Hi Eric, 
Here is the solid solution model for biotite from Lucie. We checked this model with Rob Berman about two weeks ago and it works fine. 
Cheers,
Pierre   
!===============================================================================
!====================================== PHASE COMPONENTS FOR SOLUTION MODEL BIOTI
!=======================================model of Tajcmanova et al. 2009=======
!----------------------------checked vs Perplex coding Feb,2016 (SWOT 16)
**** MINERAL DATA ****
! ---- -------------------------------------------------------------------------
br                 MG(1)H(2)O(2)                               br            use
 ST              0.0    -924920.000        64.5000         2.4630
 C3        158.40000     -0.0040760     -1052300.0      -1171.300           0.00
 VHP     0.000130000          485.0         0.0000           0.00        0.00000
 VH2         10.0000         4.0000   -0.000150000
! ---- -------------------------------------------------------------------------
phl                K(1)MG(3)AL(1)SI(3)H(2)O(12)               phl            use
 ST              0.0   -6219160.000       328.0000        14.9640
 C3        770.30000     -0.0369390     -2328900.0      -6531.600           0.00
 VHP     0.000057900          513.0         0.0000           0.00        0.00000
 VH2         10.0000         4.0000   -0.000150000
! ---- -------------------------------------------------------------------------
ann                K(1)FE(3)AL(1)SI(3)H(2)O(12)               ann            use
 ST              0.0   -5151670.000       418.0000        15.4320
 C3        815.70000     -0.0348610        19800.0      -7466.700           0.00
 VHP     0.000057900          513.0         0.0000           0.00        0.00000
 VH2         10.0000         4.0000   -0.000150000
! ------------------------------------------------------------------------------
east               K(1)AL(1)MG(2)AL(2)SI(2)H(2)O(12)              east            use
 ST              0.0   -6338170.000       318.0000        14.7380
 C3        785.50000     -0.0380310     -2130300.0      -6893.700           0.00
 VHP     0.000057900          513.0         0.0000           0.00        0.00000
 VH2         10.0000         4.0000   -0.000150000
! ------------------------------------------------------------------------------
mnbi               K(1)MN(3)AL(1)SI(3)H(2)O(12)              mnbi            use
 ST              0.0   -5462840.000       433.0000        15.2640
 C3        809.90000     -0.0592130     -1514400.0      -6998.700           0.00
 VHP     0.000057900          513.0         0.0000           0.00        0.00000
 VH2         10.0000         4.0000   -0.000150000
! ---- -------------------------------------------------------------------------
obi               K(1)MG(2)FE(1)AL(1)SI(3)O(12)H(2)    obi          use
 ST              0.0      -6800.000         0.0000         0.0000
COM    phl[2/3]ann[1/3]
!COM above with annite, not annite2 explicitly stated by Tajcmanova et al. 2009
! ---- -------------------------------------------------------------------------
tbi               K(1)MG(2)TI(1)AL(1)SI(3)O(12)    tbi          use
 ST              0.0      84000.000       11.5000         0.0000
COM    phl[1]br[-1]ru[1]
! ---- -------------------------------------------------------------------------
fbi               K(1)MG(2)FE(1)AL(2)SI(2)O(12)H(2)    fbi          use
 ST              0.0       6000.000         0.0000         0.0000
COM    east[1]cor[-0.5]hem[0.5]
!--------------------------------------------------------------------------------
!--------------------------------------------------------------------------------
**** SOLUTION DATA ****
BIOTI    (-SITE,MARGULES)    M1(1):Mg,Fe,Al,F3,Mn - M2(2):Mg,Fe,Ti,Mn - T1(2):Si,Al - v(2):OH,O
    phl              Mg - Mg,Mg - Al,Si - OH,OH
    ann              Fe - Fe,Fe - Al,Si - OH,OH
    obi              Fe - Mg,Mg - Al,Si - OH,OH
    east             Al - Mg,Mg - Al,Al - OH,OH
    tbi              Mg - Mg,Ti - Al,Si - O,O
    fbi              F3 - Mg,Mg - Al,Al - OH,OH
    mnbi             Mn - Mn,Mn - Al,Si - OH,OH
*** MARGULES PARAMETER ***
phl - ann
12    12000   0   0
phl - obi
12    4000   0   0
phl - east
12    10000   0   0
ann - obi
12    8000   0   0
ann - east
12    3000   0   0
obi - east
12    7000   0   0
!
!