Granular neoblastic zircon (ZrSiO₄) with systematically oriented granules has been proposed as evidence for extreme shock pressures (>30 GPa) and subsequent high temperatures (>1200 °C). It is widely agreed to reflect the solid-state phase transition from zircon to its high-pressure polymorph reidite and subsequent reversion to zircon. This model is based on crystallographic relationships between granules of a single type of granular zircon and does not explain the formation of other types of granular zircon textures, for example, grains with randomly oriented granules or with large, often euhedral granules. Here we report the first nano-scale observations of granular neoblastic zircon and the surrounding environment. We conducted combined microstructural analyses of zircon in the lithic clast from an impact melt dike of the Vredefort impact structure. Zircon granules have either random or systematic orientation with three mutually orthogonal directions of their c-axes coincident with [110] axes. Each 1-2 μm zircon granule is a mosaic crystal composed of nanocrystalline subunits. Granules contain round inclusions of baddeleyite (monoclinic ZrO₂) and amorphous silica melt. Tetragonal and cubic ZrO₂ also occur as sub-μm-sized inclusions (<50 nm). Filament-like aggregates of nanocrystalline zircon are present as “floating” in the surrounding silicate matrix. They are aligned with each other, apparently serving as the building blocks for the mosaic zircon crystals (granules). Our results indicate shock-related complete melting of zircon with the formation of immiscible silicate and oxide melts. The melts reacted and crystallized rapidly as zircon granules, some of which experienced growth alignment/twinning and parallel growth, causing the characteristic systematic orientation of the granules observed for some of the aggregates. In contrast to the existing model, in which this type of granular zircon is considered to be a product of reversion from the high-pressure polymorph reidite, our nano-scale observations suggest a formation mechanism that does not require phase transition via reidite but is indicative of instant incongruent decomposition, melting and rapid crystallization from the melt.

已经提出具有系统取向颗粒的粒状新胚性锆石（ZrSiO ₄）作为极端冲击压力（> 30 GPa）和随后的高温（> 1200）的证据。 °C）。人们普遍同意可以反映出从锆石到高压多晶型堇青石的固态相变以及随后的锆石还原。该模型基于单一类型的锆石颗粒之间的晶体学关系，并未解释其他类型的锆石纹理的形成，例如，具有随机取向的颗粒或具有大的，通常为正方体的颗粒的晶粒。在这里，我们报告了粒状新胚锆石和周围环境的首次纳米级观测。我们从Vredefort冲击结构的冲击熔岩堤上进行了岩屑中锆石的组合微结构分析。锆石颗粒具有随机或系统取向，其c的三个相互正交的方向-与[110]轴重合的轴。每个1-2μm的锆石颗粒都是由纳米晶亚基组成的镶嵌晶体。颗粒中含有圆形的斜晶石（单斜晶ZrO ₂）和无定形二氧化硅熔体。四方和立方ZrO ₂也以亚微米大小的夹杂物（<50 nm）出现。纳米晶锆石的细丝状聚集体以“漂浮”的形式存在于周围的硅酸盐基质中。它们彼此对齐，显然是镶嵌锆石晶体（颗粒）的基础。我们的结果表明，与冲击有关的锆石完全熔化，而形成不溶混的硅酸盐和氧化物熔体。熔体迅速反应并结晶为锆石颗粒，其中一些经历了生长排列/孪生和平行生长，从而导致观察到某些骨料的颗粒具有特征性的系统取向。与现有模型不同，在现有模型中，这种类型的粒状锆石被认为是从高压多晶型堇青石中还原的产物，通过熔体，但表明熔体立即分解，熔化并快速结晶。