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LOCATION

 

Localities where beryl is found are confined to a series of highlands more or less parallel to the coastline of southeastern Brazil (mostly in Minas Gerais province), in a region that is maturely weathered, low in elevation, and heavily vegetated, except in the NE corner, which is arid. Granitic pegmatites are abundant and widespread.

 

HISTORICAL REFERENCES

 

Eschwege, considered one of the most important writers on brazilian gem mining history, wrote that Souza had cited a vague report that Francisco Bruza de Spinosa led an expedition to the Jequitinhonha River valley as early as 1551. It seems it was this venture which brought back the first stones that were called "Brazilian Emeralds". 

Eschwege mentioned other expeditions in the 17th century which penetrated the interior and noted that all the "emeralds" brought back were green tourmalines, which were readily mistaken for the far more precious stone. 

In 1682 Paiva recorded that beryls and aquamarine had been sent to Portugal, indicating that from that date gemstones were being obtained from some inland source. More gemstones were found after of a flood of immigrants to the interior, but there is no information about where they were found. 

The first indication of sources appeared in 1812 when John Mawe, who became an important London mineral and gem dealer, provided an account of his travel into the interior and a description of gemstones he had seen there. 

Toward the close of the 19th century Brazil had become a world leader in the supply of mica. Prior to World War II an increased demand for mica, beryl and piezoletric quartz led to intensive prospecting of the interior and resulted in the finding of many more gem-bearing pegmatites. 

In modern times a transition occurred from mining gravel deposits for gemstones to mining pegmatites directly.

 

GEOLOGY

 

 

Carnaiba emerald deposit: The city of Campo Formoso is 77 Km North of Jacobina. The emerald deposits are located within an area of several kilometres diameter around the village of Carnaiba, the latter located 27 Km. SW of Campo Formoso or almost directly South about 9 Km from the village of Brejao das Grotas.  The "Rocha verde" or green rock outcrop at several points around Carnaiba from where emeralds are obtained. The formation of beryl is due to schist-beryl type deposits, which resulted from the chemical interaction between pegmatites and basic-ultrabasic rock.  These deposits are associated with two intruding proterozoic leucogranite massifs, the Serra da Jacobina volcano-sedimentary series and the older gneissic-migmatitic Archean basement. This deposit is divided into two districts: Carnaiba de Cima (Trecho Velho, Trecho Novo, Bica, Cabra pits) and Carnaiba de Baixo (Bode, Graviao, Lagarta and Formiga). 

The emerald mineralization is related to intrusive albitic (albitic-oligoclase) or quartz-albitic pegmatoids or quartz veins crosscutting the serpentinites. Two kinds of veins are distinguish: the fracture veins or frinchas and contact veins or esteiras. The pegmatoids had caused the formation of a monomineralic, F-rich, biotite-rich zone, developed symmetrically on each flank of one vein. Emerald is intimately associated with the biotite-rich zone and sometimes is found within the vein. 

In Santa Terezinha de Goiàs the pegmatite veins are absent. The pegmatites are directly linked to intrusive granitic bodies with limited local extension, or are related to important and extended pegmatite provinces. Emerald mineralization is located in biotite schists (biotitites), biotitete-talc-caarbonate and carbonate-biotite schists. The biotitites are the product of a metasomatic process which acted within the Santa Terezinha sequence.

 

MINERALIZATION

 

Emerald mineralization is located in biotite schist resulting from K-metasomatism of serpentinites, talc schists or tremolitic schist from a volcano-sedimentary sequence sequence usually metamorphosed in the greenschist facies.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

INFRARED

 

Reflectance FTIR (Fourier Transform Infrared Spectra) of gemstones have been acquired using a microscope IRscopeII (Bruker) accumulating 200 scans with a resolution of a cm-1 or better, in the range 7000-600 cm-1. Here are given only the skeletal bands which lie in the interval 1500-600 cm-1 When the samples were suitably cut in direction parallel or normal to the c axis, polarized spectra were also obtained on the pinacoid or on the basis respectively or both. According to the group theory 6A2u + 16 E1u infrared modes are predicted: the E1u modes are observed in the spectrum when the electric field is perpendicular to the c-axis (ordinary ray) and the A2u modes are observed for the extraordinary ray (E|| c). Therefore, non degenerate bands (A2u) are observed on pinacoids and show intensity changes in polarization, while degenerate bands (E1u) are detected on basis spectra and show only a lowering in intensity in polarized light.
 

Carnaiba
S.Teresinha

 

REFERENCES

 

BIBLIOGRAPHY

 

1. Giuliani G., Silva L. J. H. D. and Couto P. "Orign of emerald deposits of Brazil" Mineral. Deposita, 25, 57-64 (1990) 
2. Sinkankas J. "Emeralds and Other Beryls" Pensylvania, Chilton Book Co. p. 542-548, 1981

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