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Post by vegasjames on Jul 22, 2022 8:08:57 GMT -5
I was contacted by a teacher who I am going to give some rocks to for her teaching. She of course wants examples of igneous, metamorphic and sedimentary stones. I was thinking of what examples I could give her of each and was thinking about all the opal I have. That led to me to ask myself, which of these three does opal, a mineraloid, fall under? Or none of the above, which would require a 4th category?
Opal does not form out of molten lava and so would not be considered igneous. Opals can form in igneous rocks after the fact, but are not formed as part of the original igneous process.
It does not formed from some other rock necessarily, unless I guess you consider dissolution of silica from sandstone or clay to form the silica solutions that lead to opal formation. Although, there are also biogenic opals not formed from mineral sources. So can all opals really be considered metamorphic?
And there are some opals, but not opals that, are sedimentary. So what about the opals that are not sedimentary?
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Post by rockjunquie on Jul 22, 2022 8:20:03 GMT -5
Above my pay grade, but it would be a good bonus question for a test.  |
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Post by hummingbirdstones on Jul 22, 2022 9:04:24 GMT -5
I'd have to say none of the above, vegasjames. It's a mineraloid gel and like you've said, forms after the fact. This is a pretty good explanation of how opal forms in each of the categories. |
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Post by stardiamond on Jul 22, 2022 12:44:43 GMT -5
This reminds me of a brief discussion on the facebook lapidary forum. Someone was complaining that people were using the word mineral to describe a rock; i.e a fossil is a fossil not a mineral.
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Post by rmf on Jul 22, 2022 13:54:02 GMT -5
vegasjames We (Wife and I) have the same issue. Pyrite is a good example too. Forms in all three environments (igneous, metamorphic and sedimentary). I have seen some beautiful octahedrons of pyrite in coal in western KY., to the east cubes of pyrite in slate and schist. To the south igneous pyrite was laid down and was later converted to geothite via weathering. Then you don't want to scare them off since it is elementary school(at least here) and the three environments are a good place to start. my wife trained as an elementary teacher so she pulls me back and has to say to me "don't over think it." In WI south west corner they could not figure where the hot water came from to add the galena and sphalerite to the ordovician limestone. When I was in school the theory was just deep burial not volcanic. But who knows what the leading theory is now. It can get complicated quick.
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Post by vegasjames on Jul 22, 2022 18:23:09 GMT -5
I'd have to say none of the above, vegasjames . It's a mineraloid gel and like you've said, forms after the fact. This is a pretty good explanation of how opal forms in each of the categories. So, I guess it needs to be defined if opal, which is a gel, not a solid, is actually a type of rock to fit in to the three types of rocks (igneous, sedimentary or metamorphic).
Speaking of formation, here is part of an old post I did on another board explaining some of opal formation and its conversion to chalcedonies.
"Opal starts as a silica gel. As the molecules move closer together water is squeezed out and the gel forms in to a solid. When a certain amount of water is left (3-21%) the result is opal. When less than 3% water is left as part of the structure the opal converts in to a chalcedony. The difference between the formation of common chalcedony, jasper and agate vs chert/flint is the silica source. The silica that forms common chalcedony, jasper and agate is usually sandstone or clays as where chert/flint is formed from opal produced from biogenic silica sources and is sedimentary. Opals also vary in hardness and stability. This is in part due to water content, but also due to the silica source. In the case of clays and diatomaceous earth the opals also pick up aluminum oxide from these sources increasing the hardness and stability of the opal. Another main difference between opal and chalcedonies is that opal is generally amorphous (lacking crystalline structure) as where chalcedonies are cryptocrystalline (microcrystalline). Quartz crystals are macrocrystalline. Addressing your comment of "Quartz and opal are silicate based and form, generally, as deposits left from water evaporation", that is not really true. Quartz crystals are formed hydrothermally. Without going in to a long explanation the simple explanation is that the silica is dissolved in water under extreme temperatures and pressures allowing the water to become supersaturated. As the solution cools the supersaturation is no longer stable and the silica starts to crystallize out of the solution. Therefore, not from evaporation. As for opal, it also forms in solution, and therefore not from evaporation. As mentioned, the silica polymerizes first in to a gel still within a solution and the molecules move closer together through electrostatic forces within the solution. Again, I am not going to go in to a long explanation, but opal does not really form from directly as silicon dioxide. Instead, the pH levels involved form silicic acids that can polymerize as opal gel then in to opal. So, the evaporation of silica hypothesis makes absolutely no sense. I have been playing with making opal for years. It is pretty simple process, and again is all done in solution, not through evaporation. In fact, as an experiment I tried simply evaporating ultra-fine crushed opal in water to see if the dissolved silica in the water would form opal. The answer is no. Again, there are other factors needed to convert the silica in to opal including changes in pH. The difference between precious and common opals involves silica sphere size and arrangement. The opalization of things like shells is also a little more involved. Silica has a high affinity for calcium, which is why organic opalized items are generally calcium rich such as shells and bones. Chalcedony does not replace the organics as you claim. Instead, the item such as wood is first opalized. Then as the opal loses more water the opalized wood turns to a chalcedony-based wood such as agatized or jasperized wood. Therefore, chalcedony-based wood is much older than opalized wood."
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Post by vegasjames on Jul 22, 2022 18:28:13 GMT -5
This reminds me of a brief discussion on the facebook lapidary forum. Someone was complaining that people were using the word mineral to describe a rock; i.e a fossil is a fossil not a mineral. Seems like this would be a gray area. What about fossils made 100% of a mineral. Agatized coral or I have coral and other fossils composed 100% of quartz as examples. Most fossils are formed as mineral replacements or mineral filling. |
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Post by vegasjames on Jul 22, 2022 18:36:48 GMT -5
vegasjames We (Wife and I) have the same issue. Pyrite is a good example too. Forms in all three environments (igneous, metamorphic and sedimentary). I have seen some beautiful octahedrons of pyrite in coal in western KY., to the east cubes of pyrite in slate and schist. To the south igneous pyrite was laid down and was later converted to geothite via weathering. Then you don't want to scare them off since it is elementary school(at least here) and the three environments are a good place to start. my wife trained as an elementary teacher so she pulls me back and has to say to me "don't over think it." In WI south west corner they could not figure where the hot water came from to add the galena and sphalerite to the ordovician limestone. When I was in school the theory was just deep burial not volcanic. But who knows what the leading theory is now. It can get complicated quick. Yes, pyrite can also lead to the formation of selenite. In the case here in Southern Nevada, red selenite crystals. I was thinking about this last night, where selenite would fall as well. It is not igneous, and it is not sedimentary. The closest I can see is that it would have to be fall under metamorphic as it can form from the alteration of limestone by sulfuric acid. In school though they taught us that metamorphic rocks form from alteration under heat and pressure. Not the case here. So does metamorphism include chemical alteration of a rock?
And if so, wouldn't some igneous rocks also be metamorphic? For instance, rhyolite. Rhyolite is formed from a viscous, high silica lava flow. Therefore, the original rock was melted and mixed, thus altering the original rocks chemically leading to a transformation. In the case of rhyollite a change to a more microcrystalline silica and feldspar mix. Therefore, is is not technically also metamorphized?
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Post by stardiamond on Jul 22, 2022 18:45:07 GMT -5
This reminds me of a brief discussion on the facebook lapidary forum. Someone was complaining that people were using the word mineral to describe a rock; i.e a fossil is a fossil not a mineral. Seems like this would be a gray area. What about fossils made 100% of a mineral. Agatized coral or I have coral and other fossils composed 100% of quartz as examples. Most fossils are formed as mineral replacements or mineral filling. He considers pet wood the same as fossils; replacements. |
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Post by vegasjames on Jul 22, 2022 19:20:08 GMT -5
Seems like this would be a gray area. What about fossils made 100% of a mineral. Agatized coral or I have coral and other fossils composed 100% of quartz as examples. Most fossils are formed as mineral replacements or mineral filling. He considers pet wood the same as fossils; replacements. Petrified wood can also be opalized, so we would be back to the same original question in this case.
I also have some pyritized wood, which is not a replacement reaction, but rather formed from a direct chemical reaction.
So, this brings me back to my original point. Do all rocks, stones, minerals fall under igneous, sedimentary or metamorphic? Or should there be an additional category such as noneoftheaboveic?
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Post by vegasjames on Jul 23, 2022 20:04:18 GMT -5
No other insights?
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RWA3006
Cave Dweller 
Member since March 2009
Posts: 4,612 
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Post by RWA3006 on Jul 24, 2022 11:56:58 GMT -5
What would glass be classified as? Besides obsidian, is there any other natural glass?
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Post by vegasjames on Jul 24, 2022 18:32:28 GMT -5
What would glass be classified as? Besides obsidian, is there any other natural glass? Glasses are certain amorphous compounds without water.
There are various types of natural and synthetic glasses, and not all are silica based.
As far as natural glasses some examples are obsidian, Pele's hair, scoria, pumice, tektites and fulgurites.
Man-made glasses include soda glass, borosilicate glass, fused quartz, aluminosilicate glass, some slags, etc. These are silica based glasses, but not all glasses are silica based. Metals can also be converted in to glasses by putting them in a molten state an them cooling them at a very fast rate making them amorphous by cooling them before crystallization of the metal can occur. Metallic glasses have been known for decades.
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Post by opalpyrexia on Jul 24, 2022 21:19:51 GMT -5
vegasjames — wondering what your take might be on the association between some Australian opal deposits and the presence of gamma radiation from uranium decay. A 2010 paper in Journal of Crystal Growth suggested that the opal might have formed around uranium "seeds".
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Post by vegasjames on Jul 25, 2022 5:06:08 GMT -5
vegasjames — wondering what your take might be on the association between some Australian opal deposits and the presence of gamma radiation from uranium decay. A 2010 paper in Journal of Crystal Growth suggested that the opal might have formed around uranium "seeds". Yes, I have read various reports on this. It does not have to be uranium itself though as the nucleator, but also any of its decay elements also work. That is why only some opal, such as a lot of the hyalite opal, contains uranium.
I am working on another batch of opal right now. Did not really want to use my uranium ore (carnotite) in the process so I went down the chain and decided to go with bismuth instead. So I am using Pepto Bismol to make this batch of opal. Really beautiful color.
There are also various non-uranium elements/compounds that can be used to start the nucleation process for opal formation. I have experimented with several such as barite. Although, the uranium and uranium associated elements seem to be associated with most the precious and hyalite opals.
There are actually various elements that can be used to start the nucleation process as well as various compounds not involving metallic elements. That probably has a whole lot to do with the fact that there are around 300 types of opal. If they all formed in the same exact way with the same exact nucleator, we would have a lot less forms of opal.
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markb
Cave Dweller
Member since May 2022
Posts: 472
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Post by markb on Jul 27, 2022 22:26:06 GMT -5
vegasjamesWhat about Common Opal, found in diatomaceous earth? Isn't diatomaceous earth a sedimentary rock? Maybe someone already mentioned this before and I didn't see it. markb
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Post by vegasjames on Jul 28, 2022 0:26:09 GMT -5
vegasjamesWhat about Common Opal, found in diatomaceous earth? Isn't diatomaceous earth a sedimentary rock? Maybe someone already mentioned this before and I didn't see it. markb Yes, brought them up in my original post:
"And there are some opals, but not opals that, are sedimentary. So what about the opals that are not sedimentary?"
They would also come in to question from the part of my original post where I mention biogenic silica sources as this includes diatomaceous earth and radiolarian sources of silica.:
"It does not formed from some other rock necessarily, unless I guess you consider dissolution of silica from sandstone or clay to form the silica solutions that lead to opal formation. Although, there are also biogenic opals not formed from mineral sources. So can all opals really be considered metamorphic?"
Look at how metamorphic rocks are defined:
"Metamorphic rocks form when high temperatures and pressure act on a rock to alter its physical and chemical properties (metamorphism means 'to change form')."
Opals do not require high temps to form though. Actually, the best opal forms under low temps and high temps can actually destroy opal. And opals do not need high pressure to form either.
So yes, some opals are sedimentary. In fact, as opals lose water they start to crystalize forming in to chalcedonies such as chert/flint, which is a chalcedony formed from sedimentary opal.
What about most common opal though and even opalized wood? These are not sedimentary, are not igneous and do not seem to fit the definition of metamorphic.
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