Halloysite

Halloysite
Trammission electronic microscope (TEM) image of halloysite nanotubules (unknown scale)
General
Category	Phyllosilicates Kaolinite-serpentine group
Formula (repeating unit)	Al2Si2O5(OH)4
Strunz classification	9.ED.10
Crystal system	Monoclinic
Crystal class	Domatic (m) (same H-M symbol)
Space group	Cc
Unit cell	a = 5.14, b = 8.9, c = 7.214 [Å]; β = 99.7°; Z = 1
Identification
Color	White; grey, green, blue, yellow, red from included impurities.
Crystal habit	Spherical clusters, massive
Cleavage	Probable on {001}
Fracture	Conchoidal
Mohs scale hardness	2 - 2.5
Luster	Pearly, waxy, or dull
Diaphaneity	Semitransparent
Specific gravity	2 - 2.65
Optical properties	Biaxial
Refractive index	nα = 1.553 - 1.565 nβ = 1.559 - 1.569 nγ = 1.560 - 1.570
Birefringence	δ = 0.007
References

Halloysite is a 1:1 aluminosilicate clay mineral with the empirical formula Al₂Si₂O₅(OH)₄. Its main constituents are aluminium (20.90%), silicon (21.76%) and hydrogen (1.56%). Halloysite typically forms by hydrothermal alteration of alumino-silicate minerals. It can occur intermixed with dickite, kaolinite, montmorillonite and other clay minerals. X-ray diffraction studies are required for positive identification. It was first described in 1826 and named after the Belgian geologist Omalius d'Halloy.

Two common forms are found, when hydrated the clay exhibits a 1 nm spacing of the layers and when dehydrated (meta-halloysite) the spacing is 0.7 nm. Halloysite naturally occurs as small cylinders which average 30 nm in diameter with lengths between 0.5 and 10 μm. The cation exchange capacity depends on the amount of hydration, as 2H₂O has 5–10 m.e./100g, while 4H₂O has 40–50 m.e./100g. "Endellite" is the alternative name for the 4H₂O structure.

The formation of halloysite is due to hydrothermal alteration, and it is often found near carbonate rocks. For example, halloysite samples found in Wagon Wheel Gap, Colorado, United States are suspected to be the weathering product of rhyolite by downward moving waters. In general the formation of clay minerals is highly favoured in tropical and sub-tropical climates due to the immense amounts of water flow. Halloysite has also been found overlaying basaltic rock, showing no gradual changes from rock to mineral formation. Halloysite occurs primarily in recently exposed volcanic-derived soils, but it also forms from primary minerals in tropical soils or pre-glacially weathered materials. Igneous rocks, especially glassy basaltic rocks are more susceptible to weathering and alteration forming halloysite.