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Can any one help identify this rock? Found in the UK, found in a common garden stone mix. Hard to drill but breaks with a hammer easily. I'm assuming it's fairly common but can't identify it.

Thanks!

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tl;dr

It's a gneiss: a form of metamorphic rock. Its composition is roughly granitic.

A gneiss is a foliated metamorphic rock: a rock that changed its mineralogical composition after being subjected to high pressures and temperatures (several thousands of atmospheres and several hundreds of degrees Celsius). "Foliated" means that it has minerals that like to orient themselves in a certain direction. Your black streaks are this mineral - called biotite.

The bulk of the rock is pink and grey - feldspars and quartz. Those are the minerals you find in granite, so that's why it's roughly granitic composition. I do not know if it actually was a granite before being metamorphosed, but I have a feeling that it did for reasons beyond the scope of this answer.

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  • $\begingroup$ Indeed, the UK is made up of mostly glacial erratics, that rock has glacial and river weathering shape, and metamorphic flow striation bands. I live in a place with very typical Gneiss, the bands are much more pronounced, and the rock surface would be a bit more zebra type, it's definitely a similar to gneiss without as much banding as most gneis, so it's also standard felsic quartzic metamorphic rock that has been made by slow melting of older rock that is now near impossible to identify. $\endgroup$ – com.prehensible Sep 19 '17 at 4:18
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It looks more like a quartzite than a gneiss to me. the grains do not show the intergrowth that you would expect from a gneiss and the rock is breaking around the grains rather than through them indicating a lower grade (lower temperature/pressure) metamorphism than needed to form gneiss. The layers are likely relics of the original bedding.

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  • $\begingroup$ Good point, it can be metamorphosed sandstone, the images of quartzite look a bit different though. Gneiss very often has a more chalky and less grainy feel, more striations, less straight striations, the size of the grains on top do indicate that it has undergone heavy metamorphism and banded metamorphic can be related to gneiss, however the grain structure is a very particular and common type which should play a major role in it's description, i would be happy if someone gave a description of that felsic crystalization habit because it's very common. $\endgroup$ – com.prehensible Sep 19 '17 at 4:45
  • $\begingroup$ I like this answer more than mine actually. A "gneissic-like quartzite" would be spot on I reckon. $\endgroup$ – Gimelist Sep 19 '17 at 22:49

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