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Volume 25 / No. 5 / 1997 he Journa TGemmolog Volume 25 No. 5 January 1997 \ji U The Gemmological Association and Gem Testing Laboratory of Great Britain Gemmological Association and Gem Testing Laboratory of Great Britain 27 Greville Street, london ECl N 8SU Tel: 0171 404 3334 Fax: 0171 404 8843 President: Professor R.A Howie Vice-Presidents: E.M. Bruton, AE. Farn, D.G. Kent, R.K. Mitchell Honorary Fellows: R.T. Liddicoat Inr; E. Miles, K. Nassau Honorary Life Members: D.J. Callaghan, E.A. [obbins, H. Tillander Council of Management: C.R. Cavey, T.J. Davidson, N.W. Deeks, R.R. Harding, I. Thomson, V.P. Watson Members' Council: Aj. Allnutt, H Dwyer-Hickey, R. Fuller, j. Greatwood, B. Jackson, J. Kessler, J. Monnickendam, L. Music, J.B. Nelson, K. Penton, HG. Read, I. Roberts, R. Shepherd, C.H. Winter Branch Chairmen: Midlands - G.M. Green, North West - L Knight, Scottish - B. Jackson Examiners: A.j. Allnutt, M.Sc., Ph.D., FGA, S.M. Anderson, B.Sc(Hons),FGA, L. Bartlett, s.se., M.Phi!., FGA, DGA, E.M. Bruton, FGA, DGA, CR. Cavey, FGA, S. Coelho, B.Se., FGA, DGA, Prof. AT. Collins, B.Se., Ph.D, A.G. Good, FGA, DGA, Cl.E. Hall, B.Sc(Hons), FGA, C.M. Howe, FGA, DGA, C.H. Jones, B.Se, Ph.D., FGA, H.L Plumb, B.Sc., FGA, DCA, R.D. Ross, s.sc, KiA, DCA, P.A. Sadler, B.Sc., FGA, DCA, E. Stern, FGA, DCA, Prof. I. Sunagawa, D.Se., M. Tilley, CC, FGA, CM. Woodward, B.Se., FGA, DGA The Journal ofGemmolo.gy Editor: Dr JtR.. Harding A.$:stst;JfJt Editors:M!J.O/DohQgh~e, PiG. Rei:lo Associate Editots:$;M. Anderson (London), Dr C.E.S. Arps (Lelden), C. BQssharf{Lu;cernel, Prof..A.T. Collins (London), Dr j.W, .. Harris(Glasgow), Prof, R",A. Howie (Derbyshire), Dr hM. Ogden (Cambridge)j Dr J.E. Shigley (Santa Monica),· Prof. D.C. Smith (Paris), E. Stetn(London), Prof. L Sunagaws (Tokyo), DrM. Superchi (Milan), CM. WODdward (London) Pt;(jdtiGtiO/i Editor: M.A. B(jrland Vol 25, No.5, Jan 1997 ISSN: 1355-4565 A new colour-change effect A. Halvorsen 1 and B.B. Jensen2 1Norplan NS 0612 0510 2Mineralogical Geological Museum, 0510. ABSTRACT: Crystals of green chrome tourmaline from the Umba Valley (Tanzania) may show a wine-red colour when viewed in certain directions. This is neither pleochroism, nor a colour-change of alexandrite type. The change from green to red depends primarily on the path length of light through the stone and probably also on the content of Cr and V; in the samples tested the stones appear red when more than 15 mm thick. It is suggested that this new type of colour change be called the 'Usambara effect' . Introduction other gems. Some of these can have rare and unique colours which are not found elsewhere. e have examined many pieces of The rocks of the area are of medium to high green chromiferous tourmaline metamorphic grade and belong to the Eastern W from the Daluni area in the Umba Granulite Complex of the Precambrian Valley of north-eastern Tanzania. Several Usagaran system. According to Dirlam et al. 327 showed a startling green to orange or red (1992), the metamorphic processes which colour variation. Bank and Henn (1988) have affected the granulites have caused a described green tourmaline from Tanzania remobilization of chromophore elements such that exhibited a green/red alexandrite effect as chromium and vanadium, which have and we thought at first that this was the same concentrated in several of the Umba type of material. However, our studies gemstones and resulted in their unusual indicate that since the red colour can be seen colours and optical phenomena. Colour­ in any sufficiently thick portion of a crystal, change tourmalines, sapphires and garnets irrespective of optical direction and only with effects comparable to those seen in mildly modified by type of illumination, the alexandrite have been described from this area colour change involves other factors. (Bank and Henn, 1988; Dirlam et al., 1992). The best demonstration of the effect is Figure 1 is a general view of the chrome­ provided by two pieces which are both strong tourmaline locality from which the material blue-green when examined separately, but was collected, with the Usambara mountains which become red when superimposed, one in the background. upon the other, with no change in orientation or in the conditions of illumination. Gemmology Locality Assuming a common thickness, different shades of green may indicate some chemical The Umba Valley is one of the richest variation in the material available, so we gemstone areas in East Africa producing ruby, examined several pieces in detail, mainly sapphire, spinel, garnets, tourmaline, chrome crystals and rough fragments but including diopside, zircon, scapolite, kornerupine and some cut stones. © Gemmological Association and Gem Testing Laboratory of Great Britain ISSN: 1355--4565 E 1.622, with birefringence of 0.022. Uniaxial interference figures were observed in the samples under a polariscope. Chemistry Using the general formula for tourmaline of XY3Z6B3Si6027(0,OH)3(OH,F) various end members have previously been defined as follows: X Y Z Dravite Na Mg3 Al6 B3 Liddicoatite Ca (Li,AI)3 Al6 B3 Uvite Ca Mg3 AI5MgB3 Figure 1: General view of the area, with the Usambara mountains in the background. The Table I gives the result of microprobe tourmaline locality is in the foreground of the analyses of two samples of our material (T3 picture. and T4) together with analyses from the work of Bank and Henn (1988) on the chromiferous The following gemmological descriptions tourmalines from Tanzania which show an are based on two crystal fragments (T1 and T2), alexandrite colour-change effect. Our analyti­ 326 four pieces of rough (T3-6) and one faceted cal results are very similar to those of Bank and Henn. 327 stone (17). The weights are respectively T1 (7.735 g), T2 (4.721 g), T3 (1.788 g), A qualitative X-ray fluorescence spectrum T4 (2.161 g), T5 (4.026 g), T6 (5.840 g), from a sample supplied to S. Bergst0l at T7 (0.719 g). Norges Tekniske H0gskole indicates that vanadium content is significant, but only Specific gravity about a quarter of the chromium content, i.e. The average specific gravity of the seven specimens, determined by hydrostatic weighing, Figure 2: A drawing of a tourmaline crystal is 3.06. showing pyramidal habit. In order of degree of development - trigonal pyramid, pedion, hexagonal prism and trigonal prism. Crystal habit Like other chromiferous tourmalines previ­ ously reported from this area, the crystals tp Trigonal pyramid show pyramidal habits (Dirlam et a/., 1992). One good crystal (T1) is a fine bluish-green, Pedion about 1 cm thick and 2.5 cm in diameter, and carries three large pyramid faces, a small pedion and a fringe of thin prism faces (Figure 2). The back of the crystal is a large irregular fracture surface. Refractive index The refractive indices determined on a \/ tp = Trigonal prism crystal face of T2 and on a polished surface of hp = Hexagonal prism T7 were consistent and are co 1.644 and J. Gemm., 1997, 25, 5, 325-330 Table I: Chemical analyses of East African tourma­ According to Bank and Henn (1988) their lines tourmaline lies about half-way in the isomorphous series dravite-uvite which involves 7# 2# 3* 4* | a coupled substitution between the X and Z sites Si02 37.84 37.51 35.29 34.87 (Na, AI - Ca, Mg). In the present analyses the Ti02 .43 .36 .18 .48 sodium/calcium ratios support the assumption Al2°3 32.49 32.71 33.57 31.73 of a significant uvite component, but the MgO 11.30 10.58 11.13 11.40 aluminium/magnesium concentrations are not 1.87 1.95 2.63 CaO 1.89 consistent with a compensatory substitution of Cr 0 .34 .22 .36 .64 2 3 Mg for AI in the Z position. How then is the v o (.05) (.05) .04 .06 2 3 charge increase on the X position accommo­ Na20 1.76 1.66 1.53 1.39 FeO .06 - - - dated? Minor amounts of Li have been found in Total 86.14 84.98 84.05 83.20 the Y position of some dravites and might be present here (partial solid solution with liddi- Number of ions on the basis of 6 Si coatite). Other types of charge balance, such as Si 6 6 adjustment of the 0270H" ratio or the creation Ti 0.051 0.044 AI 6.065 6.159 of vacancies in the X position are not Mg 2.666 2.52 uncommon in tourmalines (Foit and Rosenberg, Ca .317 .324 1977) and may be operative here. We would Cr .042 .029 therefore prefer to consider this Umba valley Na .540 .515 tourmaline as a calcium-bearing dravite until Fe .008 - we understand the exact nature of the # The analyses were performed by wavelength dispersive substitutions. spectrometry using a CAMECA electron microprobe at the Mineralogical-Geological Museum in Oslo. Oxides, natural Fluorescence 327 minerals and synthetics were used as standards. Each 327 analysis represents the average of at least six single point Under short-wave (254 nm) ultraviolet illu­ 327 analyses. Analytical precision (2sigma) is better than ±1% at mination all the specimens are mustard concentrations >20wt.% oxide, ±2% in the range yellow. The same reaction was reported by 10-20wt.°/O/ ±5% in the range 2-10% and better than 10% Dunn for 'low-iron chrome-rich tourmaline' in the range 0.5-2wt.%.
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