calcite nodule (?)

stdenis_jd · May 22, 2015

Pretty sure these are yellow Calcite crystals, but what do y'all think the white parts are? I thought quartz of course, but it is only slightly harder than the calcite. Harder than copper, but softer than steel and much softer than quartz. Probably just calcium deposits that didn't crystallize. The rest I'm sure is limestone. Didn't feel so bad about not finding anything decent in the gravel pit when I found this and cracked it open :-)

stdenis_jd · May 23, 2015

I just realized this has an outside chance at being yellow fluorite as well which would be pretty cool. I must say, my scratch test could actually be inconclusive since the specimen is somewhat brittle and when I scratched it with a penny, it crumbled/fractured and didn't leave a definitive groove. Since its a nodule, not a geode, the crystals were unable to grow to termination so they're not cuboidal but are more flat and rhomboidal in shape. They don't Peel or flake like mica, but they have a similar thinness (but relatively thicker) when separated. One small crystal, roughly 7mm long and about 2mm wide broke off and was translucent and irregularly shaped but did have facets. What other way can I test for fluorite? Fluorescent light?

I'm kind of an amateur so any help would be appreciated :-) closer photos can be provided too if it helps.

DDancer · May 25, 2015

I think your probably correct in the first post on it being Calcite. Fluorite has similar fracturing though its somewhat harder than calcite. UV may work as calcite tends to glow green and yellow. A little bit of acid, maybe vinegar or some battery acid, will bubble on calcite.
You scored a really neat piece there for taking a crack at an unassuming rock

If you can some good close ups of the yellow crystals and the white body may be of help.

stdenis_jd · May 25, 2015

Thx for the reply :-) white vinegar effervesces on it but with very small bubbles and not a strong reaction. It was enough to lift off a small layer of crystal though. Hope this pics help a little: