An oligomer is a chemical compound composed of a small number of structural units (e.g., AlO₄ or SiO₄ tetrahedra) formed by a polymerization process, typically with carbon or aluminum atoms, where smaller units are bound together to form larger units. According to IUPAC, a minor change in the number of integrated units causes a significant change in the properties of the oligomers and distinguishes them from polymers that consist of large numbers of structural units. Small oligomers composed of only a few units include: monomer = 1 unit, dimer = 2 units, trimer = 3 units, tetramer = 4 units, etc. Oligomers are found in synthetic clay-related products (e.g., geopolymers) or naturally occurring clay minerals (e.g., hydroxy-interlayered clay minerals often contain oligomers in the interlayer).
Cf., geopolymer, hydroxy-interlayered minerals, polymer, polymerization

The process of forcing a solvent (typically, water) through a semi-permeable membrane from a solution with lower water activity to a solution with higher water activity by applying a pressure opposite to and in excess of the osmotic pressure. Reverse osmosis is a process commonly used to purify (i.e., to physically separate and remove dissolved ions from) water.

A clinopyroxene (space group P2/n or C2/c) with an ideal chemical composition of (Ca,Na)(Mg,Al)Si₂O₆ and this represents the solid solution between augite and jadeite. Ordering and resulting symmetry occurs between Ca and Na (in M2 sites) coupled with ordering between Mg (+Fe) and Al (in M1 sites). Omphacite commonly occurs in high-pressure, high temperature metamorphic rocks, such as eclogite which forms by subduction of oceanic crust.
See pyroxene group for additional details.

In clay science, the initial stage of consolidation compression of a clay, which is governed by the dissipation of excess pore water pressure (i.e., pore water pressure in excess of the hydrostatic water pressure). During this stage, the excess pore water pressure changes (usually approaching the hydrostatic water pressure), and the clay either produces pore water (in the case of positive excess water pressure) or adsorbs water into the pores (in the case of negative excess water pressure). During this stage, the effective stress changes, and the clay volume changes as well.
Cf., compression, secondary

An obsolete term for muscovite.

A poorly defined material, possibly muscovite with quartz and/or other phases.

An obsolete term for muscovite.

A poorly defined material, possibly muscovite with quartz and/or other phases.

A poorly defined material, possibly with muscovite, quartz and/or other phases.

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.