L6 Flashcards
Homovalent Substitutions
simple substitution of one ion with another f tha same charge
Aliovalent Substitution
substitution through coupled substitutions
Amphibole chain structure:
Double chain (like tagliatelli) chain silicate with 3 different sites: AXY
The three different sites on Amphibole:
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
exchange mechanism for amphibole
homovalent exchanges:
Mn2+ = Mg2+ = Fe2+ = Ca2+
Aliovalent exchanges include:
Na+ + Al3+ = Ca2+ + Mg2+
Na+ + Fe3+ = Ca2+ + Mg2+` [vac] + Ti4+ = Ca2+ + Mg2+
three types of Amphibole:
Monoclinic (CLINOAMPHIBOLE)
Orthorhombic (ORTHOAMPHIBOLE)
Monoclinic (CLINO AMPHIBOLE)
AMPHIBOLS- similarities to Pyroxene:
- Mg and Fe solid solution is orthorhombic
- Ca-bearing ones are monoclinic.
- Miscibility gap in between clino- and ortho-forms. (NB The equivalent to pigeonite is cummingtonite)
- Sodic amphiboles (found in igneous rocks)
Wy amphibole is different to Pyroxene
- Extra sodium site which can accept more Na and even K
- More sites means ability to take wider range of elements,
- Orthoamphiboles have two different structures and a small gap in between
- Very common to get octahedral Al and tetrahedral Al
- Tetrahedral Al exchanges for Si on the chains. This means….
[vac] + Si4+ = Na+ + Al3+
Na+ = K+
Contains OH and F
What is Hornblende
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.
Paired Silicate
SOROSILICATE
Si2O7 ^6- ion
jost important is the EPIDOTE GROUP
Ca2Al3SiO4O(OH)Si2O7
(Si2O7) = Double silicate Tetrahedra (farfalle pasta!)
SiO4 = single Silicate Tetrahedra
EPIDOTE Group
- Epidote (low grade regional metamorphic)
- Zoisite (metamorphosed marls)
- Allanite (accessory in igneous rocks)
Ring silicates:
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)
Sheet silicates:
Phyllosilicates- silicate tetrahedral share three corners
Al swaps for Si
silicate sheets separated by sheets of ions
CLAYS!
What lies between aluminosilicate sheets?
Octahedral layers containing Mg or Al:
- Mg (Brucite layers, trioctahedral clays)
- Al (Gibbsite layers, dioctahedral clays)
3 Mg2+ = 2 Al3+ + [vacancy]
Three groups of clay mineral structures:
- KANDITES (e.g. Kaolinite)
- SMECTITES (e.g. Montmorillonite)
- ILLITES (e.g. Muscovite)
KANDITES
1:1 structure
One octahedral layer alternating with one aluminosilicate layer.
Dioctahedral Kandite [Al4Si4O10(OH)8] is called kaolinite
norweigan sandwiches
SMECTITES
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
ILLITES
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.
MIXED LAYER CLAYS
most clays- interfingered with all three end members
Important properties of Clays:
- 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).
- 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 - Some ions transported in water are strongly attracted to the surfaces of clay minerals.
e. g. UO22+(aq)»_space; UO22+(clay)
(adsorption) accumulates pollutants and filters them from aquifers
HOWEVER, IF CONDITIONS (e.g. pH) CHANGE THEN THE PROCESS CAN BE REVERSED.
How to identify clays?
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.
