): synthetic crystals: circle, natural specimens, triangle, Chopelas (1999), square have become available for a number of minerals, such as forsterite, perovskite and akimotoite through the work of Gillet et al (1991), Gillet et al (2000) and Reynard and Rubie (1996), respectively. Incorporation of mode anharmonicity is important because as demonstrated in Jacobs and de Jong (2005a) and Jacobs and de Jong (2007), it significantly affects the location and Clapeyron slope of phase boundaries in the magnesium-olivine and magnesium-pyroxene system.…”

Estudio espectroscópico y DRX de afloramientos terrestres volcánicos en la isla de Tenerife como posibles análogos de la geología marciana

“…En las Cañadas el óxido 3,4,5,6,7,8,9,10,11,12,14,15;Piroxenos: 1,3,5,7,8,9,10,11,12,14,15;Óxidos (Cuarzo): 3,8,15;Olivinos: 5,7,12,15;Óxidos (Hematite): 6, 13, 15; Arcillas (Moscovita): 4, 8, 9, 10, 14. principal es la magnetita, aunque se han detectado también de manera abundante hematite y goethita (Lalla, 2014 Ecuación de ajuste 1 (Kuebler et al, 2006) Fo ( (Huang et al, 2000) Fe(2) =10406-15.649 (DB2); Fe(3) =1415-4.3554 (DB3) En las distintas zonas de estudio se han detectado especies minerales de carácter primario y secundario, así como también materiales accesorios. Los olivinos detectados son mayoritariamente forsterita en todas las zonas, lo cual confirma lo señalado por otros autores en la bibliografía especializada y por las diferentes técnicas empleadas.…”