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Generalcolors in silicified wood at Petrified Forest, AZ

16th Jul 2016 19:12 UTCNorman King 🌟 Expert

Yellow and orange goethite and red iron oxide are “everywhere” in the silicified wood at Petrified Forest National Park. Many of the tough, silicified logs are covered by a rim of punky, brown “limonite” (probably goethite), seemingly replacing the bark of the trees. I noted that one can specify the occurrence of generic manganese oxides in Mindat, which can be useful because manganese oxides tend to look alike, and few analyses have been run to determine their precise identity, especially when the brownish to blackish material is only in the matrix of a specimen rather than being the object of interest. In contrast, I found that I cannot mention generic iron oxides in the list of minerals for the Petrified Forest locality, apparently because Mindat has no such mineral in the files. Have I missed something?

In view of the pervasive presence of iron oxides at Petrified Forest, and the importance of them for the appearance of the petrified wood, it is a shame not to be able to mention them (I tried!). Shouldn’t there be such category (“iron oxides”) analogous to “manganese oxides” in Mindat? Incidentally, purple and blue colors of silicified wood at Petrified Forest are reported to be caused by presence of manganese dioxide (e.g., scienceviews.com/parks/woodcolors.html), although other studies, including primary sources (e.g., G. E. Mustoe, 2016, Origin of Petrified Wood Color: Geosciences 6[25]:1-24), indicate the situation is more complicated than that, with iron oxides in varied states of oxidation capable of producing any colors commonly seen at Petrified Forest.

16th Jul 2016 19:21 UTCDavid Von Bargen Manager

Limonite.

16th Jul 2016 20:20 UTCAlfredo Petrov Manager

I agree with David. "Limonite" covers any fine-grained (not macrocrystalline), yellow to brown or reddish iron oxides/oxyhydroxides of unknown species or mixture of species. And we often forget that other crap is often present in so-called "iron oxides" too, including sulphates of the jarosite group, silicates like hisingerite, arsenates.... So iron "oxides" are often not entirely composed of oxides.

16th Jul 2016 20:51 UTCSteven Kuitems Expert

Norman, in your adventures at the Petrified Forest did you get a chance to see any of the logs with amethyst crystals in them??

They are only occasionally seen now as early on many of these logs were blown open to try and harvest the amethyst crystals... before the Brazilian deposits were available. Think of how much was destroyed in this pursuit, just another influence of the Fe in these logs to color quartz.

Steve.

16th Jul 2016 21:57 UTCNorman King 🌟 Expert

Well, I figured "limonite" refers to a hydrated iron oxide, and it has always suggested something yellow or brown to me. In my inquiry I meant to use the term "hematite" for the red iron oxide.

From Mindat's limonite page: "Currently used for unidentified massive hydroxides and oxides of iron, with no visible crystals, and a yellow-brown streak. 'Limonite' is most commonly the mineral species goethite, but can also consist of varying proportions of maghemite, hematite, lepidocrocite, hisingerite, pitticite, jarosite group species, etc." To me, this meant that the streak is yellow to yellow-brown, so that accounts for the yellows and oranges in silicified wood. In sedimentary rocks, finely-divided hematite is red, grading to purplish often due simply to increase in grain size, so hematite accounts for those colors (although purple can have different origins). Is "limonite" the best we can do? I would expect some people to complain if I referred to "red limonite."

16th Jul 2016 22:00 UTCNorman King 🌟 Expert

Steven,

Interesting comments. No, I didn't notice any amethyst, but didn't spend any time looking for it. I did see many surfaces covered by of minute quartz crystals, and who knows (not me!) if any of those were amethyst. Sorry.

16th Jul 2016 22:55 UTCAlfredo Petrov Manager

Norman, is the "iron oxide" term you're looking for just for the purpose of adding to species lists on locality pages? In that case why not add both "limonite" and "hematite" if you want to account for both yellow and red colours? (I'm not completely certain that all red colors in oxide-rich material are due to hematite, but I guess most of it is.) Why add a single generic "iron oxides" term to a locality if you already know that both types are present?

16th Jul 2016 23:24 UTCPaul De Bondt Manager

Personally I would not add the iron oxides as they are part of the petrification processus.

Then we should also include vanadium, manganese and chromium oxide for other colours which could get messy.

Cheers.

Paul.

17th Jul 2016 02:43 UTCNorman King 🌟 Expert

Alfredo,

Good questions. I looked at the Petrified Forest locality pages again, and noted there is virtually no geological information. I clicked the external link and found almost nothing about the mineralogy and other aspects of the geology.

Aside from that, I would be happy just to have iron oxides listed (separate listings, as you suggested, would be good). But the page needs more information, so the oxides listed will mean something. This text appears on the Apache County locality page: “No description has been added for this locality. Can you add one?” OK, I will put “add more information to the Petrified Forest locality” on my list of things to do (which is now a few years long).

For now, I will point out that there are two localities in Mindat for Petrified Forest National Park, one for Apache County and one for Navajo County. These are: mindat.org/loc-52632.html (Apache Co.) and mindat.org/loc-52633.html (Navajo Co.). Before I added some locality photos a few days ago, the locality for Apache County was listed as Petrified Forest National Monument, but I changed that (to National Park). Today it occurs to me that those two localities actually should be merged into one. Can’t the county be changed to “Navajo and Apache Counties”? Then the two mineral lists can be combined–they are short but nevertheless different now. When I was there in early June I observed that the two smaller portions of the park in Navajo County have no significant geological differences with the larger portions of the park that are in Apache County. The two portions in Navajo County are separated from each other by parkland that occupies Apache County alone, but it is a continuous tract of land. I do not know the procedures for doing such a merger.

17th Jul 2016 13:26 UTCDavid Von Bargen Manager

Right now that is the way we handle localities that span two different political entities in the hierarchy. The next iteration of the locality system will be able to handle that case by having a single locality that spans the hierarchy.

17th Jul 2016 23:22 UTCTony Albini

Norman, page 27 of Frank J. Daniel's book, Petrified Wood, has a color chart with the elements that cause the colors. In many cases, several elements may cause the same color so the mineral coloring agent can be different. I bought the book at the Petrified Forest National Park. Hope this helps you.

18th Jul 2016 12:39 UTCRolf Luetcke Expert

Norm,

I just ran into the cross border situation in a cassiterite I was looking at and adding information to my label. The mine sits right on the border between Germany and the Czech Republic and mindat solved this very nicely by putting one country and a / and the other country. This is for the mine name and the locality. I thought it was a good way to do this for an ore deposit that straddles a country border.

Tony,

Very interesting with the different coloring agents that can cause the same color, very good to know.

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