The D2 PHASER is a portable benchtop X-ray diffractometer (XRD) that is used to identify clay and bulk minerals within geological samples.
Clay minerals are made up of a large class of fine-grained, layered silicates, which form due to the weathering of bulk minerals. The drilling and mining industries are particularly interested in clays, as these minerals pass on physical properties to adjacent geological formations.
In this article, the qualitative analysis of clays, using an X-ray diffraction (XRD) coupled with the D2 PHASER benchtop diffractometer, (Figure 1) is discussed in detail. The article also describes how to identify the swelling clay species.
Figure 1. The D2 PHASER benchtop diffractometer.
There are a wide variety of discrete clay species and interstratifications. However clay minerals can be roughly arranged into three major categories; smectite, kaolinite, and illite. Frequently vermiculites are considered as a fourth category. Other phyllosilicate minerals that are in demand include chlorites and micas. Although these are not explicitly clays, they are included in the investigation of clay minerals.
Smectites, among the three groups, have the capability to absorb moisture and the concomitant demonstration of volumetric expansion. Clays such as montmorillonite are classified as smectites, and are frequently called swelling clays.
Clay minerals play a big role in several drilling applications. In the hydraulic fracturing sector high concentrations of clays point to higher ductility, and can result in weak fracture formation. If swelling clays are present, it may lead to water-induced swelling during the initiation process or negative outcomes, such as self-healing during production steps.
The mineral identification is crucial to develop customized solutions for stabilizers and additives.
According to the process, described in the U. S. Geological Survey (USGS), samples were prepared as air-dried and glycolated oriented mounts. The oriented mount makes the plate-shaped clay mineral particles stretch out flat along the surface of the substrate, allowing the basal diffraction peaks to be probed using XRD symmetric scans in reflection geometry. By establishing the diffraction peak angle (in degrees 2Theta) of a diffractogram, the basal plane spacing or d-spacing can be calculated. After certain treatments, such as glycolation, the initial d-spacing and the degree of contraction or expansion allows the clay minerals to be identified. For instance, adding glycol to smectite clays causes an expansion of the basal planes, as polyol molecules intercalate between atomic layers, causing them to separate. The associated reflection in diffraction data will move to a larger d-spacing, and smaller diffraction angle as estimated by Bragg’s Law. Non-swelling clays will not exhibit this lattice expansion. The associated diffraction peaks will stay in the same location before and after glycolation. A bulk shale rock sample was ground using a micronizing mill and dispersed in water via sonication (Figure 2).
A small quantity of sodium hexametaphosphate, a dispersant, was added to help to split up flocculated clay particles and agglomerates. Bulk minerals were allowed to settle for 1 hour before the clay minerals were gathered (Figure 2).
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 the clay fraction was separated by decanting, and placed aside while the oriented mounts were prepared. This can be performed gravimetrically, but the process can be hastened with a centrifuge. Oriented mounts were prepared by placing the dispersed clay particles onto glass slides and allowing the suspension to dry. Extra sample was added in a dropwise manner until an opaque film was obtained. The dried oriented mounts were tested using XRD, and then altered by glycolation. This was performed by cautiously coating a small drop of ethylene glycol on the surface of the clay and allowing it to absorb (Figure 3).
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).
It is possible to batch-process multiple clay mounts by placing them in a warm desiccator filled with a small quantity of ethylene glycol for a long period of time. A second diffraction scan was acquired after treatment to compare with the original oriented mount. Extra mounts were prepared from a number of commercially available clay standards for experimental purposes. Using the D2 PHASER, equipped with a high-speed linear detector (LYNXEYE), data was collected in reflection geometry, which is necessary for rapid data collection.
The D2 PHASER can be operated in a mobile lab environment, featuring an on-board cooling system, integrated computer, and operating with basic domestic power. It is recommended that the scanning range should start at = 3° 2q so that it is possible to fully and clearly observe the clay peaks of interest. The total data collection time taken for these two scans is 10 minutes each. The total processing time for each sample is approximately three hours, typically unattended during separation and drying stages.
Two scans were acquired on each prepared sample, with the first scan collected on the untreated oriented slide, and the second scan collected on the totally swelled and glycolated slide. In Figure 4, the low angle diffraction data for two clay samples is displayed.
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.
In the bentonite sample, a vivid shift to a larger d-spacing could be observed, representing sample swelling. The kaolinite reflections remained untouched. For the collected clay fraction many mineral species were studied, as illustrated in Figure 5, including strong reflections from muscovite and chlorite. Although the smectite reflection is highly widened in the oriented mount, nearly to the point of being hard to verify visually, the swelled mount shows a very precise shifted reflection centered around 17.4Å, confirming 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.
This article has shown how the mobile D2 PHASER X-ray diffractometer can be effectively used to identify the presence of swelling clays. Sample preparation is performed using an uncomplicated approach involving grinding, dispersal in water, separation, and deposition of dispersed clay particles onto glass slides.
Samples were tested as oriented mounts in reflection geometry; first as deposited and air-dried, and next after adding ethylene glycol. Using a small centrifuge and a warm drying oven, this process can be accelerated to a great extent. The spotlight of this article is the smectite clay identification. A better and comprehensive clay speciation is attainable using the D2 PHASER with extra sample processing, for example multiple heating cycles, beyond the scope of this research.
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.