To plot/compute site occupancies with WERAMI it is neccessary to formulate a
rule that relates the bulk composition to the site occupancy (this is always
possible). For example suppose we want the site fraction of Na on the A site
(Z(Na,A)) of an amphibole model with endmember site occupancies as follows:
Site: A M1 M2 M4 T1
Mutliplicity: 1 3 2 2 4
1 tr Vac Mg Mg Ca Si_Si
2 ftr Vac Fe Fe Ca Si_Si
3 ts_dqf Vac Mg Al Ca Al_Si
4 fts Vac Fe Al Ca Al_Si
5 parg_dqf Na Mg Mg_Al Ca Al_Si
6 fparg Na Fe Fe_Al Ca Al_Si
7 gl Vac Mg Al Na Si_Si
8 fgl Vac Fe Al Na Si_Si
From inspection of the endmember site occupancies we deduce:
Z(Na,A) = 2*[Z(Al,T1) - Z(M,M2)] where M = Fe + Mg
then
Z(Al,T1) = 1 - Z(Si,T1)
Z(Si,T1) = (n(SiO2) - 4)/4
Z(M,M2) = ([n(MgO) + n(FeO)] - 3)/2
and substituting these last three relationships into the first we obtain
the desired "rule" (you may want to check my math, as I haven't):
Z(Na,A) = 7 - n(MgO) - n(FeO) - 1/2 n(SiO2)
which can be evaluated in WERAMI.
The only problem in getting this composition from WERAMI is the constant "7".
A trick for including such constant is include a component in the composition
that is in fact constant, and multiply the amount of this component by a
factor that gives the constant; for example, in this amphibole model n(H2O) = 1,
therefore n(H2O)*7 = 7).
The WERAMI dialog to define the above expression for Z(Na,A) is:
Select a property:
1 - Specific Enthalpy (J/m3)
2 - Density (kg/m3)
3 - Specific heat capacity (J/K/m3)
4 - Expansivity (1/K, for volume)
5 - Compressibility (1/bar, for volume)
6 - Weight (%) of a component
7 - Mode (Vol, Mol, or Wt proportion) of a phase
8 - Composition (Mol or Wt) of a solution
9 - Grueneisen thermal ratio
10 - Adiabatic bulk modulus (bar)
11 - Adiabatic shear modulus (bar)
12 - Sound velocity (km/s)
13 - P-wave velocity (Vp, km/s)
14 - S-wave velocity (Vs, km/s)
15 - Vp/Vs
16 - Specific entropy (J/K/m3)
17 - Entropy (J/K/kg)
18 - Enthalpy (J/kg)
19 - Heat Capacity (J/K/kg)
20 - Specific mass of a phase (kg/m3-solid)
21 - Poisson ratio
22 - Molar Volume (J/bar)
23 - Chemical potentials (J/mol)
8
Enter solution or compound name (left justified):
Amphibole
Compositions are defined as a ratio of the form:
Sum {w(i)*n(i), i = 1, c1} / Sum {w(i)*n(i), i = c2, c3}
n(j) = mole proportion of component j
w(j) = weighting factor of component j (usually 1)
How many components in the numerator of the composition (<16)?
4
Enter component indices and weighting factors for the numerator:
1 - H2O
2 - MGO
3 - AL2O3
4 - SIO2
5 - K2O
6 - CAO
7 - FEO
8 - NA2O
1 7
2 -1
7 -1
4 -0.5
How many components in the denominator of the composition (<12)?
Enter zero to use the numerator as a composition.
0
The compositional variable is: 7.0 H2O -1.0 MGO -1.0 FEO -0.5 SIO2