page_title: Clay mineral X-ray diffraction
section_title: Identification of Clays using XRD
hierarchical_section_title: Clay mineral X-ray diffraction / X-Ray Diffraction and Clays / Identification of Clays using XRD
caption_attribution_description: English: X-ray diffraction pattern of a Nontronite standard purchased from the clay mineral society. Red boxes represent a wide peaks attributed to nontronite. Blue box represents a sharp peak attributed to quartz present in the standard.
context_page_description: Clay minerals are one of the most diverse minerals but all have a commonalty of crystal or grain sizes below 2 µm. Chemically clays are defined by crystal structure and chemical composition. Sometimes fine grain sediments are mistakenly described as clays, this is actually a description of the “clay-size fraction” rather than the mineralogy of the sediment. There are three crystallographic clay groups: platy clays, fibrous clay minerals, and amorphous clay. Phyllosilicates are the most abundant clays and are categorized based on the layering of a tetrahedral and an octahedral layer. For most clays, the octahedral layer is centered with Al³⁺, Fe³⁺, or Mg(OH)₂, but sometimes Zn²⁺, Li⁺, and Cr³⁺ can substitute as well. Si⁴⁺ is normally the center of the tetrahedral layer but Al³⁺ will often partially substitute and create a charge imbalance. Two-layer clays are composed of a tetrahedral layer and an octahedral layer while three-layer clays contain an octahedral layer sandwiched by two tetrahedral layers. When substitution of Al³⁺ for Si⁴⁺ creates a charge imbalance, an interlayer cation will fill in between tetrahedral layers to balance the charge of the clay.
context_section_description: Basal reflections give d-spacing of the basal layer which represent the thickness of the silicate layers and the unit cell often contains multiple layers. Clay mineral peaks are can generally be distinguished by the width halfway up the peak (i.e. the full width at half maximum, FWHM). Well-defined crystalline minerals have sharp peaks while clays, which range from crystalline to noncrystalline, produce broad peaks with noticeable width on both sides. These broad peaks make it easy to pick out which peaks are contributed by clays. These peaks can be compared to known diffraction patterns for better identification but if some peaks are broader than others, it is likely that multiple clays are present. The Clay Mineral Society maintains a collection clays for the purpose of comparison to unknown clays. Because the majority of the clays available from the Clay Mineral Society are naturally formed, they can contain minerals other than the desired clay. Diffraction patterns calculated using theoretical methods do not generally match with experimental diffraction patterns, so using diffraction patterns from known samples to help identify a clay is preferable to calculation. Some minerals can be eliminated from identification using background information or prior analysis.
  Well-crystallized and pure samples are ideal for x-ray diffraction, but this is rarely the case for clay. Clay minerals are almost always mixed with very small amounts of nonclay minerals which can produce intense peaks, even when there is very little of the sample is not a clay. If additional minerals are known to be presents, attempts should be made to separate clays from nonclays otherwise additional peaks should be expected. Some common minerals associated with naturally occurring clays are; quartz, feldspars, zeolites, and carbonates organic matter is sometimes present. Synthesis of clays can reduce the presence of some of these associated materials but does not guarantee pure samples as quartz or other associated materials are still commonly produced alongside synthetic clays.
caption: X-ray diffraction pattern of a Nontronite standard purchased from the clay mineral society. Red boxes represent a wide peaks attributed to nontronite. Blue box represents a sharp peak attributed to quartz present in the standard.
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