A TERNARY SOLID SOLUTION MODEL OF NATURAL CHLORITES

Compositional variations of natural Fe-Mg chlorites are projectable onto the ternary compositional space bounded by either amesite, chamosite and clinochlore or by Al-free chlorite, chamosite and clinochlore. Al^IV greater than 1/O₁₀(OH)₈, in Al-rich chlorites, quantifies X_amesite; then, X_chamosite = (X_Fe)(1-X_amesite) and X_clin = 1 – (X_amesite + X_chamosite). X_Fe is obtained from chemical analyses of the samples. In Si-rich chlorites, by contrast, Al^IV less than 1/O₁₀(OH)₈, quantifies X_{Al-free chlorite}; X_chamosite = (X_Fe)(1-X_{Al-free chlorite}) and X_clin = 1 – (X_{Al-free chlorite} + X_chamosite). 84% of the chlorite compositions reported by Foster (1962) plot within the amesite-chamosite-clinochlore compositional space whereas the balance plot within the Al-free chlorite-chamosite-clinochlore system. The mole fraction of amesite or Al-free chlorite reflects Tschermak substitution away from the chamosite – clinochlore binary join.

The excess entropy of mixing amesite, chamosite and clinochlore to yield equivalent compositions to some non-stoichiometric natural chlorites is given below (Table 1).

Table 1: Excess entropy of mixing (S^ex) for some aluminous Fe-Mg chlorites.

Samples	Xamesite	Xchamosite	Xclinochlore	Sex (J/mol.K)	S298 (J/mol.K)
H1_High Mg	0.01	0.18	0.81	-7.2	431.70 ± 5.00
H2_High Fe	0.53	0.36	0.11	39.8	495.70±10.00
CCa-2	0.37	0.25	0.38	23.9	469.40 ± 2.90
CA	0.36	0.56	0.08	62.3	548.20 ± 3.78
CC	0.36	0.38	0.26	31.0	494.30 ± 3.40
CD	0.33	0.30	0.37	22.5	475.70 ± 3.28
CE	0.10	0.09	0.81	16.2	441.90 ± 3.04
CF	0.00	0.04	0.96	5.5	422.80 ± 2.91
Mg-Chl	0.14	0.10	0.76	10.8	437.81 ± 3.00
Fe-Chl(W)	0.38	0.44	0.18	28.6	499.14 ± 3.40
Fe-Chl(M)	0.37	0.47	0.16	40.5	515.06 ± 3.60

Sources of S₂₉₈: H1_High Mg and H2_High Fe (Hemingway et al. 1984); CCa-2 (Gailhanou et al. 2009); CA, CC, CD, CE, CF (Bertoldi et al. 2007); Mg-Chl, Fe-Chl(W), Fe-Chl(M) (Aja et al. 2015).

S^ex shows a curvilinear correlation with molar volumes of the chlorites and also decreases curvilinearly with increasing X_clinochlore. X_chamosite and X_amesite are limited in range (Table 1) and thus the curvilinear trend of S^ex is not as well resolved. Nonetheless, the curvilinear trend probably derives from pairwise interacting entropy – volume correlations of the constituent binaries of the ternary system.