Opals are microcrystalline or noncrystalline forms (Graetsch, 1994) of SiO₂ or SiO₂ ^. nH₂O. The H₂O is not structurally required. Opal-C (synonym: lussatine) refers to a form with a disordered cristobalite (C) structure, opal-CT (synonym: lussatite, common opal) refers to disordered cristobalite/tridymite intergrowths (CT), and opal-AG (synonym: precious opal) is comprised of cubic- or hexagonal-stacking of closest-packed silica spheres (amorphous spheres of equal size, ~0.0003 mm), where A = amorphous, G = gel-like. Precious opal exhibits opalescence or play of colors in reflected, white light. Opal-AG (synonym: potch opal) lacks opalescence because of disorder in the stacking of planes of the closest-packed spheres. Opal-AN (synonym: hyalite) occurs as botyroidal masses, where N = network or glass-like forms.

An obsolete term for opal-CT.
See opal

See opal.

See opal.

See cutan.

In civil engineering, an organic clay is a clayey soil containing a specific range of organic matter content (ASTM Standard D2487). Organic clay is usually distinguished by determining the change in the liquid limit of a clay that was oven-dried at 105 – 110^oC. If the ratio of the liquid limit of the oven-dried clay to that of the natural (i.e., without oven drying) clay is < 0.75, then the soil is classified as organic clay. Cf., liquid limit, organoclay, organic soil

See peloid.

A phyllosilicate, typically smectite, vermiculite, or kaolin, but also other minerals (e.g., double metal hydroxides) with sorbed organic molecules, such that the properties of the mineral are altered. Commonly, the sorption occurs between the 2:1 or 1:1 layers. The mechanism for sorption may vary depending on the organic molecule and concentration. In alkylammonium organoclays, alkylammonium cations replace inorganic cations, and these organic cations are bonded to the layers via electrostatic (= Coulombic) forces. In organophilic alkylammonium organoclays where large organic cations completely fill the interlayer, van der Waals interactions between the alkyl groups augment the Coulombic forces, which increase both adsorption and organoclay stability. Adsorptive type organoclays have small organic cations that partially fill the interlayer, are stabilized by Coulombic forces, and act as pillared materials where there are accessible areas between the pillars for additional adsorption. These pillars increase surface areas relative to untreated clay or organophilic clays. In grafted compound-type organoclays with organic compounds such as silanes in the interlayer, bonding to the clay layer is covalent. Organoclays with adsorbed neutral polymers are attached to the clay layer by weak multiple dipole-induced dipole type bonds. In organoclays prepared from charged polymers, such as proteins, the polymers are bonded by both electrostatic and dipole-induced dipole bonds. Organoclays can be used as adsorbents, thickening and thixotropic agents, in nanocomposites, and in new materials with catalytic, optical, and electronic properties (Lagaly et al., Ogawa, and Dékány, 2006).
Syn., organo-clay, clay-organic complex;
Cf., organoclay, adsorptive; organoclay, alkylammonium; organoclay, organophilic alkylammonium phyllosilicate; pillared clay; phyllosilicate

Based on organic contaminant adsorption isotherms and sorptive behavior, the adsorptive-type organoclays (generally producing nonlinear and Langmuir-type isotherms) have exchanged organic cations that act as interlayer props to hold the interlayer open. This arrangement facilitates additional organic contaminant adsorption onto the siloxane surfaces, which are relatively hydrophobic except near exchangeable cation sites (Chen, 1976; Jaynes and Boyd, 1991a). These clays are prepared from smectite by replacing inorganic exchangeable cations with small organic cations, such as tetramethylammonium or trimethylphenylammonium. Lower charge clay minerals (i.e., lower charge smectite) with adsorbed small organic cations yield organoclays that more effectively adsorb organic contaminants compared to the unmodified clay. See Boyd and Jaynes (1994).
Cf., organoclay organophilic; organoclay.

See organoclay.