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I'm currently learning how to identify rocks in hand specimen. The initial classification into igneous, metamorphic, and sedimentary is mostly ok, but I'm having a problem with further identifying sedimentary rocks—I don't know how to tell if they're calcareous or siliceous without using HCl, which seems somewhat impractical to use on every sedimentary rock I see. Is there any good way of telling?

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When learning to distinguish between siliceous and calcareous samples you should use a handlens. Look for quartz, feldspars or micas. Quartz clasts may be frosted, or you may be able to see conchoidal fracture. Calcite will be very soft compared to both feldspar and quartz, and should fail to scratch copper (or at least not scratch it very well, while a quartz rich sedimentary hand sample will scratch glass.

For really fine grained stuff, there are times when you may need to use the context of site where the sample is from if you don't have HCL. There can be sedimentary rocks that are made up of siliceous clasts that react with HCL due to calcite in the matrix, so HCL can be misleading. I've even found that some mudstones and limestones can have an organic odor.

Lastly, there are structures that can suggest if a sample is siliceous or calcareous. Styolites would suggest non-siliceous while thermal alterations like liesegang banding would suggest siliceous.

The more samples you tackle, the easier it gets.

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    $\begingroup$ "The more samples you tackle, the easier it gets." -- or, as I've occasionally heard it phrased, the best geologists have seen the most geology. $\endgroup$ – Pont Jun 7 '14 at 15:56
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Two approaches:

  1. Hardness. Which mineral will scratch the glass plate?
  2. General look. Can you see any microfossils?

Use HCL to confirm your intuition.

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Field geologists always carry a small squeeze bottle of dilute hydrochloric acid (HCl) to identify calcareous sedimentary rocks. One drop applied to the specimen is all you need and that amount will not damage your specimen. Muriatic acid is the same as HCl and is available at any hardware store and should be diluted in a 50/50 ratio with water.

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  • $\begingroup$ The education I received during my geology studies was to dilute the acid down to 10:1, water to acid. 50:50 will work, but it's overkill. $\endgroup$ – Rokman Jun 25 at 10:03

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