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Introduction |
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CHEMICAL VARIATIONS IN MULTICOLORED “PARAIBA”-TYPE
TOURMALINES FROM BRAZIL AND MOZAMBIQUE:
IMPLICATIONS FOR ORIGIN AND AUTHENTICITY DETERMINATION
Adolf Peretti (1,2,3), Willy Peter Bieri (2,3), Eric Reusser (4), Kathrin Hametner (5) and Detlef Günther (5)
(1) GRS Gemresearch Swisslab Ltd, Sempacherstr. 1, CH-6003 Lucerne, Switzerland
(2) GRS Gemresearch Swisslab Ltd, Rue du Marche 12, CH-1204 Geneva, Switzerland
(3) GRS (Thailand) CO LTD, Bangkok, Silom 919/257, 10500 Bangkok, Thailand
(4) Institute of Mineralogy and Petrography, Clausiusstr. 25, ETH Zentrum, CH-8092 Zurich, Switzerland
(5) Laboratory of Inorganic Chemistry, ETH Hönggerberg, HCI, G113, CH-8093 Zurich, Switzerland
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INTRODUCTION
Copper-bearing tourmalines (or “cuprian- elbaites”) with neon-blue colors are considered highly valuable gemstones matching prices that are normally only equaled by valuable rubies and sapphires. Part of the trade and some laboratories have introduced the new variety name “Paraíba” tourmalines for the “neon”-blue colored cuprian-elbaite. (www.gemresearch.ch/ news). Currently a large number of publications were released on copper-bearing tourmalines from various localities (see review Lit. Par11-44).
Copper-bearing tourmalines were discovered in the Paraíba State, Brazil, in the late 1980’s (Lit. Par22 and 23), in the neighboring Rio Grande do Norte State in Mulungu (Parelhas) and in Alto dos Quintos (Lit. Par20 and 28). On the African continent the tourmalines occured in Idaban State and Ofiki in Ilorin State, Nigeria (Lit. Par17, 18, 31, 32, 40 and 44) as well as in the Mavuco area in the Alto Lingonha pegmatite district of Mozambique (Lit. Par41).
Historically, tourmalines from Paraíba were discovered first, followed by Rio Grande do Norte (both in Brazil), Nigeria and Mozambique. Cuprian-elbaite occurs in different colors, such as purple, violet, blue and green. Heat-treatment of purple and violet colors can be transformed to neon-blue colors (See e.g. Lit. Par11 and 33). Cu-bearing tourmalines from Mozambique have become commercially important gems with colors such as purple, violet, blue. The copper-bearing, unheated tourmalines occur in extremely large size, fine clarity and vibrant colors (Fig. Par05). Some of the magnificent tourmalines of Paraíba-type neon-colors reached extremely large sizes exceeding 100ct (Fig. Par05). These magnificent pieces were mostly heat-treated (Fig. Par07). Large copper-bearing tourmalines of such excellent clarity are less commonly known from the Brazilian deposits. On the other hand, the color intensity of the “neon-blue” colors of Paraíba tourmalines from Brazil is almost unmatched (Fig. Par01, 09, 10 and 14d). A first detailed chemical study aimed for origin determination using LA-ICP-MS has been published (Lit. Par11 and 33).
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Since it has been proposed that the separation of tourmalines of different origin using standard gemological testing is not possible, we acquired an extensive set of data using different analytical techniques.
In this study, we concentrate on the detailed chemical profile analyses of color-zoned copper- bearing tourmalines from Brazil and Mozambique using LA-ICP-MS spectroscopy and element concentration mapping by electron microprobe. New insights on the chemical evolution during growth of copper-bearing tourmalines were obtained with important consequences for origin determination and understanding of natural chemical zoning in these tourmalines. A test procedure is proposed that enables origin determination by the combination of ED-XRF and LIBS analyses in addition to LA-ICP-MS using matrix-matched calibration saples prior analyzed by EMPA and LA-ICP-MS.
Fig. Par01 Example of a pair of “neon-blue” copper-bearing tourmaline (or “Cuprian-elbaite”) from Brazil in the mounting. This type of tourmaline is also known as Paraiba tourmaline in the trade. The tourmalines are the most valuable members of the tourmaline group. The two stones are total 15ct (7ct and 8ct). Photo courtesy P. Wild (Idar-Oberstein, Germany).
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