Clays and Layered Silicates

Clays and Layered Silicates

X-ray powder diffraction analysis of clay minerals and layered silicates is carried out along the guidelines of the United States Geological Survey. Detailed procedures for sample preparation, particle size separation, and other specific treatments to identify the types of clay can be found here. Clays used as purification agents in pharmaceutical applications are analyzed according to USP protocol <941>.

Click to enlarge.
Click to enlarge.
Recording of the basal layer peak shift from 15.5Ã to 17.5Ã upon ethylene glycol treatment of smectite (Cu-Kα rad)
Magnesium aluminum silicate identification as per USP protocol
 (X-ray Diffraction <941> Cu-Kα rad)

Geological samples, such as soils or rocks, are first crushed and pulverized for sample reduction. To prevent sample segregation and ensure representative subsamples, a small amount of the processed material is selected by either splitting the material with a riffler or by coning and quartering the material. XRD data sets are collected on a wide angle powder diffractometer in the following order:

  • Survey scan over an extended range from a random powder mount.
  • Detailed scans of the low-angle region of an oriented slide mount after drying in air and exposed to an ethylene glycol atmosphere.
  • Repeat measurement of the low-angle region after heating.

Data from a random powder mount usually provides an idea about the average composition of major and minor minerals present. Oriented slide mounts provide the means to determine the clay mineralogy of the sample. Slides are prepared from the <2um fraction of the sample, which was obtained by gravity sedimentation. An additional heat treatment can be applied as well to further determine the type of clay mineral. The effects of ethylene glycol (EG) solvation and heat treatment can be found here.

Certain applications require a quantitative breakdown of the observed mineral phases. This is accomplished with FULLPAT1 and a reference library containing diffraction data of the most common rock forming minerals and clays.

One important aspect of XRD clay mineralogy is the identification of expansive clays in aggregate material for the construction industry. Please follow this link to Chemistry of Concrete to find out more.

1S. J. Chipera and D. L. Bish, “FULLPAT: a full-pattern quantitative analysis program for X-ray powder diffraction using measured and calculated patterns”, J. Appl. Cryst. (2002). 35, 744-749
CAMET Research, Inc.
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