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GeneralManganese Nodules

18th Apr 2024 23:51 UTCLalith Aditya Senthil Kumar

Mindat does not have a page for them but does have a locality. Should manganese nodules be considered a rock type?

19th Apr 2024 00:36 UTCEd Clopton 🌟 Expert

"Nodule" is in the Mindat glossary as a mineral habit or shape, but my feeling is that it would be cumbersome and not very useful to define a rock type for each material that can form a nodule: chert, Mn oxides, howlite, turquoise, chalcedony, limonite, etc.

19th Apr 2024 02:34 UTCLalith Aditya Senthil Kumar

I though manganese oxide nodules are sometimes referred to as manganese nodules?

19th Apr 2024 02:53 UTCAlfredo Petrov Manager

They are quite distinctive in the chemistry, conditions of formation and mineralogical composition, so I agree they should be a rock type.

19th Apr 2024 05:13 UTCDalibor Matýsek

Yes. This rock type is called as manganolite. And don't forget the manganese crusts.

19th Apr 2024 15:22 UTCRalph S Bottrill 🌟 Manager

Its a tricky area. We do have concretions under chemical and biochemical rocks, and that classification includes ironstone and lime-clay concretions. It could also include flints, baryte, pyrite, azurite and aragonite concretions etc, which can all be locally large and abundant but don't really form a true mappable rock formation. These things are arguably mineral growths within sediments and soils, more than rocks. Its a bit like stalactites and other cave formations.

19th Apr 2024 15:58 UTCTony Nikischer 🌟 Manager

They are, indeed, "mineral growths" as Ralph points out. We have a number of these concretions from the Clarion-Clipperton fracture zone in the Pacific Ocean, depth about 15,000 feet (!!) The late Pete Dunn analyzed several of these for me many years ago, and they were comprised primarily of todorokite, birnessite and other Mn-rich phases.

So, the term "manganolite" makes sense, and these concretions would fit that term.

20th Apr 2024 11:04 UTCRalph S Bottrill 🌟 Manager

The main trouble with the term manganolite is that it seems to be used very commonly as a synonym for rhodonite, especially in mystic and lapidary circles, despite it not being listed in Mindat. Its referenced in Mohsen Manutchehr-Danai (2000) Dictionary of Gems and Gemology. Springer. It seems to be an anglicised version of manganolith, an old German word for rhodonite, from 1831.

I have also found manganolite used as a synonym of merwinite (in Yue et al 2018).

The original use of manganolite as a rock name seems to be from Wadsworth (1891), as quoted by Holmes (1920) and Kemp (1920), but I cannot find any record of that original reference.

I updated the manganolite page.

20th Apr 2024 11:38 UTCDalibor Matýsek

I don't know that the term "manganolite" was also used as a synonym for rhodonite. I have learned what is stated in our country, for example, in the Encyclopedic Dictionary of Geological Sciences (Prague, 1983, in Czech). There is roughly this: Manganolite is a rock, made up mainly of Mn-minerals (especially Mn oxides), According to Konta (1972) a cementation hardened sediment with a predominance of Mn minerals. The admixture of clay minerals is below 20%, the coarser clastic admixture is below 10%. The term can be specified if the mineral Mn is known.

Konta J. (1972): Quantitative system of residual rocks, sediments and volcaniclastic deposits. Charles University, Prague. (in Czech).

20th Apr 2024 11:54 UTCDalibor Matýsek

The magic of the term manganolite, as I stated above, is that it is a non-genetic term, it is simply a rock composed mostly of Mn minerals. Oceanic manganese concretions and crusts are prime examples.

20th Apr 2024 14:37 UTCRalph S Bottrill 🌟 Manager

Hi Dalibor

I think your description of manganolite is fairly consistent with our main usage in Mindat, but I think specifying the clay and coarser clastic mineral contents may prove difficult. Until we can find the original description or a formal redefinition it may be best to use it for any unmetamorphosed rock dominated by Mn minerals. Rocks dominated by rhodonite and vittinkiite etc may be best descried as metamanganolites.

However any internet search on manganolite seems dominated by pages with rhodonite/vittinkiite rich metamorphic rocks so we need to accept this as an issue.

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