Open Adit™ - Phyllosilicates

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The phyllosilicates (sheet silicates) or disilicates are minerals in which all the SiO₄ tetrahedra share corners with three others, thus forming infinitely extending layers or sheets. The fourth corner is unshared and all these unshared O ions point in the same direction. In the middle of the hexagon formed by the unshared corner there is room for an OH group. The Si to O ratio in the Silicate layer is 2 to 5, which together with the OH groups gives a formula ending with Si₂O₅(OH). The sheets are held together in stacks by metal ions and because these can bridge the sheets in a number of alignments they gives rise to the huge variety of minerals in this group. The key families include the Serpentines, the Clay minerals, Talcs, Micas and Chlorites. In spite of their diffrences, the layered structures common to all produce very similar physical characteristics. For example the minerals normally occur as tabular or platy crystals, they have one cleavage direction parallel to the layers, and because of the relatively weak bonding between the layers the are typically very soft.

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