A2M3Si4O10OH2

A2M3Si4O10OH2 is a general empirical formula used to describe a class of layered silicate minerals in the phyllosilicate group. The formula denotes two A-site cations, three M-site cations, four silicon atoms in tetrahedral coordination, ten oxygens, and two hydroxyl groups per formula unit. In this notation, A represents interlayer or charge-balancing cations, while M represents cations occupying octahedral sites in the sheet that links the silicate layers.

In mineralogical practice, A sites are commonly occupied by monovalent or divalent cations such as K+, Na+, Ca2+, and M sites are occupied by trivalent or divalent cations such as Al3+, Mg2+, or Fe2+/Fe3+. The Si4O10 portion corresponds to a pair of fused silicate tetrahedral sheets that form the fundamental two-dimensional layer. The OH2 part reflects hydroxyl groups that are associated with the octahedral sheet and contribute to the layer charge balance and hydrogen bonding within the structure.

Structure and properties: A2M3Si4O10OH2-type minerals share a sheeted architecture in which silicate tetrahedra form 2D layers, and interlayer cations balance charge and influence interlayer spacing. The presence of hydroxyl groups and potential interlayer water leads to characteristic phyllosilicate properties such as basal cleavage, variable cation exchange capacity, and, in some members, swelling behavior.

Occurrence and context: Minerals with this formula type are associated with the broader mica and clay groups and can form through alteration of other silicates or through magmatic and metamorphic processes. In practice, specific minerals exhibit variations in cation composition and hydration that reflect their formation environment.