The Silicate Class
Silicates are the largest, the most interesting, and
the most complicated class of minerals by far. Approximately 30% of all
minerals are silicates and some geologists estimate that 90% of the Earth's
crust is made up of silicates. With oxygen and silicon the two most abundant
elements in the earth's crust, the abundance of silicates is no real surprise.
The basic chemical unit of silicates is the (SiO4) tetrahedron shaped
anionic group with a negative four charge (-4). The central silicon ion
has a charge of positive four while each oxygen has a charge of negative
two (-2) and thus each silicon-oxygen bond is equal to one half (1/2) the
total bond energy of oxygen. This condition leaves the oxygens with the
option of bonding to another silicon ion and therefore linking one (SiO4)
tetrahedron to another and another, etc..
The complicated structures that these silicate tetrahedrons form is
truly amazing. They can form as single units, double units, chains, sheets,
rings and framework structures. The different ways that the silicate tetrahedrons
combine is what makes the Silicate Class the largest, the most interesting
and the most complicated class of minerals.
The Silicates are divided into the following subclasses, not by their
chemistries, but by their structures:
The Nesosilicate Subclass (single tetrahedrons)
The simplest of all the silicate subclasses, this subclass
includes all silicates where the (SiO4)
tetrahedrons are unbonded to other tetrahedrons. In this respect they are
similar to other mineral classes such as the sulfates
and phosphates. These
other classes also have tetrahedral basic ionic units
& SO4) and thus
there are several groups and minerals within them that are similar to the members of the
nesosilicates. Nesosilicates, which are sometimes referred to as orthosilicates,
have a structure that produces stronger bonds and a closer packing of ions
and therefore a higher density, index of refraction and hardness than chemically
similar silicates in other subclasses. Consequently, There are more
in the nesosilicates than in any other silicate subclass.
Below are the more common members of the nesosilicates.
See the nesosilicates' page for a more complete list.
The Sorosilicate Subclass (double tetrahedrons)
Sorosilicates have two silicate tetrahedrons that are linked by one oxygen ion and thus
the basic chemical unit is the anion group (Si2O7) with a negative six charge (-6).
This structure forms an unusual hourglass-like shape and it may be due to this oddball structure that this subclass is the smallest of the silicate subclasses.
It includes minerals that may also contain normal silicate tetrahedrons as well as the double tetrahedrons.
The more complex members of this group, such as Epidote, contain chains of aluminum oxide
tetrahedrons being held together by the individual silicate tetrahedrons and double tetrahedrons.
Most members of this group are rare, but epidote is widespread in many metamorphic environments.
Below are the more common members of the sorosilicates.
See the sorosilicates' page for a more complete list.
The Inosilicate Subclass (single and double chains)
This subclass contains two distinct groups: the single chain and double
chain silicates. In the single chain group the tetrahedrons share
two oxygens with two other tetrahedrons and form a seemingly endless chain.
The ratio of silicon to oxygen is thus 1:3. The tetrahedrons alternate
to the left and then to the right along the line formed by the linked oxygens
although more complex chains seem to spiral. In cross section the chain
forms a trapezium and this shape produces the angles between the crystal
faces and cleavage directions.
In the double chain group, two single chains lie side by side
so that all the right sided tetrahedrons of the left chain are linked by
an oxygen to the left sided tetrahedrons of the right chain. The extra
shared oxygen for every four silicons reduces the ratio of silicons to
oxygen to 4:11. The double chain looks like a chain of six sided rings
that might remind someone of a child's clover chain. The cross section
is similar in the double chains to that of the single chains except the
trapezium is longer in the double chains. This difference produces a difference
in angles. The cleavage of the two groups results between chains and does
not break the chains thus producing prismatic cleavage. In the single chained
silicates the two directions of cleavage are at nearly right angles (close
to 90 degrees) forming nearly square cross sections. In the double chain
silicates the cleavage angle is close to 120 and 60 degrees forming rhombic
cross sections making a convenient way to distinguish double chain silicates
from single chain silicates.
Below are the more common members of the inosilicates.
See the Inosilicates' page for a more complete list.
Single Chain Inosilicates:
The Double Chain Inosilicates:
The Cyclosilicate Subclass (rings)
These silicates form chains such as in the
inosilicates except that the chains link back around on themselves to form rings. The silicon to oxygen ratio is generally the same as the inosilicates, (1:3).
The rings can be made of the minimum three tetrahedrons forming triangular rings (such as in benitoite).
Four tetrahedrons can form a rough square shape (such as in axinite).
Six tetrahedons form hexagonal shapes (such as in beryl, cordierite and the tourmalines).
There are even eight membered rings and more complicated ring structures.
The symmetry of the rings usually translates directly to the symmetry of these minerals; at least in the less complex cyclosilicates.
Benitoite's ring is a triangle and the symmetry is trigonal or three-fold. Beryl's rings form hexagons and its symmetry is
hexagonal or six-fold.
The Tourmalines' six membered rings have alternating tetrahedrons pointing
up then down producing a trigonal as opposed to an hexagonal symmetry.
Axinite's almost total lack of symmetry is due to the complex arrangement of its square rings, triangle
shaped borate anions (BO3) and the position of OH groups.
Cordierite is pseudo-hexagonal and is analogous to beryl's structure except that aluminums
substitute for the silicons in two of the six tetrahedrons.
There are several gemstone minerals represented in this group, a testament to the general high hardness, luster and durability.
Below are the more common members of the cyclosilicates.
See the Cyclosilicates' page for a more complete list.
(Calcium Magnesium Iron Manganese Aluminum Borosilicate Hydroxide)
(Potassium Lithium Calcium Titanium Zirconium Silicate Fluoride)
(Barium Titanium Silicate)
(Magnesium Aluminum Silicate)
[Copper Silicate Hydroxide]
(Sodium Calcium Cesium Iron Manganese Zirconium Silicate Hydroxide Chloride)
(Hydrated Potassium Calcium Aluminum Beryllium Silicate)
Osumilite (Potassium Sodium Iron Magnesium Aluminum Silicate)
The Tourmaline Group:
(Sodium Magnesium Aluminum Boro-Silicate Hydroxide)
(Sodium Lithium Aluminum Boro-Silicate Hydroxide)
Iron Aluminum Boro-Silicate Hydroxide)
Sodium Iron Magnesium Aluminum Boro-Silicate Hydroxide)
(Potassium Sodium Lithium Iron Manganese Aluminum Silicate)
The Phyllosilicate Subclass (sheets)
In this subclass, rings of tetrahedrons are linked by shared oxygens to other
rings in a two dimensional plane that produces a sheet-like structure.
The silicon to oxygen ratio is generally 1:2.5 (or 2:5) because only one oxygen is exclusively
bonded to the silicon and the other three are half shared (1.5) to other silicons.
The symmetry of the members of this group is controlled chiefly by the symmetry of the rings but is usually
altered to a lower symmetry by other ions and other layers.
The typical crystal habit of this subclass is therefore flat, platy, book-like and display good basal cleavage.
Typically, the sheets are then connected to each other by layers of cations.
These cation layers are weakly bonded and often have water molecules and other neutral
atoms or molecules trapped between the sheets. This explains why this subclass
produces very soft minerals such as talc, which is used in talcum powder. Some members
of this subclass have the sheets rolled into tubes that produce fibers as
in asbestos serpentine.
Below are the more common members of the phyllosilicates.
See the Phyllosilicates' page for a more complete list.
(Hydrated Potassium Sodium Calcium Silicate Hydroxide Fluoride)
(Hydrated Calcium Vanadium Silicate)
(Hydrated Copper Aluminum Hydrogen Silicate Hydroxide)
- The Clay Group:
- The Chlorite Group:
(Iron Magnesium Aluminum Silicate Hydroxide)
Clinochlore (Iron Magnesium Aluminum Silicate Hydroxide)
Cookeite(Lithium Aluminum Silicate Hydroxide)
(Aluminum Silicate Hydroxide)
(Aluminum Silicate Hydroxide)
[Hydrated Calcium Silicate hydroxide]
- The Mica Group:
(Potassium Iron Magnesium Aluminum Silicate Hydroxide Fluoride)
(Potassium Lithium Aluminum Silicate Hydroxide Fluoride)
(Potassium Aluminum Silicate Hydroxide Fluoride)
(Potassium Magnesium Aluminum Silicate Hydroxide Fluoride)
(Potassium Lithium Iron Aluminum Silicate Hydroxide Fluoride)
(Calcium Aluminum Silicate Hydroxide)
(Iron Magnesium Silicate Hydroxide)
The Tectosilicate Subclass (frameworks)
This subclass is often called the "Framework Silicates" because
its structure is composed of interconnected tetrahedrons going outward
in all directions forming an intricate framework analogous to the framework
of a large building. In this subclass all the oxygens are shared with other
tetrahedrons giving a silicon to oxygen ratio of 1:2. In the near pure
state of only silicon and oxygen the mineral is quartz (SiO2). But the
tectosilicates are not that simple. It turns out that the aluminum ion
can easily substitute for the silicon ion in the tetrahedrons up to 50%. In other
subclasses this substitution occurs to a more limited extent but in the tectosilicates it
is a major basis of the varying structures. While the tetrahedron is nearly
the same with an aluminum at its center, the charge is now a negative five
(-5) instead of the normal negative four (-4). Since the charge in a crystal
must be balanced, additional cations are needed in the structure and this
is the main reason for the great variations within this subclass.
Below are the more common members of the tectosilicate subclass.
See the tectosilicates' page for a more complete list.
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