Experimental investigations into the thermodynamic properties of halogens in silicate melts
Keywords: chlorine bonding
NaCl activity
HCl degassing
chlorine solubility

The aim of this thesis is to understand the chemical behaviour of halogens in silicate melts by experimentally determining their thermodynamic activities. This includes defining how halogens become incorporated into a melt and what influences chlorine's solubility and bonding behaviour in silicate melts.
A halogen buffer has been developed using Ag/AgCl, in which Ag (as oxide) is virtually insoluble in silicate melt under the conditions of the experiment. The buffer controls the fugacity of chlorine. Experiments were performed at 5-20 kbar and 1200-1500 °C in a piston-cylinder apparatus. The effect of oxygen fugacity on Cl solubility was determined using Re-ReO2 and C-CO2 oxygen buffers. Experiments show (1) that chlorine solubility in haplobasalt at 15 kbar/1400 °C can reach 5 wt%, even at Cl2 fugacities as low as 0.0035 bar. 2) Cl concentration increases linearly with the square root of chlorine fugacity at fixed f(O2), obeying Henry’s Law up to ~3% Cl in the melt of a haplobasalt. 3) Cl solubility decreases with the fourth root of oxygen fugacity at fixed √𝑓(𝐶𝑙2). This implies that Cl- replaces O2- in the silicate framework as follows:
Cl2(gas)+[O2-](melt) = 2[Cl-](melt)+ ½O2(gas) [I]
This relationship is used to correct for the effect of oxygen and chlorine fugacity on pressure and temperature, to determine more accurately how chlorine solubility changes with these intensive variables. Results show that Cl increases with temperature and decreases with pressure at fixed conditions.
The chlorine buffering method was then applied to 44 chemically different compositions, all performed at 1400 °C, 15 kbar, and a fixed oxygen and chlorine fugacity. Results show that chlorine solubility increases with Ca and Fe and decreases with Si content. Knowing the compositional effects at fixed conditions allowed for the quantification of a chloride capacity (CCl), defined as:
𝐶𝐶𝑙= 𝐶𝑙 (𝑤𝑡%)√𝑓(𝐶𝑙2) ×√𝑓(𝑂2)4 [ii]
The chlorine capacity allows for the comparison of literature data (including those performed with water) if the temperature, pressure, oxygen fugacity and chemical composition are known. Literature data were combined with the 44 experiments performed across a wide compositional range and the pressure and temperature series of experiments. Cl content in wt% can then be expressed as a function of the different variables as:
𝑙𝑜𝑔𝐶𝐶𝑙=0.571+(4873𝑋𝐶𝑎−3803𝑋𝑆𝑖+2724𝑋𝐹𝑒+1891- 943P)/T [iii]
Water was found to be insignificant in controlling chlorine solubility at concentrations less than 4.3 wt%. [iii] can then be used to determine the conditions that HCl would degas. Results indicate that, at fixed Cl2 fugacity, pressure, and temperature most HCl loss occurs at pressures below 2 MPa. Additionally, the fraction lost as the surface is approached varies dramatically with composition as rhyolite>dacite>andesite>basalt. Constraining the thermodynamic activities also allows for determining the conditions under which Cl-rich minerals such as sodalite would form and when brines would begin to segregate from the melt.
The bonding behaviour of chlorine in silicate melts was investigated by X-ray absorption fine structure. Results indicate that Cl predominantly bonds with 2+ cations (Ca, Mg and Fe) but can also covalently bond with Si in melts that exhibit low 2+ cation contents.