Thermochemistry of fahlore (tetrahedrite) and biotite mineral solutions
Abstract
Modern adaptations of classical chemical thermodynamic theory successfully describe phase equilibria in silicate and sulfide systems. A thermodynamic formulation, and parameters describing Ag-As incompatibility in $\rm (Cu,Ag)\sb{10}(Fe,Zn)\sb2(Sb,As)\sb4S\sb{13}$ fahlores at 400$\sp\circ$C (Ebel and Sack, 1989), which incorporate large temperature dependencies of standard state properties and composition-ordering systematics, describe Ag-As incompatibility equally well in experimentally reversed equilibria at 300$\sp\circ$C. This formulation can be applied to descriptions of fahlore composition as a function of temperature and the compositions of coexisting minerals, which define Ag(Cu)$\sb{-1}$ and Fe(Zn)$\sb{-1}$ exchange properties controlling fahlore composition. The formulation and calibration for fahlore can be used to derive properties of other phases with which it equilibrates, for example bournonite-seligmannite CuPb(Sb,As)S$\sb3$ solid solutions. The fahlore model predicts miscibility gaps consistent with composition ranges observed in nature. The formulation can be successfully applied to equilbria involving As-free fahlore + pyrargyrite $\rm (Ag,Cu)\sb3SbS\sb3$ + miargyrite (Ag,Cu)SbS$\sb2$, to extract values for the Gibbs free energies of formation of $\rm (Ag,Cu)\sb{10}(Fe,Zn)\sb2Sb\sb4S\sb{13}$ endmembers at 400$\sp\circ$C. Using the formulation for fahlore thermochemistry, these equilibrium relations can be extrapolated to 300$\sp\circ$C and 200$\sp\circ$C, successfully reproducing experimental results. Application of the fahlore model phase relations in the ZnS-saturated Ag$\sb2$S-Cu$\sb2$S-$\rm Sb\sb2S\sb3$ system indicates: (1) no $\rm (Ag,Cu)\sb{16}Sb\sb2S\sb{11}$ polybasites are stable in ZnS-saturated conditions. (2) (Ag,Cu)$\sb2$S solid solutions behave very nearly ideally in the T-X$\sb{\rm Ag}$ region where they are in the fcc structural state. However, experimental data show $\rm (Ag,Cu)\sb{16}As\sb2S\sb{11}$ pearcite + fahlore + (Ag,Cu)$\sb2$S + ZnS to be a very stable assemblage. Preliminary analysis indicates that nonideal Fe-Mg mixing in biotite-phlogopite solutions may be described by a parameter $\rm W\sbsp{FeMg}{oct}=23.196kJ\cdot mol\sp{-1}.$ Biotite-olivine Fe-Mg exchange systematics in volcanic rocks illustrate a complex dependence on $\rm X\sbsp{Ti}{biotite}.$ These effects are responsible for the failure of current models for silicate liquid crystallization to successfully account for biotite stability in many mica-bearing extrusive rocks.
Degree
Ph.D.
Advisors
Sack, Purdue University.
Subject Area
Mineralogy|Geochemistry|Chemistry
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