Phyllosilicates (phyllo, Greek for "leaf"), formerly known as Sheet Silicates, 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 p:q ratio in the silicate layer is 2 to 5 giving the basic unit (Si2O5)2- or (AlSi3O10)5- where Al substitutes for Si. 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. Some members of this subclass have the sheets rolled into tubes. These tubes form fibrous structures as in Chrysotile (asbestos).
On the basis of chemistry and geometry of the octahedral layers the phyllosilicates are divided into two major sub-groups: trioctahedral and dioctahedral. In spite of their diffrences, the layered structures common to all phyllosilicates 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. Most are very important rock-forming minerals.
The main mineral groups of the phyllosilicates are:
Serpentine Group - a group of polymorphs having little substitution of Si by Al. Members include Antigorite, Lizardite and Chrysotile. The latter often forms long fibrous structures referred to as asbestos.
Clay Group - a group consisting of hydrous aluminium silicates. Examples include Kaolinite and Talc. Mg or Fe sometimes substitutes for Al. Clay is used to refer to fine-grained earthy material that becomes plastic when mixed with water.
Mica Group - form thin sheets and there is considerable substitution of Si by Al with little or no OH. The crystals are usually tabular with prominent basal planes and have either diamond or hexagonal outlines with angles of 60 or 120 degrees.
Chlorite Group - A number of minerals which have very similar chemical, crystallographic and physical properties. Without X-ray analysis it is extremely difficult to distinguish between the members. These include Chamosite, Clinochlore and Pennantite.
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Phyllosilicates (phyllo, Greek for "leaf"), formerly known as Sheet Silicates, 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 p:q ratio in the silicate layer is 2 to 5 giving the basic unit (Si2O5)2- or (AlSi3O10)5- where Al substitutes for Si. 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. Some members of this subclass have the sheets rolled into tubes. These tubes form fibrous structures as in Chrysotile (asbestos).
On the basis of chemistry and geometry of the octahedral layers the phyllosilicates are divided into two major sub-groups: trioctahedral and dioctahedral. In spite of their diffrences, the layered structures common to all phyllosilicates 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. Most are very important rock-forming minerals.
The main mineral groups of the phyllosilicates are:
Serpentine Group - a group of polymorphs having little substitution of Si by Al. Members include Antigorite, Lizardite and Chrysotile. The latter often forms long fibrous structures referred to as asbestos.
Clay Group - a group consisting of hydrous aluminium silicates. Examples include Kaolinite and Talc. Mg or Fe sometimes substitutes for Al. Clay is used to refer to fine-grained earthy material that becomes plastic when mixed with water.
Mica Group - form thin sheets and there is considerable substitution of Si by Al with little or no OH. The crystals are usually tabular with prominent basal planes and have either diamond or hexagonal outlines with angles of 60 or 120 degrees.
Chlorite Group - A number of minerals which have very similar chemical, crystallographic and physical properties. Without X-ray analysis it is extremely difficult to distinguish between the members. These include Chamosite, Clinochlore and Pennantite.
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