Hematite after(?) magnetite
Locality unknown
This irregular 21-mm-long specimen of hematite (iron oxide) has mirror-bright faces and skeletal features that are very similar to hematite pseudomorphs replacing magnetite at the Payún Matrú volcano, Mendoza Province, Argentina. The iron oxide there formed as deposits from volcanic fumaroles associated with eruptions dating to about 168,000 years ago, with some activity as recent as 500 years ago.
My problem is that I think the crystals from Payún Matrú volcano only came on the market in the early 2000s, and I’ve had this little crystal since 1971 (for my Indiana geology friends, it belonged to Carl Beck). Also, those crystals tend to have a somewhat rougher surface texture than mine.
Other localities where similar mirror-bright, skeletal crystals of hematite have been found include Mt. Calvario (part of the Etna volcanic complex on Sicily), Lohning quarry at Salzburg, Austria, and an occurrence at Nador, Morocco. As near as I can tell, the Morocco find is from the late 1990s and the Austria locality has been known since perhaps the 1950s, while minerals at Mt. Calvario have been known since the 19th Century. There are also well-known hematite localities in the Buckskin Mountains of west-central Arizona, but I think the skeletal, hoppered habits like my specimen shows are rare from there, and they also appear to be unusual at Mt. Calvario. And of course, since hematite is so common, there are numerous other localities with at least some mirror-bright crystals similar to this one.
Some of the mirror-like faces have complex plumose (feathery) textures, seen in the right image here. Patterns like that are not rare on hematite crystal faces, but I don’t know what they mean, other than to give the cop-out answer that they show patterns of crystal growth.
I wish I knew for sure where this hematite was from, but I don’t think I’ll ever know with 100% certainty. If I had to pick one of the localities listed above, based on photos on MinDat, and what I know about the timing of the finds, I’d very unconfidently choose the one in Austria. Even without knowing the locality it still makes an interesting specimen. And to be honest, hematite can make such a wild variety of forms, I’m not even sure it is a pseudomorph after magnetite. It isn’t magnetic at all, even to a neodymium magnet, so if it is a replacement it is complete. Catalog No. 7A.
My feeling is that this is straight out hematite Richard. The morphology fits (in your photo I think I see hexagonal/trigonal form), the lack of magnetism, and the fact that "most" pseudomorphs of any minerals don't have shiny faces, all point to this.
Ouch! Hard way to learn that it was not genuine. But every janitor knows water is the universal solvent. Nothing really stops water. And especially if there's any acid or alkalinity or volatiles in it, it can do even more crazy stuff. Distilled water may be the least powerful solvent, I don't know. Before I got into minerals, I had no idea what water could do deep inside the Earth.