FELDSPARS AND OTHER FRAMEWORK SILICATES

Derivation of minerals from the silica frameworks by substitution of elements for Si.

Si⁴⁺ <=> Al ³⁺ + M⁺

2Si⁴⁺ <=> 2Al³⁺ + M²⁺

THUS: (4SiO₂ = Si₄O₈):

₄

₈

K(AlSi₃)O₈
Na(AlSi₃)O₈
Ca(Al₂Si₂)O₈

K(AlSi₃)O₈ = K-feldspar (sanidine, orthoclase, microcline)

Na(AlSi₃)O₈ = Albite

Ca(Al₂Si₂)O₈ = Anorthite

Other cations also found substituting into feldspar = Ba, Sr, Eu (some Fe, etc).

Compositions are simple, basic structures are relatively simple, structures of minerals with compositions intermediate between Ab and An, and Ab and Kspar are, however, very complex!

BASIC PHASE RELATIONS - Na-Ca-K

Stable compositional range as a function of P and T defined by nature and experiments: see the triangular diagram in Fig. 13.117, p. 532 for the names and ranges of compositions observed in nature.

Range of feldspar compositions found in nature determined by the sizes and charges on Na, Ca, K.

In general, a mineral structure will accept a mixture of cations if the cations have the same charge OR are about the same size (remember Paulings Rules).

THUS:

Na+ -- K+ (same charge)
Na+ -- Ca++ (about same size)
NOT: K+ -- Ca++ (different size and different charge)

Therefore: We see LIMITED Ab-An-Kspar solid solution (sometimes termed Anorthoclase or ternary feldspar)

Exsolution: immiscibility:

Separates Na-K from Na-Ca feldspars = f(T) (see Fig. 13.123 p. 536).

Shape of the gap:

ALKALI FELDSPARS

While the same charge, the size of K and Na differ by ~ 30 %!

Shape of the miscibility gap (can get more Na in Kspar than K in albite)

- a function of size (see Fig. 13.121, p. 535)

Al - Si order and disorder: (..thermodynamics..)

Order - disorder ....(2nd law)

Disorder increases with increasing temperature - applied to feldspars, find most Al-Si disorder at high temperature. Rearrangement requires breaking strong Al-O and Si-O bonds, so that it is possible to quench in disorder.

(dG/dT)_P = -S, where G = free energy, T= temperature, S = entropy

THUS: where would monoclinic K-feldspar (e.g., sanidine) be found?
where would triclinic K-feldspar (e.g., microcline) be found?

K-FELDSPAR AND Al - Si ORDER

Al - Si ordering: Two distinct types of tetrahedral sites in monoclinic feldspar - known as T1 and T2. In detail, T1 = T1o and T1m (related by the mirror) and T2o and T2m (also related by the mirror).

In monoclinic feldspars (space group C2/m), T1o = T1m and T2o = T2m because, while Al and Si are disordered, there is an equal probability of finding Al on T1o and T1m and an equal probability of finding an Al on T2o and T2m.

SANIDINE: equal probability of Al on all T sites-> MONOCLINIC
ORTHOCLASE: probability of Al on T1 not equal T2, but T1o=T1m and T2o=T2m -> MONOCLINIC!
MICROCLINE: Al selectively on T1 sites -> TRICLINIC! (ordering destroys 2-fold and mirror).

Varying degrees of Al-Si order - varying "STRUCTURAL STATES"

Symmetry change and twinning!: Cross-hatch twinning in microcline

Destroy mirror: ALBITE TWIN
Destroy 2-fold: PERICLINE TWIN

What does the presence of twinning in a triclinic K-feldspar imply?

PERTHITE - macro-, micro-, crypto-

Exsolution example: Albite (twinned) in Kspar host.

ALBITE STRUCTURAL CHANGES AS A FUNCTION OF TEMPERATURE

At high temperatures: Albite is monoclinic:

Al - Si disorder
Na motion at high T makes the Na equivalent in size to K.

On cooling high-T albite: Na 'slows down' - framework collapses around the 'smaller' cation: ALBITE BECOMES TRICLINIC

-> albite twins!

Followed by Al - Si ordering.

EXSOLUTION

Range of stable solid solution as a function of T given by diagram. Find that K-feldspar is more tolerant of Na than vice versa Thus miscibility gap (solvus) is asymmetric.

Phase separation or unmixing: (liquid analogy: low temperature spaghetti sauce or turkey gravy)

Free energy vs composition diagram (general) see Fig. 5.22, p. 239. How does this occur ?

Spinoidal decomposition
Nucleation in the bulk
Nucleation at defects

PLAGIOCLASE FELDSPARS

Names for intermediate plagioclases: In order of increasing Ca content:

Albite - An0 - An10 (or Ab100-Ab90)
Oligoclase - An10 - An30
Andesine - An30 - An50
Labradorite - An50 - An70
Bytownite - An70 - An90
Anorthite - An90 - An100.

Anorthite: Al = Si content in tetrahedra.

Al avoidance principle: Al and Si are rigorously ordered!

Na-Ca feldspar behavior is VERY complex. Will not go into details here, just note the existance of complexity associated with cation ordering and variations in structural characteristics.

IMPORTANT POINT: In plagioclases, we are ordering two elements whose behavior must be coupled via charge compensation (NaSi) (CaAl).

Phase diagram: Liquid, L+Ssn, ssn+/- exsolution.

P= peristerite (~An2 -~An17)
B = Boggild (~An48 - ~An58) - LABRADORITE!
H = Huttenlocher (~An65 -~An88)

Labradorite - schiller due to subdivision of the feldspar into lamellae with diameters and separations ~ wavelength of light!

FELDSPAR STRUCTURES:

Albite (580K) (Si-dark blue, Al-light blue, Na-yellow)
Anorthite (350K)(Si-dark blue, Al-white, Ca-aqua)
K-feldspar (sanidine) (326K)(complete Al-Si disorder-both dark blue)

These were created on the CrystalMaker 1.1.4 interactive crystallography program . If you would like more information on the program, please contact:

David Palmer

(Technical information)