Download Mineral Groups – Silicates

Mineral groups
Rock-forming minerals
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4000 minerals have been named but only few dozen make up most of the
rocks of Earth’s crust – classified as rock-forming minerals
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Only 8 elements make up the bulk of these minerals and represent 98% of the
continental crust
Mineral Groups
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Silicates (most abundant)
Non-silicates (~8% of Earth’s crust):
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Oxides
Carbonates,
Sulfides,
Sulfates,
Halides
Native elements
Phosphides,
and hydroxides
nitrates, borates, iodates
O2(CO3)2S2(SO4)2Cl-, F-, Br(Au, Pt, Cu…)
(Cu3P…)
selenides, tellurides, arsenides, antimonides, bismuthides
chromates, selenates, tellurates, molybdenates, wolframates
arsenates, vanadates
Mineral Groups
Non-ferromagnesian
Silicates (K, Na, Ca, Al)
Ferromagnesian
Silicates (Fe, Mg)
Oxides
Carbonates
Sulfides/sulfates
Native elements
Mineral Groups – Silicates
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Silicates
– Tetrahedron
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fundamental building
block
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4 oxygen ions
surrounding a much
smaller silicon ion
SiO44- Total charge: -4
1 x cation Si4+
4 x anion O2-
Silicon-oxygen
tetrahedron
(SiO4)4-
Mineral Groups – Silicates
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Joining Silicate Structures
– How tetrahedra may be linked:
• independent tetrahedra
• single chains
• double chains
• sheets
• 3-D framework
Mineral Groups
– Silicates –
However, these Si structures are not neutral
and must be neutralized by the inclusion
of metallic cations: Fe, Mg, K, Na, Al, Ca
Si-O tetrahedra are joined together to become neutral compounds through the
addition of positively charged ions
Relative sizes and electrical charges of ions of the most common elements
Inclusion of positive ions in the crystal structure
balances the negative charge of Si-O tetrahedra,
resulting in an overall electric neutrality.
Two major groups of silicate minerals
Ferromagnesian (dark) silicates – contain Fe and/or Mg
- specific gravity 3.2 to 3.6
Nonferromagnesian (light) silicates – contain Al, K, Na, and Ca
- specific gravity about 2.7
Mineral Groups – Silicates
Olivine Group (Mg,Fe)2SiO4
dark silicates (Fe-Mg)  ferromagnesian
No cleavage
Orthorhombic crystal structure with
isolated tetrahedra (nesosilicate)
Pyroxene group (Mg,Fe)SiO3
 ferromagnesian
Pyroxenes - mineral important in the Earth’s mantle
- silicate chains bonded by Fe and Mg
- cleaves parallel to the silicate chains
- most common member: augite (cleavage angle 90 degrees)
The upper mantle of Earth is composed mainly of olivine and pyroxene.
Augite
2-directions of cleavage
(at nearly 90 degrees)
Green-olivine
Black-pyroxene
Amphibole group
Hornblende – most common member of amphibole group of minerals
- dark green to black
- cleavage angles 60 and 120 degrees
- double chains of tetrahedra
- forms elongated crystals
 ferromagnesian
Hornblende Ca2(Mg, Fe, Al)5 (Al, Si)8O22(OH)2
2-directions of cleavage
(not at 90 degrees)
Mica Group
Nonferromagnesian (light) silicates
Muscovite KAl2(AlSi3O10)(F,OH)2
- member of mica family
- light in color, pearly luster
- excellent cleavage (transparent sheets – ”window glass”)
1-direction
of cleavage
Clay (sv. lera) – complex minerals that have a sheet structure
- fine grained
- often are products of chemical weathering of other silicates
- form large percentage of surface material that we call soil (sv. jord)
- important to humans (agriculture, construction of buildings)
Most common member – kaolinite (manufacture of chinaware)
Al2Si2O5(OH)4
Mineral Groups – Silicates
Feldspar Group
K-feldspar
light silicates (K-Na-Ca, Al)
Most common mineral group (50% of Earth’s crust)
- hard
- glassy to pearly luster
- smooth shiny faces
Orthoclase
Plagioclase
(tectosilicates)
2-directions of cleavage
(at 90 degrees)
Ca/Na-feldspar
Mineral Groups – Silicates
Quartz
light silicates (pure SiO2)
no cleavage
(conchoidal fracture)
hard, resistant to weathering
Quartz
Quartz - the most important polymorph of SiO2
- 2nd most abundant mineral in the crust (after feldspar)
- the only silicate that contains only Si and O (silica)
- 3-dimensional framework of corner-sharing tetrahedra
- all bonds are strong
- hard, resistant to weathering
- no cleavage, conchoidal fracture
Major groups of silicate minerals
Complexity of structure increases
Silicon-to-oxygen ratio
Due to the varying degree of sharing of oxygen, silicon-to-oxygen ratio varies
for different arrangments of Si-O tetrahedra
Silicon-to-oxygen ratio
Non-silicate minerals
Carbonates – structurally much simpler than silicates
- contain carbonate ion CO22Constituents of sedimentary rocks
-Calcite CaCO3
-Dolomite CaMg(CO3)2
Rock dominated by calcite : limestone (kalksten)
Non-silicate minerals
Halite NaCl (table salt)
Gypsum CaSO4 . 2H2O (calcium sulfate) – water bound into structure
- plaster and building materials
Often form thick layers after ancient seas that have evaporated
CaSO4 . 2H2O
NaCl
Polymorphs of minerals
Two minerals with different structure but with the exactly same
chemical compositon are called polymorphs
Examples:
Graphite – Diamond
At ambient conditions, both diamond and graphite (two polymorphs of carbon) can be
found in nature. However, diamond has higher energy than graphite and is metastable (upon
heating in an inert atmosphere, it will convert to graphite).
SiO2
Heating: Quartz - 573 C - high quartz – 870 C – tridymit e - 1470 C – cristobalite
Compressing: Quartz – 30 kbar – coesite – 160 kbar - stishovite
Polymorphism of carbon
Polymorphs of SiO2
Quartz is just one of 11 crystalline and 2 non-crystalline polymorphs.
The basic structural element of silica is the SiO4 tetrahedron. Quartz consists
of interconnected SiO4 tetrahedra that build up a rigid three-dimensional
Network.There are many possible ways of connecting SiO4 tetrahedra
different from that found in quartz, realized in various other silica polymorphs.
Since all of them consist of a three-dimensional SiO4 network, all are classified
as network silicates.
Stishovite and Seifertite are special cases because they are not made
of SiO4 tetrahedra and accordingly are not classified as a network silicate.
Instead, each silicon atom is surrounded by 6 oxygen atoms, and the packing
of atoms is much more dense.