Hydrothermal Synthesis of Wurtzite and Sphalerite at T=350°-250°C

Abstract

Wurtzite and sphalerite were synthesized hydrothermally at 350°, 300° and 250°C using a temperature-gradient transporting method with sphalerite as a starting mineral. Fibrous wurtzite together with sphalerite was formed only in experiments at 300° and 250°C when a Zn-rich solution (1m ZnCL₂+1m NH₄CI) was used as the transporting media; only sphalerite was produced in experiments using a Zn-free solvent (1m NH₄Cl). Formation of wurtzite (vs. sphalerite) was not affected by the composition of the starting sphalerite nor by the addition of pyrrhotite and/or alabandite to the starting materials. These results agree with a thermodynamic prediction that the solubility of wurtzite is higher than that of sphalerite.
A comprehensive literature survey on the occurrence of wurtzite in hydrothermal ore deposits shows that wurtzite is not uncommon in shallow subaerial deposits and in submarine hydrothermal deposits of relatively young ages (mostly Tertiary or younger). For these'deposits, drastic physicochemical changes in the ore-forming solutions, such as rapid cooling by mixing with local meteoric or sea water, have been postulated during ore mineralizations. The wurtzite crystals often exhibit fibrous or radial textures, which would be produced by the above changes. Our ex-perimental data and the occurrence of wurtzite in ore deposits suggest that hydrothermal wurtzite is a metastable mineral that is formed by rapid crystal growth from highly supersaturated solutions.