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Copper and Tenorite Inclusions in Cuprian-Elbaite Tourmaline

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Copper and Tenorite Inclusions in Cuprian-Elbaite Tourmaline NOTES AND NEW TECHNIQUES COPPERAND TENORITEINCLUSIONS FROM PARAI'BA,BRAZIL By Franz Brandstatter and Gerhard Niedermayr Inclusions of native- copper and tenorite (CuO) were found inwayish peen elbaite from Sao Jose da Batalha, Paraiba, Brazil. Native copper forms strongly dendritic platelets (about 100 pm Long, 1 urn thick) ori- ented pa~pallelto the c-axis of the host crystal in a trigonal arrangement. The much less abundant tenorite forms 20-30 pn platy grains. Electron microprobe a~zaly~esrevealed that the tou~ftmalineis cuprian elbaite (1 wt. % CuO), with significant contents of iron (2.7-3.1 wt. % FeO) and manganese (0.9-1.4 wt. % MnO). The CuO content of tou~ftmaline around the native copper decreases toward these inclusions up to a factor of two. No such decrease in CuO was obseived for the tenorite inc/~~.rions. In 1989, unusually intense blue and green tourina- green colors of these tourmalines are mainly due to lines appeared on the international market (figure their copper (Cu]content (Fritsch et al., 1990; 1). They reportedly came from a locality called Rossman et al., 1991).Although the Cu concentra- Mina da Batalha, close to the village of Sio Jose da tion reported in these tourmalines (0.4-2.4 wt.% Batalha, in Paraiba State, Brazil (see, e.g., Koivula CuO] is among the highest ever analyzed in nomi- and Kammerling, 1989). Subsequent research nally Cu-free silicates, no information was avail- revealed that the vivid yellowish green to blue- able regarding the source of the Cu responsible for the unusual colors. ABOUT THE AUTHOR In September 1992, Mr. F. Janouselz, a mineral collector from Vienna, showed us several pieces of Both authors are mineralogists in the Museum of Natural History in Vienna (NaturbistorischesMuseum Wien), tourmaline that he had selected in Brazil a couple Mineralogisch-PetrographischeAbteilung, Burgring 7, of months previously from a lot of more than 100 A- 1074 Vienna, Austria. Dr. BrandstStter specializes in similar-appearing tourmaline crystals and frag- mineral analysis by scanning electron microscopy and electron microprobe techni$~es;Dr. ~iedermakiscurator merits. The miner from whom he purchased the of the mineral collection and head of the Staatliches material said that it came from Paraiba. All of the Edelsteininsfitut (State Gemological Institute). tourmalines contained numerous metallic yellow Acknowledgments: The authors thank Franz Janousek, specks (similar to those seen on the table of the Vienna, for bringing the problem to their attention and provid- ing information and samples for investigation, Paraiba tourmaline in figure 2). He gave us three representative pieces of this material and asked us Gems & Gemology, Vol. 30, No. 3, pp. 178- 183 @ 1994 Gemological Institute of America to determine the nature of these unusual-appearing inclusions. 178 Notes and New Techniques Gems & Gemology Fall 1994 Figure 1. The striking blues and greens of these elbaite tourma- lines from Paraiba, Brazil, are caused by the presence of copper in their structure. Stones (largest, upper right, is 26.63 ct) cour- tesy of Michael Scott; photo 0Harold d Erica Van Pelt. PREVIOUS WORK ot epigenetic exsolution. they pointed to the fact Bank et al. (1990)and Fritsch et al. ( 1990) provided that "microchemical testing with nitric acid, done brief comments on the regional geology and min- on a crystal with exposed inclusions, indicated the ing history of the Paraiba tourmalines, together presence of copper." Brandstatter and Niedermayr with detailed reports about their mineralogical and (1993)proved these inclusions to be native copper. gemological properties. Microprobe analyses Hem and Bank (19901, Fritsch et al. (19901, and proved the tourmalines to be elbaites, with MnO Rossinan et al. (1991) investigated the origin of and CuO contents up to 3.0 and 2.4 wt.%, respec- color in the cuprian elbaites. According to their tively. In these reports, no explanation was given findings, the incorporation of Cu^ into the tourma- for the source of the unusually high Cu content of line structure is the main cause for the exceptional these elbaites. Fritsch et al. (1990)mentioned typi- hues. Some modification of the elbaite colors to cal inclusions for gem tourmalines: three-phase, violet-blue and violet hues is ascribed to increasing liquid (in veils or "fingerprint" patterns), thin absorption from Mn^. growth tubes parallel to the c-axis, and some dou- bly refractive crystals. Also mentioned and illus- MATERIALS AND METHODS trated were "numerous yellowish specks" that local All analytic results mentioned here were obtained miners had assumed to be gold (see figure 11 on p. from Mr. Janouseli's three tourmaline specimens, 197 of that article). X-ray fluorescence analyses which he said were representative of the tourma- cited by the authors revealed the presence of Mn, line lot he had seen in Brazil. All of the samples Fe, Cu, Zn, and Bi, as well as some S; this implies were grayish green: a 1-crn-long crystal fragment that these metallic inclusions could be composed weighing 0.7 ct, a 0.9-ct cabochon, and a 1-crn-long of a sulfide with a certain Fe content. In a short terminated crystal that weighed 10 ct. The crystal item in the Fall 1992 issue of Gems a> Gemology, showed the striated prism faces typical of tourma- Koivula et al. reported the appearance of such line and the pyramid (1011) only. At the irregular inclusions near the surface of a cut Paraiba tourma- base of this crystal, white platy albite was inter- line and suggested that they might be the product grown with the gem host. Notes and New Techniques Gems &. Gemology Fall 1994 Figure 3. With magnification, numerous specks of a yellow metallic mineral were evident, in the Paraiba tourmalines examined. Photo by Gerliard Niedermayr; field of view, 1.5 cm. RESULTS Optical Properties. The gemological properties (refractive indices, birefringence, and specific gravi- Figure 2. On this 2.41-ct faceted Paraiba tour- ty) of these three samples are consistent with those maline, the yellow platelets are seen near the reported by Fritsch et al. (1990) for two-one yel- surface, running across the table facet. Photo lowish green, the other greenish gray-Paraiba tour- by Maha DeMaggio. malines of similar color. All three samples were grayish green color and with typical pleochroisin (e = yellowish green, co = grayish green). Refractive As a first step, the flattened crystal fragment indices were measured as follows: E = 1.622-1.624, was cut parallel to the c-axis and polished on one co = 1.642-1.644, with a birefringence of 0.020; uni- side. As a second step, the same crystal was cut axial negative. Specific gravity was 2.99. perpendicular to the c-axis and a polished thin sec- tion was made. Inclusions. When examined with magnification, Specific gravity was determined on all three all three specimens exhibited the standard suite of specimens by the hydrostatic method; optical prop- fluid, three-phase, and crystalline (e.g., tourmaline) erties, using a standard gemological refractoineter; inclusions typical for elbaite tourmalines. In addi- and pleocl~roism,with a calcite dichroscope. tion, however, we noted platelets of a yellow mate- Chemical composition was determined on two of rial with a metallic luster (figure 3).In transmitted these specimens (the cut crystal fragment and the light, the yellow metallic inclusions turned out to cabochon). Qualitative chemical analysis and be numerous minute, strongly skeletal, branching investigation of morphology and orientation of the platelets on the average 100 pi11 long and only 1 inclusions on the polished tourmaline surface (cut pin thick (figure 4). They were reminiscent of the parallel to the c-axis) were carried out on a JEOL typical branching growth characteristic of native JSM-6400 scanning electron microscope equipped copper. With the microscope and reflected light (at with a KEVEX energy-dispersive system (SEM- a magnification of about 120x) we observed a few EDS). Then five randomly located spots were irregularly scattered grains that had a different selected on both specimens for quantitative cheini- reflectance behavior in the polished tourmaline cal analysis with an ARL-SEMQ electron micro- sample that was cut parallel and perpendicular to probe. To reduce sample decomposition, we ana- the c-axis. lyzed the spots with a defocused beam (area 100 pm2) at 151zV, 20nA. The analyses were corrected Chemical Analyses. As can be seen from table 1, by standard procedures using natural and synthetic there are some differences in the contents of CuO standards. (1.01 and 1.02 wt.%) and FeO (2.71 and 3.10 wt.%) Notes and New Techniques Gems & Gemology Fall 1994 Figure 5. Note in this backscattered electron image of native copper (white)in cuprian elbaite (gray)from Paraiba that the copper Figure 4. At higher magnification, the "goldu- inclusions are oriented parallel to the c-axis of colored specks reveal a dendritic form that is the tourmaline. "Dashed lamellae" indicate the typical for native copper. Photo by Gerhard dendritic morphology of these platelets. Niederz~~ayr;field of view, 600 urn. mum content)-in the direction toward the inclu- in the two specimens we analyzed as compared to sions. We did not observe a corresponding variation the two specimens of similar color for which in the CuO content of the tourmaline around microprobe analyses were reported by Fritsch et al. tenorite inclusions. (1990).Although the CuO content of our samples is witliin the total range (0.37-2.38 wt.%) for Paraiba elbaites of different colors given in that TABLE 1. Electron microprobe analyses (in wt.%) of four article, the FeO content is significantly higher elbaite tourmalines from Paraiba, Brazil.3 (maximum FeO of 0.34 wt.%). Sample N0.b SEM analysis of the dendritic platelets proved that they are indeed native copper.
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