#foliation

 The different minerals are programmed to have different strengths; the dark minerals are 15% weaker than the white minerals, making these dark minerals the ones that will define the foliation. Those weaker minerals are typically thought to be micas, which have flat shapes due to one major cleavage direction. This is a simulation of a specific type of foliation called an S-C foliation. In this type of foliation, there are 2 different directions to the foliation defined by the two different types of minerals. The weak minerals form a foliation direction that is parallel to the shear directions, while the stronger minerals are elongated at an angle to them. That texture can be hard to pick up even in this image, but in still frames you can often find it. You can also see as the animation proceeds how the minerals are rotated by the shear, and new foliations develop as new minerals are stretched out. This video comes from a paper where they are studying the formation of a second foliation direction, at an angle to the original one. They find that it only forms under certain conditions of mineral strength and of external shear stress, which highlights the importance of having the exact correct conditions for development of that texture. -JBB Video Credit and Original Paper: https://doi.org/10.1016/j.jsg.2020.104091 S-C Fabrics Reference: https://www.alexstrekeisen.it/english/meta/fabricsc.php

Garnets like these are found in metamorphic rocks. They are produced when the rocks are being strained and sheared. Previously, it was thought that since the garnets show a spiral, the garnets themselves must be rotating; the garnet nucleated at a point and the mineral spun around, overgrowing the surrounding grains as it enlarged. However, that growth model makes a prediction; the orientation of the minerals trapped inside the crystal should be gradually changing as the garnet spins. But, it turns out, a number of papers have measured these inclusions and found that distinct orientations of minerals or foliations survive and are trapped inside the garnet; in other words, it is not the garnet that is rotating, it’s the stresses around the garnet. A recent paper led by Dr. Aerden at the university of Granada in Spain supported this argument by studying a number of spiral garnets from the Alps by taking X-Rays of all the observed grains inside large 3-d crystal structures and found that consistently, across many grains in the same sample, they could identify specific foliation directions still preserved in the garnets. Finding these directions across multiple different grains supports the idea that these garnets aren’t rolling; instead the stresses around them are changing direction, and the garnet grows in a spiral in response. These crystals, therefore, aren’t produced by spinning the crystal, they’re produced by a specific stress regime spinning around the crystal as it grows. Never would have thought that before, might even have taught this wrong previously. Cool. -JBB Image credit: Yardley et al. (1990) Atlas of Metamorphic Rocks and their Textures. Grain is ~ 5 mm in diameter. Original paper: https://doi.org/10.1016/j.jsg.2020.104054