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Origin of Color in Cuprian Elbaite from Sflo Jos6 De Batalha, Paraiba, Brazil

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Origin of Color in Cuprian Elbaite from Sflo Jos6 De Batalha, Paraiba, Brazil American Mineralogist, Volume 76, pages 1479-1484, I99I Origin of color in cuprian elbaite from Sflo Jos6 de Batalha, Paraiba, Brazil Gnoncn R. Rosslvr,tN Division of Geology and Planetary Sciences,l7O-25, California Institute of Technology, Pasadena,California 9l125, U.S.A Evrvrm.luel FnrrscH, J,lvrns E. Srncr.Bv GIA Research,Gemological Institute of America, 1660 Stewart Street,Santa Monica, California 90404-4088, U.S.A. Ansrn-q.cr Gem-quality elbaite from Paraiba, Brazil, containing up to 1.4 wt0i6Cu has been char- acterizedusing optical spectroscopyand crystal chemistry. The optical absorption spectra of Cu2t in these tourmalines consist of two bands with maxima in the 695- to 940-nm region that are more intense in the E I c direction. The vivid yellowish green to blue- greencolors of theseelbaite samplesarise primarily from Cu2*and are modified to violet- blue and violet hues by increasingabsorptions from Mn3*. IwrnooucrtoN in a small, decomposed granitic pegmatite (formerly quartz, The color of elbaite has hen extensively investigated. prospectedfor manganotantalite).The associated Most colors are determined by small amounts of transi- altered feldspar, and lepidolite have not been character- tion elements(Dietrich, 1985).In particular, blue to green ized. Most of the tourmalines occur as crystal fragments colors have been attributed to various processesinvolv- weighing less than a g.ram,although pieceslarge enough ing both Fe2*and Fe3*ions (Mattson and Rossman, 1987) to produce facetedgemstones up to 33 carats(6.6 g) have and Fe2*- Tia* chargetransfer (Mattson, as cited in Die- been found. In rare instances, small crystals are recov- trich, 1985, p. 129). Cr3* and V3t have been identified as ered. coloring agentsin greendravite and uvite (Schmetzerand These elbaite samples are found in a range of color Bank, 1979) but never in elbaite. Several investigators from violet-blue, not unlike the color of a blue sapphire, briefly mentioned Cu2+as a potential coloring agent in to green-blue,reminiscent of turquoise, and to yellowish blue and green elbaite on the basis of emission spectrog- green. The deep blue material is referred to locally as raphy analyses. For example, Warner (1935) observed "heitorite," in honor of Heitor Dimas Barbosa,founder that blue samples have a lower Cu concentration than of COGASBRA, the mining cooperative working the de- pink yellow-greenones. On the contrary, Carobbi and Pieruc- posit. Some of the complete crystals contain a deep pink cini (1946, 1947) found greateramounts of Cu in blue core sunounded by a blue zone and a thin rim. crystals. None of these investigators, however, proved that Other color-zoning patterns have been observed. this ion actually acts as a coloring agent. Staatz et al. Becauseof their attractive color, these elbaite samples (1955)mentioned the presenceof traceamounts of Cu in have proved very popular as gemstonesand have com- tourmaline but did not relate it to color. manded prices up to ten times higher than those paid for gem in size and A new find of gem-quality elbaite with unusually sat- other colors of tourmalines the same gemological urated shadesof greenand blue colors was made in 1988. clarity range.A brief description of the Para- given (1990). This discovery took place at an undescribed locality called iba tourmaline locality is in Fritsch et al. Mina da Batalha ("Mine of the Battle") near the village ofSdo Jos6de Batalha,4.5 km northeastofthe town of M.c.TERrar,s AND METHODS Salgadinho, in the state of Paraiba, northeastern Brazil Three crystal fragments and two facetedsamples, rep- (G. Becker, B. Cook, and D. Epstein, personal commu- resentativeof the color range of the tourmaline found at nication, 1989).These elbaite samplesare unique because Sio Jos6 de Batalha, were studied in detail- These five of their Cu content(up to 2.4 wto/oCuO; Bank et al., 1990; specimenswere selectedon the basis of their color and Fritsch et al., 1990),higher than any previously reported, color homogeneity from several hundred fragments of this and their spectacularcolors. Cu2* has been proposed as material. The crystallographic orientation of the crystal a coloring agentfor this material (E. Fritsch, cited in Koi- fragmentswas determined using striations (parallel to the vula and Kammerling, 1989;Bank et al., 1990).The pur- c axis, or optic axis) on the prism faces.When striations pose of this article is to characterizethe chemical com- were not present, the samples were oriented by X-ray position and optical absorption spectra of these alignment photography. Each specimen was cut with a tourmalines and to demonstrate that Cu is mainly re- microsaw to obtain a parallel-windowed section contain- sponsible for their color. ing the optic axis. The sectionswere then ground to an Elbaite from 56o Jos6 de Batalha is reportedly found appropriatethickness (0.2-5 mm) and polishedwith I -pm 0003-004x/9I /09 l 0-l 479$02.00 1479 1480 ROSSMAN ET AL.: COLOR IN CUPRIAN ELBAITE TaBLE1. Propertiesof cuprianelbaite from 56o Jos6 de Batalha,Paraiba, Brazil Specimen Birefrin- no. Color- Munsell notation R.l. gence Pleochroism S.G.- R030 Brilliant bluish green 8 BG 7/9 1.621-1 .646 0.025 o: mediumgreen-blue 3.117 (: mediumbluish green R050 Light yellowishgreen 2.5 G 716 1.619-1 .639 0.020 @: medium yellowishgreen 3.05 € : light grayish green R052 Light blue 10B 7i6 1.619-1 .639 0.020 @: mediumblue 3.11 e : light greenish blue R066 Light purplish blue 6.5PB 7/55 1.620-1.638 0.018 o: mediumpurplish blue 3.04 e : light, slight grayish green R067 Light blue 10B 7/65 1.618-1 .638 0.020 @: mediumslight grayish blue 3.065 € : medium green-blue - Color of a 1.5-mm slice containing the c axis. " Average of three measurements. alumina powder. Optical absorption spectrawere record- relative intensity to those from the pattern of a repre- ed using a Cary model I 7I spectrophotometerwith calcite sentative elbaite sample (1986 JCPDS Mineral Powder polarizers. Chemical analyses were obtained from the Diffraction File 26-964). Least-squares refinement of same specimenswith a JEOL model 733 electron micro- data for the most Cu-rich of the five specimens(sample probe. X-ray diffraction measwements to confirm the R030) yielded unit-cell dimensionsof a : 15.883(4)and identity of the flve specimens were perfoffned with an c:7.lll(l) A. The five strongestlines in this pattern automated Rigaku powder diffractometer operated at 35 are 3.9 7 8 (5 6) (220), 3 .4 4 s (76)(0 | 2), 2.93 8 (r 00)(r 22), kV and 15 mA. 2.5 6 7(9 0X0 5 I ), and 2.029(5 4)(l 52). No significant differ- The color of each of the five specimensappears ho- enceswere noted between the difraction patterns ofthis mogeneouswhen the polished sections are viewed with and the other elbaite specimensfrom this locality. The a the naked eye. Color descriptions are given in Table l, and c dimensions of the other specimenswere within 0.02 using the terminology of the Munsell Book of Color (1919) A and 0.03, A, respectively,of those for sample R030. and Kelly and Judd (1976). The color was visually deter- mined on parallel-polished slices about 1.5 mm thick Chemical analysis containing the c axis. Becauseof tourmaline's strong ple- Electron microprobe analyses(Table 2) show that these ochroism, the face-up color of faceted gemstonesmight five tourmalines contain little or no Ti, V, Cr, or Fe and be significantly different from the descriptions reported have Cu and Mn as major transition element constitu- in Table l. The colors of the slices range from light yel- ents. Analyses performed at three random locations on lowish green to light purplish blue, the sample with the each specimenshow only very slight compositional vari- highest Cu content being brilliant bluish green. ation, as might be expectedfrom the fairly uniform col- oration. It is also interesting to note that small amounts Rrsur,rs of Bi. Pb, andZnwere detectedin eachspecimen. Results Physical properties of site occupancy calculations based on charge balance present The rangesofindices ofrefraction ofthe five specimens considerations (see Table 2) indicate that Cu is measured with a GIA GEM Instruments Duplex II re- in the I crystallographic site (using the site terminology fractometer and a filtered, near-monochromatic Na- of Correns, 1969,p. 406; Deer et al., 1986). equivalentlight sourceare €: 1.618-1.621(+0.001) and visible, and near-infrared c.r: 1.638-1.646(+g.gg1). These values are typical of Ultraviolet, spectroscopy elbaite (Dietrich, 1985). All specimenshave a uniaxial absorption negative optical character.The birefringenceranges from Absorption spectrawere obtained at room temperature 0.018 to 0.025; thesevalues are toward the high end of in the range from 300 to 2000 nm. The spectra of all the recorded range for elbaite (Dietrich, 1985). These el- specimensare dominated by three features-an ultravi- baite samplesare distinctly pleochroic, with the most sat- olet absorption edge beginning at about 400 nm (more urated color seen along c.r(see Table l). Their specific intense in the E I c direction), a pair ofbroad bands in gravity, measured by the hydrostatic method in HrO, the 695- and 940-nm region (absorbingmore strongly in rangesfrom 3.05 to 3.12 (+9.91;, which is slightlyhigher the E I c direction), and a seriesof sharp bands in the than for most elbaite (2.84-3.10: Dietrich, 1985).None 1400- to 1500-nm region strongly polarized in the E ll c of the samplestested contained inclusions offoreign, pos- direction (Fig. l). Bandsin the 1400-to 1500-nmregion sibly heavier minerals, as determined by microscopic ex- arise from the first overtones of the O-H stretching mo- amination at 80 x magnification.
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