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And Vanadium-Bearing Garnets from Tranoroa, Madagascar

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And Vanadium-Bearing Garnets from Tranoroa, Madagascar NOTES & NEW TECHNIQUES NEW CHROMIUM- AND VANADIUM-BEARING GARNETS FROM TRANOROA, MADAGASCAR By Karl Schmetzer, Thomas Hainschwang, Heinz-Jürgen Bernhardt, and Lore Kiefert In June 2001, we obtained a parcel of about 30 faceted garnets from southern Madagascar with Pyrope-spessartine garnets from Tranoroa, in south- unusual coloration (see, e.g., figure 1). According to ern Madagascar, contain appreciable Cr and some- our supplier, these garnets had been mined recently what lower V contents. Although these elements are responsible for the color-change behavior of similar from a new area near Tranoroa, about 60 km south- garnets from the nearby Bekily area, the Tranoroa west of Bekily (figure 2). We do not know the samples show only a slight change in color appear- amount of this material that has been mined to ance from day or fluorescent light (brownish purple- date. Both the Tranoroa and Bekily localities belong red) to incandescent light (purplish red). Charac- to a region formed by several metamorphic belts teristic internal features in the Tranoroa garnets consisting of high-grade metamorphic rocks of include networks of rutile needles and strain patterns Precambrian age (Windley et al., 1994). More infor- caused by anomalous double refraction. Additional mation on the geology, mining, and production of inclusions are graphite, quartz, negative crystals, garnets from the Bekily area can be found in apatite, zircon, and monazite. One Cr-bearing spes- Schmetzer et al. (2001). sartine from the same area is also described. The present study was undertaken to character- ize these new garnets, which differ from those pre- viously described from Madagascar with regard to em-quality pyrope-spessartine garnets have their color and color behavior. been known from Tanzania and Sri Lanka G since the late 1970s. Recently, two important varieties of these intermediate garnets have come MATERIALS AND METHODS from the Bekily area in southern Madagascar. Pink to For this study we selected seven samples, which pinkish orange Bekily material with variable iron con- ranged from 1.52 to 4.77 ct (see, e.g., figure 1), from tents but almost no vanadium or chromium has been the parcel of about 30 faceted garnets. Six were called “malaya” garnet in the trade (Schmetzer et al., brownish purple-red in daylight and represented the 2001). Bekily garnets with variable amounts of vana- majority of the samples. We also studied one dium and lesser chromium generally show a distinct brownish orange garnet from the same parcel (see color change between day (or fluorescent) light and box A). From several other parcels totaling more incandescent light. These color-change pyrope-spes- sartine garnets have been subdivided into several groups or types according to the different colors See end of article for About the Authors. observed under variable illumination (see Schmetzer GEMS & GEMOLOGY, Vol. 38, No. 2, pp. 148–155 and Bernhardt, 1999; Krzemnicki et al., 2001). © 2002 Gemological Institute of America 148 NOTES AND NEW TECHNIQUES GEMS & GEMOLOGY SUMMER 2002 Figure 1. Chemical analysis of these pyrope-spessartine garnets from Tranoroa, Madagascar—shown here in fluorescent light (left) and incandescent light (right)—revealed appreciable amounts of Cr and V. However, they do not show the distinct color change in daylight and incandescent light that is observed in pyrope-spessartines from the nearby deposit at Bekily. The samples weigh 4.77, 3.26, and 2.81 ct. Photos by Maha Tannous. than 400 garnets from Bekily, we chose about 50 Visual Appearance and Gemological Properties. All Cr- and V-bearing samples for chemical analysis. samples revealed a homogeneous brownish purple- From these samples, we selected three representa- red color in day or fluorescent light and were pur- tive stones for comparison of their color appearance and composition with the Tranoroa garnets. The seven Tranoroa samples were tested by Figure 2. The garnets described here were mined standard gemological methods for refractive index, recently near Tranoroa, which is located about 60 fluorescence to long- and short-wave ultraviolet km southwest of the Bekily garnet deposits in radiation, and specific gravity. We examined the southern Madagascar. samples for inclusions and internal structural prop- erties using various microscopes and lighting condi- tions, both with and without immersion in methy- lene iodide. In addition, we identified solid inclu- sions by laser Raman microspectrometry using a Renishaw 1000 system. For all these Tranoroa samples, we recorded spectra in the UV-visible range with a Leitz-Unicam SP 800 spectrophotometer as well as with an Adamas Advantage SAS 2000 spectrophotometer. To determine quantitative chemical composition, we used a Cameca Camebax SX 50 electron micro- probe, with traverses of 10 point analyses each, measured across the tables of the faceted stones. The same instrument was used to analyze the three garnets from Bekily. RESULTS The results for the six brownish purple-red Trano- roa samples are listed in table 1 and discussed below. The results for the three Bekily samples are included in table 1 for comparison. NOTES AND NEW TECHNIQUES GEMS & GEMOLOGY SUMMER 2002 149 BOX A: A CHROMIUM-BEARING SPESSARTINE Figure A-1. As seen in day or fluorescent light (left) and incandescent light (right), the color appearance of this 2.84 ct Cr- bearing Tranoroa spessartine (sample G, see table 1) is distinctly different from the 1.03 ct V- and Cr-free Madagascar spessartine (0.02 wt.% V2O3, 0.02 wt.% Cr2O3, 39.34 wt.% MnO, and 2.29 wt.% FeO). Photos by Maha Tannous. Within the parcel of brownish purple-red garnets from undoubtedly associated with its high Cr content. The Tranoroa, we observed one sample that appeared brown- absorption spectrum (figure A-2) was similar to that of ish orange in day or fluorescent light and reddish orange the Cr-bearing intermediate pyrope-spessartines from in incandescent light (figure A-1). Physical and chemical the same area. The spectrum revealed a dominant properties of this garnet (sample G) are given in table 1. chromium absorption in the yellow region and weak This garnet showed distinctly higher R.I. and S.G. iron absorption bands. Consistent with the distinctly values than the other six samples. Microprobe analysis higher Mn content of the sample, the absorption bands revealed that it was different in composition from the of manganese were stronger, and thus the absorption other six garnets, with only about 5 mol.% pyrope and minimum in the blue-green to violet range was less a relatively high spessartine value of just over 88 mol.% pronounced than in the intermediate pyrope-spessar- (again, see table 1). The almandine content was rather tine samples from Tranoroa. This difference in trans- small, and chromium (1.01 wt.% Cr2O3) was again parency in the blue-green to violet region is responsible more abundant than vanadium (0.32 wt.% V2O3). The for the differences in color in daylight and incandescent sample was inert to long- and short-wave UV radiation. light observed in the two types of Tranoroa garnets. The color behavior of this sample, as compared to Microscopically, the spessartine sample was free of that of typical spessartine (again, see figure A-1), was rutile needles or other mineral inclusions. With crossed polarizers, we observed a mosaic-like pattern that showed high-order interference colors (figure A-3). Figure A-2. The absorption bands observed in the The garnet from Tranoroa is, to the best of our spectrum of the Tranoroa spessartine (sample G) knowledge, the first chromium-bearing spessartine are consistent with those of Cr-bearing pyrope- described as a faceted gemstone. spessartines. The bands are due to chromium, manganese, and iron (see figure 3 in the main text). Figure A-3. Microscopic examination of the Tranoroa spessartine revealed high-order interfer- ence colors in an irregular mosaic-like structure and strain pattern. Photomicrograph by K. Schmetzer; immersion, crossed polarizers, magnified 40´. 150 NOTES AND NEW TECHNIQUES GEMS & GEMOLOGY SUMMER 2002 plish red in incandescent light (again, see figure 1). spessartine solid-solution series. These R.I. and S.G. Refractive indices for the six samples were found to values are within the ranges found for malaya gar- vary within a small range, from 1.768 to 1.773 (table nets from the same region, although they are higher 1). Likewise, the specific gravity values fell between than the average values recorded for the samples 3.95 and 3.97, which indicates a narrow range of described in Schmetzer et al. (2001). All garnets were chemical composition within the pyrope-almandine- inert to both long- and short-wave UV radiation. TABLE 1. Physical and chemical properties of pyrope-spessartine garnets from Tranoroa and Bekily, Madagascar. Tranoroa Bekily Property A B C D E F G X Y Z Weight (ct) 4.77 1.86 3.26 2.81 2.02 1.52 2.84 0.67 0.90 0.78 Color Day or Brownish purple-red Brownish Grayish Slightly Bluish fluorescent light orange green reddish green- purple gray Incandescent Purplish red Reddish Grayish Strongly Pinkish light orange purple reddish purple purple Refractive index 1.768 1.768 1.769 1.770 1.770 1.773 1.808 1.750 1.752 1.755 Specific gravity 3.95 3.96 3.95 3.96 3.97 3.97 4.15 3.82 3.83 3.86 Microprobe analyses (wt.%)a SiO2 38.70 37.67 38.82 38.41 37.91 37.88 35.81 40.62 40.83 39.66 TiO2 0.05 0.08 0.04 0.07 0.12 0.16 0.33 0.05 0.07 0.05 Al2O3 21.98 21.75 22.00 21.69 21.46 21.33 20.24 22.92 22.91 22.50 Cr2O3 0.67 0.69 0.68 0.71 0.75 0.75
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