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Clay Mineralogy

Bricks have been made of clay for over 6000 years using the same basic principles until today. These are the plastic deformability of clay minerals in aqueous systems and the irreversible mineral reactions during ceramic firing. The plastic clay body is transformed into solid mineral phases.

Kaolinite / Fireclay: Widens the sintering interval and reduces the fired body thermal conductivity
Kaolinite / Fireclay: Widens the sintering interval and reduces the fired body thermal conductivity
 
Pyrophyllite: Reduces the plasticity and significantly increases the refractoriness
Pyrophyllite: Reduces the plasticity and significantly increases the refractoriness
Talc: Improves the moulding during extrusion with vacuum press
Talc: Improves the moulding during extrusion with vacuum press

The clay mineralogical composition of the raw materials is decisive for the quality of the brick and the efficiency of the production process. The molecular crystalline structure has a significant influence on the brick production. Sub-microscopic structures with a particle size of less than 2 µm dictate the ceramic processing parameters.

Muscovite / Sericite / Illite: Potassium acts as a flux agent and reduces the water absorption
Muscovite / Sericite / Illite: Potassium acts as a flux agent and reduces the water absorption
 
Montmorillonite: Extremely increases the plasticity and the drying and heating sensitivity
Montmorillonite: Extremely increases the plasticity and the drying and heating sensitivity
Chlorite: Shortens the sintering interval and reduces the linear drying shrinkage
Chlorite: Shortens the sintering interval and reduces the linear drying shrinkage

Mineralogical phase analysis, its interpretation and practical application are very fascinating topics. For further information please contact us. Phone: +49 551 504550. E-Mail: krakow@rohstoffconsult.de.

Clay Mineral Structure

Clay minerals are hydrous aluminium silicates. They usually consist of planar crystals with a particle size of less than 2 µm. Oxygen and hydroxide form tetrahedral and octahedral sheets with intercalated cations. All models are modified after GRIM, R. E. (1953).

Model of SiO4-tetraedras (view perpendicular to the crystallographic c-plane)
Model of SiO4-tetraedras (view perpendicular to the crystallographic c-plane)

The central tetrahedral position is normally occupied by silicon ions that can be partially substituted by aluminium ions. The central cation is surrounded by four oxygen ions at an angle of 109,5° that form an equilateral tetrahedron.

The SiO4-tetrahedrons share three mutual oxygen anions in a two-dimensional plane, with all tetrahedrons pointing in the same direction. The two-dimensional network in the crystallographic a-b plane has the chemical composition [Si2O5]2- and forms a hexagonal structure. The tetrahedral sheet has a thickness of 4,93 Å.

The SiO4-tetrahedrons are arranged in hexagonal rings  perpendicular to crystallographic c-axis
The SiO4-tetrahedrons are arranged in hexagonal rings perpendicular to crystallographic c-axis

M2/3(OH)6-octahedral sheets are the second basic element of phyllosilicate clay mineral structure. Each positively charged metal ion is surrounded by six negatively charged hydroxide-ions that form an octahedron.

Model of the M2/3(OH)6 octahedral sheet (view perpendicular to the crystallographic c-plane)
Model of the M2/3(OH)6 octahedral sheet (view perpendicular to the crystallographic c-plane)

The central octahedral position is usually occupied by Al3+ that can be substituted by magnesium and iron ions, or other ions. In dioctahedral clay minerals two thirds of the central octahedral positions are occupied by metal ions (predominantly with Al3+ and Fe3+) for charge balance. In trioctahedral clay minerals on the other hand, all central octahedral positions are occupied (predominantly with Mg2+ and Fe2+). The thickness of the octahedral sheet varies depending on the central cation:

Al3+ = 8,64 Å, Fe3+ = 9,00 Å, Mg2+ = 9,39 Å, Fe2+= 9,78 Å.

 

In contrast to the tetrahedral sheets, the octahedral sheets are also stable as independent minerals such as as Brucit Mg3(OH)6 and Gibbsit Al2(OH)6.

Tetrahedral and octahedral sheets form layers in the crystallographic c-direction, bonded by mutual oxygens. The free tetrahedral oxygens connect tetrahedral and octahedral sheets. Two thirds of the octahedral hydroxide ions are substituted by these tetrahedral oxygens. The classification of clay minerals is defined by the type of layering and the layer distance:

Kaolin-Serpentine-Group

The clay minerals of this group have one tetrahedral and one octahedral sheet packed into each layer and is therefore a 1:1 clay. The characteristic layer distance for this group is co = 7,0 – 7,3 Ångström (Å) depending on the type of mineral. The sheets are held together by dipole-dipole interactions, hydrogen bonds and Van der Waals forces. Minerals of the kaolin-serpentine group are non-swellable clay minerals and comparably stable mineral phases. A combination of 50 sheets/layers builds up one kaolinite particle.

Spatial display of the inner-crystalline non-swellable kaolinite structure.
Spatial display of the inner-crystalline non-swellable kaolinite structure.

Kaolin minerals are dioctahedral two-layer silicates with Al3+ occupying two-thirds of the central octahedral positions. Examples are kaolinite, and disordered kaolinite known as kaolinite-D with a shifted crystal lattice in the b-direction. The ideal chemical formula is:

Kaolin minerals: Al2 [(OH)4 Si2O5]

 

Serpentine minerals are two-layer silicates with all central octahedral positions occupied by Mg2+-ions. Examples are the minerals chrysotile and antigorite with the ideal chemical formula:

Serpentine minerals: Mg3 [(OH)4 Si2O5]

 

Serpentine minerals are not relevant for the brick industry but are mentioned for the understanding of the clay mineral systematics.

Pyrophyllit-Talc Group

Three-sheet silicates of the 2:1 type are formed by layers of one octahedral sheet between two tetrahedral sheets. The free oxygens of both tetrahedral sheets point towards the central octahedral sheet. In the dioctahedral pyrophyllite the central octahedral position is occupied by Al3+, while in the trioctahedral talc it is occupied by Mg2+. In both minerals the layers have no charge.

Spatial display of the inner-crystalline non-swellable pyrophyllite structure
Spatial display of the inner-crystalline non-swellable pyrophyllite structure

The bonds between the sheets are dipole-dipole interactions, hydrogen bonds and Van der Waals forces. The layers are not charged and do not show inner-crystalline swelling capacity. The characteristic layer distance for this group is c0 = 9 – 10 Å. The ideal chemical formulas are:

Pyrophyllite: Al2 [(OH)2 Si4O10]

 

Talc: Mg3 [(OH)2 Si4O10]

Illit-Mica-Group

Minerals from the illite-mica group are three-sheet silicates of the 2:1 type. In contrast to the phyrophyllite-talc group they have charged layers, because Si4+-ions are substituted by Al 3+-ions in the tetrahedral sheets. The negative charge of the layers is compensated by large potassium ions in the interlayer space. The electrostatic attraction between potassium ions and tetrahedron-octahedron layers results in a non-swellable crystal lattice with a basic distance of co = 10 Å.

Spatial display of the muscovite-illite structure. Intrusion of water molecules causes swelling at the edges of the structure and widens the layer distance c0.
Spatial display of the muscovite-illite structure. Intrusion of water molecules causes swelling at the edges of the structure and widens the layer distance c0.

Peripheral removal of potassium-ions due to weathering processes causes partial intrusion of water molecules into the interlayer space. They are responsible for effective particle disintegration, high plasticity and swelling of the crystal lattice fringes. The result is the following mineral transformation:

Muskovite – Sericite – Illite – Mixed Layer – Montmorillonite.

 

Micas differ in the type of cation in their central octahedral position. In dioctahedral muscovite, sericite and illite trivalent aluminium occupies the central octahedral position whereas in trioctahedral biotite bivalent magnesium or iron ions occupy the central octahedral position. Muscovite, sericite and illite are of high importance in the brick industry. One illite particle is formed by 10-20 elemental layers.

Muskovite: K Al2 [(OH)2 Al Si3O10]

 

Illite: Kx Al2 [(OH)2 Alx Si4-xO10]     (x < 1)

 

Glauconite is iron-rich illite with substitutions of Fe3+, Fe2+ und Mg2+ in the octahedral sheets. Transitions to trioctahedral structures are possible.

Smectite-Group

The structure of smectite can be derived from the mica structure. Smectites are tree-sheet silicates with charged layers. Compared to micas the layer charge is low, the interlayer space is filled with hydrated cations instead of potassium ions. Weak electrostatic forces between negatively charged layers and interlayer cations lead to strong inner-crystalline swelling capacity and low stability.

Spatial display of inner-crystalline swellable montmorillonite
Spatial display of inner-crystalline swellable montmorillonite

The layer distance varies between co = 12 - 18 Å depending on the type of interlayer cations and the degree of hydration. Complete disintegration of the layer grid is possible, if the interlayer space is filled with sodium ions. One smectite particle is composed of 2-5 layers.

Montmorillonite is a dioctahedral smectite with mainly Al3+ and Mg2+ -ions occupying the central octahedral position..

Montmorillonite: (Al, Mg)x [(OH)2 Si4O10] nH2O (Na, K, Ca0,5)X

Chlorite-Group

Clay minerals of the chlorite group are composed of negatively charged layers, and positively charged octahedral interlayer sheets. The octahedral interlayer sheets are mostly Mg(OH)2. Minerals of this group are named 2:1:1 type three-sheets minerals or 2:2 type four-sheet minerals. The negative charge of the layers is a result of silica-aluminium substitution in the tetrahedral sheets.

Spatial display of the inner-crystalline non-swelling chlorite structure with octahedral interlayer sheet (brucite)
Spatial display of the inner-crystalline non-swelling chlorite structure with octahedral interlayer sheet (brucite)

Chlorites are inner-crystalline non-swelling clay minerals with a layer distance of co = 14 Å. Most chlorites are trioctahedral with magnesium ions occupying the central octahedral position. Chlorites can have a wide range of chemical compositions.

Mineralogical Phase Analysis

From a geological point of view, clays are the most complicated natural mineral aggregates. Not least because of their submicroscopic particle size, a direct visual observation of these minerals is not practicable. To determine the type and portion of components of clay mineral mixture, it is necessary to combine both radiographical and thermoanalytical methods. Prevalent methods are scanning electron microscopy (SEM), X-ray fluorescence analysis (XRF), X-ray diffraction analysis (XRD), simultaneous thermal analysis (STA ) and particle-size distribution (PSD).

Phase Analysis/ X-ray diffractometry diagram (2015)
Phase Analysis/ X-ray diffractometry diagram (2015)
Phase Analysis/ Infrared Spectrum (2015)
Phase Analysis/ Infrared Spectrum (2015)
Phase Analysis/ simultaneous thermal analysis (2016)
Phase Analysis/ simultaneous thermal analysis (2016)

X-ray diffraction as methodological basis

The phase analytical determination of clay minerals is important for the technical use of clay. The clay mineralogical structure and composition has direct implications for the technical properties of the clay. This is important for the use of clay in the ceramics industry and in other fields, as well.

Quantitative mineral phase analysis of clay minerals is one of the most complex tasks in mineralogy. The work of Max von Laue has paved the way for X-ray analysis on crystals. In 1912 he discovered the diffraction of X-ray beams in crystals. This not only proved the wave-character of X-rays but also the lattice structure of crystals. He received the Nobel Prize in Physics in 1914 for his discovery. Until today, X-ray diffraction is the most important methodological basis in clay mineralogy.

Principle of X-ray diffraction

X-ray diffraction is the elastic scattering of X-rays by atoms in a periodic lattice. Resulting interference clusters are diagnostic for the crystal structure. Bragg’s law is the mathematical relation between a crystallographic structure and a diffraction image produced by X-ray diffraction. Diffraction diagrams show interference maxima (peaks) at specific angles depending on the crystal lattice. Each crystalline substance produces an individual X-ray diffractogram which can be assigned to specific mineral phases.

Structure determination with X-ray diffractometry, Thomas Splettstoesser (www.scistyle.com)
Structure determination with X-ray diffractometry, Thomas Splettstoesser (www.scistyle.com)

Analysis of Mineral Assamblages

Mineral resources are usually an assemblage of various mineral phases. X-ray diffraction does not provide quantitative results because the intensities of interference maxima do not correlate with the amount of a mineral phase. Moreover, the assignment of peaks to specific mineral phases can be complicated if peaks of different minerals overlap. This is especially the case for clay minerals because they are structurally very similar. Hence it is recommended to analyse clay minerals separately without other minerals present.

In a centrifugal field the fraction < 2µm is separated from an aqueous dispersion. The identification and differentiation of clay minerals is based on the ascertainment of the basal face interspaces dL (001 reflexes) in c-direction and the determination of the b-parameter (060-reflex). That is why the texture specimen should ideally have a parallel orientation of the single clay particles. Because some layer distances coincide and produce similar reflexes, proof of swelling capacity has to be undertaken for distinct identification. To provide this evidence, a second texture specimen is treated with ethylene glycol vapor in a vacuum desiccator. In case of an inner-crystalline swellable mineral, a diagnostic expansion of the crystal lattice will be detected.

Supplementary Infrared Spectroscopy (FTIR)

A short-coming of X-ray diffraction is that it can only reliably detect crystalline phases and is therefore not sufficient for the complete determination of type and amount of clay mineral phases in a mineral assemblage. Amorphous phases such as volcanic glass cannot reliably be detected qualitatively nor determined quantitatively. For this reason, X-ray diffraction has to be combined with an additional method. Infrared Spectroscopy has proven to be the most suitable. A short-coming of X-ray diffraction is that it can only reliably detect crystalline phases and is therefore not sufficient for the complete determination of type and amount of clay mineral phases in a mineral assemblage. Amorphous phases such as volcanic glass cannot reliably be detected qualitatively nor determined quantitatively. For this reason, X-ray diffraction has to be combined with an additional method. Infrared Spectroscopy has proven to be the most suitable.

The method is based on interactions between infrared radiation (0.7 – 500 µm) and matter. The absorbtion of defined energy values of infrared radiation causes individual molecular groups to show a characteristic vibration spectrum. Because the amount of absorbed energy depends on the number of molecules in the infrared beam, quantitative data on the mineral content can be generated. The ascertainment of the minerals amount is calculated by analyzing the extinctions of the beam on selected measurement marks. Even amorphous, microcrystalline or poorly crystallized phases can be detected and analyzed by infrared spectroscopy.

Scanning Electron Microscopy

The finer the matter, the more complicated and complex the analysis - this is the simple way to put it. With scanning electron microscopy (SEM) it is possible to visualize the finest mineral textures and microstructures of complex clay mineral aggregates. The SEM images below show the considerable differences between the various types of clay minerals. All SEM images were provided courtesy of the "Clay Archive of Mineralogical Society of Great Britain & Ireland and the Clay Minerals Society".

Kaolinite, wormlike structures/Southern Australia (www.minersoc.org/gallery.php?id=2)
Kaolinite, wormlike structures/Southern Australia (www.minersoc.org/gallery.php?id=2)
 
Chlorite, parallel fabric/Poland (www.minersoc.org/gallery.php?id=2)
Chlorite, parallel fabric/Poland (www.minersoc.org/gallery.php?id=2)
 
Smectite,
Smectite, 'house of cards'-like structure/Antarctica (www.minersoc.org/gallery.php?id=2)

Chlorite and fibrous illite/North Sea (www.minersoc.org/gallery.php?id=2)
Chlorite and fibrous illite/North Sea (www.minersoc.org/gallery.php?id=2)
 
Corrensite,
Corrensite, 'house of cards'-like structure/Central Africa (www.minersoc.org/gallery.php?id=2)
 
Kaolinite, stacked structure/Georgia USA (www.minersoc.org/gallery.php?id=2)
Kaolinite, stacked structure/Georgia USA (www.minersoc.org/gallery.php?id=2)

Dickite, parallel structure/Scotland (www.minersoc.org/gallery.php?id=2)
Dickite, parallel structure/Scotland (www.minersoc.org/gallery.php?id=2)
 
Illite, fibre-like structure/North sea (www.minersoc.org/gallery.php?id=2)
Illite, fibre-like structure/North sea (www.minersoc.org/gallery.php?id=2)
 
Smectite,
Smectite, 'house of cards'-like structure/Nevada USA (www.minersoc.org/gallery.php?id=2)