Effect of Fe Content on Raman Spectral Characteristics of Dolomite

Dolomite is a common carbonate mineral in sedimentary rocks, and dolomite rock serves as an important reservoir rock for oil and gas. However, the production of synthetic dolomite under normal temperature and pressure is not yet possible, the genesis of dolomite and dolomite reservoirs has been a difficult problem to understand in geological research. Fe ion is a prevalent impurity element in natural dolomite, and the Fe content in dolomite can serve as an indicator of the diagenetic environment, which provides valuable insights into the genesis of dolomite. The technology of Raman spectroscopy has evolved from qualitative analysis to quantitative analysis. Therefore, the quantitative characterization of carbonate mineral chemical composition can be accomplished using Raman spectroscopy. Previous studies have shown that the Raman spectra of calcite group minerals shift regularly with the change of cation composition. However, there is no systematic study on the relationship between Fe content and Raman spectrum in dolomite.

To discuss the influence of Fe content on the Raman spectrum of dolomite, and establish a test method for the determination of Fe content in dolomite by Raman spectroscopy.

Seven dolomite samples from the Maokou Formation were collected from a well in the Sichuan Basin. Based on the petrography observation under a microscope, different types of dolomite with relatively clean grains were selected for EPMA to obtain the chemical composition of dolomite, then microscopic confocal laser Raman spectroscopy was conducted in the same area to acquire the corresponding dolomite Raman spectra. In addition, Raman spectra and corresponding chemical composition data of 14 typical dolomite samples were obtained from the RRUFF database for this study. The effect of Fe content on Raman spectroscopy of dolomite was studied by analyzing the correlation between the characteristic parameters of dolomite Raman spectroscopy and Fe content.

(1) Raman peaks for two lattice vibration (T and L) and three CO₃²⁻ group internal vibration (v₁, v₃ and v₄) were observed in the dolomite Raman spectrum. Compared with dolomite, each Raman peak position of ankerite moves to a lower frequency. Moreover, the Raman shifts of T and L peaks of ankerite are reduced by 5cm⁻¹ and 13cm⁻¹ on average, and the Raman shifts of v₁, v₃ and v₄ peaks are reduced by 3cm⁻¹, 2cm⁻¹ and 2cm⁻¹ on average, respectively.　　(2) There is an obvious linear relationship between the Raman shift and their Fe content of dolomite minerals. The measured Raman shift decreases with the increase of Fe content in dolomite. Notably, the movement of the peak position of the lattice vibration mode is more obvious, with the change of Fe content in dolomite, compared with the internal vibration peak.　　(3) Because the ionic radius of Fe²⁺ is larger than that of Mg²⁺, when Fe²⁺ replaced Mg²⁺ in the dolomite lattice, the average length of the metal-oxygen bond in minerals becomes longer, and the bond energy becomes weaker. Thus, with the changing of the Raman active vibration mode, Raman shifts of each peak in the dolomite Raman spectrum decreases.

Based on the different influences of Fe content in dolomite on the Raman shift of lattice vibration peaks and internal vibration peaks in Raman spectroscopy, a testing method for determining Fe content in dolomite is preliminarily established based on the distance between L peak and v₁ peak in Raman spectroscopy. Compared with traditional methods for testing the Fe content in dolomite, this method has lower requirements for samples and can be used for non-destructive testing, and is suitable for artificial synthesis experiments of dolomite.