Back in 2010 I found a Devonian brachiopod fossil with a fibrous or needlelike mineral growth in it. I was trying to determine what this was. Last week while touring the Gallery of Natural History section of the Arizona Historical Society Museum at Papago Park (1300 N. College Ave, Tempe Arizona 85281) I saw a calcite specimen (AHS-NH#11396) from Cave in the Rock, Hardin County Illinois, USA. It appears to be composed of calcite fibers. This formation type seems similar to the one I saw in the brachiopod. So maybe this type of calcite is what formed in that Devonian brachiopod. Images of museum specimen taken October 2014.
Here is an image of the Devonian brachiopod I found in Clark County, Indiana, USA.
See the rest of the pictures and more discussion of this specimen at my blog post here:
http://louisvillefossils.blogspot.com/2010/12/fibrous-or-needlelike-mineral-growth-in.html