Opal in Brazil
A LITTLE HISTORY
The most significant opal fields in Brazil are located in the districts of Pedro Segundo and Buriti dos Montes located in the semi-arid region in north-west of Piaui State of Brazil. The area is primarily an agricultural region on a high plateau of about 700 metres and has a cool climate. The area is named after a Brazilian Emperor Don Pedro Segundo.
According to Oliveira (1998) The first opal gemstone was found in Pedro !! between the late 1930’s and the early 1940’s. The most significant deposits are in an approximately 200 square kilometre area around the capital of this municipality.
There are about 30 known active and inactive mines within the area, with the largest and most important mine being at Boi Morto farm. There is also the Tatu Mine (Armadillo Mine), which according to local folklore was so named a hunter once passing through the area saw two opals coming out of an armadillo hole!
Another piece of folklore suggests the commercial nature of the gemstones were at first not recognised, and locals used the stones as buttons.
Commercial recognition seems to have increased in the 1960’s
GEOGRAPHY AND MINING FIELDS
Pedro II
The opal mining fields surround the village of Pedro Segundo (Pedro II) in an area that approximately radiates for 20km or so. And according to Joshen Knigge, the opal occurrences occur in two modes. The first is labelled as be an “alluvial deposit” where the opal occurs as “nuggets” in amongst the stratum, and is usually dry washed and sieved on screens.
The second so-called “Primary deposit” the opal occurs as “veins” within the base of the sandstone stratum.
Buriti dos Montes
The county of Buriti dos Montes is located in the north-eastern part of the Sate of Piaui, approximately 230 kilometres east of the capital Teresina.
The area of the opal fields in Brazil is near the grey area with th letter “P” in the top right corner of the map.
The Map is from the Australian Gemmologist Volume 21, No. 1
A LITTLE GEOLOGY
Introduction to the geology of the Opal fields at Buriti dos Montes
According to Tavares Marques, et al. The orange opals in Buriti dos Montes originated from hydrothermal processes and are found mainly as veinlets and veins in the sandstones of the Serra Grande Group, sectioned by diabase (dolerite) dikes and sills of the Sardinha Formation. The opals are also found as the cementing material of breccias in the contact zone between the host rocks (sandstones) and the intrusive dykes. The reference states that the Serra Grande Group is of Silurian Period of geological age (419 - 433ma) the Sardinha Formation is of the Cretaceous geological period (66 - 145ma).
Introduction to the geology of the Opal fields at Pedro II.
The precious opal fields of Pedro II occur in an area associated with the Cabecas Formation which is believed to be of the Devonian period of geological age (approximately 360 - 397ma) . As an example, the geology at Boi Morto the Cabecas Formation consists of clastic (sedimentary), fine to medium grained whitish cross bedded sandstones siltstones and claystones up to 60 metres in thickness. These strata are underlain and intruded by a dolerite sill.
The claystones (siltstones) play a prominent role in the geology of opal formation. The opals are found in the zone of contact between the sill and and the sandstone. According to Bartoli et al, 1990) the opal occur as a result from both the contact metamorphism and also the weathering at the top of the sill which frequently controls the water table.
LITERATURE
Project evaluation of opal deposits of Pedro 11 -2015 - Ministry of mines and energy (Brazil) - Mineral resources report No. 8, in Portuguese.
Contribution to the genesis of opals from Pedro11 (Piaui), - 1994 - Geochim. Brazil, 8 (1) pp79-98 - in Portuguese
Video - The Precious Opals of Pedro Segundo - Joshen Knigge.
Brazilian Gem Provinces - Pinto, CP., Pedrosa-Soares, AC., - 2001 - Australian Gemmologist Volume 21, No. 21, p15
Orange opals from Buriti dos Montes, Piaui: Solid inclusions as genetic guides. - 2015 - Marques, GT., Lima de Costa, M., Gomes, ER., - Geosciences, - R Esc.Minas, 68(1), p53-59.
The Noble opal Deposits of the region of Pedro 11 in the state of Piaui (North-Eastern region of Brazil - 1988 - Deocleciano Bittencourt Rosa, Thesis, Doctor of INPL (Institute National Polytechnique de Lorraine) Geoscience and Raw Materials. - In French.
TYPES AND VARIETIES OF OPAL FROM Brazil
A piece of light (White) precious opal rough (left) and a cabochon cut piece of Precious light crystal opal from Brazil on the right. This opal from Pedro 11 district.
Orange body coloured transparent “Fire opal” from Brazil. Some of this material is suitable for faceting gemstones 9lower left. Whilst cabochons of the material produce an interesting gemstones.
This material is from Buriti dos Montes.
Eleven small and approximately 7 x 5mm light opal cabochons. Light and crystal precious opal from Brazil.
A LITTLE GEMMOLOGY
From a gemmological viewpoint, Brazilian light, white and crystal opal poses a challenge for the gemmologist. This is one of the enigmas with precious opal. The Brazil precious opal (with POC) has been said to be in many ways very similar to precious opal from the Andamooka opal fields in South Australia (Ralph Segnit, pers. coms). It has also been stated that the gems from Brazil are mined in a so called ‘sedimentary environment’ however as noted above there is much discussion as to whether this is so.
In most instances a separation can be made between opal from overseas locations and those of Australian origin. The Brazil light opal shows some characteristics of both origins. A distinguishing factor remains the reaction seen with standard ultra-violet examination as the Brazil material is non fluorescent and non phosphorescent.
As tested at Opal Academy:
Light precious opal:
RI: Average - 1.453 (1.450-1.460, 5 items)
SG: Average 2.12
UV: Inert to both long wave (LW) and short wave (SW).
Orange Fire opal:
RI: Average 1.436
SG: 2.04
UV: Inert to both long wave (LW) and short wave (SW)
