As one of the main constituent materials of the earth's crust, the microstructure of rock is of great significance for understanding geological processes, resource exploration and engineering construction. According to the origin of rocks, they can be divided into three categories: magmatic rocks, sedimentary rocks, and metamorphic rocks. On the Earth's surface, sedimentary rocks account for 75% of the earth's surface, while at the depths of the earth's crust, they are mainly composed of magmatic and metamorphic rocks, accounting for about 95% of the earth's crustal volume. The failure and instability process of rock is essentially the result of the initiation, propagation and penetration of microcracks in the process of stress, and is the macroscopic reflection of the accumulated deformation and failure of rock microstructure. The initial micro-defects in the rock, such as holes, micro-fractures, and particle boundaries, are randomly distributed, resulting in the random distribution of new cracks when loaded. To some extent, the size of the initial microdefect plays a decisive role in the mesoscopic crack propagation and macroscopic failure instability of the rock.

In recent years, with the application of scanning electron microscopy technology in rock experiments, many scholars have been able to conduct mesoscopic experimental research on rock samples, and have achieved certain results. In this paper, the ZEM series desktop scanning electron microscope was used to select granite, sandstone, gneiss, mudstone and other samples from three types of rocks, and the different crack propagation characterization photos obtained were characterized by the characterization of different types of rocks under specific actions, aiming to deeply understand the microscopic mechanism of rock failure instability, and provide theoretical support and technical guidance for rock engineering and geological disaster prevention.

Granite is a type of magmatic rock, which is formed by the cooling and solidification of magma deep underground in the earth's crust. According to their consolidation location and formation mode, magmatic rocks can be divided into two categories: intrusive rocks and ejective rocks.

Figure Occurrence of magmatic rocks

Intrusive rocks are rocks formed by the cooling and solidification of magma inside the earth's crust as it goes deep underground. Magma rises along a weak zone in the Earth's crust but consolidates before it reaches the surface, a process known as intrusive activity. Ejecta rocks are rocks formed by magma rising along tectonic fissures in the earth and ejecting out of the earth's surface through volcanic channels, also known as volcanic rocks. When magma solidifies underground, it will form different forms, such as rock foundations, rock walls, rock beds, rock basins, rock caps, rock veins, etc. The occurrence of volcanic rocks is related to their eruption patterns, including central eruptions, fractured eruptions, and fused eruptions.

Figure Characterization of granite

From the microstructure of ZEM series benchtop scanning electron microscope, granite is mainly divided into the flat morphology of quartz grains, the laminated and stepped morphology of potassium feldspar grains. The initiation, propagation and penetration of microcracks in granite samples during the stress process were observed, and the cracks showed an irregular distribution pattern. Among them, compared with quartz particles, potassium feldspar has lower tensile strength, which is a weak mineral particle, and the cracks will expand more in the potassium feldspar particles, forming small-scale fractures, and if the force direction is parallel to the layer, it will form a cascading morphology, and vice versa, it will produce a stepped morphology.

Figure Sedimentary sequences

Sandstone and mudstone are a kind of sedimentary rocks, they are the most abundant type of rocks on the surface, under normal temperature and pressure conditions, by weathering, biological processes and some volcanic detrital materials, through a series of geological processes such as transportation, sedimentation and diagenesis formed rocks. The formation environment includes rivers, lakes and oceans, which are deposited and eventually compacted into rocks under weakened hydrodynamic conditions.

Figure Characterization of different grains of sandstone

As can be seen in the above figure, the microstructure of sandstone is mainly granular, and the failure characteristics of sandstone samples are mainly manifested as fracture and fragmentation between sand grains, and cracks propagate along the boundary of sand grains. With the decrease of particle size, the degree of particle compaction increases, the pores between the coarse-grained sandstone particles are clearly visible, the pore scale of medium-grained sandstone decreases significantly, and the fine-grained sandstone particles are tightly arranged and the pores are small.

Figure Characterization of mudstone grouting

In the process of stressing cement-bearing rock samples, the binding force between the cement matrix and the aggregate is broken, and the cracks propagate along the pores in the cement matrix. The failure characteristics of the dry mudstone samples are mainly manifested in the detachment and separation between the particles, and the cracks show a twisted and staggered shape. During the failure of the mudstone grouting sample, the combination between the grouting material and the mudstone matrix was broken, and the crack showed the characteristics of propagation along the grouting interface.

Figure Gneiss

Gneiss is a kind of metamorphic rock, the rocks that have been formed on the earth, including magmatic rocks, sedimentary rocks and metamorphic rocks, with the continuous change of the earth's crust, their physical and chemical conditions change, resulting in changes in the mineral composition, structure and structure of the rock under the condition of solid state, and the final formation of rocks is called metamorphic rock. According to the main factors and geological conditions of metamorphism, it can be divided into dynamic metamorphic rocks, contact metamorphic rocks, regional metamorphic rocks and mixed lithification, and gneiss belongs to regional metamorphic rocks.

Figure Characterization of gneiss

From the microscopic characterization of gneiss, it can be seen that during the failure process, the interface between the mineral particles is separated and fractured, and the cracks show the characteristics of staggered and reticulated, and are arranged in a directional manner to form a "gneiss-like structure". Compared with the usual granite, the minerals in the rock mass are arranged in a directional manner, and some particles are elongated, and the microgranite inclusions are also elongated, but there is no plastic deformation inside the particles, which can be distinguished from ductile deformation.

It is not difficult to find that there are great differences in the degree and scale of initial damage of different types of rocks, which are related to the structure and diagenesis of rocks, and the heterogeneity caused by different structures of rocks plays a decisive role in the deformation characteristics and final failure instability of rocks under load. The preset large-scale structure plays a controlling role in the formation location and propagation direction of the macroscopic main crack. In this study, we observed the failure characteristics of different types of rocks under specific actions, and revealed the controlling effect of micro-defects on rock failure instability. ZEM series benchtop scanning electron microscope has important application value in the study of rock microstructure, and provides theoretical support and technical guidance for rock engineering design and geological disaster prevention.

ZEPTOOLS ZEM18 Benchtop Scanning Electron Microscope

ZEPTOOLS is a scientific instrument company with fully independent intellectual property rights. Since the 1990s, our R&D team has been committed to providing excellent instruments for nanoscience research. At present, the company has a number of product lines including PicoFemto series of in-situ TEM measurement system, in-situ SEM measurement system, ZEM series of benchtop scanning electron microscope, JS series of step profiler, nano-displacement stage, two-dimensional material transfer stage, probe stage and cryogenic system, grating Ruler etc., which have gained a high degree of attention at home and abroad, and filled the gaps of the country's high-end precision instruments in the field of a number of blank.