Deltalumite, a new natural modification of alumina with spinel-type structure

16 17 Titanium diboride (TiB 2 ) is a minor but common phase in melt pockets trapped in the 18 corundum aggregates that occur as xenoliths in Cretaceous basaltic volcanoes on Mt 19 Carmel, N. Israel. These melt pockets show extensive textural evidence of immiscibility 20 between metallic (Fe-Ti-C-Si) melts, Ca-Al-Mg-Si-O melts and Ti-(oxy)nitride melts. The 21 metallic melts commonly form spherules in the coexisting oxide glass. Most of the observed 22 TiB 2 crystallized from the Fe-Ti-C silicide melts, and a smaller proportion from the oxide 23 melts. The parageneses in the melt pockets of the xenoliths require fO 2 ≤ IW -6, probably 24 generated through interaction between evolved silicate melts and mantle-derived CH 4 +H 2 25 fluids near the crust-mantle boundary. Under these highly reducing conditions boron, like 26 carbon and nitrogen, behaved mainly as a siderophile element during the separation of 27 immiscible metallic and oxide melts. These parageneses have implications for the residence 28 of boron in the peridotitic mantle, and for the occurrence of TiB 2 in other less well-29 constrained environments such as ophiolitic chromitites. 30 31 Always consult and cite the final, published document. See http:/www.minsocam.org or GeoscienceWorld Revision 3, 2/2020 parageneses, reflecting crystallization from both Fe-Ti silicide melts, and coexisting silicate 57 melts. The occurrence of the boride in situ in these aggregates provides textural evidence 58 and context that can give further insights into the behaviour of boron under highly reducing 59 conditions in the uppermost mantle. 60 61Occurrence 62 63

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Parageneses of TiB2 in corundum xenoliths from Mt. Carmel, Israel: Siderophile behavior of boron under reducing conditions

Griffin

Gain

Saunders

et al. 2020

American Mineralogist

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Why Tolbachik Diamonds Cannot be Natural

Litasov

Kagi

Bekker

et al. 2021

American Mineralogist

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Taking into account recent publications, we provide additional comprehensive evidence that type Ib cuboctahedral diamonds and some other microcrystalline diamonds from Kamchatka volcanic rocks and alluvial placers cannot be natural and undoubtedly represent synthetic materials, which appear in the natural rocks by anthropogenic contamination. The major arguments provided in favor of the natural origin of those diamonds can be easily disproved. They include the coexistence of diamond and deltalumite from Koryaksky volcano; coexistence with super-reduced corundum and moissanite, Mn-Ni silicide inclusions, F-Cl enrichment and F/Cl ratios, and carbon and nitrogen isotopes in Tolbachik diamonds, as well as microtwinning, Mn-Ni silicides, and other inclusions in microcrystalline diamond aggregates from other Kamchatka placers. We emphasize the importance of careful comparison of unusual minerals found in nature, which include type Ib cuboctahedral diamonds and super-reduced phase assemblages resembling industrial slags, with synthetic analogs. The cavitation model proposed for the origin of Tolbachik diamonds is also unreliable since cavitation has only been shown to cause the formation of nanosized diamonds only.

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Deltalumite, a new natural modification of alumina with spinel-type structure

Cited by 2 publications

References 14 publications

Parageneses of TiB2 in corundum xenoliths from Mt. Carmel, Israel: Siderophile behavior of boron under reducing conditions

Parageneses of TiB2 in corundum xenoliths from Mt. Carmel, Israel: Siderophile behavior of boron under reducing conditions

Why Tolbachik Diamonds Cannot be Natural

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