FMF - Friends of Minerals Forum, discussion and message board

[John Hodgson]

I am hoping that someone may have information on how this specimen formed or what it was formed on. The only thing I know about the location is that it is from Hunan China. The layer of marcasite covers the entire bottom of the specimen and is very uniform in thickness. It also is not flat and in one spot is about 2" deep. Thank you.

[Matt_Zukowski]

You asked how this formed. One way to get this specimen is as follows. Consider a situation where there is a hole or void space (we call this a vug) deep in the ground. This hole could have opened because of ground movement or dissolution of whatever was there before. This hole is filled with fluid which originally precipitated marcasite (that is why the marcasite is there) but then stopped. Then, for whatever reason, the fluid chemistry in the hole changed so that it became supersaturated with respect to calcite. We see nice calcite crystals because they were able to grow into the hole unimpeded. Then, before the calcite completely filled the hole, calcite precipitation stopped, and we are left with what you have today.

[Tobi]

And by the way, this is a really really good specimen!

[John Hodgson]

Thanks for the reply. The marcasite has a crystalline surface. Does this mean the marcasite formed with a void on both sides and then the calcite formed on top of the marcasite? Thank you.

[James Catmur]

The Marcasite may have formed on another mineral that has since been dissolved. So the face you see may be a cast of the mineral that is no longer there. Look very closely at at the faces and see if they are typical of Marcasite or of another mineral.

[Bob Morgan]

James' hypothesis is entirely possible, and the small marcasite crystals would not be altered to other shapes at contact. Particularly if the other mineral were calcite the interaction at contact surfaces might develop dissolution channels that enable normal crystal shapes of the small marcasites. This happens with carrollite contact surfaces with calcite.

Another thought: I've collected a calcite with crystals on the bottom from being developed in a pool and layered stalagmitic structure on top from drip action. Could something like that be involved even if with different minerals?

I'm no chemist so couldn't come up with deposit conditions in solution that might contribute to something like this. The two minerals are composed of entirely different atoms.