FMF - Friends of Minerals Forum, discussion and message board

[bugrock]

I recently purchased a marcasite specimen from the Silvane Collection,
labeled Santander, Spain. On the base of this piece is a coarse druse of light
colored crystals. Under magnification these are roughly cube shape
with cloudy white center and clear outer faces. This can best be appreciated under
magnification. I wonder if anyone knows what mineral this is. Perhaps calcite
but I also thought dolomite or fluorite possible. Perhaps this is difficult to answer
without an image but in case someone is very familiar with the location I pose the
question.

Thanks

[Jordi Fabre]

Not Fluorite, but maybe Calcite or Dolomite. Without a photo is hard to tell you.
Calcite from the Reocín Mine, Reocín, Cantabria, Spain, is usually a little bit more colored than Dolomite, but not always. Please take a look on this web page, maybe you will find there some image similar to your specimen: https://mti-cantabria.blogspot.com/2008/04/mina-de-reocn.html

Jordi

[bugrock]

I think Jordi has supplied the link that answers the question. Based on the site suggested the crystals at the base of my marcasite specimen are very much like the dolomite image including the clear outer edges of the crystals.

Thanks

[guppyman]

Marcasite is also known as white pyrite. But, I have heard that it is not real pyrite.

I am not a expert though...

[John S. White]

This probably does not need to be said, but in view of the comment above I will say it anyway - pyrite and marcasite, in spite of having the same chemical composition, are two distinctly different mineral species, they are polymorphs of iron sulfide. Pyrite is isometric (or cubic) and marcasite is orthorhombic. In spite of this, distorted or malformed crystals of either can be difficult to tell apart, but this is relatively rare.