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”.

A poorly crystallized (i.e., lacks long-range atomic order) hydrous aluminosilicate of approximate composition of (OH)₃Al₂O₃SiOH, with a natural-samples range of Al₂(OH)₃(SiO₂)_1.0-1.2(H₂O)_2.3-3.0. The Si/Al ratio is near 0.5. The structure consists of nanotubes, often occurring in closest packing arrangements about 2nm in diameter and to several micrometers in length, typically forming bundles 10 to 30 nm across. The morphology makes imogolite potentially useful in industry for contaminant sorption, gas storage, as an oxidation catalyst, and as an electron emitter. Imogolite has a gibbsite-like structure with Si tetrahedra spanning the vacant octahedral sites, and because of the mismatch in size between the vacant site and the tetrahedron, the gibbsite-like sheet rolls. Imogolite forms from weathered volcanic ash, but may also occur in podzolized soils and in pumice. 
Cf., allophane

Incongruent dissolution is where the release of constituents from the dissolving phase into the solution does not reflect the stoichiometry of the original phase.
Cf., congruent melting, congruent dissolution, incongruent melting

A reaction where the solid phase reacts to form a mixture of liquid plus crystals, with neither having the composition of the original solid.
Cf., congruent melting, incongruent dissolution

An obsolete, local term for a halloysite from Lawrence County, Indiana, in beds to 3 meters thick; may have been used as a rock name for these beds.

Hardening of a rock or sediments by the effects of temperature, pressure, cementation, etc.

a) the phenomenon by which minerals are formed in another environment in space or time from that where they are now found.

b) The term is also used when some element of a pre-existing mineral structure is inherited by another mineral via the phenomenon of transformation.
See also neoformation, transformation