Download Sheet silicates (phyllo)

Geol432 Mineralogy & Mineral Optics
15-1 Lecture
11/28/06
Sheet silicates (phyllo)
Objectives
• Learn the major groups and minerals in sheet silicates, their crystal chemistry, structure and
geological significance.
Sheet silicates
Sheet silicates account for about 10 vol.% of the Earth’s crust: Micas occupy 5 vol.% of the
Earth’s crust, clays occupy another 5 vol. %.
In sheet silicates, SiO4 tetrahedra are linked into infinitely extending sheets. Each tetrahedron has
three bridging oxygens (Si:O = 1:2.5 = 2:5). Sometimes Al substitutes for Si in the tetrahedrallycoordinated position, so that (Si+AlIV) = 2:5. Therefore, the chemical formula of phyllosilicates
contain the unit Si2O5 or Si4 O10 or AlSi3 O10, or (Si,Al)4 O10.
Most members of the phyllosilicates contain hydroxyl, with the OH group located in the center of
the six-fold rings of tetrahedra, at the same height as the non-bridging apical oxygens in the SiO4
tetrahedra.
On the basis of the chemistry and geometry of the octahedral sheets, the phyllosilicates are
divided into two major groups: Dioctahedral group and trioctahedral group. Charge balance
needs to be maintained between the cations in the octahedral sheet and the tetrahedral sheet. In
dioctahedral group, two out of three octahedral sites are occupied by trivalent ions (for example,
Al3+, 2x3+ = 6 +). In trioctahedral group, all three octahedral sites are occupied by divalent ions
(for example, Mg2+, 3x2+ = 6 +).
Phyllosilicates can also be divided into four major groups according to the arrangement of
tetrahedral layer (t), octahedral layer (o), and cation layer.
Trioctahedral
Dioctahedral
o
Al(OH)3 gibbsite
Mg(OH)2 brucite
t-o
Mg3(Si2O5)(OH)4 serpentine
Al2(Si2 O5)(OH)4 kaolinite
t-o-t
Mg3(Si4O10)(OH)2 talc
Al2(Si4 O10)(OH)2 pyrophillite
t-o-t +
cation
KMg3(AlSi3O10)(OH)2 phlogophite
KAl2(Si4 O10)(OH)2 muscovite
t-o-t +
o
(Mg,Al,Fe)3(Si,Al)4 O10(OH)2(Mg,Al,Fe)3(OH)6
chlorite
Because of the dominance of infinitely extending sheet of SiO4 tetrahedra in the structure, most
phyllosilicates have a platy of flaky habit and one prominent cleavage. They are generally soft, or
relatively low specific gravity, and may show flexibility or even elasticity of the cleavage
lamellae.
Polytypism is found in many phyllosilicates, including serpentine, mica, and chlorite. It is a
special kind of polymorphism. It occurs when two polymorphs differ only in the stacking of
identical, two-dimensional sheets. For example, lepidolite 1M and lepidolite 2M2 are polytypes.
Li
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