FMF - Friends of Minerals Forum, discussion and message board

[John Nash]

A question for the crystallographically-inclined. I recently obtained my second large Faraday calcite specimen. The hematite-involved calcite crystals from the Faraday Mine seem quite unique. They are often twinned and form some very complex shapes. There seem to be multiple stages of growth on many. I wonder if anyone could educate me on how/why some of this might actually have happened, particularly the strange bas relief patterns on the left face of the main crystal, the raised, swirling patterns, the twinning, the preferential deposits of hematite, etc. What was different about the Faraday environment that produced these interesting crystals? Any thoughts would be appreciated.

John Nash
Edina, MN

[Pete Richards]

Twinning of the sort shown by these crystals (twinning by reflection across a plane perpendicular to the threef-fold c-axis) is actually the most common form of twinning in calcite. At some localties, virtually every crystal is twinned in this manner. Twinning represents a "mistake" in the growth of the crystal, and usually occurs very early in growth. It is not clear why twinning occurs, though rapid growth and impurities in the environment are often suggested as influences.

These Faraday crystals often have some areas coated wtih hematite or limonite and others that are not coated, and the boundary between the two areas is often sharp, as is the case with this crystal. It is tempting to imagine a crystal half-submerged in water, with the submerged part becoming coated while the upper part does not. I have not seen enough specimens to know if this idea actually makes sense or not....

The wavy surface of the uncoated part suggests dissolution, but the faces on this part actually seems to stick out beyond the parts of the same faces that are coated with hematite, so there would have to have been additional growth on the uncoated parts prior to dissolution. A pretty complex, rather ad hoc hypothesis!

I presume that the hematite coating does not extend under the wavy parts of the crystal, though it is hard to tell from the images.