Stress Distribution During Cold Compression of Rocks and Mineral Aggregates Using Synchrotron-based X-Ray Diffraction

Summary

We report detailed procedures for compression experiments on rocks and mineral aggregates within a multi-anvil deformation apparatus coupled with synchrotron X-radiation. Such experiments allow quantification of the stress distribution within samples, that ultimately sheds light on compaction processes in geomaterials.

Abstract

We report detailed procedures for performing compression experiments on rocks and mineral aggregates within a multi-anvil deformation apparatus (D-DIA) coupled with synchrotron X-radiation. A cube-shaped sample assembly is prepared and compressed, at room temperature, by a set of four X-ray transparent sintered diamond anvils and two tungsten carbide anvils, in the lateral and the vertical planes, respectively. All six anvils are housed within a 250-ton hydraulic press and driven inward simultaneously by two wedged guide blocks. A horizontal energy dispersive X-ray beam is projected through and diffracted by the sample assembly. The beam is commonly in the mode of either white or monochromatic X-ray. In the case of white X-ray, the diffracted X-rays are detected by a solid-state detector array that collects the resulting energy dispersive diffraction pattern. In the case of monochromatic X-ray, the diffracted pattern is recorded using a two-dimensional (2-D) detector, such as an imaging plate or a charge-coupled device (CCD) detector. The 2-D diffraction patterns are analyzed to derive lattice spacings. The elastic strains of the sample are derived from the atomic lattice spacing within grains. The stress is then calculated using the predetermined elastic modulus and the elastic strain. Furthermore, the stress distribution in two-dimensions allow for understanding how stress is distributed in different orientations. In addition, a scintillator in the X-ray path yields a visible light image of the sample environment, which allows for the precise measurement of sample length changes during the experiment, yielding a direct measurement of volume strain on the sample. This type of experiment can quantify the stress distribution within geomaterials, which can ultimately shed light on the mechanism responsible for compaction. Such knowledge has the potential to significantly improve our understanding of key processes in rock mechanics, geotechnical engineering, mineral physics, and material science applications where compactive processes are important.

Introduction

The rationale behind the method presented in this article is to quantify the stress distribution within rock and mineral aggregate samples during compression and subsequent compaction. Understanding the compaction in rocks and mineral aggregates is of great importance to reservoir and geotechnical engineering^{8,17,18,19,20,28,33}. Compaction acts to reduce....

Protocol

1. Sample Preparation

1. Choose the test and/or reference sample; this can be either a rock core (step 1.2) or a mineral aggregate (step 1.3), depending the focus of the experimental study.
   NOTE: The following method is certainly not the only way to prepare good quality samples (e.g., other machines can be used). However, the sample preparation adopted in the present study is fully illustrated to achieve the goal of accurate replication.
2. Rock core samples
   1. Saw a small rectangular slab from a larger sample rock block. Then surface grind the sample slab such that all six surfaces of the slab are flat and perpe....

Results

We show one representative result example from an XRD experiment (experiment SIO2_55) run in the multi-anvil press at 6BM-B on a compound quartz aggregate^5,6 and novaculite core sample⁶. The grain sizes of the quartz aggregate and novaculite are ~4 µm and ~6–9 µm, respectively^5,6. Selected diffraction spectra collected during this experiment are ill.......

Discussion

We present the detailed procedure for carrying out XRD experiments using the multi-anvil cell at 6-BM-B. Perhaps the most critical, and yet most challenging, steps in the above protocol involve optimizing the quality of the sample. Such importance on sample quality applies to almost all rock and mineral deformation experiments. Firstly, it is critical for the end surface of the rock cores to be flat, with both ends parallel to each other and at the same time, perpendicular to the cylindrical surface. That will ensure the.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Rotatory Tool Workstation Drill Press Work Station with Wrench	Dremel	220-01
MultiPro Keyless Chuck	Dremel	4486
Variable-Speed Rotatory Tool	Dremel	4000-6/50
Super small Diamond Core Drill - 2.5 mm	Dad's Rock Shop	SDCD
Coolant	NBK	JK-A-NBK-000-020	Grinding Fluid Concentrate US 5 gal / 20 L
commercial software package and codes for instrument control and data acquisition	IDL EPICS and SPEC		installed on the computer at the beamline
CCD Camera	Allied Vision	Prosilica GT	installed at the beamline