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Interesting crystal
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Joined: 11 May 2017
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Location: BC

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PostPosted: May 11, 2017 23:03    Post subject: Interesting crystal  

Greetings all,
As a newbie to minerology I don't really know where to start with this. I got the Audubon Society Field Guide to Rocks and Minerals from the library but was unsuccessful in identifying this specimen. When I tried googling some of the minerals they result in such a wide range of images that I'm not sure how to even approach identifying specimens. I took some pictures of one my rocks. Could you help me identify it?

I took these pictures at different angles. On one of the pictures you can see what I'm guessing are some pyrite flecks. I tried what I could to test the hardness. I was able to scratch copper but not glass or quartz. I found this in what was probably someone's discarded rock collection that we found on a property that we were renting so there's no telling where it came from.

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Volkmar Stingl

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PostPosted: May 11, 2017 23:26    Post subject: Re: Intersting crystal  

This is calcite.
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PostPosted: May 12, 2017 00:27    Post subject: Re: Intersting crystal  

If you want to confirm that it is calcite, and hopefully have some fun with some experiments, you can test the rock for hardness and solubility in an acid. For a hardness test you can try to scratch the rock with your fingernail (hardness 2.5), a copper penny (hardness 3), and a nail (hardness 5.5). Calcite has a harness of 3 so if you can't scratch it with your fingernail, find it difficult to scratch with a penny, and can scratch it with a nail, then you have material about the hardness of calcite.

Calcite is a carbonate that is particularly reactive with an acid. The best acid to use for this test is dilute HCl or muriatic acid (~10%). If you put a bit of calcite in this acid it will "fizz" readily. You can use a weaker acid called acetic acid (which is what vinegar is) to test calcite, but because it is a weaker acid you may need to crush a small bit of powder from your specimen to test (crushing increases the surface area of the material which helps speed the reaction) and/or heat the vinegar (hot vinegar reacts more vigorously than cold vinegar). Geologists carry around a small bottle of dilute HCl with them to test for calcite (and some other carbonates) when out in the field. Tums antacid tablets are made out of crushed calcite - they react with (neutralize) acid in your gut which helps with indigestion.
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