Natural diamond form by self-redox of ferromagnesian carbonate - Drs. Jinfu Shu and Ho-Kwang Mao
MARCH 15, 2018
A team of scientists including Drs. Jinfu Shu and Ho-Kwang Mao from HPSTAR report a new mechanism for natural diamond formation, a self-reduction reaction, indicating that external reductant, liquids or melts are not necessary for natural diamond formation. This work is published in recent PNAS.
Natural diamonds are mainly formed under high pressure and high temperature conditions, which can be produced in the Earth’s deep interior or by collisions between celestrial bodies.
Most natural diamonds formed at depths of 150 to 250 kilometers in the Earth's mantle. Under high pressure and temperature in the Earth’s deep interior, carbon-containing fluids reduce minerals and replaced them with diamonds via a reduction-oxidation reaction between C–O–H-bearing fluid and rocks, or between carbonate melt and reductant (for instance, metallic iron). This means that the formation of natural diamonds requires the reduction of carbon to its bare elemental form by the presence of external reductant.
The team — including Drs.Ming Chen, Xiande Xie and Dayong Tan of Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Dr. Jinfu Shu of HPSTAR, and Dr. Ho-Kwang Mao of HPSTAR and CIW found that diamond can be produced directly from their new study of shocked ferromagnesian carbonate at the Xiuyan impact crater, China.
From their analysis, ankerite, a ferromagnesian carbonate, will decompose and reduce to diamond via a self-reduction- oxidation reaction without the melts, fluids, and other reductant.
During this process, ferrous iron changed to ferric iron in ferromagnesian carbonate and behaved as a reductant.
“Ferropericlase (Fe, Mg)O is a major mineral in the lower mantle. It appears that CO2 reduced from carbonates might react with ferropericlase to form diamonds”, said Dr. Jinfu Shu.
“This suggests that diamonds could be ubiquitously present as a dominant host for carbon in the Earth’s lower mantle where the carbonates are abundant and pressures and temperatures are sufficiently high”, added Dr. Ming Chen.
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科学网： http://news.sciencenet.cn/htmlnews/2018/2/403975.shtm
天然金刚石在高温高压条件下形成，主要途径包括星球撞击和巨大星球内部地质作用。地球内部的金刚石形成均涉及了流体相或熔体和另一种还原剂的存在，尚未发现碳酸盐在没有外部还原剂条件下发生亚固态分解形成金刚石的现象。广州地球化学研究所与北京高压科学研究中心通过对我国岫岩陨石撞击坑中岩石和矿物的冲击变质效应分析，发现铁白云石这种铁镁碳酸盐在撞击产生的高温高压下发生亚固态自氧化还原反应生成金刚石，同时伴随着二价铁氧化为三价铁。反应过程中既没有发生碳酸盐熔融，也没有流体和另一种还原剂的参与，该研究为了解地幔中碳的赋存形式提供了重要依据。