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