# Experimental and empirical investigation of the silver sulfide-silver selenide system and discrimination of topaz rhyolites by major-element composition.

#### Abstract

This dissertation comprises an investigation of the Ag$\sb2$S-Ag$\sb2$Se system and the discrimination of the topaz rhyolites by major-element composition, and consists of a brief introduction and four chapters. Each chapter has a separate objective and was or will be published as an independent paper.

Solid solutions in the system Ag$\sb2$S-Ag$\sb2$Se. Chemical and x-ray diffraction analyses of synthetic samples indicate the presence of two solid-solution series at ambient conditions. One series extends from Ag$\sb2$S to approximately $\rm Ag\sb2S\sb{0.4}Se\sb{0.6}$ and is monoclinic. The second series ranges from $\rm Ag\sb2S\sb{0.3}Se\sb{0.7}$ to the Ag$\sb2$Se and is orthorhombic. Members of both series characterize the compositional gap between the two solid solutions.

Calorimetric analysis of the system Ag$\sb2$S-Ag$\sb2$Se. It was demonstrated that, within the compositional range of the two series, each compound is a single phase, rather than an intergrowth of end members. Calorimetry confirmed the continuous nature of each solid solution showing that the transition temperatures vary smoothly with composition. Determination of the entropy of the transitions was consistent with the model of two solid solutions.

Silver sulfide-selenide minerals from Mexico, evidence for two solid solutions from acanthite to naumannite. Electron probe microanalysis of ores from the Mexican silver districts of Guanajuato and Zacatecas provided compositions ranging from pure silver sulfide to pure silver selenide. The analyses probably correspond to the two solid-solution series which characterize the system Ag$\sb2$S-Ag$\sb2$Se. In the light of such experimental data, the full range of intermediate compositions in nature should not be unexpected.

Discrimination of topaz rhyolites by major element composition, a statistical routine for geochemical exploration. The recognition of topaz-bearing, calc-alkaline, and peralkaline rhyolites at an early stage of an exploration program may be of both geologic interest and of economic significance.

Multiple discriminant function analysis classified each sample statistically as a member of the calc-alkaline, topaz or peralkaline groups, using only major element composition. In effect, major element composition can serve as a proxy for fluorine content either when no fluorine analysis is available or when it is suspected that fluorine was not preserved in the rock due to loss of fluorine during emplacement or subsequent processes.

#### Subject Area

Geochemistry; Mineralogy

#### Recommended Citation

Ponce, Benjamin F, "Experimental and empirical investigation of the silver sulfide-silver selenide system and discrimination of topaz rhyolites by major-element composition." (1995). ETD Collection for University of Texas, El Paso. AAI9536833.
http://digitalcommons.utep.edu/dissertations/AAI9536833

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