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The origin of needle-like rutile inclusions in natural gem corundum: A combined EPMA, LA-ICP-MS, and nanoSIMS investigation

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Published/Copyright: July 17, 2017
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American Mineralogist
From the journal American Mineralogist Volume 102 Issue 7

Abstract

Trace-element chemistry and microscopic observations of included gem corundum (α-Al2O3) suggests a new model of syngenetic growth of oriented rutile inclusions rather than the usual interpretation of their growth through exsolution. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is now a robust method for measuring trace elements in gem-quality corundum (ruby and sapphire). Nonetheless, the corundum structure is relatively unforgiving for substitutional components and typically only a small handful of minor to trace elements are measured by LA-ICP-MS (Mg, Ti, V, Cr, Fe, Ga). Less commonly, trace elements such as Be, Zr, Nb, Sn, La, Ce, Ta, and W are found in natural corundum. Their concentrations are typically correlated with high contents of Ti and silky or cloudy zones in the corundum that contain a high concentration of needle-like rutile or other oxide inclusions. Three metamorphic-type sapphires from Sri Lanka, Madagascar, and Tanzania were studied here using LA-ICP-MS, electronprobe microanalysis (EPMA), and nanoSIMS to document correlations between the various trace elements and their distribution between the corundum and included, oriented rutile TiO2 needles. NanoSIMS and EPMA measurements show concentration of Be, Mg, Fe, V, Zr, Nb, Ce, Ta, and W in the rutile needles. The relative atomic concentrations of Mg and Ti from LA-ICP-MS measurements suggest the corundum-rutile intergrowth grew as a mechanical mixture of the two phases as opposed to rutile formation through exsolution from the corundum host. This scenario is also suggested for the three magmatic-type sapphires studied here based on the presence of glassy melt inclusions in close association with included, oriented oxide needles. The preservation of a glassy melt inclusion requires fast cooling, whereas exsolution of the oxide inclusions would require slow cooling and annealing at a temperature lower than sapphire formation. The studied sapphires suggest the likely origin of the oriented, needle-like rutile inclusions to be syngenetic epitaxial coprecipitation of both rutile and corundum. The interpretation of such oriented oxide inclusions has important implications for understanding the geological formation conditions based on trace element data or using such data to separate sapphires and rubies based on their geographic origin.

Keywords: Corundum; sapphire; exsolution; immiscibility; gemology; trace element chemistry; melt inclusions; nanoSIMS; LA-ICP-MS; EPMA; rutile inclusions

Acknowledgments

The authors thank Chi Ma of Caltech for assistance with EPMA measurements and Yunbin Guan of Caltech for working with us on the nanoSIMS experiments. We owe a debt of gratitude to F. Lin Sutherland and an anonymous reviewer for their constructive comments and suggestions as well as the Associate Editor Beda Hofmann for handling this manuscript and for his suggestions to improve our work. The study was funded by an R.T. Liddicoat Postdoctoral Fellowship from the Gemological Institute of America for one author (A.P.). Many thanks are owed to Nathan Renfro, Vincent Pardieu, and Ziyin Sun from the Gemological Institute of America and John Emmett from Brush Prairie, Washington, for helpful discussions.

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Received: 2016-9-23
Accepted: 2016-12-28
Published Online: 2017-7-17
Published in Print: 2017-7-26

© 2017 by Walter de Gruyter Berlin/Boston

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https://doi.org/10.2138/am-2017-5965
Keywords for this article
Corundum; sapphire; exsolution; immiscibility; gemology; trace element chemistry; melt inclusions; nanoSIMS; LA-ICP-MS; EPMA; rutile inclusions

Articles in the same Issue

  1. Editorial
  2. A new high JIF for American Mineralogist (by all early indications), why you shouldn’t care, and a note on values
  3. Highlights and Breakthroughs
  4. Sapphire, a not so simple gemstone
  6. Radon emanation coefficients of several minerals: How they vary with physical and mineralogical properties
  7. Actinides in geology, energy, and the environment
  8. Cabvinite, Th2F7(OH)⋅3H2O, the first natural actinide halide
  9. Special collection: apatite: a common mineral, uncommonly versatile
  10. Cathodoluminescence images and trace element compositions of fluorapatite from the Hongge layered intrusion in SW China: A record of prolonged crystallization and overprinted fluid metasomatism
  11. Special Collection: Nanominerals and Mineral Nanoparticles
  12. Structural characterization of marine nano-quartz in chalk and flint from North Sea Tertiary chalk reservoirs for oil and gas
  13. Special collection: Geology and geobiology of lassen volcanic national park
  14. Secondary minerals associated with Lassen fumaroles and hot springs: Implications for martian hydrothermal deposits
  16. Phillipsite and Al-tobermorite mineral cements produced through low-temperature water-rock reactions in Roman marine concrete
  18. The origin of needle-like rutile inclusions in natural gem corundum: A combined EPMA, LA-ICP-MS, and nanoSIMS investigation
  20. Dehydration studies of natrolites: Role of monovalent extra-framework cations and degree of hydration
  22. Mineralogical and compositional features of rock fulgurites: A record of lightning effects on granite
  24. Formation of basic lead phases during fire-setting and other natural and man-made processes
  26. Revisiting the nontronite Mössbauer spectra
  28. Experimental evidence for the survival of augite to transition zone depths, and implications for subduction zone dynamics
  30. Hydrothermal alteration of monazite-(Ce) and chevkinite-(Ce) from the Sin Quyen Fe-Cu-LREE-Au deposit, northwestern Vietnam
  32. Diagenetic F-rich ferroan calcite and zircon in the offshore Scotian Basin, eastern Canada: Significance for understanding thermal evolution of the basin
  34. Addibischoffite, Ca2Al6Al6O20, a new calcium aluminate mineral from the Acfer 214 CH carbonaceous chondrite: A new refractory phase from the solar nebula
  35. Letter
  36. 17O NMR evidence of free ionic clusters Mn+ CO32− in silicate glasses: Precursors for carbonate-silicate liquids immiscibility
  38. New Mineral Names
  40. Erratum
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