A trioctahedral member of the true mica group. The ideal end-member formula is KFe²⁺₃AlSi₃O₁₀(OH)₂. The ideal end-member composition is unlikely to occur because of tetrahedral- octahedral misfit (the inability of the tetrahedral sheet to link with the octahedral sheet at certain compositions), with at least 10 % Fe³⁺ required to minimize misfit. Annite occurs in granites, granitic pegmatites, greisens, and some alkalic (syenite) rocks. Annite forms most commonly in the 1M polytype and belongs to the collective name “dark” mica.
Cf., biotite

An obsolete term for biotite.

See feldspar, plagioclase feldspar.

See alkali feldspar.

Antibacterial clays are clays that kill bacteria by various means, including but not limited to the chemical transfer of toxins, elimination of nutrients, or physical disruption of metabolic functions.
Cf., active principle, aesthetic medicine, geotherapy, healing clay, medicinal clay, pelotherapy, zeta potential

A property of materials where there is an antiparallel alignment of electric moments that cancel out over the entire crystal. The “ferro” aspect of the word is a misnomer because iron is not a usual constituent; the word derives from analogy to antiferromagnetism.
Cf., antiferromagnetism

A property of material where magnetic moments have antiparallel spins so that there is complete canceling and hence no magnetic attraction or repulsion effects. Antiferromagnetism coupling effects are removed by heating to the Néel temperature, and the material reverts to paramagnetism.

A platy serpentine with lath-like characteristics. Diffraction studies have shown an atomic superstructure arrangement approximately along the [100] direction. The origin of the superstructure is the tetrahedral repeat unit involving tetrahedral reversals in + or – directions along the c axis to form a wave-like structure. Antigorite structures have been shown by single- crystal X-ray diffraction to have, for example, a wave structure involving a tetrahedral repeat of 17 and one with two waves of tetrahedral repeats of 16, although other wavelengths are possible (from 12 to 21). For the 17 tetrahedral repeat, there is a half-wave width of 8 tetrahedra and 9 tetrahedra (Capitani and Mellini, 2004). At one reversal in each unit cell, there are 4- and 8-fold tetrahedral rings, but only 6-fold tetrahedral rings at the other reversal. Between reversal points, the tetrahedral rings are 6-fold. The generalized formula is ideally: M_3m-3T_2mO_5m(OH)_4m-6 where M = six-coordinated cations, T = Si, Al, and m = tetrahedral repeat along the superstructure direction, usually the [100]. M is predominantly Mg, but may have Fe (mostly Fe₂₊), Ni, Cr, and Al. Because each superstructure type affects the composition, these forms are not sensu stricto polymorphs of each other or between lizardite and chrysotile. Each half wave has a curvature, either concave up (+ c axis) or concave down (- c axis). The interlayer in an ideal serpentine is occupied by hydrogen bonds, which link adjacent 1:1 layers. In antigorite, linkage across this region is by tetrahedra, and polytypism cannot describe the stacking structure. For m = 17, the space group symmetry is Pm, for m = 16, the space group is C2/m. Antigorite is commonly found in igneous or metamorphic serpentinites. Serpentine rock is mostly comprised of antigorite and lizardite.
Cf., carlosturanite, chrysotile, lizardite

A poorly defined material, possibly a mica.

An obsolete term for chlorite filling cavities in igneous rocks, possibly chamosite.