A dioctahedral member of the true mica group. The end-member formula is KAl(Mg,Fe³⁺)□Si₄O₁₀(OH)₂, where □ = vacancy. Typical range in composition is: ^viR²⁺ / (^viR²⁺ + ^viR³⁺) ≥ 0.25, ^viAl / (^viAl + ^viFe³⁺) = 0.5 – 1.0, Mg / (Mg + ^viFe²⁺) > 0.5 (Rieder et al., 1998).

A surficial AlOH group (see silanol group), which is able to incorporate or dissociate protons.
See silanol grou

“In general, silicates which contain tetrahedrally coordinated aluminum are called aluminosilicates in contrast to silicates containing octahedrally coordinated aluminum for which the term aluminum silicates is used…” (Liebau, 1985, p. 13). Although this is the classic definition of aluminosilicate, many texts use the term inconsistently. For example, kyanite and andalusite, traditionally considered part of the “aluminosilicate group” with sillimanite, do not have tetrahedrally coordinated Al, although they are closely related (structurally and geologically) to each other and sillimanite. Clearly, taking the first part of definition by Liebau (1985) as the basis of the definition, the second part should not be limited to aluminum in octahedral coordination only. Phyllosillicates are generally considered aluminosilicates, because most phyllosilicates do have Al substitution for Si, but not all do (and those minerals without tetrahedral Al present, but with Al in other polyhedral coordinations are more properly referred to as “aluminum silicates”).

An obsolete varietal term for manganoan muscovite and manganoan illite.

Known only in the Russian literature and not an official mineral name; the accepted name is tosudite.

A (trioctahedral) platy serpentine mineral of ideal composition of Mg₂Al(Si,Al)O₅(OH)₄. Fe₂₊, Mn, Cr, Ni, and vacancies may substitute for Mg and ^viAl in natural samples. Natural occurrences are rare and have been noted from the Saranovskoye chrome deposit, North Urals Mountains, Russia; Chester, Massachusetts, USA; Mount Sobotka, Poland; Postmasburg, South Africa; Lake Asbestos Mine, Black Lake, Quebec, Canada; and Antarctica. The Quebec occurrence involves a rodingitized granite within a serpentinized peridotite. Stacking disorder is common but, where regular layer stacking occurs, it is most often based on a distorted 2H₂ layer sequence (space group C1).
Cf., kellyite, zinalsite.

An obsolete term for muscovite.

An obsolete term for tobelite.

An obsolete term for tobelite.

A term describing a non-crystalline phase where the constituent components (i.e., atoms, ions, molecules) are without long-range order, i.e., not related by translation periodicity. There are many variations of such order or lack of order, for example, glasses commonly occur where atoms are arranged as coordination groups of tetrahedra (e.g., four oxygen ions in coordination about a silicon ion). Such tetrahedra share vertices to define local structure similar to that of crystalline phases; however, such groups of atoms are not related by long-range translation periodicity. This type of short-range order is much different than a simple random positioning of atoms. Amorphous character can be described by the nature of the analysis, e.g., such that the material may produce an X-ray diffraction pattern without discrete maxima, hence, “X-ray amorphous”.
Cf., non-crystalline, crystalline