Corundum, inclusions.5, Diamond inclusion, Mantle, Trace element, Nickel, gems

Mineral inclusions of corundum are reported from diamonds from alluvial deposits of tributaries of the Rio Aripuanã, Juina, Brazil. We present the first recorded occurrence of sapphire as an inclusion in diamond and expand on the database of ruby and white corundum inclusions. Ruby inclusions are found to occur both as isolated and touching grains with aluminous pyroxene and associated with ferropericlase. Mineral chemistry and phase relations place the origin of such ruby-bearing diamonds within the lower mantle at ~770 km. Mineral associations involving other corundum inclusions were not observed; hence, their depth of origin is less certain. Compositions of corundum samples were characterised by electron and ion microprobe. Given the scarcity of literature data, corundum samples from a variety of other geological settings were also analysed. Samples comprised corundums associated with granitic emplacement, metasomatism, amphibolite-facies and granulite-facies rocks, gem and industrial synthetic origins and carmine-coloured corundums recovered from kimberlite drill cores. In addition to variable amounts of Cr, Fe, Ti, Mg and Si, measurable quantities of other transition elements and high field strength elements were also detected. Corundums from similar geological settings show very similar compositions and are easily distinguishable from other settings. Irrespective of locality, rubies from Norwegian, Tanzanian and Kenyan amphibolite-facies rocks are compositionally indistinguishable. Additionally, corundums from metasomatised zones associated with contact metamorphism from Arizona and Japan were very similar, particularly characterised by unusually high abundance of mobile Zr and Nb (tens of ppm). All Juina inclusions are particularly distinguishable from other corundums by high concentrations of Ni (18–171 ppm weight), typically at least an order of magnitude enriched over the same corundum varietal types from elsewhere. Furthermore, the sapphire inclusion exhibited much larger ratios of Ga and Ge to HFSE elements compared to otherwise similar samples, and ruby inclusions are distinguished by high Mg/Fe ratios (0.27–1.56 by weight). Compositional differences between inclusions in diamonds and corundums from other settings in addition to corundum’s physical and chemical durability suggest that with the employment of rapid identification tools such as energy dispersive spectrometry (EDS) and laser-ICPMS, corundum has promise as an indicator of diamond prospectivity.

gems, diamond, Russia, Yakutia, inclusions, large diamonds, garnet

Large diamonds (10–200 carats) from the Udachnaya, Mir, and Aikhal kimberlite pipes of Yakutia contain mineral inclusions similar to those found in smaller diamonds. Most of these inclusions, particularly from the outer portions of the diamonds, are garnets of harzburgitic paragenesis, with fewer belonging to lherzolitic and eclogitic groups, along with some chromite diamond inclusions (DIs). A distinctive feature of the harzburgitic and some lherzolitic garnets is their sinusoidal rare-earth element (REE) patterns, in contrast to the normal light REE-depleted patterns seen in eclogitic garnets. These sinusoidal REE patterns suggest a complex petrogenetic history for the garnets, involving initial partial melting followed by metasomatic enrichment—especially of light and middle REEs—by fluids likely rich in C-O-H-N-S rather than carbonatitic compositions, as evidenced by negative Ba and Sr anomalies. These findings challenge the widely held assumption that most DIs are syngenetic with their host diamonds. Instead, the preserved geochemical signatures in harzburgitic garnet DIs reveal a history that predates diamond encapsulation, providing strong evidence that these inclusions are protogenetic rather than syngenetic.

corundum, Vietnam.12, gems

Fluid inclusions have been studied in rubies from marble-hosted deposits in the Luc Yen mining district, North Vietnam. Raman and Infrared spectroscopy combined with microthermometry investigations on primary and secondary fluid inclusions provided evidence of CO₂–H₂S–COS–S₈–AlO(OH)-bearing fluids with native sulphur and diaspore daughter minerals, without visible water. Diaspore appeared as a nonvisible film of 2–3 µm coating the wall of the fluid inclusion cavity and native sulphur was generally nucleated during Raman irradiation. The presence of diaspore and COS in the fluid inclusions indicates that water was present in the paleofluid. The mole fractions of H₂O and CO₂ are around 10⁻² and the expected concentration of water in the fluid inclusions is in the 1–10 mol.% range. Crush-leach identified sulphates and chlorides that are assigned to the presence of anhydrite and Na–Ca–Cl salts found by SEM in the ruby crystals. The CO₂-rich inclusions which do not coexist with an aqueous phase by immiscibility process demonstrate that ruby grew from this CO₂-rich and water-poor fluid at equilibrium with Na–Ca–Cl salts. Thermal reduction of evaporitic sulphates based on an initial assemblage of anhydrite, calcite and graphite, originating from the metamorphism of organic matter is proposed to explain the original fluid chemistry of these marble-hosted rubies. The marbles acted as a closed system and the carbonic composition of the parent fluids in ruby indicates that aluminium can be transported in CO₂-rich fluids at high P–T.

Burma, Mogok, inclusions, gems

In 2000, an unfamiliar gemstone was purchased in Mogok. It subsequently proved to be the rare borosilicate poudretteite, a mineral that previously had been identified only as tiny crystals from Mont Saint-Hilaire, Quebec, Canada. This article presents a complete gemological description of this unique gemstone and furthers the characterization of this mineral by advanced spectroscopic and chemical analytical techniques.

corundum, geology.11, Australia.12, chemistry.2, , gems

JES: This study was undertaken to get a better understanding of the conditions under which rubies found in palaeodrainage deposits along the Cudgegong–Macquarie river system in central eastern New South Wales, Australia, originally formed. Sixty gem samples collected along the two drainages were investigated to document their mineralogical and chemical properties, as well as their inclusions. The rubies had up to 0.6 wt.% Cr₂O₃ and up to 0.5 wt.% Fe₂O₃, and contained syngenetic inclusions such as Al-rich diopside (with up to 21 wt.% Al₂O₃), scapolite (meionite), and anatase. Associated sapphires had up to 0.8 wt.% Fe₂O₃, with one sample containing an unusual nepheline–anorthoclase inclusion. The chemical composition of the rubies contrasts with that of other Mesozoic–Cenozoic basalt-associated corundum deposits in Australia, suggesting they originated from a different eruptive source. However, their composition is similar to that of rubies from the eastern Thailand basalt fields (but not from other occurrences in Southeast Asia), which are thought to have a metamorphic origin deep within the earth. It is suggested that the rubies formed at temperatures between 1,000 and 1,300 °C, which are high equilibration temperatures for proposed lithospheric granulite sources. Further work will be required to definitively establish the geologic source for the Cudgegong–Macquarie ruby suite.

gems, beryl, Mozambique, Maria, alkali-carbonic solution, emerald, fluid inclusions, nahcolite

The compositions of fluid inclusions in emerald and quartz from the Maria deposit in Mozambique were investigated using microthermometric and Raman microprobe techniques. The inclusions in emerald belong to the Na–Ca–Mg–(HCO₃)⁻–(CO₃)²⁻–Cl–H₂O system and are saturated carbonic acid brines. Nahcolite is the primary daughter mineral phase observed, with average concentrations of 25 wt.% nahcolite and 5 wt.% NaCl. Additional daughter phases identified include Mg-calcite, magnesite, calcite, and aragonite. Many fluid inclusions show a necked-down morphology, with groups of nahcolite crystals often comprising up to ~50 vol.% of these inclusions. Zones rich in fluid inclusions containing numerous birefringent solids appear to be characteristic of emeralds from the Maria deposit. Estimated conditions for emerald formation are between 400 and 500 °C and pressures of 3 to 4 kbar.

gems, advanced testing, inclusions, inclusion spectra, cathodoluminescence, CL

From a study of inclusions in gemstones using cold-cathode cathodoluminescence (CL) microscopy and microspectrophotometry, ten examples containing various inclusions are described. Visual microscopic examinations of the luminescing inclusions, supported by photomicrographs and spectral curves, provide valuable information about the inclusion types. Strongly luminescing, even small colourless inclusions such as apatite, fluorite, or diopside can be easily detected, though they might otherwise go unnoticed. Additional details on both inclusions and host material can be obtained through CL spectra recorded in the 380–950 nm range. Because of the limited electron penetration depth into solids, all CL investigations must be conducted where inclusions intersect the surface of a host gemstone facet. Distinct colour differences between the CL emission of host and inclusion aid in their differentiation. This method also helps distinguish between inclusions that are otherwise difficult to separate by standard optical microscopy. Cold-cathode CL is a non-destructive and rapid technique that employs commercially available instruments and requires no coating or special treatment of samples.

corundum, synthetic corundum.7, gems

A relatively new production of red corundum, reportedly from Bangkok, has been offered for sale in recent years. Although the producers claimed that these stones were red diffusion-treat-ed corundum (i.e., a shallow coloration induced by the lattice diffusion of chromium), the surface layer is actually a synthetic ruby overgrowth on natural colorless to near-colorless corundum. The overgrowth layer, after recutting, typically ranged from < 0.1 to 0.3 mm thick. This material can be distinguished from red lattice diffusion–treated corundum by the presence of a boundary plane and two different types of inclusion features: those that occur in the natural corundum core and those present only in the synthetic overgrowth. This product demonstrates that as treatment techniques continue to evolve, the distinction between natural, treated, and synthetic corundum may become increasingly difficult to define.

corundum, geology.11, sources.12, India.12, gems

Southern Kerala and the adjacent state of Tamil Nadu are historically important regions for gem production in southern India. A wide variety of gems—including beryl, chrysoberyl, corundum, garnet, quartz, spinel, topaz, and zircon—are found in primary deposits (mainly in pegmatites emplaced in granulite facies metamorphic rocks), or in associated alluvial deposits. Pink sapphire is found at Melankode near Balaramapuram, and at Rose Mala near Tenmala. Electron-microprobe data indicate that the pink sapphire is nearly pure Al₂O₃ (98.43–99.48 wt.%). Cr₂O₃ and FeO are each present in amounts up to 0.12 wt.%. Abundant CO₂-rich fluid inclusions occur in the sapphires. The estimated temperatures of formation of the Melankode pegmatite are in the 650–750 °C range. The crystallization age of the pegmatite (based on U–Pb and Pb–Pb dating of gem zircons) has been calculated by others to be about 513 million years (My), whereas granulite-facies metamorphism in the same region is dated at about 540 My. Thus, pegmatite emplacement postdated the main period of granulite metamorphism. Both the pegmatite formation and regional metamorphism occurred during the Pan-African Orogeny (a geologic event beginning about 600 My that led to the assembly of the Gondwana supercontinent). Pink sapphires from these two Indian deposits are similar to material from a number of localities in southern Madagascar (Betroka, Ilakaka, Andranondambo), as well as from the Ratnapura district in Sri Lanka. These occurrences are cited by the authors as additional evidence for the juxtaposition of southern Madagascar, Sri Lanka, and southern India when these areas were brought together as part of Gondwana. Gem deposits in all three areas are within or closely proximal to a regional shear zone that extended at the time across Madagascar, southern India, and Sri Lanka, and terminated in Antarctica.

corundum, Sapphire, Zircon, Basalt, Cenozoic, U-Pb dating, Laos.11, gems

Sapphire from the Ban Huai Sai alluvial gemfield, Laos, derives from local Cenozoic basalts. Syngenetic mineral inclusions in the sapphire include zircon, cheralite, albite, and possibly euxenite-(Y). Sensitive high resolution ion microprobe (SHRIMP) analysis of zircon inclusions shows enrichment in Hf, Y, and rare earth elements (REE), with REE reaching some of the highest abundances recorded in such zircons (up to 5785 ppm). This, along with Eu depletion and the associated inclusion mineralogy, suggests that the sapphire crystallized from highly evolved alkaline melts prior to corrosive transport in basalt magma. U–Pb isotope (SHRIMP) determinations on zircon inclusions yield measured zircon ages of 1.2–1.3±0.3 Ma, interpreted as the time of host sapphire crystallization, and falling within the error range of a K–Ar age on basalt from the same province. The proposed crystallization age for Ban Huai Sai zircon/sapphire aligns with that determined for a zircon–sapphire–magnetite xenolith from Khao Wau, Thailand (1–2 Ma). These findings support a model of sapphire genesis linked to processes associated with young alkali basaltic magmatism in Southeast Asia.