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GeneralMn w/o Fe

24th Jan 2013 17:46 UTCD Mike Reinke

Since it is near zero out, and won't be warm anytime soon, I'm reading, in this case, 'The Geochemistry of Iron', trying to, anyway. Most of it is over my head, but a few points of note. A paper in it from 1956 mentions how, in igneous rocks Fe and Mn are almost always found together, but that why Fe is not deposited when Mn is a 'puzzle in the geochemistry of Mn.' (I know this info isn't recent, but you take your chances ordering books from other libraries.) Has this puzzle been solved since then? Thanks.

24th Jan 2013 18:28 UTCRichard Gunter Expert

Hi Mike:

Not quite as cold here on the west coast. When the book mentions igneous rocks do they include a section on topaz rhyolites? The Fe-Mn mobility in these can vary between Ruby Mountain, Colorado where the Spessartine-Almandine has 29.5% Mn and the Bixbyite rhyolites of Utah where there are; an Mn-Fe oxide (Bixbyite) phase, an Fe-Ti oxide (Pseudobrookite) phase and an Fe oxide (Hematite, some with Ti) phase co-existing. The Mexican topaz rhyolites have nether mobile Fe or Mn and the oxide phase is often Rutile. More Fe-rich igneous rocks can be quite complex in their Fe-Ti mobility but few have extensive Mn.

Hope this helps a little.

24th Jan 2013 19:24 UTCD Mike Reinke

Richard,

Yeah, there are maybe 3 days a year or weather north of Chicago is 'better' than yours!

This book is a collection of papers. One paper, in the chapter on the deposition of iron made the observation above, and it made me wonder how we have Mn silicates at all, that aren't all tainted w/ iron, since it is everywhere. Them rhodonites and others can be purdy!

So the book hasn't mentioned rhyolite. At least not yet. Some of these chemistry books are too deep overall, but I glean from the introductions and conclusions and the easier chapters a few tidbits. Though it is 'old' it is still 60$ on amazon, used. I Iike libraries!

In the Mexican rhyolites, what is the iron locked up with, since it is not mobile? (If i understand you correctly.)

24th Jan 2013 19:59 UTCRichard Gunter Expert

Hi Mike:

It is sunny and 5 degrees C here. We may get rain but the sunny days in winter make up for it.

From the samples I have seen the original Fe-Ti oxides in the rhyolite from the San Luis Potosi area were still embedded in the original, rather silicic, igneous rock and were not mobilized into the gas cavities. It may be timing mechanism. These rhyolite domes are small and probably cooled quickly, though mobile Rutile is not common. An interesting texture from these rhyolites is the coating of the Rutile needles, but not the Topaz, in Opal or glass. I cannot photograph small enough to post the coated Rutile in detail but almost all of the Ruilte needles on my posted matrix Topaz are coated.

As far as Mn silicates are concerned I often wonder how Franklin/Sterling Hill managed to separate the Fe from everything else. The contamination of Rhodonite is less difficult to explain that iron-free Sphalerite with unreacted inclusions of Franklinite. I would have expected at least a small halo of Fe-rich black Sphalerite. The massive Rhodonite from Rivers Inlet in my post has no iron but it is a metasediment, where separation of Fe and Mn can be complete.

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