In clay science and materials science, microstrain is a local strain caused by a local deviation of the lattice parameters from the mean value. Microstrain originates by 1) atomic substitutions where the ionic radius of the substituting ion differs from the original ion, 2) missing atoms or ions in the structure, 3) interactions with neighboring crystallites having slightly different lattice parameters, e.g., owing to twinning, an inhomogeneous mixture of crystallites with similar lattice parameters. In X-ray diffraction patterns, microstrain causes peak broadening. A microstrain expression is defined as the root-mean-square of the lattice parameters. In physics, mechanics, and many engineering disciplines, microstrain is a strain expressed in terms of parts per million (10^-6), where strain is defined as a ratio of the change in the distance/dimension to the original distance/dimension, and hence it has no unit and is dimensionless.

An obsolete term for a Cr-bearing kaolinite.

See reyerite group.

An element or chemical compound that is normally crystalline and that has been formed as a result of geological processes (quot Nickel, 1995). A mineral forms by a naturally occurring process (e.g., “geological” process); phases formed by the interaction of individuals (even if outdoors under conditions not fully controlled by the individual) are not minerals. Nickel (1995) discussed exceptions to the requirements, such as the equivalence of extraterrestrial and “geological” processes, metamict (non-crystalline) minerals, mercury (a liquid mineral), and others (quot Guggenheim et al., 2006). Some naturally occurring processes, but without a geological component, such as compounds that form biologically (e.g., oxalate crystals in certain plants, marine animal shells), are not minerals. Synthesized materials are not minerals, but may be referred to as “synthetic minerals” (e.g., “synthetic diamond”, “synthetic halite”) because the use of “synthetic” negates the naturally occurring/geological aspects specifically. Likewise, “biomineral” is acceptable for similar reasons as synthetic mineral.
See crystalline,
Cf., biologically controlled mineralization, biologically induced mineralization, biomineralization

A natural solid with insufficient long-range atomic ordering to be classified as a mineral. For example, limonite (FeO . OH . nH₂O) is often considered an amorphous “mineral” or mineraloid.

A poorly defined material, possibly interstratified biotite and vermiculite.

A modulated 2:1 layer silicate with a continuous octahedral sheet and a tetrahedral sheet that forms linked hexagonal 6-fold tetrahedral rings along strips along the [010] direction (Guggenheim and Eggleton, 1986). Some of the tetrahedra are partially inverted to form a chain along the [010], and this chain links adjacent 2:1 layers. There are two varieties of minnesotaite that are based on strip widths and chemical composition: a P cell is Mg-rich and is formed where 10 tetrahedra span 9 octahedra along the [010] whereas a C cell, which is Fe-rich, forms with 9 tetrahedra spanning 8 octahedra. The ideal chemical composition for the P cell is (Fe,Mg)₃₀Si₄₀O₉₆(OH)₂₈ and (Fe,Mg)₂₇Si₂₆O₈₆(OH)₂₆ for the C cell. Early workers incorrectly considered minnesotaite as the Fe analogue of talc. Minnesotaite occurs in low grade metamorphic silicate iron formations.

Or symmetry plane, used to describe a repetition of features whereby identical points occur an equal but opposite distance along any line perpendicular from this imaginary plane. Consequently, an object is “bilateral” in that it shows a matching of features (also referred to as “reflection”) but a change in “handedness”, e.g., most people, standing with arms by their sides and feet side-by-side, have a mirror (bilateral or reflection) relationship between the left side and the right side.
Cf., symmetry, center of symmetry, rotation symmetry

An obsolete term.
See Southern bentonite.

A Fe-,Mg-rich chlorite, heat treated in air for one-hour at 550^oC to produce a chlorite-like structure (Guggenheim and Zhan, 1999) with a strong d(001) peak (14 Å) and weak or absent higher order 00l peaks. The chlorite-to-modified chlorite reaction allows the identification of mixtures of 7 Å phases (e.g., kaolin minerals) and Fe-,Mg-rich chlorite after heating samples of clay mixtures that may contain chlorite with moderate to high amounts of Fe by revealing the possible presence of 7Å peaks in an oriented clay mineral aggregate.