Santa Terezinha de Goiás district, Goiás, Brazil

A large Brazilian source of emeralds, Santa Terezinha, was discovered in 1981 about 275 km north of Goiana in the state of Goias.

Emerald mineralization is developed in the talc schists alternating with the hematite schists and quiartzites, which are crumpled into large isoclinal folds. Rich and extensive emerald-bearing zones with thickness of several meters have been traced to a depth fo 100 m. The emeralds are mined at several sections. The richest emerald area is a periclinal closure of the fold structure, where the most of the pegmatite bodies are concentrated. In the upper section, a large quantity of emeralds was extracted from clayey eluvial deposits of an ancient weathered crust, where emerald is enclosed in a dense limonite envelope.

The emerald crystals are of limited size and seldom exceed 1 cm in length. Faceted stones of good quality usually have a weight of less than 1 ct. Emerald is characterized by high content of chromium, iron, magnesium and sodium. Density may reach 2.87 g/cm³. Pleochroism is dark and bluish green to pale yellowish green. The most common inclusions are pyrite, carbonates (dolomite, ferroan magnesite), with lesser talc, magnesioferrite, chrome spinels and mica (Cassedanne and Sauer, 1984).

Ref: E.Ya. Kievlenko (2003) Geology of gems, p. 92

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Brazil
  • Goiás
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