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I was wondering if you have any ideas on what kind of rock this might be? I know that it doesn't effervesce with HCl (no reaction with hydrochloric acid at all) and the size and shape of each grain is <1mm. The texture is smooth. The rock is also scratched by glass and weighs 189.5 g.

enter image description here

Any help would be greatly appreciated. Thanks!

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    $\begingroup$ Please read the guide for asking rock identification questions and edit your question to add as many details as you can. You've made a good start with describing the physical characteristics, but higher-quality photos (with a scale indicator) and details of where you found it would be useful additions. $\endgroup$ – Pont Mar 7 '18 at 19:48
  • $\begingroup$ Possibly rhyolite - an extrusive igneous rock. geology.com/rocks/rhyolite.shtml $\endgroup$ – wanderweeer Mar 8 '18 at 11:00
  • $\begingroup$ @Kurt are you sure it's not Andesite? $\endgroup$ – Eevee Mar 8 '18 at 13:14
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    $\begingroup$ It really depends on the strength of the acid. What is the molarity of acid you used? $\endgroup$ – Eevee Mar 8 '18 at 13:19
  • $\begingroup$ @Eevee - Can't really be sure without knowing the mineral composition. Just hoping to give the OP a starting point for further research. $\endgroup$ – wanderweeer Mar 8 '18 at 23:19
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Your rock is either a granite, a granodiorite, or a tonalite.

If you look carefully, the grey bits are a bit translucent and look waxy. This is quartz. The white stuff sparkles slightly when you look at it in strong light from different angles. This is feldspar. The black stuff is either biotite mica or hornblende amphibole, but it's impossible to tell from the image.

The reason it's one of the three mentioned above is because it is impossible from the image alone to identify which kinds of feldspars are in the rock, but it doesn't really matter. You can just call it a granite and it will be good enough.

The rock is also scratched by glass

I suspect that what you're seeing is the glass being smeared on the rock, and not the rock being scratched by glass. Wipe the mark off, do you still see a scratch? Is the scratch on everything, or only the white stuff?

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Looks like deorite. It’s used around railroad tracks in my area. https://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/diorite.html

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