FMF - Friends of Minerals Forum, discussion and message board

[lidaasteria]

We've seen calcite that has oil in it (thus turning it brown/black) but nothing quite like we found yesterday.

It was a large limestone boulder with a small black cavity in it (about the size of a pea). Wanted to investigate, so we took the sledge hammer to it. When we broke it open it had a cavity (pocket, vug) about the size of a football inside it (cylindrical shaped, almost like a large bone fossil?)...it was holding black oily crystals.

Most of the crystals were small (crusting) but some were quite large. I've attached a couple pics of one of the large ones I pulled out. I used a soil-pipe cutter to trim it down to a manageable, display size. Also, cleaned the worst of the oil off with some mineral spirits and a toothbrush.

I know it's definitely dogtooth calcite, via an acid test, but I've got a couple questions about it.

First off, is it rare? Personally I haven't seen anything like it, but that's not saying much.

What causes this to happen?

[Susan Robinson]

The calcite does not have the "dog tooth" habit, which is a very pointed, sharp scalenohedron, and your crystals are more rounded in shape. The oil is probably a natural occurrence in the midwest Mississippi Valley Type deposits in sedimentary rocks. Gasoline, mineral spirits, will easily take the oil off of the surface of the crystals.

[lidaasteria]

Thanks for pointing that out...you're definitely right. The picture below is the dogtooth we usually find.

[lidaasteria]

I've got another question....Is this what is referred to as "black calcite"? Or is that actually calcite that formed with some other trace mineral?

[Darryl]

I don't know what is usually referred to as "black calcite" but I have seen calcite crystals with coatings and or inclusions of manganese or manganese oxides that are black. I don't have any photos at hand but do have some samples at home and if I remember to bring them back to the work trailer on the next job, I will see if I can get some photos to submit.

Wish I could offer more information on the nature of the manganese but what I stated is all I know.

[Roger Warin]

Hello,
Organic products are not exceptional in the rocks, even these phases are incompatible. If their molecular weight is low, they are soluble in gasoline. But they can often be unsaturated polyaromatic derivatives and limits of graphenes.
Take the test of combustion on a small sample size.
Roger.

[Pete Richards]

[lidaasteria]

After soaking, and scrubbing in both mineral spirits and gasoline, most of the black coloration remains. I'm guessing the hydrocarbons are actually inside the crystals, as opposed to just filling cracks and such.

Pretty interesting stuff:)

Thanks for all of the information...Learning a little more every day.