Dominé Florent, Lauzier Thomas, Cabanes Axel, Legagneux Loïc, Kuhs Werner F, Techmer Kirsten, Heinrichs Till
CNRS, Laboratoire de Glaciologie et Géophysique de l'Environnement, 38402 Saint Martin d'Hères, cedex, France.
Microsc Res Tech. 2003 Sep 1;62(1):33-48. doi: 10.1002/jemt.10384.
Current theories of snow metamorphism indicate that sublimating snow crystals have rounded shapes, while growing crystals have shapes that depend on growth rates. At slow growth rates, crystals are rounded. At moderate rates, they have flat faces with rounded edges. At fast growth rates, crystals have flat faces with sharp edges, and they have hollow faces at very fast growth rates. The main growth/sublimation mechanism is thought to be by the homogeneous nucleation of new layers at or near crystal edges. It was also suggested that the equilibrium shape of snow crystals would be temperature dependent: rounded above -10.5 degrees C, and faceted below. To test these paradigms, we have performed SEM investigations of snow samples having undergone metamorphism under natural conditions, and of snow samples subjected to isothermal metamorphism at -4 degrees and -15 degrees C in the laboratory. In general, current theories predicting crystal shapes as a function of growth rates, and of whether crystals are growing or sublimating, are verified. However, the transition in equilibrium shapes from rounded to faceted at -10.5 degrees C is not observed in our isothermal experiments that reveal a predominance of rounded shapes after more than a month of metamorphism at -4 and -15 degrees C. Some small crystals with flat faces that also have sharp angles at -15 degrees C, are observed in our isothermal experiments. These faces are newly formed, and contradict current theory. Several hypotheses are proposed to explain their occurrence. One is that they are due to sublimation at emerging dislocations.
当前的雪变质理论表明，正在升华的雪晶呈圆形，而正在生长的雪晶形状则取决于生长速度。在生长速度较慢时，晶体呈圆形。在中等生长速度下，晶体有带圆边的平面。在快速生长速度下，晶体有带尖锐边缘的平面，而在非常快速的生长速度下，晶体有中空的面。主要的生长/升华机制被认为是通过在晶体边缘或其附近新层的均匀成核。还有人提出，雪晶的平衡形状将取决于温度：在-10.5摄氏度以上呈圆形，在其以下呈多面形。为了验证这些范式，我们对在自然条件下经历过变质作用的雪样，以及在实验室中于-4摄氏度和-15摄氏度下进行等温变质作用的雪样进行了扫描电子显微镜研究。总体而言，预测晶体形状作为生长速度以及晶体是在生长还是升华的函数的当前理论得到了验证。然而，在我们的等温实验中未观察到在-10.5摄氏度时平衡形状从圆形到多面形的转变，该实验表明在-4摄氏度和-15摄氏度下经过一个多月的变质作用后，圆形形状占主导。在我们的等温实验中观察到一些在-15摄氏度时具有平面且也有尖锐角的小晶体。这些面是新形成的，与当前理论相矛盾。提出了几个假说来解释它们的出现。一种假说是它们是由于位错出现时的升华所致。
