Analyzing the Crystal Structure
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The Identification of the New Gem Mineral Pezzottaite THE CHALLENGE OF THE IDENTIFICATION OF A NEW MINERAL SPECIES: EXAMPLE "PEZZOTTAITE" Adolf Peretti (1 ), Thomas Armbruster (2), Detlef Gunther (3), Bernard Grobety (4 ), Frank C. Hawthorne (5), Mark A. Cooper (5), William B. Simmons (6), Alexander U. Falster (6), George R. Rossman, (7), Brendan M. Laurs (8) (1) GRS Gemresearch Swisslab AG, Hirschmattstr. 6, P. 0 . Box 4028, CH-6003 Lucerne, Switzerland (2) Chern. Miner. Kristallogr., University of Berne, Freiestr. 3, CH-3012 Berne, Switzerland (3) Laboratory of Inorganic Chemistry, ETH Honggerberg, HCI, G113, CH-8093 Zurich, Switzerland (4) Department of Geosciences, University of Fribourg, Perolles, CH-1700 Fribourg, Switzerland (5) Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 (6) Geology & Geophysics, University of New Orleans, New Orleans, Louisiana, LA 70148, USA (7) Division of Geological & Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA (8) Gemological Institute ofAmerica, Carlsbad, CA 92008, USA (Authors (1)-(4) are referred in this article as the "Swiss research team" and authors (5)-(8) are referred in this article as the "US-Canadian !'esearch team" IDENTIFYING A NEW GEM MINERAL In 2002, a new gem mineral of commercial importance XRFanalysis - direct measurement by Laser Ablation was discovered. In accordance with the need for all Mass Spectroscopy was also used (see Box 48 and new mineral discoveries to be scientifically Fig. P16), as well as conventional methods used for characterized (see Nickel and Grice, 1998), the chemical analysis (Box 4A). In addition to challenges gemological community anxiously awaited the results in the analysis of chemical composition, determination of tests to positively identify the new mineral of different atomic positions in the crystal structure (Hawthorne et al., 2003, Hawthorne et al., submitted was not trivial. A combination of various analytical and Laurs et al., 2003). This period of analysis techniques was necessary (Figs. P3 and P17) to brought into play the question: Exactly what finally elaborate the structural differences to beryl procedures are necessary for the positive (Figs. P1 and P2 and Box 58). It was discovered that characterization of a new mineral? the unit-cell of pezzottaite had unusually large dimensions, which is best described as a These principal steps of identification are illustrated in superstructure of conventional beryl (Figs. P1, P2 and the following case study for identifying a new mineral P6). Also, the number of atoms necessary to define of the beryl group - pezzottaite. the crystal structure turned out to be unusually high in comparison with other beryl- group minerals (Boxes 1 The identification of this new mineral provided two and 5). major challenges: first, the determination of the exact chemical composition, and secondly, the identification ANALYZING THE CRYSTAL STRUCTURE of its crystallographic structure (the geometrical arrangement of the atoms in three dimensions (see The secrets of the nature of a new mineral are so Box 1, Figs. P2 and P6). From special technique of small we cannot even identify them with the help of analysis (Boxes 4 and 5A), the exact stoichiometric microscopic magnification. One way to explore this chemical formula, and the space group of this new atomic world is with X-rays (Figs. P3 and P17). mineral, have to be determined (Box 1). As soon as the mineral was characterized (Boxes 1-6), the X-rays have a very short wavelength (with dimensions differences between already existing minerals had similar to atoms), allowing them to penetrate and also to be investigated (see Nickel and Grice, 1998 interact with atoms in a mineral structure. As atoms and Box 58) and a decision had to be made regarding group together at very short distances, X-rays are whether a new mineral had been found, and, if so, diffracted when passing through a crystal and change what would be its new name? (Hawthorne et al., 2003 their direction when they interact with them. and Box 6). In the case of pezzottaite, these analysis Furthermore, diffracted X-rays interfere with each provided major challenges. As pezzottaite contains other, just like water waves emerging from two ships. light elements - such as hydrogen, lithium, and The interaction between atoms and X-rays depends, beryllium, which cannot be directly analyzed by among other factors, on the geometrical arrangement quantitative methods commonly used for mineral of the atoms and can be interpreted with X-ray analysis such as electron microprobe (EMPA) or diffraction diagrams. and computer calculations. A 01 New Gem Mineral Species Pezzottaite: Comparison between Beryl and Pezzottaite THE SUPERSTRUCTURE: THE DIFFERENCE BETWEEN THE TWO MINERALS "BERYL" AND "PEZZOTIAITE" Fig. P1-2 Ball-and-stick models of beryl (Fig. P1) and pezzottaite (Fig. P2). A projection of the crystal structure in the direction of the c-axis is shown in order to explain some details of the ring systems in these two minerals. The size and position of the unit-cell (as seen perpendicular to the c-axis) is also shown. Green balls = aluminium cations (AI), blue = beryllium (Be), yellow= lithium (Li), red =caesium (Cs), grey = oxygen (0), black =silicon (Si). The structure of beryl is characterized by six-membered rings of Si04 tetrahedra alternating along the c-axis with twelve-membered rings composed of alternating Be04 and AI06 polyhedra. In the mineral pezzottaite, an arrangement of beryl-like rings is found, but there are two different types of twelve-membered rings: (1) Those identical to the corresponding rings in beryl, and (2) those assembled of Li04, Be04 and AI06 polyhedra. To allow for the ordered (Be, Li) distribution in the structure of pezzottaite, the unit-cell had to be enlarged relative to beryl (compare P1 and P2). Both models are represented in hexagonal settings for better comparison. It must be remembered, however, that pezzottaite has a rhombohedral crystal lattice and beryl has a hexagonal lattice. In trigonal/rhombohedral symmetry, the highest symmetry element is a three-fold rotation axis, whereas in hexagonal symmetry a six-fold rotation axis is required. Fig. P3 Single crystal X-ray diffractometer One of the instruments used for the analysis of the crystal structure of the new mineral pezzottaite. Critical parts of the instrument include an X-ray generator (1), a multichannel X-ray detector (2), and utilizing the latest computer software for visualization and analyzing signals (not shown). X-ray can be used to compute the geometrical arrangement of atoms (Figs. P1,2 <!_nd P6) in a mineral. The result le~ds to the determination of crystal structure details (Box SA). , diffraction pattern of a single crystal consists of Finally, the unit-cell and the space group can be symmetrically arranged spots, where the . spot determined (Boxes 1 and 5A). This is a crystallog(aphic separation determines the periodicity of the lattice code that identifies the crystallographic nature of a (unit-cell dimensions) and the arrangement of the mineral. spots provides clues to the symmetry of the structure. 02 ~ N,ew Gem Minernl Species Pezzottaite: Materials and the IMA Application MATERIALS The new gem material was first introduced to the international mineralogical community at the Tucson show in February 2003, where it was recognized as a mineral of unusu~l gemological properties not referenced in world literature (see Laurs et al., 2003). For details on materials tested, see Hawthorne et al. (submitted) and Laurs et al. (2003). Three of the totally tested samples are officially registered reference materials (Box 6). They are deposited in the USA, Canada and in Switzerland. The samples analyzed by the Swiss team are shown in Figs. P4-P5, P?-8, P12-13 and Box 7. Three samples were investigated by chemical and crystallographic investigation. These include O.Scm-sized samples obtained from the large mother piece (Fig. PS). One of the small fragments entered the collection as a type locality specimen (Box 6). THE APPLICATION TO THE COMMISSION ON NEW MINERALS AND MINERAL NAMES The Swiss research group (authors 1-4) filed an Fig. P4 A reference set of pezzottaite including a application to the Commission on New Minerals and faceted sample of 4.85 ct, 2 pezzottaite eat's eyes, 2 Mineral Names of the International Mineralogical Association tabular rough crystals (35.38ct, 29.41ct), 11arge (CNMMN, http://www.geo.vu.nl/users/ima-cnmmn/), crystal fragment with matrix (35.22 ct), 2 tabular stating that they had characterized a new mineral. The pezzottaite crystals with host rock, and one with chairman of CNMMN informed the Swiss team that he tourmaline intergrowth (27.51 ct). On public display at had just received a proposal for the same mineral the Natural History Museum of the University of from a different research group (US-Canadian, Fribourg (Switzerland), GRS collection. authors 5-8). He further stated that both proposals were complementary to each other. An agreement was established that both groups' information and data should be merged and a new joint proposal should be submitted. Subsequently, CNMMN members decided unanimously that the new mineral should be accepted with the name 'pezzottaite'. Box 1 to 6 summarized original data of the merged IMA application submitted by: Hawthorne F.C., Cooper M.A., Peretti A. , Simmons W.B., Armbruster T. , Rossman G.R., Gunther D., Laurs B.M., Grobety B. (2003) Fig. P5 Pezzottaite tabular crystal of 29.41ct and a broken part of it (lower right), which served as the Check-list for new mineral proposals: sample used for the Swiss team's detailed chemical Pezzottaite. Proposal submitted to the and crystallographic investigations. The lower right piece parted in 3 different fragments which are International Mineralogical Association, 8 pp. currently at the University of Berne (see Box 6), the Application No. 2003-022. University of Fribourg, and the SFIT (Switzerland).