lin sem project

Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627

Quartz is one of the most common minerals in clastic rocks (sandstone, siltstone and mudstone). From the source region to deposition site, quartz grains can be transported through various geological forces, for example, river water, wind and glacier. Due to the different mechanism of these transportation methods, characteristic morphologies can be left on the surface of quartz grains. In addition, post-depositional diagenesis is also able to alter the surface morphology of quartz. It has been shown that the surface morphology of quartz is useful in inferring the origin of clastic rocks (Krinsley and Doornkamp, 1973).

The electron microscopic observations of quartz grains are mainly about the surface morphologies, which include recognitions (presence or absence) of the following features: 1) conchoidal breakage patterns; 2) cleavage plates; 3) upturned plates on cleavage or crystal faces; and 4) the degree and nature of alteration of these features (Krinsley and Doornkamp, 1973). For example, quartz grains of eolian origin tend to be rounded, and with dish-shaped concavities; while, subaqueous flow transported quartz grains, more often, exhibit chemically etched or mechanical V-forms and curved grooves.

This study aims to use Scanning Electron Microscopy (SEM) to study the surface morphology of quartz grains to distinguish their potential depositional environments. Samples used for this study include sandstones/siltstones collected from the Tibetan Plateau and sand particles from EES203 class with unknown origin. Two kinds of samples were prepared: 1) sandstone powders, grinded using mortar and pestle; 2) bulk sandstone pieces with one flat surface cut by rock saw.

1. Light Microscopy. Sandstone samples were cut into thin sections for observations under optical microscope. This is to help select specific samples for SEM observation. Samples with more than 50% (volume) quartz grains, and mean grain diameter larger than 0.02 mm were chosen.

2. Sputtering Coating. All samples were air-dried before coating. Due to the fact that the grain size in powders are rather variable, and the “flat” bulk sandstone surfaces are still uneven in micro-scale, 60 seconds gold coating were applied for better conduction. Even so, excessive charging was common when observing in SEM.

3. SEM observations (SE2 and BSE). Since the main target of the SEM observation here is to look at the surface morphologies, this study account heavily on SE2 mode, considering that secondary electron images give the best resolution and depth of field. However, as mentioned above, charging is always a problem, so backscattered electron mode was also used for observation when charging was a big issue.

4. X-Ray Microanalysis. The elemental spectrum analysis was occasionally used to identify the mineralogy of grains observed: quartz (mainly Si and O) or calcite (mainly Ca, O, and C).

5. Colorization. Selected images were colorized using Adobe Photoshop for better visualization.

6. Image Analysis. Images of the sand particle sample was analyzed using ImageJ software to obtain the size/roundness information of sand particles.

Due to the fact that, sometimes, it is difficult to tell apart quartz from calcite, just based on morphologies (SE2 images). It is also hard to use BSE images to distinguish them, as quartz (SiO₂) and calcite (CaCO₃) does not show big elemental difference. X-Ray microanalysis was used occasionally to ensure that grains observed are quartz. As shown in Figure 1, the three high-lighted crystals have similar morphologies: a group of cleavage parallel to the top surface. However, X-Ray microanalysis indicates that the bigger crystal on the left is a quartz grain, while, the two smaller crystals on the left are both calcites.

Figure 1. X-Ray microanalysis. Note that it is hard to distinguish between the quartz (left big one) and calcite (right two smaller ones).

Four different types of quartz grains were observed under SEM (Figure 2): 1) Quartz crystal with well preserved crystal facets (6 facets constituting a prism) (Figure 2A). Note that the lower face is a conchoidal breakage face; 2) Quartz crystal with cleavages. Cleavages are only observed on smaller crystals (<20 mm) (Figure 2B); 3) Rounded quartz grains, suggesting mechanical collisions (Figure 2C); 4) Amorphous quartz grain, suggesting dissolutions and/or new crystal precipitation (Figure 2D).

I observed several different surface morphologies that can be used to recognize the transportation mechanisms, some of the most typical ones are shown in Figure 3. Dish-shaped concavity is an index feature for aeolian sands (Figure 3A). These deep depressions are probably caused by mechanical chipping during particularly powerful sand storms (Krinsley and Doornkamp, 1973). On the contrary, the occurrence of capping layer (Figure 3B) suggests deep burial diagenesis after deposition on Earth surface. The thin layer of capping silica can only be precipitated when the surrounding temperature and pressure reach to a certain level. Although adhering particles (Figure 3C) can occur in several different environments, such as glacial, subaqueous environments, or during diagenesis. Its co-occurrence with capping layer suggests this surface morphology was probably produced during diagenesis. Conchoidal fracture (Figure 3D) is not a good index surface morphology, however, this feature can not be produced during diagenesis or aeolian transportation. More likely environments are subaqueous or glacial settings.

Apart from characterized dish-shaped concavity as shown in Figure 3A, another main feature of aeolian sands is the size distribution and roundness of sand particles. I used the particle analysis command in the ImageJ software to quantitatively constrain these information. Figure 4 shows the processes of particle analysis in ImageJ. Table 1 summarizes the statistical data of individual grains of the sand particle sample. The distribution of quartz sand size and the high roundness clearly indicate that these sand particles are of aeolian origin.

Figure 4. Particle analysis using ImageJ software. Left, original SEM image; middle, image after threshold was adjusted; right, outlined individual sand particles.

Conclusions

The SEM observations of the surface morphologies of two types of samples indicate different origins of the quartz grains. The sandstone samples collected from the Tibetan Plateau was probably deposited in fluvial (subaqueous) environment (Figure 3D); however, these samples might experience deep burial diagenesis, indicated by silica precipitation on quartz surface (Figure 3B). The other sand particle sample from EES 203 class is probably aeolian sand, considering their great roundness (Table 1) and characterized dish-shaped concavities (Figure 3A).

Acknowledgement

Great thanks are given to Brian Mclntyre for his enthusiastic guidance and help during the entire period of course and project.

Reference

Krinsley, D.H., Doornkamp, J.C., 1973. Atlas of quartz sand surface textures. Cambridge University Press.