Sunday, December 19, 2010

Fibrous or Needlelike Mineral Growth in Orthospirifer

While sorting Devonian brachiopod specimens for upcoming posts on beekite and internal coral growths I came across this oddity.  It appears to be a needlelike or fibrous mineral that grew inside the fossil. 

Looking at the list of Indiana minerals that are needlelike are: aragonite (calcium carbonate), strontianite (strontium carbonate --RARE), and geotite (iron oxide in water).  Indiana minerals that are fibrous are gypsum (calcium sulphate in water).  So I think it is either gypsum or aragonite.  Any suggestions as to what mineral this might be?

The material does not respond to long or short wave UV.

This Orthospirifer brachiopod was found in the Jeffersonville Limestone of Clark County, Indiana.

Additional photographs show specimen plus it magnified 20x with a loupe, 40x & 100x with a microscope.


Anonymous said...

Let me try this again...

The xls are probably not calcite(I haven't seen acicular calcite???). Other minerals to consider are some sulfates- barite(BaSO₄) and less common celestite(SrSO₄), and the carbonate cerrusite(PbCO).

Anonymous said...

My first guess would be gypsum or aragonite. Aragonite will dissolve with effervescence in vinegar and very rapidly in hydrochloric acid (HCl). If you have HCl (a 10% solution is lots) and a watch glass, powder a small bit of the crystals and add a few drops of HCl in the watch glass. Heat the HCl to boiling on a hotplate, then let it cool slowly. When the HCl evaporates, gypsum will leave a ring of microscopic, acicular crystals on the watch glass.

If these tests are negative, then it may be something more exotic; barite has already been suggested, zeolite minerals are another possibility.


Dave said...

Considering your local geology, I would agree with Howard and say Gypsum or Aragonite. The fact that it didn't respond to LW or SW Ultraviolet light is odd though. Try the vinegar approach and see what happens.

Dave said...

The more I look at the crystals, the more I'm leaning towards Gypsum. Celestite and Strontianite are more regularly arranged, even when fibrous. Does it feel furry or spiky when you run your finger over the stuff? Gypsum is flexible, Aragonite is less so.

Kentuckiana Mike said...

Thanks all for the feedback.

I will try to have the material identified soon.

I did not feel the crystal material but once seeing it on the glass slide, it reminded me of fiberglass insulation residue.