A class of reagents characterized by a central silicon atom coordinated tetrahedrally to four R groups. Such compounds are commonly used to bond an inorganic substrate, such as clay minerals, to a polymer. In the most reactive form, two or three of the R groups are chlorine atoms and the remaining are organic groups. The chlorine atoms in these compounds are very reactive with water and hydrolyze readily to form condensation polymers with Si-O-Si backbones, or the chlorine atoms can react with hydroxyl groups on clay mineral surfaces with loss of HCl. A less reactive form has two or three of the R groups as methoxy or ethoxy groups with the remaining R group being another nonlabile organic group. These silanes can be hydrolyzed to form similar polymers or reactions with hydroxyls on clay minerals, but generally require heat and vacuum to drive the condensation reaction. These agents allow the surface of hydroxyl containing compounds to be converted to hydrophobic or reactive surfaces depending on the character of the organic R group.

A surficial SiOH group, which is able to incorporate or dissociate protons. A ferronol group (FeOH) is an additional surface group that behaves similarly to an aluminol or silanol group.
See aluminol group

Silica refers to SiO₂ chemical composition only and the term does not connote structure. The term commonly is used to describe a mineraloid or a glass, as in a silica-rich glass. Thus, silica does not specifically refer to quartz or opal (but opal has a structure involving small, non- crystalline silica spheres).
Cf., mineraloid

Silica refers to the chemistry only, SiO₂, and not a specific structure or phase.
Cf., amorphous; cristobalite, tridymite, opal

A synthetic, poorly crystalline/amorphous SiO₂ product composed of spheres of <<1 m in diameter, and thus with a large specific surface area. Silica fume is a by-product of the Si and FeSi alloy production and used as an additive for concretes or geopolymers. Cf., cement, geopolymer

Lung fibrosis caused by the inhalation of dust containing crystalline silica (SiO2) in any of its forms (e.g., quartz, cristobalite, tridymite, coesite, stishovite).

A general term for any non-consolidated, clast-rich material that consists of >50% primary particles with diameters of 4-63 μm (American geologists), 2-63 μm (European geologists), or 2-50 μm (American soil scientists).
See siltstone

A sedimentary rock with primarily silt-size components; siltstone rocks may show sedimentary structures, such as flow structures and cross-bedding. Siltstones are often, but not always, chemically cemented.
See mud, mudstone, silt

Obsolete term for an iron-rich red clay used as a pre-history ocher in the Black sea region.

Bonding of powdered material by solid-state reactions at temperatures lower than melting.