Chromium in Corundum: Ultra-high Contents Under Reducing Conditions

Sarah E.M. Gain, William L. Griffin, Martin Saunders, Vered Toledo

Research output: Contribution to journalArticle

Abstract

An exploration project run by Shefa Yamim (A. T. M.) Ltd has recovered a variety of gemstone minerals from Cretaceous pyroclastic vents and associated alluvial deposits at Mt Carmel, Israel [1]. Among these are several types of corundum (Al2O3), including rubies with <2 wt% Cr2O3 and sapphires in a variety of colours from yellows through to greens, blues and purples, with a range of chemical impurities e.g. Ti, Fe, V, Ga. The most scientifically interesting type of corundum is the inclusion-rich ‘Carmel SapphireTM’, which contains a variety of mineral phases; some of these have only been seen in meteorites previously, e.g. tistarite (Ti2O3) [2], and others have not previously been described, e.g. carmeltazite (ZrAl2Ti4O11) [3]. These minerals indicate very low oxygen fugacities, at least 7 log units below the Iron-Wustite buffer (DIW-7), and are interpreted as reflecting the presence of CH4+H2-rich fluids [1,4]. These discoveries have led to a new understanding of fluid transfer and redox conditions in the crust and mantle. Here we describe another variety of Cr-rich corundum (Fig. 1) with Cr concentrations up to 32 wt.% Cr2O3, representing a composition in the solid solution series between corundum and eskolaite (Cr2O3), and considerably more Cr-rich than previously known examples. These crystals are a deep purple (Fig. 1), but while purple in corundum usually is due to a combination of Ti and Cr, in this case the crystals are Ti-free and contain much higher concentrations of Cr. The cores of the crystals have relatively low Cr concentrations (1-2 wt.% Cr2O3) and the Cr concentration increases towards the rim. In the highest-Cr areas, the material consists of subgrains with small but distinct variations in Cr content (Fig. 1a, 2). On the surface of the illustrated crystal there are abundant balls (<10µm to 100’s of µm) of native Cr; Transmission Electron Microscopy (TEM) studies show that these are associated with chromium nitride CrN (carlsbergite; Fig. 2), otherwise known only from iron meteorites. Electron Energy Loss Spectroscopy (EELS) analyses show that the valence of the Cr changes from Cr3+ in the corundum (both low-Cr and high-Cr types) to Cr2+ in the carlsbergite and finally Cr0 in the chromium metal. The coexistence of all three valence states suggests that the oxygen fugacity was constrained by the CrO/Cr buffer, and that Cr was undergoing a crystallographically-controlled disproportionation, Cr2+ à Cr3+ + Cr0 The oxygen fugacity implied by this reaction lies at ca DIW-5, less reducing than the conditions inferred from the Ti3+-bearing, but Cr-free, assemblages in the Carmel Sapphire. These unusual high-Cr rubies thus appear to represent an earlier stage in the crystallization of the Mt Carmel magmas.
Original languageEnglish
Pages (from-to)2484-2485
Number of pages2
JournalMicroscopy and Microanalysis
Volume25
Issue numberS2
DOIs
Publication statusPublished - Aug 2019

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Corundum
chromium
Chromium
aluminum oxides
Meteorites
Minerals
minerals
Sapphire
Crystals
Oxygen
sapphire
oxygen
Bearings (structural)
buffers
iron meteorites
Iron
crystals
valence
Israel
Fluids

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Gain, Sarah E.M. ; Griffin, William L. ; Saunders, Martin ; Toledo, Vered. / Chromium in Corundum: Ultra-high Contents Under Reducing Conditions. In: Microscopy and Microanalysis. 2019 ; Vol. 25, No. S2. pp. 2484-2485.
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abstract = "An exploration project run by Shefa Yamim (A. T. M.) Ltd has recovered a variety of gemstone minerals from Cretaceous pyroclastic vents and associated alluvial deposits at Mt Carmel, Israel [1]. Among these are several types of corundum (Al2O3), including rubies with <2 wt{\%} Cr2O3 and sapphires in a variety of colours from yellows through to greens, blues and purples, with a range of chemical impurities e.g. Ti, Fe, V, Ga. The most scientifically interesting type of corundum is the inclusion-rich ‘Carmel SapphireTM’, which contains a variety of mineral phases; some of these have only been seen in meteorites previously, e.g. tistarite (Ti2O3) [2], and others have not previously been described, e.g. carmeltazite (ZrAl2Ti4O11) [3]. These minerals indicate very low oxygen fugacities, at least 7 log units below the Iron-Wustite buffer (DIW-7), and are interpreted as reflecting the presence of CH4+H2-rich fluids [1,4]. These discoveries have led to a new understanding of fluid transfer and redox conditions in the crust and mantle. Here we describe another variety of Cr-rich corundum (Fig. 1) with Cr concentrations up to 32 wt.{\%} Cr2O3, representing a composition in the solid solution series between corundum and eskolaite (Cr2O3), and considerably more Cr-rich than previously known examples. These crystals are a deep purple (Fig. 1), but while purple in corundum usually is due to a combination of Ti and Cr, in this case the crystals are Ti-free and contain much higher concentrations of Cr. The cores of the crystals have relatively low Cr concentrations (1-2 wt.{\%} Cr2O3) and the Cr concentration increases towards the rim. In the highest-Cr areas, the material consists of subgrains with small but distinct variations in Cr content (Fig. 1a, 2). On the surface of the illustrated crystal there are abundant balls (<10µm to 100’s of µm) of native Cr; Transmission Electron Microscopy (TEM) studies show that these are associated with chromium nitride CrN (carlsbergite; Fig. 2), otherwise known only from iron meteorites. Electron Energy Loss Spectroscopy (EELS) analyses show that the valence of the Cr changes from Cr3+ in the corundum (both low-Cr and high-Cr types) to Cr2+ in the carlsbergite and finally Cr0 in the chromium metal. The coexistence of all three valence states suggests that the oxygen fugacity was constrained by the CrO/Cr buffer, and that Cr was undergoing a crystallographically-controlled disproportionation, Cr2+ {\`a} Cr3+ + Cr0 The oxygen fugacity implied by this reaction lies at ca DIW-5, less reducing than the conditions inferred from the Ti3+-bearing, but Cr-free, assemblages in the Carmel Sapphire. These unusual high-Cr rubies thus appear to represent an earlier stage in the crystallization of the Mt Carmel magmas.",
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Chromium in Corundum: Ultra-high Contents Under Reducing Conditions. / Gain, Sarah E.M.; Griffin, William L.; Saunders, Martin; Toledo, Vered.

In: Microscopy and Microanalysis, Vol. 25, No. S2, 08.2019, p. 2484-2485.

Research output: Contribution to journalArticle

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T1 - Chromium in Corundum: Ultra-high Contents Under Reducing Conditions

AU - Gain, Sarah E.M.

AU - Griffin, William L.

AU - Saunders, Martin

AU - Toledo, Vered

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N2 - An exploration project run by Shefa Yamim (A. T. M.) Ltd has recovered a variety of gemstone minerals from Cretaceous pyroclastic vents and associated alluvial deposits at Mt Carmel, Israel [1]. Among these are several types of corundum (Al2O3), including rubies with <2 wt% Cr2O3 and sapphires in a variety of colours from yellows through to greens, blues and purples, with a range of chemical impurities e.g. Ti, Fe, V, Ga. The most scientifically interesting type of corundum is the inclusion-rich ‘Carmel SapphireTM’, which contains a variety of mineral phases; some of these have only been seen in meteorites previously, e.g. tistarite (Ti2O3) [2], and others have not previously been described, e.g. carmeltazite (ZrAl2Ti4O11) [3]. These minerals indicate very low oxygen fugacities, at least 7 log units below the Iron-Wustite buffer (DIW-7), and are interpreted as reflecting the presence of CH4+H2-rich fluids [1,4]. These discoveries have led to a new understanding of fluid transfer and redox conditions in the crust and mantle. Here we describe another variety of Cr-rich corundum (Fig. 1) with Cr concentrations up to 32 wt.% Cr2O3, representing a composition in the solid solution series between corundum and eskolaite (Cr2O3), and considerably more Cr-rich than previously known examples. These crystals are a deep purple (Fig. 1), but while purple in corundum usually is due to a combination of Ti and Cr, in this case the crystals are Ti-free and contain much higher concentrations of Cr. The cores of the crystals have relatively low Cr concentrations (1-2 wt.% Cr2O3) and the Cr concentration increases towards the rim. In the highest-Cr areas, the material consists of subgrains with small but distinct variations in Cr content (Fig. 1a, 2). On the surface of the illustrated crystal there are abundant balls (<10µm to 100’s of µm) of native Cr; Transmission Electron Microscopy (TEM) studies show that these are associated with chromium nitride CrN (carlsbergite; Fig. 2), otherwise known only from iron meteorites. Electron Energy Loss Spectroscopy (EELS) analyses show that the valence of the Cr changes from Cr3+ in the corundum (both low-Cr and high-Cr types) to Cr2+ in the carlsbergite and finally Cr0 in the chromium metal. The coexistence of all three valence states suggests that the oxygen fugacity was constrained by the CrO/Cr buffer, and that Cr was undergoing a crystallographically-controlled disproportionation, Cr2+ à Cr3+ + Cr0 The oxygen fugacity implied by this reaction lies at ca DIW-5, less reducing than the conditions inferred from the Ti3+-bearing, but Cr-free, assemblages in the Carmel Sapphire. These unusual high-Cr rubies thus appear to represent an earlier stage in the crystallization of the Mt Carmel magmas.

AB - An exploration project run by Shefa Yamim (A. T. M.) Ltd has recovered a variety of gemstone minerals from Cretaceous pyroclastic vents and associated alluvial deposits at Mt Carmel, Israel [1]. Among these are several types of corundum (Al2O3), including rubies with <2 wt% Cr2O3 and sapphires in a variety of colours from yellows through to greens, blues and purples, with a range of chemical impurities e.g. Ti, Fe, V, Ga. The most scientifically interesting type of corundum is the inclusion-rich ‘Carmel SapphireTM’, which contains a variety of mineral phases; some of these have only been seen in meteorites previously, e.g. tistarite (Ti2O3) [2], and others have not previously been described, e.g. carmeltazite (ZrAl2Ti4O11) [3]. These minerals indicate very low oxygen fugacities, at least 7 log units below the Iron-Wustite buffer (DIW-7), and are interpreted as reflecting the presence of CH4+H2-rich fluids [1,4]. These discoveries have led to a new understanding of fluid transfer and redox conditions in the crust and mantle. Here we describe another variety of Cr-rich corundum (Fig. 1) with Cr concentrations up to 32 wt.% Cr2O3, representing a composition in the solid solution series between corundum and eskolaite (Cr2O3), and considerably more Cr-rich than previously known examples. These crystals are a deep purple (Fig. 1), but while purple in corundum usually is due to a combination of Ti and Cr, in this case the crystals are Ti-free and contain much higher concentrations of Cr. The cores of the crystals have relatively low Cr concentrations (1-2 wt.% Cr2O3) and the Cr concentration increases towards the rim. In the highest-Cr areas, the material consists of subgrains with small but distinct variations in Cr content (Fig. 1a, 2). On the surface of the illustrated crystal there are abundant balls (<10µm to 100’s of µm) of native Cr; Transmission Electron Microscopy (TEM) studies show that these are associated with chromium nitride CrN (carlsbergite; Fig. 2), otherwise known only from iron meteorites. Electron Energy Loss Spectroscopy (EELS) analyses show that the valence of the Cr changes from Cr3+ in the corundum (both low-Cr and high-Cr types) to Cr2+ in the carlsbergite and finally Cr0 in the chromium metal. The coexistence of all three valence states suggests that the oxygen fugacity was constrained by the CrO/Cr buffer, and that Cr was undergoing a crystallographically-controlled disproportionation, Cr2+ à Cr3+ + Cr0 The oxygen fugacity implied by this reaction lies at ca DIW-5, less reducing than the conditions inferred from the Ti3+-bearing, but Cr-free, assemblages in the Carmel Sapphire. These unusual high-Cr rubies thus appear to represent an earlier stage in the crystallization of the Mt Carmel magmas.

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