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 A33BB22SiSi33OO1212Usually 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: TopazTopaz 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: HemimorphiteHemimorphite, 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