This thesis is about thermal etching of quartz single crystals with boron oxide melt and thermal etchings on inorganic polymeric single crystals of orthosilicates, willemite (Zn2SiO4) and phenakite (Be2SiO4), where isolated [SiO4] groups are polymerized by corner-sharing with other tetrahedral groups, such as [ZnO4] and [BeO4]. On the thermal etching of quartz, experiments were performed on quartz (10 0), (0001), (10 1) and (11 1) from 500¢J to 700¢J. Three types of etch figures were recognized by scanning electron microscopy: isolated dislocation etch pit, aligned etch pits and flat etch pits. The effects of defect specification and £\-£] displacive phase transformation of quartz on its development of thermal etch figures were evaluated. By doing so, boron oxide melt was proved to be a useful etchant on the studies of defect types and dynamics of quartz. As for the thermal etching of phenakite type silicate, we conducted thermal-cycle etching of willemite at 1250¢J, hydrochloric and hydrofluoric acid etchings of willemite and phenakite at room temperature, and boron oxide melt etching of willemite and phenakite at 700¢J. Surface premelting, anisotropic lattice etching and defect etching were found to play important roles on the thermal etching of willemite. Impurity segregation at dislocation outcrops on willemite (0001) should occur in the first thermal cycling in order to nucleate hillocks at the centers of the hexagonal dislocation etch pits. Reflection IR spectroscopic analysis indicated the surface premelt has the same structural units as willemite, although the subsequent crystallization follows a silica rich path. A silica-rich surface coverage impedes the etching of crystal plane underneath. There is significant polygonization and cleaving-healing of willemite single crystal upon thermal cycling according to transmission electron microscopy observation. Phenakite has remarkable chemical and thermal etching resistance in comparison to its isostructure willemite due to site energy difference of Be and Zn in coordination number 4.

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The Vazante Northern Extension is a continuation of the structurally controlled, hypogene nonsulfide zinc deposit from the Vazante Mine, Minas Gerais, Brazil. The deposit is hosted in Neoproterozoic carbonates of the Serra do Poço Verde Formation located in the Brasília Fold Belt along the western margin of the São Francisco Craton. The Northern Extension deposit is hosted within a shear zone that strikes 200 to 225 and dips 30º-70º NW and is associated with substantial folding that occurred in the same deformation (D2) event and later transcurrent faulting. The shear zone is composed of a tectonic-hydrothermal breccia that can be subdivided into four types: Type 1 Breccia– distally located from the willemite (Zn2SiO4) ore, weakly brecciated dolostone with some Fe-carbonate alteration; Type 2 Breccia–Strong Fe-carbonate alteration, intense brecciation; Type 3 Breccia–hematite replacement breccia, usually proximal to the Type 4 Breccia; Type 4 Breccia–willemite ore breccia, containing three generations of hypogene willemite (Zn2SiO4) as well as hematite (Fe2O3), and franklinite (ZnFe2O4). In the southern part of the deposit, late sulfide-rich veinlets are observed cutting the hypogene zinc silicate mineralization, and contain galena, sphalerite, native silver, covellite, and stromeyerite (Cu1-xAg1-xS). Geochemical studies conducted on the tectonic-hydrothermal breccia showed a strong regional signature enriched in Ag, Cd, Pb, and Zn. The presence of willemite as opposed to sphalerite and the ubiquity of hematite suggest that the zinc silicate mineralization has formed from the mixing between a low sulfur, acid, oxidizing metalliferous basinal brine similar to the fluids found in MVT deposits with a cooler, more oxidizing, meteoric fluid. / Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2012-02-29 16:45:36.41