gems, diamond, inclusions

With its first-ever dedicated session and with the sponsorship of the Gemological Institute of America (GIA), gemology made significant progress towards recognition as a formal sci- ence by the mainstream geo-sciences community during this fall’s Geological Association of America’s (GSA) historic 125th Anniversary Annual meeting held October 27-30, 2013, in Denver, Colorado.

corundum, Geology.11, Madagascar.12, gems

Madagascar is one of the principal producers of gem corundums recovered from continental basaltic fields. The main deposits are the secondary deposits of Ambondromifehy from the Antsiranana Province in the northern part of the country (sapphire-bearing palaeoplacer and placer), Soamiakatra–Mandrosohasina from the Antananarivo Province in the central part (primary ruby deposit, and ruby and/or sapphire-bearing placer and palaeoplacer), and Vatomandry from the Toamasina Province in the eastern part of the island (ruby- and sapphire-bearing paleoplacer). Soamiakatra is the only known primary deposit where ruby is found in metagabbro and pyroxenite xenoliths, which were entrained and brought up to the upper crust by the Ankaratra volcanic event. Petrographic studies have demonstrated the existence of two different conditions of ruby formation at the boundary of the eclogite domain (HT ~1100 °C, HP ~20 kbar) and granulite facies (HT ~1100 °C, BP <15 kbar). In contrast, the sapphires originated from alkaline mafic magmatic chambers at the lower continental crust–mantle boundary. They occur within syenite and anorthoclasite xenoliths in the basalts or are associated with them. These alkali basalts transported corundum and their host rocks to the surface; they are linked with asthenosphere upwelling and thinning of the lithosphere underneath the Ankaratra Plateau during the Oligocene–Quaternary. The mineralogical study of ruby and sapphire permitted us to establish the relationships that exist between their chemical composition, their colour, and the nature of their solid inclusions. Iron is the main chromophore of sapphire, and the Fe²⁺/Fe³⁺ ratio in its structure, in addition to the quantity of titanium and chromium available in the fluid, determines the colour of the sapphire and the hue of the polychrome sapphires. Chromium is the second most important chromophore for pink, violet-blue, and some blue-green to light-blue sapphires. For ruby, chromium is the chromophore, but iron and vanadium interfere to produce red-brown to purplish-blue crystals. The deeply coloured, high-quality rubies are characterized by a Fe₂O₃/Cr₂O₃ ratio less than 1. The chemical diagrams used for the genetic classification of corundum indicate a metamorphic origin for all rubies from the central and eastern regions of Madagascar: rubies are hosted either by mafic and/or ultramafic rocks or by metasomatites formed by fluid–mafic rock alteration (“plumasite” or biotitite). Two processes have been identified to explain the origin of sapphires from the different regions: (i) a dominant magmatic process linked to syenites (anorthoclasite), and (ii) a metamorphic process (probably of skarn type). The oxygen isotopic compositions of the corundum corroborate these two origins. Solid inclusions such as pyrochlore, samarskite, uraninite, and anorthoclase in the sapphires confirm their syenitic origin. The presence of pentlandite, talc, sillimanite, titanite, and phlogopite in the rubies testifies to a metamorphic paragenesis.

corundum, Cambodia.12, Vietnam.12, gems

While trapiche emerald is well known, trapiche corundum (especially blue sapphire) is considerably rarer. The authors examined 33 blue sapphires from Vietnam and 27 from Cambodia. The majority showed trapiche structure, but non-trapiche specimens were included for comparison. The trapiche sapphires typically featured a brown core and brown to yellowish brown arms separating the sections of blue crystal; dotted and needle-like inclusions were also seen. Iron and titanium were both present in the samples, but the iron was concentrated in the trapiche core while the titanium was evenly distributed for the most part. The authors also compared the visual characteristics of trapiche rubies and sapphires, as well as trapiche vs. star phenomena. The trapiche structure could point to a multi-stage growth process that starts at the core, with the arms and blue crystal segments growing concurrently but at different rates.

corundum, inclusions.5, gems

gems, inclusions, quartz

Photomicrographs of inclusions in quartz. The every day mineral quartz can be found all over the world. It takes a long time to form. Over millions of years, a growing quartz crystal sometimes incorporates other minerals that develop simultaneously, thus forming so called inclusions. This book presents photos of fascinating inclusions in quartz, often beautiful like a dream. It wants to give you an idea of the fantastic inner life of quartz crystals and might also be an animation for your own engagement with this phenomenon. The every day mineral quartz so gets a whole new perspective.

corundum, geology.11, gems

corundum, geology.11, inclusions.5, Myanmar (Burma).12, trapiche ruby, gems

After a comparison of the geologic occurrences of trapiche emerald and trapiche ruby, and a discussion of the various theories proposed for their formation, this article presents the results of an investigation of the chemical compositions, and the solid and fluid inclusions, of six trapiche rubies from Mong Hsu. The study used advanced analytical techniques such as energy- and wavelength-dispersive spectrometry and “cold” cathodoluminescence. The authors identify 12 minerals not previously reported as inclusions in trapiche rubies or as components of their skeletal “arms.” The rubies show a chemical zonation that can be linked to variations in the chemical composition of the growth fluid. The chromium content, in particular, appears to be related to variations in the circulation of metasomatic fluids rich in REE (rare-earth element) fluorides.

gems, beryl, emerald, Brazil, inclusions

corundum, inclusions.5, Thailand.12, Chantaburi, star sapphire, gems

After a brief overview of some basic facts concerning asterism, the characteristics of black star sapphires from Thailand are described. As in star corundums from other sources, these stars were found to be diffraction phenomena created by needles or lamellae of rutile or hematite-ilmenite. These inclusions can be oriented either parallel or perpendicular to the second-order prism faces.. Six-rayed star sapphires from Bang-kha-cha [also spelled Bang Kacha] show bluish white or, less commonly, “golden” yellow stars. The bluish white stars are caused by Ti-bearing hematite (as are similar black stars from Australia), whereas the nature of the mineral(s) forming the yellow stars is unknown. In the bluish white stars, the band of light (chatoyancy) can be oriented both parallel and perpendicular to the second-order prisms, but they are always parallel to these prisms in the yellow stars.. Rarely, both colors of stars are combined in the same specimen and form 12-rayed star sapphires. The authors studied nine such sapphires and found that the bluish white stars were always oriented perpendicular to the prism faces, whereas the yellow stars were parallel to these faces. The article is illustrated with four excellent photographs and two diagrams, which make the rather complicated phenomena easily understandable.

corundum, sources.12, Thailand.12, Bo Ploi, inclusions.5, gems

Three main types of fluid inclusions occur in Bo Ploi sapphires: (1) vapor-rich CO₂ with a density up to about 0.86 g/cm³; (2) multiphase inclusions containing several daughter minerals, hypersaline brine, and a CO₂-rich vapor phase; and (3) (silicate?)-melt inclusions with immobile vapor bubbles in an isotropic or weakly anisotropic phase of low relief, with bubbles becoming mobile when heated above 800 °C. These observations suggest a magmatic origin for the sapphires, although they did not crystallize as phenocrysts within the basalts that transported them to the surface.