Raman spectroscopy provides insight into carbonate rock fabric based on calcite and dolomite crystal orientation

Carbonate rocks record the oldest forms of life on Earth, and their geologic reconstruction requires multiple methods to determine physical and chemical processes before conclusions of ancient biosignatures are made. Since crystal orientation within rock fabric may be used to infer geologic settings, we present here a complementary Raman method to study the orientation of calcite (CaCO₃) and dolomite [CaMg (CO₃)₂] minerals. The relative peak intensity ratio of the carbonate lattice E_g modes T and L reveals the crystallographic orientation of calcite and dolomite with respect to the incident light polarization. Our results for calcite show that when the incident laser light propagates down the crystallographic a/b axis: (1) the L mode is always greater in intensity than the T mode (I_T < I_L), and (2) the spectra are most intense at 45° and least intense at 90° polarization angles measured from around the c axis. Our results for dolomite show that (1) I_T > I_L when the incident light propagation is down the crystallographic c axis and (2) I_T < I_L when the incident light propagation is down the crystallographic a/b axis. This study reveals mineral orientation variation related to deposition and paragenesis within limestone and dolostone samples. The method presented yields information related to growth and deformation during diagenetic and metamorphic alteration and may be used in research seeking to identify the fabric parameters of any calcite or dolomite containing rock. The compositional and structural data obtained from Raman mapping is useful in structural geology, materials science, and biosignature research.