NaAlSi3O8 = AlbiteNot required for Geology 306 students!
Derivation of minerals from the silica frameworks by substitution of elements for Si.Si4+ <=> Al 3+ + M+
2Si4+ <=> 2Al3+ + M++
THUS: (4SiO2 = Si4O8):
KAlSi3O8 = K-feldspar (sanidine, orthoclase, microcline)
NaAlSi3O8 = Albite
CaAl2Si2O8 = Anorthite
Other cations substituting into feldspar = Ba, Sr, Eu (some Fe, etc).
Ccompositions are simple, basic structures are relatively simple, structures of minerals with compositions intermediate between Alb and An, and Alb and Kspar are
very complex!
Stable compositional range as a function of P and T defined by nature and experiments: triangular diagram.
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.
THUS:
Na+ -- K+ (same charge)
Na+ -- Ca++ (about same size)
NOT: K+ -- Ca++ (different size and charge)
THUS: see LIMITED Ab-An-Kspar solid solution (sometimes termed Anorthoclase or ternary feldspar)
Exsolution: immiscibility:
Separates Na-K from Na-Ca feldspars = f(T).
Shape of the gap:
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
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
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.
Varying degrees of Al-Si order - varying "STRUCTURAL STATES"
Symmetry change and twinning!: Cross-hatch twinning in microcline
What does the presence of twinning in a triclinic K-feldspar imply?
PERTHITE - macro-, micro-, crypto-
Exsolution example: Albite (twinned) in Kspar host.
At high temperatures: Albite is monoclinic:
On cooling high-T albite _ Na 'slows down' - framework collapses around the 'smaller' cation: ALBITE BECOMES TRICLINIC
-> albite twins!
Followed by Al - Si ordering.
Range of stable solid solution as a function of T given by diagram. Find that K-feldspar is more tolerant of Na than v.v. Thus miscibility gap (solvus) is asymmetric.
Phase separation or unmixing: (liquid analogy)
Free energy vs composition diagram (general) see p. 239. How does this occur ?
Names for intermediate plagioclases: In order of increasing Ca content:
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, are ordering two elements whose behavior must be coupled via charge compensation (NaSi) (CaAl).
Phase diagram: Liquid, L+Ssn, ssn+/- exsolution.