An interlamellar arrangement of alkylammonium cations with the positively-charged (polar) end groups (e.g., NH₃⁺) of the alkylammonium cations attached to the interlayer silicate surfaces of a 2:1 phyllosilicate. Kinks in the alkyl chains shift the (non-polar) alkyl group tails between other alkylammonium alkyl chains to form a pseudotrilayer with a characteristic spacing of ~22Å as determined from the thickness of three alkyl chains (Lagaly and Weiss, 1975; Lagaly, 1994).
Cf., alkylammonium layer charge method, alkylammonium organoclay bilayer, alkylammonium organoclay monolayer, organoclay

Obsolete name replaced by rectorite.
See rectorite

An “alligator hide/skin” texture relates to a macro-texture observed on exposed and weathered clay surfaces. The texture evolves from polygonal shrinkage cracks that develops from bentonites with low dialatancy.
See also popcorn texture.

A semiordered hydrous aluminosilicate, Al₂O₃(SiO₂)_1.3-2.0 2.5-3.0(H₂O), that is closely related to imogolite because both seem to be incipient 1:1 minerals structurally and chemically, although it has been suggested that allophane may be an incipient montmorillonite. The ratio of SiO₂ to Al₂O₃ is usually 1.3 to 2.0 for allophane, but has been reported as low as 0.83, a greater variation than in imogolite. Allophane shows curved walls with a more spherical morphology (30 – 50 Å spherical diameters) than imogolite, which is tube-like. Allophane is common in soilsderived from volcanic ash, but may be present in soils derived from basic igneous rocks in tropical climates or in podzol soils derived from more acidic rocks. Allophane may precipitate in hot springs rich in silicic acid and aluminum. Allophane is white or colorless when moist, but earthy when dried.
Syn., disordered allophane
Cf., imogolite

An alteromorph is an alteration product of a crystal that preserves the external anhedral form of the precursor mineral. The alteromorph may have a different chemical composition and/or crystal structure than the precursor crystal. The alteromorph can result from replacement in the solid state or by fluid interactions.
Cf., centripetal replacement, pseudomorph

Alumina refers to Al₂O₃ chemical composition only and the term does not imply a structure. The term commonly is used to describe a mineraloid or a glass, as in a alumina-rich glass. Thus, alumina does not specifically refer to corundum or other alumina-rich phase (e.g., boehmite, diaspore).
Cf., mineraloid

A dioctahedral member of the true mica group. The end-member formula is KAl(Mg,Fe³⁺)□Si₄O₁₀(OH)₂, where □ = vacancy. Typical range in composition is: ^viR²⁺ / (^viR²⁺ + ^viR³⁺) ≥ 0.25, ^viAl / (^viAl + ^viFe³⁺) = 0.5 – 1.0, Mg / (Mg + ^viFe²⁺) > 0.5 (Rieder et al., 1998).

A surficial AlOH group (see silanol group), which is able to incorporate or dissociate protons.
See silanol grou

“In general, silicates which contain tetrahedrally coordinated aluminum are called aluminosilicates in contrast to silicates containing octahedrally coordinated aluminum for which the term aluminum silicates is used…” (Liebau, 1985, p. 13). Although this is the classic definition of aluminosilicate, many texts use the term inconsistently. For example, kyanite and andalusite, traditionally considered part of the “aluminosilicate group” with sillimanite, do not have tetrahedrally coordinated Al, although they are closely related (structurally and geologically) to each other and sillimanite. Clearly, taking the first part of definition by Liebau (1985) as the basis of the definition, the second part should not be limited to aluminum in octahedral coordination only. Phyllosillicates are generally considered aluminosilicates, because most phyllosilicates do have Al substitution for Si, but not all do (and those minerals without tetrahedral Al present, but with Al in other polyhedral coordinations are more properly referred to as “aluminum silicates”).

An obsolete varietal term for manganoan muscovite and manganoan illite.