Solubility of fluorite in haplogranitic melt of variable alkalis and alumina content at 800°-1000°C and 100 MPa

Abstract

To study the behavior of fluorite in SiO2-Al2O 3-Na2O-K2O-H2O melts, we conducted both crystallization and dissolution experiments. In the first case, fluorite crystallized from melts enriched in Ca relative to F. In the second case, ground fluorite (<10 μm) was partially dissolved into melts of initially F-free or F-doped glass. Experiments were conducted at 800°-1000°C and 100 MPa for 7 to 240 hours using a series of finely ground subaluminous and peralkaline haplogranitic glasses. Electron microprobe (EMP) analysis of the run products (glass + crystals) documented increasing fluorite solubility with melt alkalinity. These molar relationships can be described by CF/2 = -1.3 ASI + 2.3 at 900°C and CF/2 = CCa = -2.1 ASI + 3.8 at 1000°C, where ASI is the aluminum saturation index measured in experimental glasses as Al/(Na + K + 2Ca), CF/2 is one half the molar concentration (%) of F, and CCa is the mol% of Ca in the glass. Fluorite solubility may be defined as either CF/2, CCa, or both. Solubilities at 800°, 900°, and 1000°C for the melts with ASI between 0.58 and 0.59 approach log linear with 104/T(K) (slope = -0.45). The joint effect of F and temperature on fluorite solubility is significant: solubility rises from 0.28 mol% at 800°C (4.96 mol% doped F) to 0.99 mol% at 900°C (2.17 mol% doped F). Solubilities at 800°, 850°, and 900°C for the melts with the same initial F contents (4.96 mol%) approach log linear with 104/T(K) (slope = -0.33). Copyright 2005 by the American Geophysical Union.