Transcript Lecture 16 Silicates I mod 11

Pyrope
Lecture 16
Systematic Description of Minerals
Part 3:
Silicates I: Introduction to Silicates,
Nesosilicates, and Sorosilicates
Predominance of Silicate Minerals
in the Earth’s Crust
CRUST MOSTLY Oxygen O and SilicoN Si
27% of all known minerals are silicates
40% of common minerals are silicates
>90% minerals in the earth’s crust are silicates
Silicon Tetrahedra – the basic
building block of silicate minerals
The Si-O bond – 50% covalent, 50% ionic
Electrostatic Valence (e.v., measure of bond strength)=Z/CN=4/4 =1
Each tetrahedral oxygen shares a -1 charge with the tetrahedral silicon
and has an extra -1 charge to share with another cation
Four (4) oxygens in each tetrahedron, so total charge -4
Polymerization of
Silicon Tetrahedra
Oxygens can share electrons with two silicons
Adjacent silicon tetrahedra
can share corners, but
because of the high repulsive
charge of Si+4 cations, they
will not share edges or faces.
These shared corners are
called bridging oxygens.
Role of Al in Silicate Minerals
Al+3 may occur in tetrahedral [4] (substituting for Si+4)
or octahedral [6] coordination
Ionic radius of Al+3 = 0.39Å (4-fold) (Si+4=0.26Å)
= 0.54Å (6-fold)
Ionic Al:O Radius Ratio (4-fold) =0.39/1.36=0.286
(Upper limit of tetrahedral coordination RR=0.225)
Ionic Al:O Radius Ratio (6-fold) = 0.388
(Upper limit of octahedral coordination RR=0.414)
Bond strength: e.v. = 3/4 for Al+3 in tetrahedral coord.
= 3/6=1/2 in octahedral coord.
O-coordination and Bond Strength of Other
Common Cations in Silicate Minerals
Electostatic
Valence w/ O-2
big
medium
small
1/8 - 1/12 Weak
1/6 - 1/8
1/3 – 1/4
2/6 = 1/3
2/6 = 1/3
2/6 = 1/3
3/6 = 1/2
4/6 = 2/3
3/6 = 1/2
3/4
4/4 = 1 Strong
Note size trend for all, dual coordination for Al+3 , and silicate cation labels XYZ
Silicate Mineral Classification
(based on arrangement of SiO4 tetrahedra)
Silicate Mineral Classification
(based on arrangement of SiO4 tetrahedra)
Nesosilicates
Inosilicates
Sorosilicates
Inosilicates
Cyclosilicates
Phyllosilicates
Tectosilicates
Nesosilicates (independent tetrahedra)
• X2(SiO4) Unit Composition X often +2 valence
• Isolated, but tightly packed (SiO4)4- tetrahedra
• Forms silicate minerals with:
High density and hardness
Equi-dimensional habits
Poor cleavage
• Low degree of Al substitution
with Si
Olivine X = Mg+2 or Fe+2
Common Nesosilicates: Olivine
(Mg,Fe)2SiO4
High-T igneous mineral, common in mafic and ultramafic rocks;
commonly alters to serpentine
Vitreous olive green (Mg) to black (Fe)
Equigranular to prismatic habit; poor cleavage
Optics: Colorless, biaxial (positive if Mg++, negative if Fe++), mod. high
relief (n~1.7), high 2V,  ~.05 (2nd order IF colors)
Complete solid solution between Mg and Fe
Common Nesosilicates: Zircon
Zircon is ZrSiO4. Hafnium is almost
always present in quantities ranging from
1 to 4%. The crystal structure of zircon is
tetragonal. The natural color of zircon
varies between colorless, yellow-golden,
red, brown, and green.
Zircon usually contains radioactive Uranium
and Thorium, and is frequently used to date
plutonic rocks.
In Petrology we will visit the Bemco Mining
prospect in Cranberry Lake, NJ, on the
National Registry as a site for strategic
elements Uranium and Thorium in Zircon
Common Nesosilicates: Garnet
(Mg,Fe,Mn,Ca)3(Fe3+,Cr,Al)2Si3O12
As mod-T metamorphic mineral formed from Al-rich source rocks and
Grossular
ultramafic mantle rocks (eclogites)
Equigranular, euhderal to subhedral habit; poor cleavage
Optics: Colorless, isotropic, high relief (n~1.7-1.9)
Complex solid solution with the following end-member compositions and
their characteristic colors:
Pyrope Mg3Al2Si3O12 – deep red to black
Almandine Fe3Al2Si3O12 – deep brownish red
Spessartine Mn3Al2Si3O12 – brownish red to black
Almandine
Grossular Ca3Al2Si3O12 – yellow-green to brown
Andradite Ca3Fe2Si3O12 – variable-yellow, green, brown, black
Uvarovite Ca3Cr2Si3O12 – emerald green
Andradite
B-site
Aluminum
octahedral
Garnet A3B2Si3O12
Usually B is Aluminum, A divalent
Almandine Fe3Al2Si3O12
A-site Fe++,
Mg++, Ca++,
Mn++ in
distorted
octahedra
Common Nesosilicates: The Aluminosilicates
Kyanite, Sillimanite, Andalusite
Al2SiO5
Moderate to high grade metamorphic minerals formed from Al-rich source
rocks
Al in octahedral or a mix of octahedral to tetrahedral sites.
Kyanite – Vitreous bluish bladed tablets
w/ single perfect cleavage; H: 5-7
Sillimanite – Vitreous brown to green
clustered prisms w/ single cleavage dir.
Andalusite – Vitreous flesh-red, reddish brown
square prisms; H: 7.5
Penet. twins, forming a cross
Common Nesosilicates: Staurolite
Fe2Al9O6(SiO4)4(O,OH)2
Moderate to high grade metamorphic mineral formed from Al-rich source
rocks
Resinous to vitreous (dull when altered) reddish-brown to brownish black 6sided prisms; commonly forms penetrating twins
Optics: Biaxial(-), yellow pleochroic, high relief (n~1.75), 2V=82°-88°
Common Nesosilicates: Sphene (Titanite)
CaTiO(SiO4)
Common accessory mineral in felsic igneous rocks and in some
metamorphic rocks
Resinous to adamantine, gray, brown, green, yellow or black lens crystals;
distinct diamond-shaped cleavage; H: 5-5.5
Optics: Biaxial(+), yellow pleochroic, very high relief (n~2.0), 2V=27°,  =
0.13
Common Nesosilicates: Topaz
Topaz Al2SiO4(F,OH)2, Orthorhombic prismatic
terminated by pyramidal and other faces, the basal
pinacoid often being present. Perfect basal {001} cleavage
The fracture conchoidal to uneven. Hardness 8, specific
gravity 3.4–3.6, and a vitreous luster.
Color wine or straw-yellow. They may also be white, gray,
green, blue, pink, or reddish-yellow and transparent or
translucent.
Sorosilicates (double tetrahedra)
(Si2O7)-6
• Double silicon tetrahedra linked by one bridging oxygen
•
Sorosilicates commonly also contain independent silica tetrahedra (SiO4)-4
• Typically monoclinic symmetry
Si2O7
Epidote
Common Sorosilicates: Epidote Group
Zoisite/Clinozoisite – CaAl3O(SiO4)(Si2O7)(OH)
Epidote – Ca2(Fe,Al)Al2O(SiO4)(Si2O7)(OH)
Common accessory and alteration mineral in igneous rocks and is a common phase in
various grades of metamorphic rocks
Zoisite – Orthorhombic; Clinozoisite and Epidote – Monoclinic
Physical Properties: prismatic vitreous crystals to very fine resinous massive granules;
H: 6-7
Zoisite
Zoisite: Gray, greenish brown (pink-thulite)
Clinozoisite: Gray, pale yellow, pale green,
colorless
Epidote: Pistachio green to yellow green,
Optics:
Zoisite: Biaxial(+), high relief (n~1.7), 2V=0-70°,  ~ 0.005
Epidote
Clinozoisite: Biaxial(+), high relief (n~1.7), 2V=14-90°, ~0.010
Epidote: Biaxial(-), high relief (n~1.75), 2V=74-90°, ~0.015-.051, green-yellow
pleochroic;
Common Sorosilicate: Vesuvianite
(aka Idocrase)
Ca10(Mg,Fe)2Al4(SiO4)5(Si2O7)2(OH)4
Common mineral found in thermally metamorphosed
limestone with garnet, wollastonite (Ca-pyroxene), and
diopside (Mg-Ca-pyroxene)
Vitreous to resinous, green to brown, columnar to
granular crystals, commonly striated parallel to
columns; H: 6.5
Common Sorosilicates: Hemimorphite
Hemimorphite, is a sorosilicate,
Zn4(Si2O7)(OH)2.H2O from the upper parts
of zinc and lead ores, chiefly associated
with Smithsonite.
Hemimorphite most frequently occurs as
the product of the oxidation of the upper
parts of Sphalerite (ZnS) bearing ore
bodies, accompanied by other secondary
minerals which form the so-called iron cap
or gossan. Hemimorphite is an important
ore of zinc and contains up to 54.2% of
the metal.
The first guide mentioned this origin
during the Mine Field Trip.
2011 Field Trip
Looking at the Ore Body
In the Mine