Cookeite is a mineral species of the silicate group and the phyllosilicate subgroup, part of the chlorite family, with the formula LiAl4(Si3Al)O10(OH)8.

[1] This soft, low-density mineral of variable color has a crystalline structure made up of alternating layers LiAl2(OH)6 and Al2O4(OH)2Si8O12 having several polytypes.

[3] The topotype deposit is located in the Mount Mica Quarry, Paris, Oxford County, Maine, USA.

[4] Cookeite is a white mineral with green, brown, golden, or pinkish hues of varying intensity.

It is flexible but inelastic, with perfect cleavage on the {001} plane and micaceous fracture[5] (formation of thin sheets or flakes).

It is therefore composed of the following elements: Impurities often found in cookeite include iron, manganese, magnesium, calcium, sodium, and potassium.

[6] The silicon in the silicate layers may be partly substituted by aluminum, boron or beryllium, with a fairly constant (Al,B,Be)/Si ratio.

Similarly, the tetrahedral sites of the six-membered (Si,Al)6O18 rings are of mixed silicon and aluminum occupancy (yellow in the figure).

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 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.

[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.

It is often associated with the following minerals: albite, lepidolite, microcline, petalite, quartz, spodumene and tourmaline.Cookeite is a product of silicate alteration.

A study of samples collected in the Ardlethan stanniferous field in Australia showed that cookeite forms in the porous interstices of granitic breccias containing quartz, sulfides (such as pyrite), cassiterite, tourmaline, and fluorite.