Petrified wood

A fast process

Technically, nothing here is a “mineral,” because it is all a consequence of human activity, but less technically, this is petrified wood on a steel railroad spike. The iron in the spike has leached into the surrounding wood (presumably once part of a railroad tie) which is now all mineralized.

Most of the mineralization is various iron oxides and hydroxides: goethite, hematite, magnetite, and the mixtures called limonite and turgite (the latter is a name that is discouraged, since the material is ill-defined), but I’d be willing to bet there are some stranger things in there too, at least at a microcrystalline level. Many obscure iron oxides and hydroxides exist (lepidocrocite, hisingerite, maghemite, for example) and if any other elements were present, all bets are off – dozens of possible minerals could be there. Small crusts of white gypsum coat parts of the surface, along with white to yellow cryptocrystalline quartz (chalcedony), so there’s at least calcium, sulfur, and silica to play with along with the iron and oxygen. Jarosite, hydrated potassium-iron sulfate, is a common precipitate in the contaminated water of Butte’s Berkeley Pit, and would not be surprising in this material as well. Whatever the composition in detail, there is no more wood here – it is all mineral matter, much of which is essentially rust.

The parallel layering probably reflects the original grain of the wood, but it might be possible that it is more an expression of the wave fronts of iron-rich water spreading away from the spike. That iron, in oxide and hydroxide form, replaced the wood.

How long does it take for mineralization, petrification, fossilization, whatever you want to call it, to develop? In this case, probably less than 120 years and possibly as quickly as 50 years or so. This is a spike from Butte, Montana USA, where the first railroad arrived in 1881. It was lying along the old Butte Anaconda & Pacific Railroad bed, and the BA&P did not begin operations until 1894.

The Butte Anaconda & Pacific was the railway that hauled ore from the mines of Butte to the smelter at Anaconda, Montana. I found the spike in 2003 about a half mile from the Orphan Girl Mine, so it might be from the underground rail system, which would perhaps more easily explain the mineralized wood if the tie was under mineral-rich water in the mine, but I think it is from the BA&P, perhaps a result of ephemeral soaking and drying.

Even though it’s a consequence of human activity, this specimen also shows how geologic processes can occur. You can see similar mineralization in progress (without so much iron) at Yellowstone National Park, where dead trees standing in mineralized ponds or shallow groundwater have their lower trunks partially converted to white silica (maybe quartz, maybe chalcedony, maybe (most likely) some form of opal) in just a few years or at most a few decades.

Above: Dead lodgepole pine trees near the parking lot of Fountain Paint Pots in the Lower Geyser Basin of Yellowstone. The dead trees soak up the silica-rich water, turning the lower portion of the trees white and nearly hard as rock in the initial stages of mineralization. Photo by Diane Renkin, Yellowstone National Park (public domain).