Clay minerals include a large group of fine-grained, layered silicates, which result from bulk mineral weathering. Clays are of specific interest for the drilling and mining industries because of the physical properties that they contribute to the surrounding geological formations.
In this article, the qualitative analysis of clay by X-ray diffraction (XRD) with the D2 PHASER benchtop diffractometer (Figure 1) has been covered, specifically for the identification of swelling clay species.
Figure 1. The D2 PHASER benchtop diffractometer.
Even though there are several distinct clay species and interstratifications, the classification of clay minerals can be done in three major groups - kaolinite, illite, and smectite. Vermiculites are normally considered as the fourth group. Chlorites and micas are other phyllosilicate minerals of interest that are included in clay mineral analysis even though neither of them are clearly clays.
Of the three key groups, smectites are differentiated by the ability of moisture absorption and the associated demonstration of volumetric expansion. Smectite group members such as montmorillonite, are mostly referred to as swelling clays.
As aforementioned, clay minerals are a major concern in a number of drilling applications. For instance, in the hydraulic fracturing industry, high clay concentrations show increased ductility and can result in poor fracture formation. Furthermore, swelling clays can result in water-induced swelling at the time of the initiation or negative impacts like self-healing at the time of production.
Identifying these minerals is important for the development of customized solutions for stabilizers and additives. Swelling clay identification using a mobile benchtop diffractometer and some simple laboratory equipment is demonstrated in this article.
Overview and Experimental Procedure
Based on the procedure outlined by the U. S. Geological Survey (USGS), samples were prepared in the form of air-dried and glycolated oriented mounts. The plate-shaped clay mineral particles are made to lie flat along the surface of the substrate, enabling probing of the basal diffraction peaks using XRD symmetric scans in reflection geometry.
Computing of the basal plane spacing (or d-spacing) can be done by determining the diffraction peak angle (in degrees 2θ) of a diffractogram. The degree of contraction or expansion and the initial d-spacing after specific treatments like glycolation, enables clay mineral identification such as swelling clays.
For instance, adding glycol to smectite clays causes expansion of the basal planes as intercalation of polyol molecules occur between atomic layers, resulting in them falling apart.
The associated reflection in diffraction data will shift to a larger d-spacing and smaller diffraction angle as predicted by Bragg's Law. As Bragg’s Law suggests, the related reflection in diffraction data will move to a higher d-spacing and a smaller diffraction angle. Non-swelling clays will not show this lattice expansion. The related diffraction peaks will stay in the same location prior to and subsequent to glycolation.
A micronizing mill was used to grind a bulk sample of shale rock. The resulting sample was then dispersed in water using a sonication process (Figure 2). A tiny amount of sodium hexametaphosphate, a dispersant, was added for disintegrating flocculated clay particles and agglomerates. Before collection of the clay minerals (Figure 2), bulk minerals were allowed to settle for 1h.
Figure 2. Finely ground geological samples are dispersed in water via sonication (left). The bulk and clay mineral fractions are divided by gravimetric separation (right), and the clay minerals are collected by decanting. The markings on the beaker are for tracking the progress of separations.
The supernatant with clay fraction was separated by decanting for oriented mouth preparation. This can be performed gravimetrically, but the application of a centrifuge can accelerate the process. In order to prepare the oriented mounts, dispersed clay particles were deposited onto glass slides and the suspension was allowed to dry.
Analysis of the dried oriented mounts was performed using XRD. Then, glycolation was performed to modify the oriented mounts by meticulously applying a small ethylene glycol droplet to the clay surface and then allowing absorption (Figure 3).
It is possible to batch-process a number of clay mounts by arranging in a warm dessicator filled with a small quantity of ethylene glycol for a number of hours. After treatment, a second diffraction scan was performed in order to compare with the original oriented mount. For demonstration purposes, additional mounts were prepared from a number of commercially available clay standards.
Figure 3. Prepared clay slides are placed in a sample holder with adjustable height (top) for accurate positioning within the diffractometer. Clay specimens are analyzed as a dry oriented mount (bottom left) and again following the addition of ethylene glycol (bottom right).
Data collection was performed in reflection geometry with the D2 PHASER having a high-speed linear detector (LYNXEYE), which is required for speedy data collection. The D2 PHASER system can be operated in a mobile lab environment characterizing an on-board cooling system, integrated computer, and operating with standard domestic power.
The scanning range must commence at ≤ 3° 2θ for making sure that the peaks of interest are completely and appropriately observed. The total data collection time for these two scans is 10 minutes each. The total processing time is around 3h for each sample, which is mostly unattended at the time of drying and separation.
For each prepared sample, two scans were obtained. The first scan was obtained on the oriented slide that was not subjected to treatment. The second scan was obtained in the completely glycolated and fully swelled slide.
Figure 4 shows low angle diffraction data for two clay samples. For the bentonite sample, a significant shift to a larger d- spacing is observed, which indicates sample swelling. There is no impact on the kaolinite reflections.
Figure 4. Diffraction data for two clay samples - bentonite and kaolinite - as both oriented mounts and glycolated specimens. The clear shift in low angle data for the bentonite sample indicates expansion along the c-axis. The kaolinite sample does not swell with the addition of glycol; consequently, the reflection is observed at the same location.
For the clay fraction collected, a number of mineral species were observed as shown in Figure 5, which includes strong reflections from muscovite and chlorite. Even though the smectite reflection is very much broadened in the oriented mount to the point of being tough for visual confirmation, the swelled mount shows a precise shifted reflection centered around 17.4Å, which validates the presence of swelling clays in this sample.
Figure 5. Diffraction data for a clay fraction collected from shale rock. Chlorite and muscovite reflections are easily detected and do not shift upon glycolation. The broad smectite reflection is difficult to observe in the oriented mount but appears as a stronger, shifted reflection after the addition of ethylene glycol
The ability of using the mobile D2 PHASER X-ray diffractometer for identification of the presence of swelling clays is shown in this article. Sample preparation is a simple procedure involving grinding, dispersal in water, separation, and depositing onto glass slides.
The analysis of samples was performed as oriented mounts in reflection geometry; once as deposited and air-dried, and the next time, after the addition of ethylene glycol. This process can be significantly accelerated using a warm drying oven and a small centrifuge.
The article focuses on smectite clay identification. Using the D2 PHASER, more comprehensive speciation can be achieved with further sample processing such as multiple heating cycles, which is beyond the scope of this article.
This information has been sourced, reviewed and adapted from materials provided by Bruker AXS Inc.
For more information on this source, please visit Bruker AXS Inc.