NGT32-2/3/4-01
1953, 1953
The FeS-ZnS system, a geological thermometer.
32
2, 3, 4
61-147

In the first part of this pa per, the FeS - ZnS equilibrium diagram is presented. Laboratory equipment and methods used for the establishment of the diagram are described. The end members of the system as well as the α and β mix-crystals are studied. The ZnS α-β inversion mechanism has received special attention, and the relationship between inversion temperature and mix-crystal composition is established. The increase in the ZnS lattice dimensions due to the replacement of Zn by Fe is explained by assuming that the mix-crystal bonds acquire a more ionic character as their FeS content increases. Thermodynamic calculations are applied for determinations of the equilibrium curves below 400° C.

The influence of pressure upon the FeS solubility in the ZnS lattice has been calculated, and the results in part checked by means of laboratory experiments. The FeS activity was investigated and was found to decrease very little from stoichiometric FeS to FeS2. However, it drops sharply, when S is added beyond the amount indicated by the FeS2 formula. Studies were conducted of the unmixing of FeS from mix-crystals. It was also found that small amounts of Mn and Cd have little or no influence on the solubility of FeS in ZnS.

In the second part, chemical, spectrographic and x-ray methods for determination of the composition of sphalerite ore specimens are described. Results of the analyses together with the temperatures deduced from the composition of the samples are presented in tabular form. Same of the localities from which specimens were obtained are described with a view to ascertaining the pressure conditions prevailing during ore deposition. Discussions whether the mix-crystals contain as much FeS as prescribed by the (P, T) conditions of ore formation, are aften of little value unless coupled with field work and microscopic studies. While FeS in many cases seems to have been present in ample quantities to satisfy the equilibrium requirements, Mn, Cd, and the trace elements never occurred in sufficient amounts to saturate the ZnS lattice. The only reliable index of temperature of formation is FeS. The distribution of the other elements is not necessarily related to this temperature. Possible applications of the FeS- ZnS system to geological problems are outlined.