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This amphibolite-facies micaschist features a stunning staurolite sigma porphyroclast centered on an older garnet core. The characteristic sigma geometry proves that the rate of marginal recrystallization was higher than the clast’s rotation speed during deformation. Within this two-mica pelitic system, these rigid clasts often work in tandem with the surrounding mica foliation to develop S-C planes, providing a clear kinematic indicator of the ductile shear sense during the metamorphism.

Found this Allanite (red-brown at PPL) crystal inside an Epidote crystal today. Both are part of Epidote supergroup, but Ep is basically Ca and Al, while Aln swaps out some Ca atoms for LREE ones (Ce, La, Nd, Y).

Aln is famous for being packed with LREE. It’s actually used as an ore to extract elements like cerium and neodymium, which are super important for permanent magnets, batteries, catalysts, etc. Though it’s typically extracted from highly fractionated magmatic rocks like pegmatites or skarns. However, it also plays an important role in metamorphism, because Aln in subducted rocks like this orthogneiss acts as a LREE reservoir. When it melts or breaks down at great depths, it releases its trace elements to metasomatize and enrich the overlying mantle wedge, giving arc magmas their distinct geochemical signature.

It also acts as a geochronometer because of its radioactive elements (specifically U and Th) in its structure. Those halos at PPL are sign of the destruction of the crystal lattice caused by the radioactive decay.

The rock is an orthogneiss from the Serie dei Laghi in the Western Alps