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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 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: Can wind weather limestone?

« on: 07/11/2018 22:08:39 »

Interesting question.
In the US west, limestones commonly are the cap-rocks on ridges and mesas, mainly due to the dry conditions and relatively low permeability of limestone. Sandstones tend to weather more quickly due to wind and higher porosity.
Contrast this to the eastern US, where sandstone commonly forms the cap-rock; limestone tends to dissolve more quickly in the wetter climate leading to karst geography.

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Geology, Palaeontology & Archaeology / Re: Can one test volcanic rocks to determine the age of a past eruption?

« on: 04/11/2018 18:12:42 »

The last photo you posted, if that is a boulder, certainly looks like lahar. The USGS describes " Lahar is an Indonesian term that describes a hot or cold mixture of water and rock fragments that flows down the slopes of a volcano and typically enters a river valley."- basically a mudflow from a volcano that is usually full of fragments.
Interesting that the volcano is considered "extinct" if there is still magma below and hot springs- maybe "dormant"?
Yes, it can be age dated. The most accurate way to date the rocks is radiometric age-dating, which requires a lab (or university) that specializes in that sort of work, and probably best to have the sample taken by a professional. Depending on the age of the rock, K-Ar might be best.

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Geology, Palaeontology & Archaeology / Re: What is this ridged fossil found in Somerset?

« on: 04/11/2018 17:58:03 »

These appear at first glance to be symmetrical ripples, but there are other possibilities. The Fossil2 photo seems to  show ripples disrupted by some sort of later channel.
I'm intrigued by the green in photo Fossil9- is that mineral? or organic (like lichen)? If it's mineral, then I suspect the rock is carbonate. Can you tell if the rock is sandstone (will feel like sandpaper)? If it's carbonate (limestone), it should bubble if you drop acid (10% HCl or equivilent), especially on the unweathered edges.

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Geology, Palaeontology & Archaeology / Re: Did glaciation produce this smooth, undulating rock surface?

« on: 06/10/2018 00:25:29 »

Bruce- welcome to the forum
Definitely glacial polish. The striations are one of the indicators- note that they are all parallel, even on the smaller, rounded outcrop below the main outcrop. You are correct that the polish is from abrading grit at the bottom of the ice that moves past the bedrock under great pressure. A recent article in "Geology" (GSA publication) noted that the polish is actually partially due to a micrometers-thick layer that was smeared onto the rock surface. Basically, the bedrock (granite) is ground up by the moving ice, and pieces of ground up granite at the bottom of the ice not only abrade and smooth the bedrock, but then the really fine particles are "smeared" in a thin layer onto the surface by the pressure of all that ice above. The article convincingly contends that glacial polish is the result of this thin layer, which then helps protect the bedrock from further erosion.
One note: glaciers don't move back and forth. They generally only move one direction- while glaciers may retreat, that is due to the ice melting, not to the glacier moving back uphill.

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Geology, Palaeontology & Archaeology / Re: Can one test volcanic rocks to determine the age of a past eruption?

« on: 05/10/2018 23:57:34 »

The rocks in your photo certainly could be volcanic- but I couldn't make our enough detail to tell for sure. Did you see any crystals in the rocks? Andesites commonly have both black and white crystals.
As to the age, probably Potassium-Argon would give you the best radiometric readings to determine age if it's volcanic. Why do you think this is "last" eruption? Volcanic lava flows are notoriously local and tend to fill low spots in the topography, so this may have been covered by even younger flows.

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Geology, Palaeontology & Archaeology / Re: What is the name for this form of arch-like geological structure with fractures?

« on: 16/09/2018 01:01:43 »

Great photo. I've been to the Azores, but never to Corvo. My recollection is that Corvo (and the rest of the Azores) are volcanic, and that Corvo is a caldera, with both ash, welded ash and some basalt units. I think Ophiolite is correct in calling this a geomorphic feature- I would term it an "arch". Since the island is composed of recent volcanic rocks that have undergone very little strain, this feature is not an anticline. I seem to remember some discussion of this feature being the remnants of a lava tube, the fractures being "joints" that form during contraction of the lava due to cooling.

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Geology, Palaeontology & Archaeology / Re: What kind of metal in this Alum Shale?

« on: 16/09/2018 00:39:50 »

Pyrite, or Fool's Gold. The cubic form is the real give-a-way. Black shales like this are commonly deposited in anoxic conditions, making iron sulfides more likely to form than iron oxides.

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Geology, Palaeontology & Archaeology / Re: What are these glassy rock samples found in West Yorkshire?

« on: 20/08/2018 21:37:19 »

Chris and Bored Chemist got it right- slag.

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Geology, Palaeontology & Archaeology / Re: Which rocks contain metallic flakes resembling silver and gold?

« on: 22/08/2017 00:03:55 »

are you sure they are metallic? If so, I agree that pyrite is the best guess for gold colored flakes. silver colored could be specular hematite. One simple test to try to scratch them with a knife to determine hardness and to see if they leave a colored streak. Both minerals range from 6 to 6.5 on the Mohs scale, which is about the same hardness as a knife blade.
If not metallic, several of the mica minerals can shine brightly like gold/silver, including sericite, phlogopite and biotite.

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Geology, Palaeontology & Archaeology / Re: Why are the Hawaiian and Cascade volcanoes roughly 100 km apart?

« on: 10/04/2017 18:36:14 »

All of the Cascade volcanoes, including the "inland" volcanoes, are considered active with potential to erupt. Check the USGS CVO (Cascades Volcano Observatory) website for more information https://volcanoes.usgs.gov/observatories/cvo/cascade_volcanoes.html
Almost all of the Cascade volcanoes have erupted within the past 4000 years (which is an extremely short geologic time span). There has been very recent activity, including earthquakes and/or ground movements at Ranier, Hood. South Sister, Baker, and Mt. St. Helens.

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Geology, Palaeontology & Archaeology / Re: what have I got here ya think?

« on: 20/09/2016 23:44:14 »

Looks like petrified wood from the photos. I'm guessing it's from a dead tree

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Geology, Palaeontology & Archaeology / Re: What are these minerals found in Sweden?

« on: 13/08/2016 01:07:22 »

Galena it is then!

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Geology, Palaeontology & Archaeology / Re: Is a mid-life crisis driven graduate degree in geology a wise career move?

« on: 03/08/2016 01:10:56 »

 My nephew went back to pick up his undergraduate degree in geology a couple of years ago at age 40. He was successful in the finance industry, but hated his job. He'll graduate next year with a degree in Geology and in GIS. He's survived on grants, scholarships and student loans. I've been trying to mentor him and have even been able to find him a couple of summer jobs to help keep him afloat. Really proud of him for his efforts!
My sister tells me "You've ruint him!" - but he has a real knack for geology and absolutely loves what he's doing now. He's looking at Grad programs and is inclined towards working in the oil industry (which just goes to show what little influence I've had). Actually, his timing may be perfect- if oil rebounds in a couple of years he'll get plenty of job offers.
I love volcanology, and considered working in that field, but was lured away by the siren song of mineral exploration. My advice- find what interests you most, love your work, and you'll be successful!
Welcome to the Forum!!!

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Geology, Palaeontology & Archaeology / Re: what is this very heavy rock found in the ocean in NYC?

« on: 02/03/2016 21:20:30 »

My guess is some sort of industrial slag.

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Geology, Palaeontology & Archaeology / Re: What is this small black piece of stone?

« on: 24/10/2015 19:46:05 »

Cubic form of the crystals indicates this is a pyrite nodule. I assume it feels a bit heavier than a regular garden stone of the same size.  Black color is probably goethite oxidizing from the pyrite.

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Geology, Palaeontology & Archaeology / Re: What is the "rock cycle"?

« on: 27/08/2009 04:29:18 »

Where to start?  The rock cycle = geology.

The basic idea is that rocks are recycled in the earth's crust and mantle.  Rocks exposed at the surface are subject to mechanical and chemical weathering.  The erosion creates sediments.  Sediments accumulate, are buried and lithify, forming sedimentary rocks.  

Metamorphic rocks are created with the addition of heat and/or pressure.  The original rock constituents change, or metamorphose, to form new minerals and rock.

Adding even more heat and pressure- the rocks melt to form igneous rocks.  Igneous rocks may be erupted on the surface- volcanic- or may cool slowly underground- plutonic.

Igneous/metamorphic zones are uplifted, exposed to erosion, and the cycle begins all over again.

That's a very simplistic view of the classic rock cycle.  

Plate tectonics adds another dimension- new crustal rocks are created at mid-oceanic ridges, accumulate sediments (sedimentary rocks) as they traverse the ocean basins and are consumed at continental edges (subduction zones).  At subduction zones, parts of the basin volcanic/sedimentary package is mashed into the continental edge as ophiolite sequences, and part melts during subduction- creating further igneous and metamorphic rocks, as well as uplift.  Uplifted rocks are subject to erosion, beginning the cycle anew. Again, a very simplistic view of a very complex process.

How does the rock cycle help life?  Without the rock cycle, air, water and surface relief wouldn't exist.  Most of the atmosphere was derived from eons of volcanic activity.  Continents and ocean basins wouldn't exist.  And all the minerals necessary for life, and for human activity, were formed in the rock cycle.

Does life affect the rock cycle?  Absolutely.  Bacteria, fungus, lichen, algae, plants, etc all help with chemical and mechanical weathering.  Algae created atmospheric oxygen, which allowed oxidation and deposition of certain minerals.

Again, this is barely scratching the surface- if you have a more specific question on the rock cycle, the answer can be much more focused.