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Gem Materials: Opportunites for research and cross-fertilization with education and culture

G. E. Harlow, Department of Earth & Planetary Sciences. American Museum of Natural History, New York, NY 10024-5192 U.S.A. .

Introduction: Gem materials are natural minerals and rocks whose intrinsic beauty and durability permit their fashioning into human adornments and objets díart. The use of these geomaterials are as old as human culture, and so their potential for study crosses material science, geoscience, gemology, archaeology, history, art history, technology, and economics. The intrinsic attractiveness of the substances offers a ìhookî or bait for exciting students and the public while informing about the source of their beauty and the wonders of the different stories. The message yields an understanding of the very nature of matter, its optical and physical properties, that rareness relates to the geology of the sources, and that gem-use history is a complex intersection of the evolution of human populations, mineral exploration and technology. However, in a perverse denial of this potential, academic science has mostly avoided extensive study of gem materials, perhaps in fear of not being taken seriously by ìplaying withî the glitter and glamour. Whatever the reason, the bonus for contemporary researchers is the possibility of many exciting avenues of research: the mineralogy and geochemistry of rarely-preserved but significant geological events, the origin of color & light scattering, crystal perfection, the challenge of material sourcing, property modifying technologies and their detection, and more. What follows is a brief review of some areas of gem research that show the successes and opportunities from the authorís biased perspective in a museum career.

Diamonds: The noteworthy exception to the divide between geoscience and gem materials is this adamantine substance. Vast knowledge has been and is being learned about the interior of Earth through diamonds, their inclusions, and, significantly, the byproducts of diamond mining. Diamonds record complex mantle processes in their ìdendochronlogy,î sample both primitive and organic reservoirs of carbon, preserve unique aliquots of the upper and lower mantle as inclusions (e.g., [1], [2]) which have led to new insights about crystal chemistry (e.g., K in cpx: [3]). The connections of culture and history to science and technology led me to create a major exhibition on diamond where the science and connections were presented successfully to an audience drawn by the lure of fabulous jewelryóthe exhibition was very successful [4].

Rubies and sapphires from Mogok, Myanmar: Recent studies of the trace elements of the corundum gems has permitted categorization of origin affinities on the basis of Ga-Cr-Ti-Fe systematics [5]. Traditionally, the marble-hosted rubies from the Mogok Stone Tract in northern Myanmar (Burma), the most renowned source, are classified as metamorphic whereas the sapphires are igneous. Discovering alkalifeldspar moonstone is hosted in the same marble as the rubies on a visit in 1998 has led to an ongoing project which supports an older interpretation indicating magmatic interactions with metamorphics to form metasomites [6]. Recent work shows that matrix rubies are always associated with mizzonite ±davyne±nepheline±diopside and the"ìijolite" host for sapphires is certainly not a primary melt. Potential inferences include that the rubies may be as young as the latest Eocene intrusives rather than nearly as old as the Proterozoic sediments forming the Mogok Belt marbles, and they may not be simply the product of recrystallized sediments. Resulting sample collections have yielded new blue davyne, magnesiosadanagaite, and true herderite.

Jadeite jade: This jadeite rock or jadeitite is very uncommon and is restricted to primary occurrences in bodies of subduction-related serpentinite along major fault zones. Recent studies [7] indicate jadeitites crystallize from hydrous fluids derived from the dewatering of subducted slabs at high P/T and manifest complex crystallization patterns that are reproduced (at least w.r.t. cathodoluminescenceñCL) in samples from most deposits. Jadeitite is exposed by back arc lateral faulting that permits thrusting or diapirism of host serpentinite sufficiently fast to avoid breakdown reactions. Jadeite jade is thus an important indicator of subduction and collisional processes; the material merits significant study and attention. Jadeite jade is also interesting because its extreme toughness does not derive from a felted fine-grained texture as with nephrite and synthetic compositesóidentifying the mechanism that suppresses crack propagation is another focus of research.

References: [1] Taylor L.A.. et al. (2000) Int. Geol. Rev., 42, 959-983. [2] Stachel T. et al. (2000) Contrib. Min. Pet., 140, 16-27. [3] Harlow G.E. (1997) Am. Min., , 259-269. [4] Harlow G.E. (1998) The Nature of Diamonds, Cambridge U. Press. [5] Sutherland F.L. (1998) J. Gemmology 26, 65-85. Garland M.I. (2001) This Volume. [6] Iyer L.A.N. (1953) M. Geol. Surv. India, 82. [7] Harlow G.E. (1994) J. Met. Geol., 12, 49-68.; Johnson C.A. and Harlow G.E. (1999) Geology, 27, 629-632. Sorensen S.S. and Harlow G.E. (1999) Abs. Programs GSA, 31, A-101.

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I found this article very informative.
January 27, 2005 | Unregistered CommenterDigital Rust

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