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Geology, Palaeontology & Archaeology / Re: How to genetically classify a weathering mineral?

« on: 19/11/2021 21:58:26 »

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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Geology, Palaeontology & Archaeology / Re: How to genetically classify a weathering mineral?

« on: 16/11/2021 23:50:15 »

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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Geology, Palaeontology & Archaeology / Re: What kind of rock is this?

« on: 15/09/2021 02:15:50 »

Looks like a goethite concretion. Iron is leached from elsewhere in the rock and commonly forms concretions in sedimentary rock. These may have originally been pyrite concretions.

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Geology, Palaeontology & Archaeology / Re: What makes the sea floor smooth?

« on: 15/09/2021 02:05:43 »

Plus, as the new oceanic crust moves away from the ridge, it also cools. It becomes harder and less buoyant, so it tends to sink.

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Geology, Palaeontology & Archaeology / Re: What is this lumpy rock found at Lyme Regis?

« on: 15/09/2021 01:58:06 »

coprolite??

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Geology, Palaeontology & Archaeology / Re: What produces this strange chipped pattern on the surface of this stone?

« on: 15/10/2020 23:47:12 »

Trumped?  We shall overcomb!
Hard rock suggests limestone- I still vote for coral!

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Geology, Palaeontology & Archaeology / Re: What produces this strange chipped pattern on the surface of this stone?

« on: 12/10/2020 22:03:04 »

The polygon patterns make me think this is organic in origin, possibly fossilized coral?

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Geology, Palaeontology & Archaeology / Re: Is this coloured material a mineral?

« on: 31/03/2020 16:54:53 »

Yes, at ambient temperature. I don't see any evidence for "boiling hot outgassing". It's common for desert varnish to coat just the exposed side of rocks.

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Geology, Palaeontology & Archaeology / Re: Is this coloured material a mineral?

« on: 27/03/2020 20:48:12 »

Desert varnish or desert patina is thin (micrometers thick) layer of iron and manganese oxides and clays that coat the surface of rocks on the surface in arid environments. Commonly red to brown to black.

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Geology, Palaeontology & Archaeology / Re: Is this coloured material a mineral?

« on: 17/03/2020 20:14:19 »

Looks like desert varnish

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Geology, Palaeontology & Archaeology / Re: Metallic quartz: how did nature create it?

« on: 22/12/2019 22:07:45 »

I beg to differ.

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Geology, Palaeontology & Archaeology / Re: Metallic quartz: how did nature create it?

« on: 20/12/2019 01:38:19 »

Just to clarify, most metallic minerals are compounds, usually either sulfides (or more broadly, suldfide, sulfate or sulfosalt) or oxides. Native (elemental) metals do occur. but are much more rare- we've all seen pictures of gold nuggets. Most native metals are soft and easily deformed.
As to why quartz and metallic minerals are often found together- most metals go into solution easily in hydrothermal waters, which also commonly contain dissolved silica (quartz)- both of these tend to precipitate under the same PT conditions. Also, as magmas crystallize, they become more silicic (quartz rich) with "left over junk" (i.e. metals) that don't fit well into the lattices of the solidifying crystals. Often these end up together in the last phases of the cooling igneous rocks (good example is porphyry copper deposits).

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Geology, Palaeontology & Archaeology / Re: Are altered/modified minerals classifeds as new minerals?

« on: 26/11/2019 18:20:03 »

I'm not sure exactly what you are asking. Old minerals are commonly destroyed and new minerals formed during both alteration (where minerals are altered or changed due to hydrothermal activity) or metamorphism (where one mineral is metamorphosed into new minerals due to heat and pressure).

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Geology, Palaeontology & Archaeology / Re: Metallic quartz: how did nature create it?

« on: 26/11/2019 18:13:51 »

My first guess would have been magnetite or specular hematite (iron oxide minerals), but your spectral analysis with the  high titanium content suggests ilmenite may be present as well. Is the metallic mineral strongly or weakly magnetic?

Metallic minerals are commonly precipitated in veins. The metals are carried in superheated hot water solutions (hydrothermal) and can precipitate when pressures decrease enough to cause the water to flash to steam, or upon mixing with colder groundwaters.  Most rocks contain Alumino-silicate minerals (AlSiO), so those elements are commonly the most abundant- which I think is reflected in you spectral analysis.

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Geology, Palaeontology & Archaeology / Re: What can you tell me about this rock I found?

« on: 15/10/2019 00:50:25 »

conchoidal fracture, glassy surface, vesicles: this is either slag (especially likely if you live near an industrial area) or obsidian (volcanic glass)

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Just Chat! / Has anyone heard from JimBob lately?

« on: 08/08/2019 21:25:23 »

Missing the old curmudgeon's lively chats. Any word on how he's doing?

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Geology, Palaeontology & Archaeology / Re: Are Earth's continents still moving?

« on: 19/07/2019 21:23:21 »

alancalverd is correct about rates of movement changing over time. And while the theory of continental drift has been replaced by plate tectonics, continents do indeed exist and have been persistent over long periods of time. Continental crustal material (sial) is much thicker and lighter (due to magmatic crystallization and segregation) than the heavier, thinner oceanic crust (sima). So continents tend to "float" higher on the mantle- which also makes them more difficult to subduct or destroy. Parts of most of the continents are billions of years old, while the maximum age of almost all the oceanic crust is less than 200 million years.

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Geology, Palaeontology & Archaeology / Re: What is this sparkly rock that contains golden flecks?

« on: 15/05/2019 21:53:21 »

Most likely it is some sort of mica.
To add to Bored Chemist's post- gold is very soft and pyrite is relatively hard. Try scratching it with a knife. (mica is also soft, but will flake)

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Geology, Palaeontology & Archaeology / Re: Help needed with Desert/Sand Rose also called Selenite Rose

« on: 11/03/2019 18:59:53 »

Welcome to the forum Sunandsand123. You found some nice specimens.
Known as "desert roses", "sand roses" and "selenite roses", these are actually gypsum (calcium sulfate dihydrate- calcium sulfate with water molecules attached). These form in arid environments, and as you've discovered are very fragile.
Evan-au correctly pointed out that using water (hydrating) the rosettes will cause them to break down more quickly- so try to keep them dry in place where they won't be jostled around.
Barite (baryte) roses form from barite (barium sulfate) which tend to be both harder and more durable than gypsum roses.

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Geology, Palaeontology & Archaeology / Re: Can you help me identify a rock?

« on: 17/02/2019 23:31:23 »

Any possibility of a picture?