Lecture 16 Systematic Description of Minerals Part 3: Silicates I: Introduction to Silicates,...
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Transcript of Lecture 16 Systematic Description of Minerals Part 3: Silicates I: Introduction to Silicates,...
Lecture 16
Systematic Description of Minerals
Part 3: Silicates I: Introduction to Silicates, Nesosilicates, and Sorosilicates
Pyrope
Predominance of Silicate Minerals in the Earth’s Crust
CRUST MOSTLY Oxygen O and SilicoN SiCRUST MOSTLY Oxygen O and SilicoN Si27% of all known minerals are silicates27% of all known minerals are silicates40% of common minerals are silicates40% of common minerals are silicates>90% minerals in the earth’s crust are silicates>90% minerals in the earth’s crust are silicates
Silicon Tetrahedra – the basic building block of silicate minerals
The Si-OThe Si-O bond – 50% covalent, 50% ionicbond – 50% covalent, 50% ionic
Electrostatic Valence (e.v., measure of bond strength)=Z/CN=4/4 =1Electrostatic Valence (e.v., measure of bond strength)=Z/CN=4/4 =1
Each tetrahedral oxygen shares a -1 charge with the tetrahedral silicon Each tetrahedral oxygen shares a -1 charge with the tetrahedral silicon and has an extra -1 charge to share with another cationand has an extra -1 charge to share with another cation
Four (4) oxygens in each tetrahedron, so total charge -4Four (4) oxygens in each tetrahedron, so total charge -4
Polymerization of Silicon Tetrahedra
Adjacent silicon tetrahedra Adjacent silicon tetrahedra can share corners, but can share corners, but because of the high repulsive because of the high repulsive charge of Sicharge of Si+4+4 cations, they cations, they will not share edges or faces. will not share edges or faces. These shared corners are These shared corners are called bridging oxygens.called bridging oxygens.
Oxygens can share electrons with two silicons
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
ElectostaticElectostaticValence w/ OValence w/ O-2-2
1/8 - 1/121/8 - 1/121/6 - 1/81/6 - 1/81/3 – 1/41/3 – 1/42/6 = 1/3 2/6 = 1/3 2/6 = 1/32/6 = 1/32/6 = 1/32/6 = 1/33/6 = 1/23/6 = 1/24/6 = 2/34/6 = 2/33/6 = 1/23/6 = 1/23/43/44/4 = 14/4 = 1
WeakWeak
StrongStrong
big
medium
small
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 Sorosilicates Cyclosilicates
Inosilicates Inosilicates 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 hardnessEqui-dimensional habitsPoor cleavage
• Low degree of Al substitution with Si
Olivine X = MgOlivine X = Mg+2+2 or Fe or Fe+2+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 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
Grossular Ca3Al2Si3O12 – yellow-green to brown
Andradite Ca3Fe2Si3O12 – variable-yellow, green, brown, black
Uvarovite Ca3Cr2Si3O12 – emerald green
AlmandineAlmandine
Grossular
Andradite
Garnet AGarnet A33BB22SiSi33OO1212
Usually B is Aluminum, A divalentUsually B is Aluminum, A divalentAlmandine Almandine Fe3Al2Si3O12
B-site Aluminum octahedral
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 tabletsw/ single perfect cleavage; H: 5-7
Sillimanite – Vitreous brown to green clustered prisms w/ single cleavage dir.
Andalusite – Vitreous flesh-red, reddish brownsquare prisms; H: 7.5
Penet. twins, forming a cross
Common Nesosilicates: StauroliteFe2Al9O6(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 6-sided 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)
• Double silicon tetrahedra linked by one bridging oxygen• Sorosilicates commonly also contain independent silica tetrahedra (SiO4)-4
• Typically monoclinic symmetry
EpidoteEpidote
Si2O7
(Si2O7)-6
Common Sorosilicates: Epidote GroupZoisite/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: 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
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;
Epidote
Zoisite
Ca10(Mg,Fe)2Al4(SiO4)5(Si2O7)2(OH)4Common 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 Sorosilicate: Vesuvianite (aka Idocrase)
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