Education
Emerald is the green to bluish green variety of beryl, a mineral species that also includes aquamarine as well as beryls in other colors.
It falls within the chemical class of silicates, a subclass of cyclosilicates. The name emerald is linked to its peculiar gram-green color, as a result of the presence of chromium, iron and vanadium as impurities, replacing aluminum in its crystalline structure. The intensity of the color will depend on the content of these elements in the chemical structure of beryl (Souza et al.1987).
Emerald has hardness ranging from 7.5 to 8 on a scale ranging from 1 to 10. High hardness is very important for those minerals used for gemological purposes. Pleochroism is an optical phenomenon in which the mineral has different colors when viewed from different angles. The pleochroism of emeralds ranges from yellowish green to bluish green. The relative density of emeralds ranges from 2,670 to 2,780 (Sinkankas 1981). The relative density values already measured in crystals from the Belmont Mine were 2.710 (±0.020) (Sauer 1982).
It is rare for an emerald to not have inclusions which are of different types. Descriptive analysis and genetic classification of inclusions are of fundamental importance to provide the natural or synthetic origin of a gemstone.
It is known that gems from the region of Itabira / Nova Era is characterized by a large amount of fluid inclusions. The emerald of this region presents a predominance of fluid inclusions in relation to crystalline ones (Souza 1988; Machado 1994).
This fact is clearly observed in the emerald of the Belmont Mine. Similar to other deposits in the Itabira / Nova Era region (Souza 1988). The internal cavities or growth tubes are normally oriented, characterizing the so-called “rain effect” (Schwarz 1987). These cavities are filled with liquid, gas and solid phases, which can be biphasic to polyphasic. This phenomenon is easily observed when the sets of these inclusions are seen along the orientation direction in the crystal (Photomicrographs 01).
Photomicrograph 01: Detail of growth tubes oriented in a preferential direction called “Rain Effect” in a Belmont emerald, unique characteristic of the minerals from this region. (100x and 200x magnification).
Sources
Although emeralds have been found in many parts of the world. The rarity of this Beryl variety makes it, together with diamond and alexandrite, the most coveted gemstones in the world, reaching high values.
In addition to Brazil and Colombia in South America, emeralds have been found in Russia, Afghanistan, Pakistan, India, Austria, Australia, Zambia, Zimbabwe, Tanzania, Madagascar, Nigeria, South Africa, USA, Norway, Spain and Egypt (Figure 01). In the Industry, most emerald crystals come from Colombia, Brazil and Zambia.
In Brazil, emeralds are mined in the states of Goiás (Campos Verdes), Bahia (Campos Formoso/Pindobaçu and Anagé) and Minas Gerais (Itabira/Nova Era, in São Domingos do Prata and Esmeraldas de Ferros). Other occurrences can be mentioned, such as Tauá (in Ceará); Monte Santo (Tocantins); Tenente Ananias (Rio Grande do Norte).
The main companies producing this mineral good are: Belmont Mineração and Canaã Indústria e Comércio located in Itabira (MG); Itaobi in Campos Verdes (GO), Brazil. And Gemfields in Zambia.
Figure 01: Map indicating emerald occurrences in the world.
Geological formation
There are mainly two types of genetic classification for emeralds: the Colombian type, with the typical example of Colombian mines and the classical or shale type, with beryllium source through pegmatite contribution or not, covers most of the known deposits worldwide. Examples of occurrences of this type are in Brazil.
Shale deposit emeralds are normally disseminated in dark shale and results from the chemical interaction of residual solutions of granitic rocks, enriched in beryllium, with basic/ultrabasic rocks that supply the iron, chromium and vanadium chromophores.
Historically, emerald deposits have been classified into three broad types. The first and most abundant deposit type, in terms of production, is the desilicated pegmatite related type that formed via the interaction of metasomatic fluids with beryllium-rich pegmatites, or similar granitic bodies, that intruded into chromium- or vanadium-rich rocks, such as ultramafic and volcanic rocks, or shales derived from those rocks. A second deposit type, accounting for most of the emerald of gem quality, is the sedimentary type, which generally involves the interaction, along faults and fractures, of upper-level crustal brines rich in Be from evaporite interaction with shales and other Cr- and/or V-bearing sedimentary rocks.
Mining
Artisanal mining is widely performed for the extraction of gemstones. Emeralds are no different. Usually, workers use rudimentary equipment to open galleries with no technical planning for the work and for the disposal of the materials called sterile waste. The mineral recovery and the operating conditions are often inefficient and unsafe. People involved in the mining work rarely use proper protective equipment. Consequently, affecting the quality of life of these employees over time.
Investments in mining technology are important to improve operational efficiency, safety, and production workflow. Mining Engineers must develop a mining plan according to the geological model of the mineral assets of economic interest. Their assessment is important to determine the type of mine to be implemented and the methods to be used to operate the mine.
Photo 01: Gallery excavated through artisanal work.
The chosen equipment should be compatible with the characteristics of the mine to be developed as well as the geometry and dimensions planned by engineering. Equipment manufacturers are often developing new technologies in order to optimize the mining activity.
Both open pit and underground mines are developed through the use of standards established by local mineral legislation. Aiming at a pleasant, safe and efficient work environment.