Hydroxy-interlayered minerals are 2:1 layer phyllosilicates with Al³⁺ and possibly Mg²⁺ and Fe³⁺ complexes in the interlayer. These complexes are formed by polymerization of usually Al3+ cations octahedrally coordinated by (OH)^– and/or H₂O. Complexes are limited to one to five octahedra without forming a continuous sheet or Keggin ions (Al₁₃), although it is possible that additional H₂O may occur adjacent to the 2:1 layer. The permanent layer charge of HIMs per formula unit is believed to vary from 0.2 (equivalent to low-charge smectite) to 1.0 (equivalent to true mica). HIMs commonly form in upper soil horizons from smectite, vermiculite, or mica during the acidic weathering of soils. The fixed interlayer complexes reduce the ability of swelling and shrinking and thus soil fertility. These phases are commonly identified by X-ray diffraction and cation exchange capacity measurements.
Cf., chemical weathering, hydroxy-interlayer, hydroxy-interlayering, hydroxy- interlayered smectite, hydroxy-interlayered vermiculite, Keggin ion, pillared clay

Hydroxy-interlayered smectites are 2:1 phyllosilicates with a permanent layer charge of approx. 0.2 – 0.6 per formula unit and with fixed interlayer complexes. These complexes are composed of cations, mainly Al³⁺, but possibly Mg²⁺ and Fe³⁺, octahedrally coordinated by H₂O and OH^–. Presumably, the high-charge Al³⁺ fixes the complex, which thus limits swelling or shrinking capabilities. Depending on the degree of hydroxy-interlayering (proportion of exchangeable cations replaced by fixed complexes), the transition between smectite and hydroxy-interlayered smectite is poorly defined. Smectite and HIS phases are commonly interstratified.
See hydroxy-interlayered mineral.
Cf., hydroxyl interlayer, hydroxyl interlayering, hydroxy-interlayered vermiculite, smectite, vermiculite

Hydroxy-interlayered vermiculites are 2:1 layer phyllosilicates with a permanent layer charge of 0.6 – 0.9 per formula unit and with fixed complexes in the interlayer. These complexes are composed of cations, mainly Al³⁺, but possibly Mg²⁺ and Fe³⁺, octahedrally coordinated by H₂O and OH^–. Presumably, the high-charge Al³⁺ fixes the complex, which thus limits swelling or shrinking capabilities. Depending on the degree of hydroxy-interlayering (proportion of exchangeable cations replaced by fixed complexes), the transition between vermiculite and hydroxy-interlayered vermiculite is poorly defined. Vermiculite and HIV phases are often interstratified.
See hydroxy-interlayered mineral.
Cf., hydroxy interlayer, hydroxy interlayering, hydroxy-interlayered smectite, smectite, vermiculite

An obsolete term for annite.

An obsolete term for biotite.

In micromorphology of soils, hypo-coating is a pedofeature that consists of a layer of material that is related and immediately adjacent to the surface of, for example, voids, grains, or aggregates.
Syn., neo-cutan;
Cf., coating, cutan, quasi-coating

A poorly defined material, possibly altered cordierite and zeolite.

An identical point (a point whose environment is exactly the same as the environment around each other point) placed on an array. A set of identipoints within an atomic structure shows the scheme of repetition, or translational periodicity, of an array.
Cf., array, structure, Bravais lattice

Illite was a term proposed in 1937 by Grim et al. as a “group” name for the mica-like, clay-size mineral constituents in argillaceous sediments. Bailey et al. (1984) indicated that illite, as a species, must meet the following characteristics: (1) the structure is not expansible, (2) the 2:1 layer is dioctahedral, and (3) there are compositional criteria. Rieder et al. (1998) gave a representative formula and typical ranges as: K_0.65Al_2.0v(Al_0.65Si_3.35)O₁₀(OH)₂ where ^viR²⁺/(^viR²⁺ + ^viR³⁺) < 0.25, v = vacncy, and ^viAl/(^viAl + ^viFe³⁺) > 0.6 and this differs from muscovite, a closely related mica with an end-member formula and ranges of KAl₂(AlSi₃)O₁₀(OH)₂ where ^ivSi: 3.0 – 3.1, ^viAl: 1.9 – 2.0, K: 0.7 – 1.0 (interlayer cations > 0.85), ^viR²⁺/(^viR²⁺ + ^viR³⁺) < 0.25, and ^viAl/(^viAl + ^viFe³⁺): 0.5 – 1.0. The interlayer deficiency observed in illite is considered an important part of the definition. Rieder et al. defined illite as a “series” name, and series names designate that additional research may be warranted. The clay-size aspects and an occurrence in argillaceous sediments are not considered acceptable criteria to define a mineral. When illite is being referred to as a mineral species, it is advisable to make a clear statement to that effect. Non-pure illite, i.e., material containing an expansible component, is referred to as “illitic”.

A phase or mixture of two phases (the status remains unknown) generally showing a non-regular interstratification of illite-like layers with smectite-like layers. The ratio of smectite-like layers (i.e., one 2:1 layer and one interlayer capable of swelling) to illite-like layers (i.e., one 2:1 layer) may vary. Regularity in stacking may be found at specific ratios of layers (I = illite-like, S=smectite-like), for example, ISISIS… or IIISIIISIIIS…, but these patterns of regularity are not of a sufficient long-range nature to designate a separate species. Commonly abbreviated as “I/S” or “I-S”.