Mineralogy 23.10.08(Class1)

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LECTURE OUTLINE

Minerals: Building blocks of rocks

RULE 1: The anions around a cation define a coordination polyhedron.

The best-known example is the silica tetrahedron. The distance between

cations and anions is determined by the sum of their ionic radii. The

ratio of their radii determines the coordination number, or number of

anions surrounding the cation.

RULE 2: Electrostatic valency principle: The total strength of the valency

bonds that reach an anion from all its neighboring cations is equal to the

charge of the anion.

EXAMPLE:

For example, in olivine, (Mg,Fe)2SiO4, the charge on the Si ion is +4, on Mg

or Fe +2 and on O -2. Since 4 oxygens surround a silicon, there must be

a charge of -1 on each oxygen to be balanced by the Mg and Fe atoms.

RULE 3: Cation coordination polyhedra tend to be linked (share anions) at

corners first, then edges, and faces last of all because of the electrostatic

repulsion between cations. The repulsion is especially great for cations with

large charge and small coordination number, like Si. Silica tetrahedra almostnever link any other way than at their corners.

RULE 4: In a crystal containing different cations, cations with large charge

and small coordination number tend not to share polyhedral elements.

Think of Bowen's Series: among the ferromagnesian minerals, olivine

(isolated tetrahedra) tends to form first, followed by pyroxene (single chains),

amphibole (double chains) and biotite (sheets). The silica tetrahedra don't

link until they have no other choice.

RULE 5: In crystals the number of structurally distinct sites tends to

be small

CN Polyhedron Example

3 Triangle C in Calcite

4 Tetrahedron Si in Quartz

6 Octahedron Mg in Forsterite

8 Cube Ca in Fluorite

12 Dodecahedron Native gold

PACKING OF ATOMS IN CRYSTALS

Most compact lowest porosity Relatively less compact higher porosity

CUBIC Close Packing :

Spinel(MgAl2O4), Halite(NaCl)HEXAGONAL close packing:

Corundum, ilmenite

Minerals: Building blocks of rocks By definition a mineral is

Naturally occurring

Homogenous solid

Definite chemical

composition

Ordered internal

molecular structure

Inorganic solid

A solid aggregate of minerals

Granite

Quartz Biotite

Feldspar

C lassification of Minerals

± Oxides CHROMITE, CORRUNDUM

± Sulfides CHALCOPYRITE, PYRITE

± Sulfates GYPSUM

± Native Elements GOLD, SILVER

± Carbonates CALCITE, DOLOMITE

± Halides GYPSITE, HALITE

± Phosphates APATITE,

Minerals are classified on their chemistry, particularly on the anionic elementor polyanionic group of elements that occur in the mineral.

NON-SILICATES

SILICATES

LINKING OF SiO4 TETRAHEDRA

Linking of SIO4 Tetrahedra

Mineral Groups

± Silicates ±

LINKING OF SiO4 TETRAHEDRA AND CLEAVAGE

Neso slicate: Olivine- M2SiO4, e.g. Forsterire(Mg2SiO4) 1:4

Si :O ratio

Soro silicate : Epidote - Ca2(Fe+3, Al) Al2(SiO4)(Si2O7)(OH) 2:7

Cyclosilicate: Ber yl - Be3 Al2Si6O18 1:3

Ionosilicate(single): Pyroxene- M2M1Si2O6, e.g. Diopside(CaMgSi2O6) 1:3

Ionosilicate (Double): Amphibole- Tremolite(CaMg5Si8O22(OH)2) 4:11

Phyllosilicate: Mica, Chlorite- e.g. Annite(KFe3 AlSi3O10(OH)2) 2:5

Tectosilicate: Quartz, Feldspar- Albite(Na AlSi3O8) 1:2

SILICATES

The olivine structure is based on isolated SiO4 tetrahedra

(blue) which link chains of (Fe,Mg)O6 octahedra. There

are two octahedral cation sites: M1 (yellow) and M2

(orange). Both sites accomodate Fe2+ and Mg2+ cations

and there is complete disorder of Fe and Mg over the M1

and M2 sites. Olivine is orthorhombic and therefore will

show parallel or symmetric extinction under crossed

polarized light. M1-O distance~2.101 A, M2-O~2.135A.

M1 radius=0.781A, M2 radius=0.812A

Mg(M1)Mg(M2)SiO4

(Forsterite) Mg=0.72A

Fayalite(2Fe2Mg)

Fe=0.78A

Tephroite (2Mn2Mg)

Mn=0.83A

Motecellite (CaMg)

Ca=1.0A

K irschsteinite (CaFeSiO4),

Ni2SiO4

Isomorphous substitutions and speciation of

orthorhombic (EOLIVINE

NESOSILIC ATE (OLIVINE)

Glaucochroite (CaMnSiO4)

M1= Fe,Mg, Ni; M2 = Ca,Mn

OFe,Mg

STRUCTURE OF OLIVINE

Tetrahedra

(Mg,Fe)O6

Octahedra

OLIVINE

GARNET

The single chain silicates have a basic structural unit

consisting of linked SiO4 tetrahedra that each share 2of their oxygens in such a way as to build long chains

of SiO4. The basic structural group is thus Si2O6

with an Si:O ratio of 1:3. The most important

inosilicates are the pyroxenes. These have a general

structural formula of: XYZ2O6

where X = Na+, Ca+2, Mn+2, Fe+2, or Mg+2 fillingoctahedral sites called M2

Y = Mn+2, Fe+2, Mg+2 , Al+3, Cr+3, or Ti+4 filling

smaller octahedral sites called M1

Z = Si+4 or Al+3 in tetrahedral coordination

SINGLE CHAIN SILICATES : THEIR PHYSICAL AND CHEMICAL PROPERTIES

C axis

PyroxenequadrilateralCALCIC GROUP

NON CALCIC

Basal Section with intersecting cleavages at 87-88o

Relationship among the optical and cr ystallographic

planes in the oriented pyroxene cr ystals

Projection along

the C-axis

Structure of AMPHIBOLE

tetrahedra

Pyroxene

Amphibole

CLEAVAGE IN INOSILICATES

Sheet Silicates: the Mica's and

Clay Minerals Mica and clay minerals are P hyllosilicates

± Sheet or layered

silicates with ± Two dimensional

polymerization of

silica tetrahedra

± Common structure isa Si205 layer

sheets of silica tetrahedraSi2O5Phyllosilicates

1-direction

of cleavage

Muscovite

non-ferromagnesian

Mica Group and Clay Minerals

Structure of Phyllosilicates

Octahedral layer

± Layer of octahedral

coordinated

magnesium (brucitelayer) or

Aluminum (gibbsite

layer)

± Makes up the other basic structural unit

Kaolinite: Al2Si2O5(OH)41:1 tetrahedral ² octahedralsheets

The Major Clay Mineral Groups

Kaolinite group:

± 1:1 TO clay minerals

Mica (illite) group:

± 2:1 TOT clay minerals ± Expandible clays:

Smectite- montmorillonite

complex 2:1 clay minerals

Chlorite ± Fe- and Mg-rich TOT clays

All the corners of a SiO4 tetrahedron are linked with the corners of the

neighboring tetrahedron resulting in a Si : O = 1:2

Name Crystal System Density (g/cm 3)Refractive Index

(mean)

Stishovite (Si in 6-fold

coordination)Tetragonal 4.35 1.81

Coesite Monoclinic 3.01 1.59

Low (E) Quartz Hexagonal 2.65 1.55

High ( F) Quartz Hexagonal 2.53 1.54

Kaetite (synthetic) Tetragonal 2.50 1.52

Low (E) Tridymite Monoclinic or Orthorhombic 2.26 1.47

High ( F) Tridymite Hexagonal 2.22 1.47

Low (E) Cristobalite Tetragonal 2.32 1.48

High ( F) Cristobalite Isometric 2.20 1.48

E Ftransformation involves small

change in structural and physical

properties and hence is ³displacive´

transformation

All other transformations involve large

change in structural and physical

properties and hence are

³reconstructive´ transformation.

IMMISCIBLE

Movement

direction of the

boundar y at high

temperature

Feldspar Group (Si+4 replaced by Al+3 and the charge is balanced by

Ca+2, K+1 or Na+1)

Alkali feldspar : Solid solution between K AlSi3O8 (Sanidine, orthoclase,

microcline) and Albite (Na AlSi3O8)Plagioclase : Solid solution between Albite (Na AlSi3O8) and Anorthite

(Ca Al2Si2O8)

Mineralogy 23.10.08(Class1)

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