^ GemmologyThe Journal of Volume 28 No.7 July 2003 The Gemmological Association and Gem Testing Laboratory of Great Britain ~ ~. ~ Gemmological Association , ~ '.~ , and Gem Testing Laboratory ~, :~ of Great Britain • 27 Greville Street, London ECIN 8TN Tel: +44 (0)20 7404 3334 Fax: +44 (0)20 7404 8843 e-mail: [email protected] Website: www.gem-a.info President: Professor A.T Collins Vice-Presidents: N. W. Deeks, A.E. Farn, RA Howie, D.G. Kent, RK. Mitchell Honorary Fellows: Chen Zhonghui, RA Howie, K. Nassau Honorary Life Members: H . Bank, D.J. Ca llaghan, E.A [obbins, H . Tillander Council of Management: T J. Davidson, RR Harding, I. Mercer, J. Monnickendam, M.J. 0'Donoghue, E. Stern, I. Thomson, Y.P. Watson Members' Council: A J. Allnutt, S. Burgoyne, P. Dwyer-Hickey, S.A Everitt, J. Greatwood, B. Jackson, L. Music, J.B. Nelson, P.J. Wates, CH. Winter Branch Chairmen: Midlands -G.M. Green, North West -D. M. Brady, Scottish - B. Jackson, South Eas t - CH. Winter, South West - RM. Slater Examiners: A J. Allnutt, M.5e., Ph.D., FGA, L. Bartlett, B.5e., M.Ph il., FGA, DGA, S. Coelho, BS e., FGA, DGA, Prof. AT Co llins, BSe., Ph.D, A.G. Good, FGA, DGA, J. Greatwood, FGA, S. Greatwood, FGA, DGA, G.M. Green, FGA, DGA, G.M. Howe, FGA, DGA, S. Hue Williams MA, FGA, DGA , B. Jackson, FGA, DGA, G.H. Jones, BSe., PhD., FGA, Li Li Ping, FGA, DGA, M.A Medniuk, FGA, DGA, M. Newton, BSe. , D.Phil., CJ.E. Oldershaw, BSe. (Hans), FGA, DGA, H.L. Plumb, B.5e., FGA, DGA, N .R Rose, FGA, DGA, RD. Ross, BSe., FGA, DGA, E. Stern, FGA, DGA, S.M. Stocklmayer, BSe. (Ha ns), FGA, Prof. I. Sunagawa, D.5e., M. Tilley, GG, FGA, CM. Woodward, B.5e., FGA, DGA, Yang Ming Xing, FGA, DGA The Journal of Gemmology Editor: Dr RR Harding Assistant Editors: M.J. O'Donoghue, P.G. Read Associate Editors: Dr CES Arps (Leiden), G. Boss ha rt (Zurich), Prof. AT Collins (London), Dr J.W. Harris (Glasgow), Prof. RA. Howie (Derb yshire), Dr J.M . Ogden (Hildeshei m), Prof. A.H. Rankin (Kingston u pon Thames), Dr J.E. Shigley (Carls bad), Prof. D.C Smith (Paris), E. Stern (London), Prof. I. Sunagawa (Tokyo), Dr M. Superchi (Milan) Production Editor: M.A Burland Vo! 28, No. 7, Jul y 2003 ISSN: 1355-4565 An anorthite-ruby-pargasite- picotite assemblage Dr Karl Schmetzer1, Dr Heinz-Jürgen Bernhardt2 and Professor Edward J. Gübelin3 1. Taubenweg 16, D-85238 Petershausen, Germany 2. Central Microprobe Facility, Ruhr-University, D-44780 Bochum, Germany 3. Haldenstrasse 4, CH-6002 Lucerne, Switzerland ABSTRACT: The mineral assemblage of cabochon-cut samples, probably from Myanmar, was determined by a combination of X-ray powder diffraction and electron microprobe analysis. The matrix consists of colourless plagioclase, which contains between 93 and 96 mol.% anorthite. The inclusions were determined as rubies, green chromium-bearing pargasites and black picotites. The possible origin of the samples is discussed. Keywords: anorthite, Cr-pargasite, electron microprobe analyses, ruby 385 Introduction n the mid-1990s, cabochons composed The following study was undertaken to of ruby crystals in a transparent matrix characterize the various components of this I of feldspar were briefly described by mineral assemblage. Koivula et al. (1994). These cabochons (Figure 1) were purchased by a German gem merchant in northern Thailand and were said to originate from Myanmar by local dealers. During further trips to Thailand, our supplier was able to obtain some similar cabochons with a colourless matrix and green inclusions (Figure 2); also some of the cabochons contained red and green minerals together as inclusions in a transparent matrix. In the mid-1990s cabochons of similar appearance (Figure 3) were also found elsewhere in the Asian market (Tay Thye Sun, pers. comm., 1995). Cathodoluminescence of Figure 1: Ruby crystals in a matrix of colourless this particular material was described anorthite. Size of the sample on the left: 10.5 x 8.3 recently by Ponahlo (2002). mm; photo by M. Glas. © Gemmological Association and Gem Testing Laboratory of Great Britain ISSN: 1355-4565 Figure 2:Pargasite crystals in a matrix of colourless anorthite. Size of the sample on the left: 213 x 163 mm; photo by M. Glas. JM Figure 3: Ruby and pargasite crystals in a matrix Materials and methods of colourless anorthite. Size of the sample about 17 x 13 mm, photo by Tay Thye Sun. For the present study about 40 cabochons in the range of about 3 to 20 ct were examined. For mineralogical phase determination, all four components of the cabochons were examined by microscope and by X-ray powder diffraction. Four samples were selected for electron microprobe analyses. After repolishing the originally slightly curved (almost plane) backs of these cabochons, analyses were made at 98 points in the different components of the mineral assemblage, namely 68 of the colourless to whitish matrix, 18 of the red component, eight of the green mineral and four of the black crystals. Results X-ray phase determination Examination of the four components of the mineral assemblage by X-ray powder diffraction showed the following phases to be present (see Figure 4): J. Gemm., 2003, 28, 7, 385-391 Occasionally, the ruby crystals appear as if several platelets of different diameters are piled up to form an irregular stack. The green amphiboles are developed as long prismatic crystals (Figure 7). Cross sections typically consist of four {110} faces, some with two smaller {010} prisms (Figure 8). The spinels are small opaque grains or clusters with metallic lustre and sporadic development of small octahedral crystal faces (Figures 4 and 9). Figure 4: Mineral assemblage with ruby, green Chemical compositions pargasite and black picotite in a matrix of The matrix consists of a transparent, colourless anorthite. Photo by E.J. Giibelin, colourless to light grey plagioclase feldspar. magnified 60 x. Microprobe analyses (Table I) showed that plagioclase is almost pure anorthite within • colourless to whitish matrix: plagioclase the compositional range of 93 and 96 mol.% feldspar anorthite and 7 to 4 mol.% albite, thus showing only small variation. • red inclusions: corundum (ruby) The red corundum crystals (rubies) show • green inclusions: amphibole a relatively large variation of chromium contents from 0.14 to 1.80 wt.% Cr2Os (Table II). • black inclusions: spinel However, iron contents are small, titanium 387 contents are extremely low and vanadium is Microscopic observations below the detection limit of the electron The crystals forming the colourless microprobe. plagioclase matrix occasionally reveal parallel striations due to polysynthetic The analyses of the green amphiboles twinning. Healing feathers are also common. (Table I) show a limited variability and reveal The ruby crystals show tabular habit with an aluminium-rich calcic amphibole with a dominant basal pinacoids (0001) and appear composition close to pargasite: as lath-like cross sections on the curved 2+ surfaces of the cabochons (Figures 5 and 6). NaCa2(Mg,Fe )4AlSi6Al2022(OH)2 Figure 5: Ruby crystals in a matrix of colourless Figure 6: Mineral assemblage with tabular ruby anorthite. Size of the sample 19.0 x 13.4 mm; crystals in a matrix of colourless anorthite. Photo photo by M. Glas. by K. Schmetzer, magnified 50 x. An anorthitè-ruby-pargasite-picotite assemblage Table I: Compositional ranges of feldspars (Fl-3), amphiboles (A l -3) and spinels (51-3) shown bymicroprobeanalyses of selected samples. Wt. % F1 F2 F3 Wt.% A1 A2 A3 Wt.% 51 52 53 Si02 43.77 43.35 43.09 Si0 2 41.78 41.59 42.75 Si02 0.06 0.07 0.05 AI203 35.20 35.64 35.95 AI20 3 19.22 19.30 18.43 AI20 3 30.47 32.74 38.27 Ti02 0.02 0.05 0.04 Ti0 2 0.14 0.13 0.10 Ti02 0.05 - 0.05 Cr203 0.02 0.05 0.06 Cr203 0.88 0.67 0.15 Cr203 31.44 29.44 25.11 Mg O -- - MgO 14.85 15.01 15.43 MgO 4.21 4.51 6.36 FeO * 0.03 0.04 0.10 FeO * 5.09 4.90 5.11 FeO * 30.86 29.95 27.61 MnO - 0.04 - MnO 0.07 0.09 0.06 MnO 0.77 0.65 0.63 CaO 19.20 19.18 19.44 CaO 13.26 13.21 13.57 ZnO 0.63 0.74 0.70 Na 20 0.78 0.53 0.42 Na 20 2.50 2.47 2.54 CaO 0.13 0.10 0.07 K20 -- - KzO 0.50 0.55 0.49 NazO 0.03 0.01 - KzO - - - Sum 99.02 98.88 99.10 Sum 98.29 97.92 98.63 Sum 98.65 98.21 98.85 Number of ions on the basis of 8 a Numberof ions on the basis of 23 a Numberof ions on the basis of 4 a Si 2.046 2.028 2.013 Si 5.918 5.916 6.032 Si 0.002 0.002 0.001 Al 1.939 1.964 1.980 Al 2.082 2.084 1.968 Al 1.152 1.226 1.374 Ti 0.001 0.002 0.001 SumT 8.000 8.000 8.000 Ti 0.001 - 0.001 Cr 0.001 0.002 0.002 Al 1.133 1.151 1.097 Cr 0.798 0.740 0.605 Mg - - - Ti 0.015 0.014 0.011 Mg 0.201 0.214 0.289 Fe 0.001 0.002 0.004 Cr 0.098 0.075 0.017 Fe 0.828 0.796 0.704 Mn - - - Mg 3.144 3.181 3.241 Mn 0.021 0.Q18 0.016 Ca 0.961 0.961 0.974 Fe 0.605 0.583 0.603 Zn 0.015 0.017 0.016 Na 0.071 0.048 0.038 Mn 0.008 0.011 0.008 Ca 0.004 0.004 0.002 K -- - Sum C 5.003 5.015 4.977 Na 0.002 0.001 - Ca 2.018 2.013 2.050 K -- - Na 0.689 0.682 0.695 Sum 3.024 3.018 3.008 K 0.091 0.099 0.089 Sum A+ B 2.798 2.794 2.834 * Total iron as FeO; - m eans below detection Table II: Compositional range and mean of ruby obtained from microprobe analyses.