Overview

Cookeite generally crystallizes in the monoclinic crystal system. Its space group can be C2,^[7] Cc^[8] (monoclinic) or C1^[9] (triclinic).

Its structure consists of a stack of layers containing metallic elements and aluminosilicate layers. The coordination polyhedron of the metallic elements is an octahedron, whose vertices are O^2- anions or hydroxyl (OH)^- groups. The aluminosilicate layers contain an octahedrally coordinated aluminum layer and two aluminosilicate layers formed by six-membered (Si,Al)₆O₁₈ rings of (Si,Al)O₄ tetrahedra. The layers are connected by hydrogen bonds.^[10]

There are two types of octahedral layers, denoted O and O', which differ in their degree of filling. Cookeites belong to the "di, trioctahedral" chlorites, i.e. they possess:

• a layer of O octahedra, with all sites occupied by metallic elements (trioctahedral layer);
• a less dense layer of O' octahedra, only two-thirds of whose sites are occupied (dioctahedral layer). Unoccupied octahedral sites are called "vacant sites". This layer is surrounded by two layers of six-membered rings of aluminosilicate tetrahedra, noted T, to form T-O'-T layers.

• 
  Trioctahedral layer O, of chemical composition LiAl
  ₂(OH)
  ₆.
• 
  Dioctahedral layer O', of chemical composition Al
  ₂O
  ₄(OH)
  ₂.
• 
  Aluminosilicate layer T-O'-T. The black circle indicates six-membered rings.

The O layer contains the metal cations Al³⁺ and Li⁺. The distribution of these cations is not ordered in the structure: octahedral sites can host either Al³⁺ or Li⁺, but the occupation of the sites by these cations varies from cell to cell in a non-periodic fashion. For this reason, octahedral sites are referred to as "mixed occupancy" sites. The occupancy of a site by different chemical species is described by its occupancy percentages, i.e. by the probability of finding a certain chemical species there. For example, the trioctahedral O layer of r-cookeite shown above has three different sites,^{[nb 1]} two of which are predominantly occupied by aluminum (orange octahedra) and the third by lithium (green octahedra). Similarly, the tetrahedral sites of the six-membered (Si,Al)₆O₁₈ rings are of mixed silicon and aluminum occupancy (yellow in the figure).

The structure of cookeite can be seen as an alternating stack of brucite-type layers and talc-type layers.^[10] Indeed, brucite Mg(OH)₂ consists of Mg₃OH₆^[11] layers, and talc Mg₃(Si₄O₁₀)(OH)₂ consists of Mg₂O₄(OH)₂Si₈O₁₂ layers.^[12] In cookeite, Li⁺ and Al³⁺ cations take the place of Mg²⁺ cations in brucite-type layers (O layers) and talc (O' layers).

The stacking of layers is not identical in all cookeites: there are several polytypes, distinguished by the length of the stacking period (lattice parameter c for monoclinic cookeites).

Cookeite r

Structure of the r-cookeite, projected along the b-direction (perspective view). Yellow: Si, orange: Al, green: Li, blue: O, grey: H. The six-membered ring structure of the T layers is particularly visible in the left-hand layer.

Cookeite "r" is a rare lithium-rich cookeite with the formula Al₂(Al₂Li)(Si_3,04Al_0,96)O₁₀(OH)₈. It crystallizes in the space group Cc, with Z=4 form units per conventional cell.^[8] Its cell parameters are ɑ= 5,158 Å, b = 8,940 Å, c = 28,498 Å and β = 96,6° (unit cell volume V = 1 305,41 ų) and its volumetric mass density is 2,66 g/cm³.

Its structure consists of an alternating stack of two types of layers, noted O and T-O'-T, parallel to the plane (ɑ, b).

The O layers, with a composition LiAl₂(OH)₆⁺, are made up of octahedrons (Al,Li)(OH)₆ connected by their edges. All octahedra are of mixed aluminum and lithium occupancy. O-layer cations are distributed over three non-equivalent sites by symmetry. Two-thirds of the sites are preferentially occupied by Al³⁺ à 91 % (orange octahedrons in the figure opposite), a third of the sites contain 82 % of Li⁺ (green octahedra). The average (Al,Li)-OH bond length is 1.95 Å for octahedra containing predominantly aluminum and 2.11 Å for the others.^[8]

The T-O'-T layers, with a composition Al₂O₄(OH)₂Si₈O₁₂, are made up of an O' layer of octahedrons AlO₄(OH)₂ containing exclusively aluminium, surrounded by two T layers of tetrahedrons (Si,Al)₄O₁₀, structured in the form of six-membered rings (Si,Al)₆O₁₈ in which the tetrahedrons (Si,Al)O₄ layers have a mixed occupancy of silicon and aluminum. The Al³⁺ cations in the O' layers are symmetrically distributed over two non-equivalent sites, with an average Al-O bond length of 1.92 Å. The cations (Si⁴⁺,Al³⁺) layers are distributed over four non-equivalent sites, with an average (Si,Al)-O bond length of 1.66 Å. There are two T-O'-T layers in the r-cookeite: the first is located at the edge of the cell, around the (x, y, z=0) coordinate plane, and the second is located in the center of the cell, around the (x, y, z=1/2)^{[nb 2]} coordinate plane. These two layers are identical and equivalent through the translatory mirror c with coordinates (x, y, z=1/4).^{[13][nb 3]} Since the O' layer has only one vacant site in the cell, when the layers are stacked, the vacant sites of the two layers are shifted in the (a, b).

Other cookeites

A cookeite with the formula (Al_1,85Fe_0,13)(Li_0,65Al_1,96)(Si_3,4Al_0,6)O_10,35(OH)_7,65 from the Djalair bauxite deposit in Central Asia has been described in triclinic space group C1 (Z = 2 form units per cell), with cell parameters = 5.14 Å, = 8.90 Å, = 14.15 Å, α = 90.55°, β = 96.2° and γ = 90° (cell volume V = 643.49 ų). It has a volumetric mass density of 2,69 g/cm³. Its structure is noted as IIa in the nomenclature of chlorite polytypes.^[9]

The cell parameter of cookeite IIa is half that of r-cookeite: the difference between these two structures is due to the stacking of the O and T-O'-T layers. Within the layers themselves, there is no notable structural difference between this cookeite and the r-cookeite. However, the dioctahedral O' layers of cookeite IIa are all identical and ordered so as to have the vacant sites one above the other in the stacking direction of the layers, whereas in r-cookeite, due to the translatory mirror c, the vacant sites of the O' layers do not overlap.

Note: there are two space groups in the triclinic system: P1. The symmetry of this cookeite is P1; its description in the unconventional space group C1, inducing an artificially doubled cell in the (ɑ, b) plane with respect to the primitive conventional cell, makes it easier to compare its structure with that of monoclinic cookeites.