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 number of Na cations on the A site
(n(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 (or rather 
Matthias Konrad-Schmolke, Potsdam, deduces):

   n(Na,A) = n(Na,total) - n(Na,M4)

   n(Na,M4) = 2 - n(Ca,M4)

   n(Ca,M4) = n(CaO)

   n(Na,total) = 2 n(Na2O)

then 

   n(Na,A) = 2 n(Na2O) + n(CaO) - 2

which can be evaluated in WERAMI.

The only problem in getting this composition from WERAMI is the constant "-2". 
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 -2 n(H2O) = -2.

The WERAMI dialog to define the above expression for n(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)?
3

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

8 2
6 1
1 -2

How many components in the denominator of the composition (<12)?
Enter zero to use the numerator as a composition.
0

The compositional variable is: 2.0 NA2O +1.0 CAO -2.0 H2O