Abstract
Carbonate rocks are widely distributed in nature and are one of the main reservoir rocks for oil and gas. Microbes play an important role in the formation of carbonate rocks. Compared with the cemented part with fewer microorganisms (abiotic minerals), the nucleation and growth of the clotted part with more microorganisms (biotic minerals) are affected by the metabolic activities of microorganisms, so it has a unique morphology and crystal structure. However, there are still some confusing questions such as ones regarding the unique thermal decomposition characteristics and geochemical characteristics, which have rarely been studied. Here, X-ray diffraction (XRD) was used to determine the mineral composition of the cemented and clotted part in the same thrombolite. Then, thermal analysis and stable isotope geochemistry were used to explore the thermal stability and stable carbon isotope. The results of X-ray diffraction (XRD) show that the mineral components of the cemented and clotted part in the thrombolite are calcites, but their crystallinity is different. The crystallinity of the clotted part in the thrombolite is higher than that of the cemented part. Thermal analysis results show that the activation energy and the thermal stability of the clotted part in the thrombolite are relatively higher. In addition, the stable carbon isotope results show that, compared with the chemical precipitated calcite in the cemented part, the microorganisms in the clotted part absorb more 12C through photosynthesis, which results in more 13C in the clotted part, and the carbon isotope δ13C value is higher. This study helps to distinguish the crystallographic, thermodynamic and geochemical characteristics of cemented and clotted parts in thrombolite, and may be helpful to deepen our understanding of abiotic and biotic minerals.
Highlights
Microbial carbonates are widely distributed in nature and are important storage locations for oil and gas resources [1,2,3]—carbonate rocks have high application potential and economic value.Microbes play an important role in the formation of microbial carbonate rocks
The results show that microbial calcite exhibits higher thermal stability and activation energy, which is mainly due to the higher crystallinity of microbial calcite
X-ray diffraction (XRD) analyses of the cemented and clotted parts showed that the corresponding Full Width at Half Maximum (FWHM, unit: degree) value of the 104-crystal plane was different, which indicates that the crystallinity of the cemented and clotted parts was completely different
Summary
Microbes play an important role in the formation of microbial carbonate rocks. They provide a place for the nucleation of carbonate minerals, and change the microenvironment of mineral precipitation through their unique metabolic activities, attracting many researchers to carry out research work [4,5,6]. Crystals 2020, 10, 1017 limestone and how to effectively distinguish the abiotic minerals from biotic minerals has attracted widespread attention. Few studies have been conducted on other characteristics of microbial limestones, such as the crystallinity and thermal stability of the cemented and clotted parts in the same thrombolite. What is the difference between the cemented and clotted parts, and what are the characteristics that can distinguish the abiotic and biotic minerals?
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