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Trapiche Tourmaline The Journal of Gemmology2011 / Volume 32 / Nos. 5/8 Trapiche tourmaline The Gemmological Association of Great Britain The Journal of Gemmology / 2011 / Volume 32 / No. 5–8 The Gemmological Association of Great Britain 27 Greville Street, London EC1N 8TN, UK T: +44 (0)20 7404 3334 F: +44 (0)20 7404 8843 E: [email protected] W: www.gem-a.com Registered Charity No. 1109555 Registered office: Palladium House, 1–4 Argyll Street, London W1F 7LD President: Prof. A. H. Rankin Vice-Presidents: N. W. Deeks, R. A. Howie, E. A. Jobbins, M. J. O'Donoghue Honorary Fellows: R. A. Howie Honorary Life Members: H. Bank, D. J. Callaghan, T. M. J. Davidson, J. S. Harris, J. A. W. Hodgkinson, E. A. Jobbins, J. I. Koivula, M. J. O’Donoghue, C. M. Ou Yang, E. Stern, I. Thomson, V. P. Watson, C. H. Winter Chief Executive Officer: J. M. Ogden Council: J. Riley – Chairman, S. Collins, B. Jackson, S. Jordan, C. J. E. Oldershaw, L. Palmer, R. M. Slater Members’ Audit Committee: A. J. Allnutt, P. Dwyer-Hickey, E. Gleave, J. Greatwood, G. M. Green, K. Gregory, J. Kalischer Branch Chairmen: Midlands – P. Phillips, North East – M. Houghton, North West – J. Riley, South East – V. Wetten, South West – R. M. Slater The Journal of Gemmology Editor: Dr R. R. Harding Deputy Editor: E. A. Skalwold Assistant Editor: M. J. O’Donoghue Associate Editors: Dr A. J. Allnutt (Chislehurst), Dr C. E. S. Arps (Leiden), G. Bosshart (Horgen), Prof. A. T. Collins (London), J. Finlayson (Stoke on Trent), Dr J. W. Harris (Glasgow), Prof. R. A. Howie (Derbyshire), E. A. Jobbins (Caterham), Dr J. M. Ogden (London), Prof. A. H. Rankin (Kingston upon Thames), Dr K. Schmetzer (Petershausen), Dr J. E. Shigley (Carlsbad), Prof. D. C. Smith (Paris), E. Stern (London), Prof. I. Sunagawa (Tokyo), Dr M. Superchi (Milan) Production Editor: M. A Burland The Editor is glad to consider original articles shedding new light on subjects of gemmological interest for publication in The Journal of Gemmology. A guide to the preparation of typescripts for publication in The Journal is given on our website, or contact the Production Editor at the Gemmological Association of Great Britain. Any opinions expressed in The Journal of Gemmology are understood to be the views of the contributors and not necessarily of the publishers. ©2011 The Gemmological Association of Great Britain The Journal of Gemmology / 2011 / Volume 32 / No. 5–8 Measurement and interpretation of growth patterns in chrysoberyl, including alexandrite Dr Karl Schmetzer Abstract: Procedures for identifying growth planes, growth zones and twin planes in optical biaxial gemstones are described and the most commonly observed interfacial angles are tabulated, using chrysoberyl as an example. The strong pleochroism of the chromium-bearing chrysoberyl variety alexandrite was found to be a useful indicator for locating the positions of the crystallographic axes, and the optic axes are determined using interference figures under crossed polarizers. Examples of the typical features of growth zoning in natural alexandrites from Russia, Sri Lanka and Brazil are shown. Twinned chrysoberyls from Madagascar are described in detail. Keywords: alexandrite, Brazil, chrysoberyl, crystal habit, Madagascar, optic properties, pleochroism, Russia, Sri Lanka, twinning Introduction overlapping trace element patterns — and and applied to the distinction of natural Origin determination has become the determination of the internal growth and synthetic rubies. Further examples of an increasingly important requirement patterns of these uniaxial gemstones characteristic growth patterns of optically in the gem trade during the last decade, provides additional criteria to use for uniaxial gemstones are also given in the especially for larger rubies, sapphires and distinguishing between samples from literature, especially in connection with emeralds. In addition to the ‘traditional’ different natural sources. With the the description of new sources of ruby examination of inclusions, trace element exception of a few significant cases, and sapphire and with the description of chemistry, e.g. by X-ray fluorescence or growth patterns should be measured various synthetic quartz varieties. laser ablation inductively coupled plasma and assessed only in combination with In contrast, the description of mass spectrometry (LA-ICP-MS), can a stone’s other properties and not as a characteristic growth patterns in optically provide the necessary data to determine single technique. the provenance of a gemstone. The determination of the internal Above: Growth structure and pleochroism However, with the increasing number growth patterns of optically uniaxial in twinned alexandrite from Lake Manyara, of new sources of gem-quality stones gemstones, e.g. ruby, sapphire, emerald, Tanzania; the twin boundary separates a first on the international market such as blue amethyst and citrine, has been described individual with growth planes parallel to two i (011) faces and a second individual with growth sapphires from Madagascar or Tanzania, in detail by Kiefert and Schmetzer (1991 faces parallel to i (011) and b (010). View inclusion and chemical information a,b,c). The general technique was also parallel to the a-axis of both parts of the twin, may be insufficient — especially due to comprehensibly described by Smith (1996) immersion, 25 ×. ©2011 The Gemmological Association of Great Britain Page 129 The Journal of Gemmology / 2011 / Volume 32 / No. 5–8 Measurement and interpretation of growth patterns in chrysoberyl, including alexandrite biaxial gemstones is limited. For flux- grown Russian synthetic alexandrites, the c internal growth patterns were determined c Z green to bluish using immersion microscopy of rough and green a, b, c faceted samples (Schmetzer et al., 1996). i crystallographic More recently, a detailed description of i axis characteristic growth patterns in natural Russian alexandrites originating from the optic axis emerald mines in the Ural mountains has been published (Schmetzer, 2010) a b optic plane b and includes practical guidance for a b a the recognition of such characteristic Y a, b, i patterns in Russian samples. However, a X yellow crystal faces general overview of growth patterns in red to to orange purple X, Y, Z alexandrites from various natural sources pleochroism, and their determination is still missing. incandescent light General considerations In optically uniaxial gemstones, the determination of the complete growth pattern is quite straightforward. Because Figure 1: Orientation of the optic plane and the optic axes relative to the three crystallographic axes a, b and c in optically biaxial chrysoberyl; the optic plane is represented by the ac-plane of the crystal. the crystallographic c-axis is parallel to Left: clinographic projection, view almost parallel to the a-axis; right: parallel projection, view parallel the optic axis, a growth plane can easily to the b-axis. be determined by measurement of its inclination to the optic axis. The complete growth pattern can be seen by rotating the sample with the c-axis (coincident with the optic axis) as rotation axis of the sample holder. In optically biaxial gemstones, on the other hand, neither optic axis can be expected to be parallel to any one of the three crystallographic axes (a-, b- and c-axes) and, therefore, the measurement A B of an angle of a growth plane relative to one of the optic axes or a rotation of the gemstone with one of the optic axes as rotation axis gives only limited information and doesn’t show the complete growth pattern of a sample. Consequently, the determination of growth patterns in optically biaxial gemstones can be considered as a trial and error process, which, depending upon the orientation of the table facet C D with respect to the crystallographic axes, may be very quick (comparable Figure 2: This series illustrates the variation of interference figures in alexandrite from Hematita, to the procedure for optically uniaxial Brazil, during a slight rotation of the stone; tilting the alexandrite from a position, in which the gemstones) or somewhat more time optic axis is inclined to the direction of view (A) progressively towards positions in which the angle consuming. For all biaxial gemstones it between the optic axis and the microscope axis is progressively diminished, moves the interference is necessary to rotate a faceted sample rings towards the centre (B and C); in (D) the optic axis is exactly parallel to the microscope axis. through an angle of 360° in a number of Consequently, this procedure is applied in a search for both optic axes and for the determination of different orientations in the sample holder. crystal orientation in faceted samples by optical means. Immersion, crossed polarizers, 30×. Page 130 The Journal of Gemmology / 2011 / Volume 32 / No. 5–8 Measurement and interpretation of growth patterns in chrysoberyl, including alexandrite The observed structural properties and formed by the crystallographic a- and the centre of the sample (Figure 2 B to D). their orientation within the gemstone, e.g. c-axes (Figure 1). The position of an optic If the geometry of the stone is such relative to the table facet, can then be axis in this plane is found by placing the that it is possible to find both optic noted. In subsequent steps, an orientation stone in the sample holder in different axes, its orientation relative to the three of the alexandrite in selected settings initial orientations and rotating the stone crystallographic axes can be determined. and subsequent rotations will give the 360° in each position under crossed With a starting position in which the basic information to determine the most polarizers. This uses the vertical axis of crystallographic b-axis is parallel or almost characteristic structural properties. In the sample holder as rotation axis. A parallel to the rotation axis of the sample other words, each setting and rotation position with a view somewhat inclined holder, it is possible to observe both optic of the gemstone in the sample holder to the optic axis is indicated by a typical axes within one single rotation of the yields information to solve part of a pattern of interference rings (Figure 2A).
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