#foliation

[zegalba]

foliation by suze woolf

[tofreezetime]

sieved sun

[earthstory]

Development of Foliation by Shear  A foliation in a rock is a fabric defined by a set of minerals that are either flat or needle-like, which forms while a rock is being stressed and metamorphosed. One way to produce a foliation is by flattening – a force like that found in mountain building that simply squashes everything. Alternatively, if rocks are being sheared, as happens deep beneath fault zones where one plate of rocks is sliding beneath another, there is a rotation component that stretches minerals out in such a way that it can also generate a foliation. This is a video of a simulation showing development of a foliation in a metamorphic rock.

 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

[drrockclub]

Leucogranite

Pleasant to look at outcrop along the road in Alice Springs. A leucogranite (white/bright granite) dykes intruded along foliation plane of the gneiss.

[philoursmars]

L’île de Port-Cros, le parc national où j’allai cet aprèm avec Brigitte et Sylviane : un énorme tamaris ( le tamaris, c’est l’arbre, je précise bien....), d’étranges plis ou peut-être plutôt d’étranges foliations, le village et des roches métamorphiques (micaschistes)

[zegalba]

foliation by suze woolf

[earthstory]

Spiral Garnet

This is the view under a microscope, with cross-polarized light, of one of the coolest features mineralogists can find in a rock; a snowball garnet. The mineral garnet does not transmit light under these settings, so all of the dark material is a single grain of garnet. You can see how this classic crystal example literally spirals around inclusions of a variety of other minerals in this shot. A newly-published paper has given me new insight into how this structure actually occurs.

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

[drrockclub]

Mineral sculpted by rock movement

This pyroxene mineral has been rotated and slightly broken up by sliding of the rock mass. The evidence of movement is preserved by linear patterns in the background, this linear alignment of minerals is called foliation.

Image was taken with petrographic microscope.