Tomkins Andrew G, Wilson Nicholas C, MacRae Colin, Salek Alan, Field Matthew R, Brand Helen E A, Langendam Andrew D, Stephen Natasha R, Torpy Aaron, Pintér Zsanett, Jennings Lauren A, McCulloch Dougal G
School of Earth, Atmosphere and Environment, Monash University, Melbourne, VIC 3800, Australia.
CSIRO Mineral Resources, Microbeam Laboratory, VIC 3169, Australia.
Proc Natl Acad Sci U S A. 2022 Sep 20;119(38):e2208814119. doi: 10.1073/pnas.2208814119. Epub 2022 Sep 12.
Ureilite meteorites are arguably our only large suite of samples from the mantle of a dwarf planet and typically contain greater abundances of diamond than any known rock. Some also contain lonsdaleite, which may be harder than diamond. Here, we use electron microscopy to map the relative distribution of coexisting lonsdaleite, diamond, and graphite in ureilites. These maps show that lonsdaleite tends to occur as polycrystalline grains, sometimes with distinctive fold morphologies, partially replaced by diamond + graphite in rims and cross-cutting veins. These observations provide strong evidence for how the carbon phases formed in ureilites, which, despite much conjecture and seemingly conflicting observations, has not been resolved. We suggest that lonsdaleite formed by pseudomorphic replacement of primary graphite shapes, facilitated by a supercritical C-H-O-S fluid during rapid decompression and cooling. Diamond + graphite formed after lonsdaleite via ongoing reaction with C-H-O-S gas. This graphite > lonsdaleite > diamond + graphite formation process is akin to industrial chemical vapor deposition but operates at higher pressure (∼1-100 bar) and provides a pathway toward manufacture of shaped lonsdaleite for industrial application. It also provides a unique model for ureilites that can reconcile all conflicting observations relating to diamond formation.
钙长辉长无球粒陨石可以说是我们从矮行星地幔获得的唯一一大组样本，通常比任何已知岩石含有更丰富的钻石。有些还含有六方金刚石，其硬度可能比钻石还高。在这里，我们使用电子显微镜来绘制钙长辉长无球粒陨石中共存的六方金刚石、钻石和石墨的相对分布。这些图谱表明，六方金刚石往往以多晶颗粒的形式出现，有时具有独特的褶皱形态，在边缘和交叉脉中部分被钻石+石墨取代。这些观察结果为钙长辉长无球粒陨石中碳相的形成方式提供了有力证据，尽管有很多推测和看似相互矛盾的观察结果，但这个问题尚未得到解决。我们认为，六方金刚石是由原生石墨形状的假晶置换形成的，在快速减压和冷却过程中，超临界C-H-O-S流体促进了这一过程。钻石+石墨是在六方金刚石形成之后通过与C-H-O-S气体持续反应形成的。这种石墨>六方金刚石>钻石+石墨的形成过程类似于工业化学气相沉积，但在更高的压力（约1-100巴）下进行，并为工业应用中制造成型六方金刚石提供了一条途径。它还为钙长辉长无球粒陨石提供了一个独特的模型，可以调和所有与钻石形成相关的相互矛盾的观察结果。
