Stevensite is a trioctahedral smectite with an ideal composition of (M⁺_2y ^. nH₂O) (Mg_3-y v_y) Si₄O₁₀(OH)₂ where M is the exchangeable cation in the interlayer (given here as univalent), v represents y vacancies, and n is variable. Stevensite is poorly defined, but the lack of Al, some octahedral vacancies, the high Si content, and the high Mg content are important characteristics. Brindley (1955) described the stevensite structure as two-layer with talc and saponite interstratifications, Faust et al. (1959) suggested that stevensite is a defect structure with two components of talc and talc-like domains, and Shimoda (1971) found stevensite to be composed only of swelling components like smectite minerals. Christidis and Mitsis (2006) studied a Ni-rich stevensite that appeared to confirm the smectite character of one layer type with turbostratic stacking and ethylene glycol intercalation capabilities. Stevensite has been shown to commonly alter from sepiolite, and a Ni-rich stevensite is reported from a supergene alteration of an ophiolite complex in Greece. A significant Al content would allow such a phase to be defined as saponite.

See hydrotalcite group.

See zeolite.

An obsolete term, possibly for nontronite or chlorite from Moravia.

A modulated 2:1 layer silicate with a continuous octahedral sheet and a modulated tetrahedral sheet (Eggleton, 1972). The tetrahedral sheet forms approximately hexagonal islands with 6-fold tetrahedral rings and island connectors are formed by an inverted single 6-fold ring with a near trigonal configuration. The islands are comprised of seven near- hexagonal tetrahedral 6-fold rings. The inverted rings also link adjacent layers across the interlayer. Trigonal rings also form by paired trigonal rings joined by the apical oxygen atoms along Z. The primarily ferrous form of stilpnomelane, often informally referred to as “ferrostilpnomelane”, has a structural formula of K₅Fe²⁺₄₈(Si₆₃Al₉)O₁₆₈(OH)₄₈ ^. 12H₂O, whereas the analogous “ferristilpnomelane” has a structural formula of K₅Fe³⁺₄₈(Si₆₃Al₉)O₂₁₆ ^. 36H₂O, although neither end-member forms have been found. The Mg-rich stilpnomelane is called lennilenapeite and the Mn-rich mineral is franklinphilite. Older literature incorrectly equates parsettensite as the Mn-rich version. Stilpnomelane occurs in many geological environments, including greenshist facies rocks, iron deposits, metal sulfide deposits, and in weathering profiles.

See b-fabric.

A mathematical expression that describes how a spherical particle, nominally less than 20 micrometers, settles in a viscous fluid. This equation is used in clay science, although clay particles are generally flat plates and not spherical, and thus fall at slower velocities than spheres of equal density as calculated using Stokes’ law.
Cf., equivalent spherical diameter

An obsolete local name for a mixture of smectite and a Ca-rich phase(?), or Ca-montmorillonite, from Stolpen, Saxony, Germany.

See ceramic.

Plural of stratum.
See stratum.