29 February, 2016

Swelling clays and X-ray diffraction

Introduction

Recently, I have been asked by a friend to guide him through a crash course in X-ray diffraction and the identification of swelling clay minerals. Swelling clay minerals are a major issue in formation damage for petroleum/natural gas production but also for natural buildings stones and construction. Since I am ever so often asked how to identify swelling clays by friends or colleagues, I figured that it would make a great blog!

Warning: I keep this very simple. No exceptions, no cutting edge tricks, no rare minerals. Every lab will do things a tiny bit different. So this might not be the exact way you saw it elsewhere. If you are unexperienced and want to do it yourself, you should consult the sources listed at the end of the blog post before proceeding.

Clay minerals

Geoscientists use the term clay in two different contexts that may cause confusion. First, clay sensu lato is any material with a particle diameter < 2.0 µm. So, anything with a grain size smaller 2.0 µm is clay in a physical sense. Second, and more important for the clay scientists, clay sensu strictu is a group of phyllosilicates (Fig. 1) composed of tetrahedral and octahedral sheets. The sheets are the building blocks of layers. There are 1:1 layers and 2:1 layers: meaning a tetrahedral sheet + an octahedral sheet, and an octahedral sheet sandwiched in between two tetrahedral sheets.
Fig. 1: An overview of the classification of clay minerals. Source: Bergaya and Lagaly (2006) General introduction: Clays, clay minerals, and clay science. In: Handbook of Clay Science, Edited by F. Bergaya, B.K.G. Theng and G. Lagaly, Developments in Clay Science, Vol. 1
Based on the site occupancy of the octahedral sheet clay minerals can also be subdivided into di- and trioctahedral. While the first have 2 out of 3 octahedral positions occupied with a cation, the later have all 3 positions occupied with a cation, e.g. Al, Mg, Fe, Li or others. Natural swelling clay minerals are hydrated 2:1 clays with an expandable interlayer such as smectite, interstratified smectite in "another mineral", and vermiculite.

Certain clay minerals have an electrical charge due to substitutions in the octahedral sheet. The charge is neutralised by additional cations (e.g. Na, Ca, Mg) located in the interlayer space between each 2:1 layer stack (Fig. 2). Water (and other useful substances) can enter into the interlayer. The clay swells.
Fig. 2: Basic structure of Montmorillonite, a dioctahedral smectite and hydrous 2:1 swelling clay mineral. Source: USGS Laboratory for X-ray powder diffraction
Determining swelling clay minerals

The clay minerals have to be separated from the rest of the rock. This is usually done by gentle crushing, followed by sedimentation in Atterberg cylinders or by centrifugation. When there is no need to determine the trace element contents or exchangeable cation composition of the clay interlayer the dispersion can be enhanced by adding ammonium (pH control) or sodium pyrophosphate. It is important to keep the suspension in a neutral pH range to prevent flocculation. The < 2.0 µm or any other size fraction can be separated based upon the Stokes Law. So, we are actually working with an equivalent diameter. Doing this we obtain a clay suspension.

A few drops of the suspension are pipetted onto a glass slide and allowed to dry. This is the orientated, air-dried sample. It is measured using X-ray diffraction from 2° to 25° or to 35°2theta. Afterwards, the glass slide is placed into a small container to saturate the interlayer with ethylene glycol vapour - or other substances. The intercalation of EG in the interlayer space (Fig. 2) expands the clay lattice. This can be detected by repeating the XRD measurement. This step must be done fast (~ half an hour to one hour) because the EG will easily escape. The third step is calcination above 500°C but several other temperature steps may be used to gain more detailed information. This will cause the interlayer space to collapse. We repeat the XRD measurement a final time.

How does it looks like in the end?

The best method to immediately see and compare the results is by putting all three measurements into one image. Most XRD devices will have their own internal computer program for data comparison. Alternatively, this can be done using commercial or non-commercial software packages. For illustration purposes I drew three XRD traces (Fig. 3) of one of my own samples (a smectite) treated according to the above explanations and measured as air-dried, EG-solvated and calcined sample. The measurement is shown from 4 to 24° 2theta. There is a clear expansion of the d001 peak from roughly 14.3 to 17.3 Å due to the intercalation of the ethylene-glycol. The calcined sample shows the characteristic collapse of the interlayer. A careful review of the peak positions will confirm that there are no other minerals.
Fig. 3: XRD traces of an orientated, air-dried, EG-solvated, and calcined sample of a smectite. Own sample and data.

References:
Bergaya and Lagaly (2006) General introduction: Clays, clay minerals, and clay science. In: Handbook of Clay Science, Edited by F. Bergaya, B.K.G. Theng and G. Lagaly, Developments in Clay Science, Vol. 1.
Hillier (2003) Quantitative Analysis of Clay and other Minerals in Sandstones by X-Ray Powder Diffraction (XRPD)
Moore and Reynolds (1997) X-Ray Diffraction and the Identification and Analysis of Clay Minerals, 2nd edition.

Internet resources for more detailed instructions: 
USGS - A Laboratory Manual for X-Ray Powder Diffraction
The Cutting Edge - Teaching Clay Science 

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