Effect of fluorine on the viscosity of diopside liquid

The effect of F on the shear viscosity of diopside liquid has been investigated at 1 atm and 1300 to 1500°C using the concentric cylinder method. To obtain glass starting materials, 100‐g batches of compositions along the CaMgSi₂O₆‐F₂O_-1 join—CaMgSi₂O₆, CaMgSi₂O_5.875F_0.25, CaMgSi₂O_5.75F_0.5, and CaMgSi₂O_5.5F(denoted Di, DiF0.25, DiF0.5, and DiFl, respectively)‐were melted in air. F volatilization was significant during batch melting but not during subsequent viscometry determinations. Electron‐microprobe analyses of these glasses permit comparison of viscosities as a function of F content.

The exchange of F for oxygen in these liquids reduces viscosity over the range of compositions investigated in this study. The viscosity reduction as a function of F/(F + O) exhibits a positive curvature (i.e., ∂²(log₁₀η)/∂(F/(F+ O))² > 0), as is the case for albite melt (Dingwell, 1987a), but the viscosity‐reducingeffect of F on diopside melt is much less than that observed for albite melt (Dingwell et al., 1985).

Raman spectra of quenched melts (glasses) on the CaMgSi₂O₆‐F_2O-1 join (Luth, 1988a) and liquidus phase equilibria in depolymerized systems (Foley et al., 1986; Luth, 1988b) have indicated that the addition of F (as F₂O_-1) increases the activity of SiO₂ in these liquids, and thus Luth (1988a, 1988b)has hypothesizedth at F (as F₂O_-1) might increase depolymerized melt viscosities (due to the increase in SiO₂ activity). Instead, the present study shows that the exchange of F for oxygen, the F₂O_-1 component, reduces the melt viscosity of diopside melt.

The polymerization of diopside melt may be quantified using the NBO/T calculation (see Mysen, 1988) at 2.0, whereas anhydrous chemical analyses of the most basic, depolymerized, natural melts yield NBO/T values of 1.4 (Mysen, I 987). Thus the present study shows that the viscosity‐reducing effect of F on silicate melts persists to compositions more depolymerized than any naturally occurring silicate melt. It is therefore proposed that F will reduce the viscosity of all low‐pressure (H₂O‐poor) silicate melts of geologic interest.