An alkaline activated clay is a clay treated with Na₂CO₃ (soda activation) to increase the proportion of Na-exchanged smectite or the addition of alkaline solutions to increase the pH of the clay aqueous suspension. In either case, bacterial growth is promoted (ageing).
Cf., activated clay, ageing

A method to determine the layer charge of phyllosilicates with 2:1 layers, primarily those phyllosilicates with interlayer-cation exchange capability, interstratified complexes, and micas. The layer charge is determined based on changes in basal spacing [d(001) value] as derived from various alkyl-chain lengths (C6-C18) of alkylammonium cations, following exchange in solutions with a series of n-alkylammonium hydrochloride salts (Lagaly and Weiss, 1969; Lagaly and Weiss, 1975). Alkylammonium-exchanged, low-charge smectites produce smaller basal-spacing values than higher-charge smectites or vermiculites. Greater expansion is produced by n-alkylammonium salts with longer alkyl chain lengths. Layer charge is estimated from the monolayer to bilayer transition alkyl-chain length. See Lagaly (1994) for further description, problems, and history.
Cf., alkylammonium organoclay bilayer, alkylammonium organoclay monolayer, alkylammonium organoclay paraffin complex, alkylammonium organoclay pseudotrilayer, organoclay

An interlamellar arrangement of one layer of alkylammonium cations with the C-C-C plane of the alkyl chains (n < 8) parallel to the (001) plane of the 2:1 layer to produce a characteristic 13.6-Å spacing of a phyllosilicate. The positively-charged end groups (e.g., NH₃⁺) of the alkylammonium cations are attached to alternate siloxane surfaces in the interlayer.
See Lagaly and Weiss (1975); Lagaly (1994).
Cf., alkylammonium layer charge method, organoclay

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. The alkyl chains are tilted from a perpendicular orientation with respect to the (001) surfaces, and this orientation produces basal spacings of>22Å. The angles between the alkyl chain axes and silicate 2:1 layers increase with layer charge and range from 50 to 60° in vermiculites and to 90° in micas.
See Lagaly and Weiss (1969; 1975); Lagaly (1994).
Cf., alkylammonium layer charge method, alkylammonium organoclay bilayer, alkylammonium organoclay monolayer, alkylammonium organoclay pseudotrilayer, organoclay

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