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519 Views 4 Replies Last post: Jun 10, 2013 6:40 AM by MikeHardman RSS
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Jun 10, 2013 12:11 AM

Can anyone tell me what sort of rocks these are?

These rocks where found in Indiana while leveling a hillside.

There was two but one was dropped and broke.

On the inside of the one that broke, in the middle was the crystal layers.

Was just wondering what kind of rock this is.

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    Jun 10, 2013 12:32 AM (in response to ddhoggatt)
    Re: Can anyone tell me what sort of rocks these are?

    A couple of questions about the pale mineral:

    1. Can you scratch it with your finger nail?

    2. Does it taste salty? (or if you don't fancy licking it, put a small piece in a mug and add boiling water, stir it well, and tell me if it dissolves or not)

     

    And whereabouts in Indiana are you? (town / lake / etc.)

     

    Mike

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        Jun 10, 2013 6:40 AM (in response to ddhoggatt)
        Re: Can anyone tell me what sort of rocks these are?

        Thanks for the answers.

        The pale mineral is gypsum (a hydrated calcium sulphate), in the form of selenite.

        http://en.wikipedia.org/wiki/Gypsum

         

        It has various modes of occurrence, including veins, which may be of greater or lesser extent; in your case it seems 'lesser'. The presence of a localized concentration of gypsum is probably the cause of the overall appearance as a large blob. Essentially it is a concretion.

         

        Concretions form by subtle migration of chemicals through sediment/rock before and during its lithification, resulting in a local concentration; this usually affects the cement (which occurs between the sedimentary particles), and it often uses an anomaly such as a fossil as a nucleus. In your case, I think the concentration of different chemical may have been partly primary (due to gypsum being deposited there during sedimentary processes), but it could also have originated as cracks being filled with the gypsum as they formed. Whatever the origin of the gypsum, it affected the cement in the whole concretion, which has (much more recently) affected the way it weathers - giving it almost onion-like layers (which run across the primary depositional layering).

         

        Although I am not familiar with the geology of Indiana, I note that gypsum is mined in the SW of the state:

        R. R. French, 1966, Geology and Mining of Gypsum in Southwestern Indiana,

        Proc. Indiana Academy of Science, Vol.76

        http://journals.iupui.edu/index.php/ias/article/view/6704

        You can download the PDF.

        That mentions anhydrite as well as gypsum; it is another form of calcium sulphate.

         

        See also thees on concretions:

        - http://en.wikipedia.org/wiki/Concretion

        - http://www.desertusa.com/mag98/oct/papr/geo_conc.html

         

        Mike

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