L6

Question 1

Homovalent Substitutions

Answer 1

simple substitution of one ion with another f tha same charge

Question 2

Aliovalent Substitution

Answer 2

substitution through coupled substitutions

Question 3

Amphibole chain structure:

Answer 3

Double chain (like tagliatelli)
chain silicate with 3 different sites: AXY

Question 4

The three different sites on Amphibole:

Answer 4

Site 1: (called X or M4, small) has an optimum radius around 0.84 Å.
(can hold:Mg, Fe2+, Mn2+)

Site 2: (called Y or M1,M2,M3; bigger) have optimum radii around 1 Å.
(can hold Ca2+)

Site 3: (called A, huge) has an optimum size about 1.3 Å.
(holds Na+ and K+)

A0-1X2Y5(Si,Al)8O22(OH,F)2

A,X and Y are the ions in-between, while the other bit is the double chain structure

Question 5

exchange mechanism for amphibole

Answer 5

homovalent exchanges:
Mn2+ = Mg2+ = Fe2+ = Ca2+

Aliovalent exchanges include:
Na+ + Al3+ = Ca2+ + Mg2+
Na+ + Fe3+ = Ca2+ + Mg2+` [vac] + Ti4+ = Ca2+ + Mg2+

Question 6

three types of Amphibole:

Answer 6

Monoclinic (CLINOAMPHIBOLE)

Orthorhombic (ORTHOAMPHIBOLE)

Monoclinic (CLINO AMPHIBOLE)

Question 7

AMPHIBOLS- similarities to Pyroxene:

Answer 7

1. Mg and Fe solid solution is orthorhombic
2. Ca-bearing ones are monoclinic.
3. Miscibility gap in between clino- and ortho-forms. (NB The equivalent to pigeonite is cummingtonite)
4. Sodic amphiboles (found in igneous rocks)

Question 8

Wy amphibole is different to Pyroxene

Answer 8

1. Extra sodium site which can accept more Na and even K
2. More sites means ability to take wider range of elements,
3. Orthoamphiboles have two different structures and a small gap in between
4. Very common to get octahedral Al and tetrahedral Al
5. Tetrahedral Al exchanges for Si on the chains. This means….
   [vac] + Si4+ = Na+ + Al3+
   Na+ = K+
   Contains OH and F

Question 9

What is Hornblende

Answer 9

It’s the amphibole equivalent of augite, i.e. Ca-rich amphibole with Mg and Fe2+.
But…
Amphibole can substitute tetrahedral Al for Si and this accepts extra Na on the A site.
Hornblende = Na, Ca, Mg, Fe amphibole.

Question 10

Paired Silicate

Answer 10

SOROSILICATE
Si2O7 ^6- ion

jost important is the EPIDOTE GROUP
Ca2Al3SiO4O(OH)Si2O7

(Si2O7) = Double silicate Tetrahedra (farfalle pasta!)

SiO4 = single Silicate Tetrahedra

Question 11

EPIDOTE Group

Answer 11

• Epidote (low grade regional metamorphic)
• Zoisite (metamorphosed marls)
• Allanite (accessory in igneous rocks)

Question 12

Ring silicates:

Answer 12

Three-tetrahedral ring (Benitoite, Catapleiite)
Three-fold axis- trigonal

Four-tetrahedral ring (Axinite)
Four-fold axis - tetragonal

Six- tetrahedral ring (Beryl - emerald, Cordierite
Three or six fold axis - trigonal or hexagonal

Nine- tetrahedral ring (Eudialyte Group minerals)
Three-fold axis (trigonal)

Question 13

Sheet silicates:

Answer 13

Phyllosilicates- silicate tetrahedral share three corners

Al swaps for Si
silicate sheets separated by sheets of ions
CLAYS!

Question 14

What lies between aluminosilicate sheets?

Answer 14

Octahedral layers containing Mg or Al:

• Mg (Brucite layers, trioctahedral clays)
• Al (Gibbsite layers, dioctahedral clays)

3 Mg2+ = 2 Al3+ + [vacancy]

Question 15

Three groups of clay mineral structures:

Answer 15

• KANDITES (e.g. Kaolinite)
• SMECTITES (e.g. Montmorillonite)
• ILLITES (e.g. Muscovite)

Question 16

KANDITES

Answer 16

1:1 structure

One octahedral layer alternating with one aluminosilicate layer.

Dioctahedral Kandite [Al4Si4O10(OH)8] is called kaolinite

norweigan sandwiches

Question 17

SMECTITES

Answer 17

2:1 structure

One octahedral layer alternating with two aluminosilicate layer.

Dioctahedral smectite [Al4Si4O10(OH)8] is montmorillonite.

Smectites have an interlayer gap where water and small ions (e.g. K+) can reside - when wet, increases in volume!
(Swelling clays)

English sandwiches

Question 18

ILLITES

Answer 18

2:1 structure
like smectites but the region between the 2:1 layers are filled with a formal cation layer

structure is also common to micas, but clays have more water.

Question 19

MIXED LAYER CLAYS

Answer 19

most clays- interfingered with all three end members

Question 20

Important properties of Clays:

Answer 20

1. ABILITY TO SWELL when placed in wet environments (smectites and mixed-layer clays with smectite inc.). - can cause DESSICATION CRACKS and SUNBSIDENCE(buildings on clay-rich sediments subside in drought or if too much water is abstracted from subsurface aquifers).
2. CLAYS ACT AS REGULATORS OF SUBSURFACE WATER COMPOSITION. Because smectites do not hold cations firmly, they can exchange them with subsurface waters.
   e.g. K+(clay) + Na+(aq) = Na+(clay) + K+(aq)
   This exchange buffers the compositions of subsurface waters
3. Some ions transported in water are strongly attracted to the surfaces of clay minerals.
   e. g. UO22+(aq)&raquo_space; UO22+(clay)

(adsorption) accumulates pollutants and filters them from aquifers

HOWEVER, IF CONDITIONS (e.g. pH) CHANGE THEN THE PROCESS CAN BE REVERSED.

Question 21

How to identify clays?

Answer 21

not Optically
SEM (Secondary Eectron Microscope reveals different morphologies of clays,
but X-Ray diffraction is most commonly used (smectites have dif. XRD patterns when wet and dry and are the only ones to have this.