Mineral/fossil...or Both?

[goldenorb]

Hi all,

I would love to know what this is. It is a 'rock' I found in my wife's garden and it was dug out among many fossilized pieces of barnacles, shells, and other such coral pieces native to this area, sw fla. But I noticed the unusual array of crystal-like structures on this piece and wondered if it is an ancient coral formation or really some type of mineral that created these strange shaped 'towers'. Any help would be appreciated. Thanks in advance, the pics enlarges for more detail

Gino

ps: My first post, hello to all. I've always loved collecting fossils and rocks, and am glad I found this forum tonight, and NOT just for the ID

[fig rocks]

Hi Gino, welcome to the forum from Alberta, Canada! :coldb: Someone here should be able to tell you what that is.

[TourmalineGuy]

It's definitely a mineral specimen. It looks like Calcite to me, but its hard to tell without more information about it. Do you know how to do a hardness test on it?

[goldenorb]

Yes, I do. Thanks, didn't think about that Tourmaline. But then what, will that isolate what it is, or just place it in a certain class that other rocks lie within on the MOHS scale? I do use the MOHS scale with it, right? (Am artist so know this way)

Gino

[Nicholas]

I'm not quite sure, but it is a beautiful mineral specimen.

[tracer]

i wouldn't be surprised to be informed that it's calcitic crystals which grew on old coral.

[TourmalineGuy]

yeah, mohs hardness test. It isn't completely diagnostic, but it is a great start at narrowing down different minerals.

Try scratching with your fingernail, then a penny, then a knife blade, finally a steel file. Those are some simple ones you can try. Once something scratches the surface, no need to go higher in hardness.

Fingernail is roughly 2. (I'm guessing its harder than this because it survived the garden in nice shape)

Penny is about 3.5

Knife blade (or glass plate) is 5.5

Steel file is 7.

Calcite has a hardness of 3, so if it is calcite, then the penny should scratch it and the fingernail won't. Here's a pretty good reference

http://www.rockroost.com/Mohs-hardness-scale-tips.shtml

One note: often mohs hardness is misread, because sometimes the object doing the scratching is actually breaking apart and leaves a fine powder than can appear like a scratch. Make sure to blow clean the "scratch" and checking with a magnifying glass always helps to make sure it was actually scratched.

Let us know, and we can go from there!

[goldenorb]

*Tracer, I was thinking that too, but am too ignorant to say for sure. Am just learning about rocks this past week, (on a more serious level, such as ID'ing) although I've worked with many types through the years. Figured coming to the experts was a more viable approach.

*Tourmaline, it's dark outside (which you would know, being in Sarasota, I'm just below you)in her garden (where the stone is) so will try the test on it in morning.

*Thank you for the warm welcome Fig Rocks. Yes, I thought this forum would contain knowledgeable people.

*And Thanks Admin/Nicholas, I too thought the piece was very unusual, and worthy of finding out more about it. Sadly, as you can see in the photos, of couple of the 'towers' were broken off when I found it. I think I may go back to the area where I (originally) found it and see if the lost pieces may be near....

Gino

Edited December 1, 2009 by goldenorb

[tracer]

calcite is a fairly stable polymorph of the same material the barnacles, shells and other stuff you referenced are formed from. the area sounds like it there may have been substantial sedimentary calcium carbonate deposited at some time in the past, and evaporitic conditions and voids in sediment could have resulted in the type of crystalline growth you've discovered. (did that sound plausible, ya'll? somebody tell me if that was ok?!)

[goldenorb]

It does make sense to me Tracer. And one of the most puzzling things that has had me wondering was HOW those shapes were formed, and I believe you just answered that question. Thank you VERY much,

Gino

[Coco]

Hi,

I'm sure it is calcite. We find it on this form in South of Paris, France.

The calcite is attacked(affected) by all the acids, and it has a clear cleavage (when it breaks, it is always in the same axes).

Coco

[Shamalama]

99% sure it's Calcite, the crystal shape, color, cleavage and environment found in all look to fit. It's a nice piece too, will it clean up any more? Welcome to the forum!

[kknight]

Kind of looks like dolomite too. Similar to calcite but it has some magnesium mixed in atomically. (CaMg(CO3)2) An acid test will help tell the difference. If it effervesces easily it is calcite. Dolomite does not effervesce as easily.

[goldenorb]

Is the acid used oxalic, like artists used to use with their paints?

I've just read on a quartz site how this is used to clean the iron streaks off of the crystals?

[tracer]

Is the acid used oxalic, like artists used to use with their paints?

I've just read on a quartz site how this is used to clean the iron streaks off of the crystals?

don't use oxalic acid without reading up on the safety hazards. and no, don't use it for this test. link

[Bill]

Vinegar will do. Break a very tiny but clean piece off the back and drop it in vinegar, If it fizzes it's calcite. Or drip a tiny amount of vinegar on a clean area, you may need a magnifying glass for the second option.

[Frank Menser]

Calcite crystals No doubt. Saw lots of that stuff when I lived there. It can be a bit confusing as it can occur as anything from that shade to a clear rootbear color. Occasionaly it will form psuedomorphs of Dolomite. Here is a piece from Broward County.

[Boo]

I found this rock by Ten Sleep, Wyoming -- wondered if it was agatized coral but think it was formed in ancient geothermal area or pockets deep in the earth in such a region.