Thermochemistry of polybasite-pearceite solid solutions

Details Thermochemistry of polybasite-pearceite solid solutions Thermochemistry of polybasite-pearceite solid solutions

Oct 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.4363h&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 58, Issue 20, pp.4363-4375

Computer Science

1

Scientific paper

A model is developed for the thermodynamic mixing properties of polybasite-pearceite (Ag,Cu) 16 (Sb,As) 2 S 11 solid solutions. In this model the configurational entropy is formulated for the assumption that Ag and Cu display long-range, nonconvergent, ordering between three crystallographically distinct sites and that As and Sb mix randomly on one type of site. The nonconfigurational Gibbs energy is described using a Taylor series of second degree in composition and ordering variables. Model parameters are calibrated from constraints on 75°C miscibility gaps derived from unmixing experiments on (Ag,Cu) 16 (Sb .5 As .5 ) 2 S 11 grains ( X Pb - Pr Cu 0.32) and from 75-350°C Ag-Cu exchange experiments (evacuated silica tubes; variable mass ratio) between polybasite-pearceites and one- and two-phase subassemblages in the Ag 2 S-Cu 2 S subsystem. The resulting model is consistent with the distribution of polybasite-pearceite compositions observed in nature. It is also consistent with the inferences that 1. (1) only slight negative departures from ideality are associated with the Cu for Ag substitution ( -0.13 kJ/gfw for polybasite and -0.22 kJ/gfw for pearceite on a one Cu + Ag formula basis), 2. (2) positive deviations from ideality due to the As-Sb substitution in polybasite-pearceite ( 1.0 kJ/gfw on a one As-Sb site formula basis) are virtually identical to those in tetrahedrite-tennantite fahlores ( and 1993), and 3. (3) the Gibbs energy of formation of Ag 16 Sb 2 S 11 polybasite is 4.58 ± 1.4 kJ/gfw more positive than the equivalent combination of -Ag 2 S and Ag 3 SbS 3 pyrargyrite at 400°C.

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