Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: Universal_learner on 13/11/2021 07:36:35
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I am programming a mineral guide for geology students for Android devices. It has sheets where I put a picture, name, group, formula, environment of formation and physical-chemical properties of each mineral.
I am classifying the environments of formation from:
-Wenk, H. R., & Bulakh, A. (2016). Minerals: their constitution and origin. Cambridge University Press.
That proposes this classification:
- Endogenic: Magmatic, metamorphic.
- Exogenic: Sedimentary, supergenic.
- Endogenic/Exogenic: Hydrothermal.
I am following this classification. My concern is how to classify minerals as chrysotile, that forms as a weathering result of mafic rocks.
I guess it should be exogenic. It does look that this environment is not supergenic, so I am wondering if I can classify it as sedimentary, as weathering is the first part of sedimentary cycle. So the field would be "Sedimentary: weathering of mafic rocks."
Or I should add a group in the list to include alteration or weathering environment?
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I really got no clue to what you ask.
Honestly, i do not even understand it.
Just wished to Welcome you to the TNS Forum...so Welcome!
Hope you will be real successful & All the Very Best for your App.
Ps - if u figure out the answer to your own question, could you Please post it in here, for the benefit of future readers.
Thanks & cya.
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I would say this is a question of "taxonomy" not "genetics" (rocks don't have genes)
I think the OP is right that minerals as a result of weathering would be an exogenic classification, but I am not sure... I think the best member to answer this would be @Bass
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maybe taxonomic origins?
Your choice of the mineral chrysotile is probably not the best example for your question. Chrysotile is a serpentine mineral, which forms by metamorphism of mafic igneous rocks- which would make it endogenic metamorphic
There are minerals that form at the surface, such as oxides, carbonates, hydroxides, etc. An example are iron oxide and hydroxide minerals (hematite, goethite, limonite) which form due to oxidation of other iron bearing minerals or iron (think rusting nail). These form by supergene processes near the surface of the earth. I would classify these as exogenic supergenic.
Hope that helps?
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@Zer0 Thank you!
I have 180 minerals in my app. Each sheet has a field: environments of formation. @Bass It has multiple environments, not only one. In this field I have for example for chrysotile two environments:
- Metamorphic: Contact and regional of ultramafic rocks (serpentinites).
- ??: Alteration of mafic rocks in surface.
Then I create questions with code for the students as:
- What of this minerals forms in the environment [??] Alteration of mafic rocks in surface?
- a) Pirite
- b) Chrysotile
- c) Enargite
- d) Calcite
@Bass Maybe chrysotile is not a good example. I have other minerals that also form in surface as an alteration/weathering product. Some of them form on soils:
Diaspore
- Sedimentary??: bauxites.
- Metamorphic: low degree of pelitic rocks.
If the mineral is formed in diagenesis I am classifying it as Sedimentary even it is a neoformation mineral from preexisting ones.
As an alteration product the genetic origin of chrysotile is not magmatic, it is an alteration product of magmatic rocks in surface. This applies to many clays and other minerals that are neoformation minerals from preexisting ones in surface rocks or soils as montmorillonite.
They are exogenous, they are not supergenic, so, they would be sedimentary?
@Bass When a rock is weathered in surface, there is not allways an enrichment. I understand supergenic proccess as an enrichment and usually related with ores and gangas formation by interaction of basin or magmatic waters with preexisting rocks. A clay in a soil is not ganga of any ore, it is not an enrichment product, it forms because the preexisting mineral is not stable in the surface conditions.
Might I add a field "Surface alteration" or "Weathering"?
What do you think it would be better for the students?:
Chrysotile
- Supergenic: Alteration of mafic rocks in surface.
- Sedimentary: Alteration of mafic rocks in surface.
- Surface alteration: Alteration of mafic rocks in surface.
- Weathering: Alteration of mafic rocks in surface.
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Just to clarify something, my code, if it choses randomly a sedimentary mineral as the correct, the other three options are minerals that don't form in any sedimentary environment. It is important to have a consistent database so the quiz have sense.
ps I am gonna say the users to sugest modifications of the databse in twitter and facebook, but if you want I can post a few questions when I made the first review of the database. I migth have doubts in some of the 180 minerals. Thank you for taking some time to answer.
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You are most welcome Universal...btw, Nice nickname.
🙂
Good Job Chiral!
👍
Bass..i luv ur signature line..lol
🤭
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I think this is really a matter of semantics, or perhaps language translation?
Supergene processes are simply geologic processes that involve weathering near the surface of the earth- oxidation is the most notable, but enrichment may also be a supergene process. Freedictionary defines supergene processes as "processes of the physical and chemical conversion of mineral matter in the upper parts of the earth’s crust and on its surface through the action of the atmosphere, hydrosphere, and living organisms at low temperatures. They consist in chemical decomposition, solution, hydrolysis, hydration, oxidation, and carbonization"
Most mineral alteration is not caused by supergene processes- much of it is caused by heat, pressure and hydrothermal (hot waters) circulation in the rocks.
The case of bauxite (aluminum ore) is caused by supergene processes: Aluminum does not oxidize easily in surface conditions, so as aluminum rich (especially feldspar rich) rocks chemically weather in the tropics, almost everything besides Fe and Al go into solution and are washed out, leaving a layer enriched in Al.
The case of chrysotile is not caused by supergene processes, and is also not sedimentary. Chrysotile is formed by metamorphism of mafic igneous rocks (mostly oceanic rocks when they get shoved into continents) and is one of main components of serpentine. These rocks form deep underground and eventually are exposed at the surface due to erosion, making chrysotile endogenic, metamorphic.
I applaud you for putting together your classification scheme. I agree that diagenetic minerals (example dolomite) should be classified sedimentary.
I hope this helps- please feel free to contact me about any other minerals.
I think "weathering" is less confusing than "surface alteration", because alteration can be caused by many geologic processes and is not limited to only surface conditions.
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Thanks. I will classify as supergenic every mineral exogenic that is not sedimentary. I come from uploading it to the store. It is published for Android devices as Minerals180.