6 - Lecture notes for Clay Mineralogy

Required reading:

Moore and Reynold,pages 104-137

Brindley and Brown, pages 102-144

Classification of hydrous layered silicates

Clay minerals are part of the larger class of silicate minerals: the phyllosilicates. Included in the phyllosilicate family are the larger true micas, which include the familiar minerals muscovite and biotite and the brittle micas, which includes the less-familar mineral margarite. We have learned much of what we know about clay minerals from the macroscopic (i.e., single crystal) study of the true micas. The true micas will be included in our discussion because they are well characterized and serve as a good model by which to understand clay structures.

Clay components.

Tetrahedral sheets are composed of individual tetrahedrons which share every three out of four oxygens. They arranged in a hexagonal pattern with the basal oxygens linked and the apical oxygens pointing up/down. The resultant sheet composition is T ₂ O ₅ where T is the common tetrahedral cations of Si, Al and sometimes Fe³⁺ and B.

The graphic below shows various views of tetrahedral sheets. The top three illustrate a sideview using three different motifs (space-filling spheres, ball and sticks, and polyhedra). The bottom row shows a top view perspective illustrating the slightly distorted hexagonal linking pattern that results when the basal oxygen share corners.

Octahedral sheets are composed of individual octahedrons that share edges composed of oxygen and hydroxyl anion groups with Al, Mg, Fe³⁺ and Fe²⁺ typically serving as the coordinating cation. These octahedrons too, are arranged in a hexagonal pattern.

Other cations include Li, Ti, V, Cr.... and also vacancies. Note that in the top view a slightly distorted hexagonal patter appears with dimensions very similar to the tetrahedarl sheet.

The minerals gibbsite Al(OH)₃ and brucite Mg(OH)₂ are very similar in structure to the octahedral sheets found in many clay mineral structures. The difference being that all the coordinating anions are hydroxyls in gibbsite and brucite.

Dioctahedral versus Trioctahedral sheets.

In order to maintain electric neutrality each cation site of an edge sharing octahedral sheet must be divalent (i.e., every site is filled). The ratio of divalent cations to oxygens is 1:2 and is known as a brucite-like or -type sheet. The fundamental unit for the octahedral sheet consists of three octahedrons. In this case, where all three of the cation sites are occupied, the arrangement is referred to as a trioctahedral structure. Click here to view the calculated X-ray powder diffraction pattern of brucite.

When trivalent cations (3⁺) occupy the edge sharing hexagonal sheet then the cation to oxygen ratio is 1:3 (in order to maintain electric neutrality). This leaves every third site empty, meaning only 2 out of 3 sites are occupied. This arrangement is referred to as a dioctahedral structure and sometimes called a gibbsite-like or -type sheet.

The tetrahedral, dioctahedral, and trioctahedral sheets are the fundamental building blocks for phyllosilicates. The principle criteria used for classification of phyllosilicates is based the sheet types in the structure.

Let's look at the criteria for classification of phyllosilicates and this will show us how these basic units can be put together to form clay minerals.

Hierarchy of criteria for classification of phyllosilicates

1. The type of tetrahedral-octahedral sheet combinations. Possibilities include:

1:1 (tetra:octa) or (TO)
2:1 (tetra:octa:tetra) or (TOT)
2:1:1(tetra:octa:tetra):octa or (TOT:O)

Some definitions:

Plane = A two-dimensional construct, that defines a family of atoms within the crystal lattice. For example the plane of basal oxygen atoms or the plane of octahedral atoms will often be referred to in our discussions. More formally, planes can be defined by Miller indices.
Sheet = A sub-structure consisting of a network of corner sharing tetrahedra or edge sharing octahedra.
Layer = The combined sheets fundamental to the phyllosilicate type.

Examples of correct references are therefore:

A plane of silicon atoms.
A tetrahedral or octahedral sheet
A 1:1 or 2:1 layer type.

Examples of incorrect references include:

The tetrahedral plane
The octahedral layer
The 2:1 sheet

We try to be precise when using the terms plane, sheet, and layer in clay science (but truth-be-told, every clay scientist has/will likely mis-used the terms at some point in their career).

2. The cation content of the octahedral sheet in the 1:1 or 2:1 layer type (i.e., trioctahedral or dioctahedral).

Sheet nomenclature

Cation type
Cation / Oxygen # of site occupied

Trioctahedral (brucite-like)
divalent
1:2 3 out of 3

Dioctahedral (gibbsite-like)
trivalent
1:3 2 out of 3

Dioctahedaral stuctures can have two possible distributions of cations relative to the symmetry of the unit cell. The empty octahedral site (M1) has a larger volume than the occupied sites (M2). The empty site can be situated on the mirror plane (trans-vacant) or one of the occuppied sites can be situated on the mirror plane (cis-vacant). This is illustrated in a figure from Tsipursky and Drits (1984).

3. Magnitude of the layer charge (most often applied to the 2:1 and 2:1:1 structures).

Sheets may be electrically neutral or they may bear a net negative charge. Charge imbalances usually come about by isomorphous substitution or vacancies.

Most commonly :

[4] Si⁴⁺ ---> [4] Al ³⁺
[6] Al³⁺ ---> [6] Mg ²⁺ Implied is the site of charge imbalance ( [4] vs [6] )
[6] Fe³⁺ ---> [6] Fe ²⁺

Also possible are vacancies:

[6] Mg²⁺ ---> [6] [ ] ⁰

All 2:1 layer structures can be defined in terms of their unit cell composition. The tetrahedral sheet of most unit cells is composed of eight (8) tetrahedral cations and twenty (20) oxygens (T₈O₂₀). It is common practice to report the formula unit composition in terms of the half-unit cell (T₄O₁₀). With silica as the tetrahedral cation (Si₄O₁₀), the sheet is electrically neutral. By substitution of Al for Si we get AlSi ₃O₁₀. In this case the amount of charge (x) needed to balance the net negative charge is equal to one (i.e., x = 1).

4. The interlayer composition.

Neutrality is restore by a compensating cation or ionic group in the interlayer space. Things that can go into the interlayer space include:

K⁺, Na⁺, Ca²⁺, Mg²⁺, Cs⁺, etc .......
NH₄⁺, organic cations, hydroxyl sheets
Water (i.e., aqueous complexes), polar organics (pesticides, fertilizers, protiens,....).

5. Polytype (a special case of polymorphism)

Stacking directions of the layer types convention for notation for polytype structures:

NS_a

where:

N = number of layers in the unit cell
S = Symmetry of the polytpe

T = trigonal

M = monoclinic

H = hexagonal

Tc = triclinic

Or = orthorhombic
a = number of different possible sub-arrangements

Turbostratic = random pile of playing cards.

Examples:

1M - simple a/3 shift in the same direction
2Or - successive 180° rotations
3T - Spiral stacking with 120° rotations (all clockwise or counter clockwise)
6H - Spiral stacking with 60° rotations (all clockwise or counter clockwise)
2M₁- Clockwise 120° rotation followed by a counterclockwise 120° rotation
2M₂-- Clockwise 60° rotation followed by a counterclockwise 60° rotation

Examples from the true mica lepidolite are shown below. Click here for more polytype examples.

Lepidolite 1M
Lepidolite-2M₂
Lepidolite-3T

6. Chemical composition

Isomorphous subsitution allows for solid solutions chemical compositions. If a particular mineral contains an element with higher than average concentrations (e.g., Cr).. but not enough to constitute a new mineral name, than the mineral name modified by that element (e.g., Cr-muscovite).

7. Type of component layers and nature of stacking for mixed-layer clays.

Sheet nomenclature	Cation type	Cation / Oxygen	# of site occupied
Trioctahedral (brucite-like)	divalent	1:2	3 out of 3
Dioctahedral (gibbsite-like)	trivalent	1:3	2 out of 3