Clay Minerals

Definition: Clay minerals are the characteristic minerals of the earths near surface environments.  They form in soils and sediments, and by diagenetic and hydrothermal alteration of rocks.  Water is essential for clay mineral formation and most clay minerals are described as hydrous alumino silicates.  Structurally, the clay minerals are composed of planes of cations, arranged in sheets, which may be tetrahedrally or octahedrally coordinated (with oxygen), which in turn are arranged into layers often described as 2:1 if they involve units composed of two tetrahedral and one octahedral sheet or 1:1 if they involve units of alternating tetrahedral and octahedral sheets.  Additionally some 2:1 clay minerals have interlayers sites between successive 2:1 units which may be occupied by interlayer cations, which are often hydrated.  The planar structure of clay minerals give rise to characteristic platy habit of many and to perfect cleavage, as seen for example in larger hand specimens of micas.  

The classification of the phyllosilicate clay minerals is based collectively, on the features of layer type (1:1 or 2:1), the dioctahedral or trioctahedral character of the octahedral sheets (i.e. 2 out of 3 or 3 out of 3 sites occupied), the magnitude of any net negative layer charge due to atomic substitutions, and the nature of the interlayer material.

The basis on which clay minerals are classified is shown below; see Hillier (2003) for a more detailed introduction to clay mineralogy.

LAYER TYPE Layer charge (q) Group Subgroup Species (e.g.)
1:1 q≈0 Kaolin-Serpentine Kaolin Kaolinite
Serpentine Berthierine
2:1 q≈0      q≈1 Increasing layer charge Pyrophyllite-talc Pyrophyllite Pyrophyllite
Talc Talc
Smectite (q≈0.2-0.6) Di-smectite Montmorillonite
Tri-smectite Saponite
Vermiculite (q≈0.6-0.9) Di-vermiculite Di-vermiculite
Tri-vermiculite Tri-vermiculite
Mica (q≈1.0) Di-mica Illite, Muscovite
Tri-mica Biotite
 q variable Chlorite Di-chlorite Sudoite
Tri-chlorite Chamosite
Sepiolite-Palygorskite Sepiolite Sepiolite
Palygorskite Palygorskite
Variable q variable Mixed-layer Di-mica-di-smectite Rectorite
Tri-chlorite-tri-smectite Corrensite

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Analyses: Some types of clay minerals such as mixed-layered clay minerals can only be identified precisely by techniques such as XRD.  Although it is not unusual to have to use a variety of techniques such as XRD, infrared spectroscopy, and electron microscopy to characterise and more fully understand types of clay minerals present in a sample.  We have extensive experience of the identification of clay minerals in both soils and rocks.  Our XRD work is based is backed up by our ability to compare clay mineral diffraction data with calculated diffraction data.  This is a particularly important technique for the precise identification of mixed-layer clay minerals.  Our track record in the Reynolds Cup round robin on quantitative clay mineral analysis is testimony to the quality of our work on the identification and quantification of clay minerals. We also have wide experience of the use of electron microscopy to study the texture and petrographic relationships of clay minerals. A useful gallery of clay mineral images, showing some of the different morphologies in which clay minerals can occur can be found HERE.


Hillier, S., 2003, Clay Mineralogy, in GV Middleton, MJ Church, M Coniglio, LA Hardie, and FJ Longstaffe eds., Encyclopaedia of sediments and sedimentary rocks: Kluwer Academic Publishers, Dordrecht., p. 139-142.