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Post by rockjunquie on Oct 16, 2013 18:56:39 GMT -5
I have been trying to define rocks for myself and for my blog and business. I ran into trouble right off the bat. Here's is what I thought I knew until I read a few contradictory things. Quartz is the parent of a family. Chalcedony, agate and jasper are children of said parent. NOW- I find sources that say that actually, quartz is a parent to chalcedony and agate and jasper are a child to chalcedony. I'm more confused than when I started and here I thought I had finally figured it out. I know some of you here are really good at these things. I have looked at what I thought were good and reliable sources. Please help me to come to some kind of consensus about this. Thanks in advance for your time and help.
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Sabre52
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Post by Sabre52 on Oct 16, 2013 20:42:30 GMT -5
OK, been so busy with horse stuff I haven't been around much lately but I'll try to tackle this one *S*
Quartz comes in four basic forms 1. Amorphous with no crystal structure like glass/obsidian 2. Megaquartz like in quartz crystals 3. Massive quartz which is big interwoven crystals that show little structure ie rose quartz and 4. Microcrystalline quartz ( cryptocrystalline) which has microscopic crystals.
Pure unincluded microcrystalline quartz is chalcedony this pure chalcedony comes in several colors ie carnelian etc where the various colors are caused by mineral salts in the pure chalcedony.
True agate is microcrystalline ( chalcedony) quartz with banding. These can be of marine ( ie Dryheads) or volcanic origin ( ie Lakers)
Many other forms we call agate are chalcedony with various inclusions either of an earthy nature ( as in jasper/ agate) or crystalline nature. ie. dendritic agate, moss Agate, plume agate etc. Jasper is chalcedony with earthy, opaque, fibrous inclusions from volcanic origins. Jasper is opaque while agate is translucent to some degree.
Technically chert is included, microcrystalline quartz from marine sediments like limestone deposits.
Flint is the same thing except from chalk deposits. Pretty much the same identical composition as chert though or agate for that matter as all are still included microcrystalline quartz.
Of course, all the scientific jargon is terribly confused by the rampant misnaming of almost every quartz mineral.
For example "picture jasper" varieties are usually silicfied argillites ( mud sediments), not jasper.
"Morgan Hill Poppy Jasper" is a chert of marine origin.
Everything called "flint" from Texas or Flint Ridge is marine chert as it comes from limestone not chalk like true flint
Leopardskin Jasper is rhyolite and many things that are sold as varieties of agate are in fact included chalcedony with some amount of opaque jasper included, which would make them more of a jasper/agate ie some of the polka dot agate and much of the agatized wood we see.
So in the most basic of terms, Quartz is the parent of all chalcedony, agate, jasper, flint and chert. All are micrcrystallne quartz family materials of various forms, patterns and origins. Clear chalcedony could be thought of as the parent for all the agate, jasper, flint and chert with all the kids natures being determined by the amount of included mineral salts and other sorts of inclusions in the chalcedony......Mel
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Post by rockjunquie on Oct 16, 2013 20:52:47 GMT -5
Perfect Mel!!!! Just what I was looking for! That made a lot of sense. If only I could have found one measly source to put it so well. Thank you!
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Post by wireholic on Oct 16, 2013 20:57:17 GMT -5
Wow - that was a whole college course! Great info! Thank you
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Tommy
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Post by Tommy on Oct 16, 2013 21:35:19 GMT -5
Wow... this thread should be stickied :-)
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gemfeller
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Post by gemfeller on Oct 16, 2013 21:50:17 GMT -5
Mel has done an extraordinary job of boiling a complex subject down to bare bones. I especially like his comment that "Of course, all the scientific jargon is terribly confused by the rampant misnaming of almost every quartz mineral." I often refer to this excellent site when questions come up about this "simple" but unbelievably complex mineral; it's a wonderful source: www.quartzpage.de/Rick
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Post by Jugglerguy on Oct 16, 2013 22:11:45 GMT -5
Mel, I was going to say the same thing, but you beat me to it! Not really. I'm still trying to get all of this to make sense, but that helped a bunch. Thanks!
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Post by Deleted on Oct 17, 2013 17:24:05 GMT -5
Sabre52, that is awesome! Quick question. Flint/Chert are from limestones. What is the process here. How does the silicates replace the carbonates? Are are they mixed like in serpentized marbles?
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Post by Jugglerguy on Oct 17, 2013 18:00:28 GMT -5
I was wondering the same thing about chert Scott. I understand that limestone is sedimentary and made of mostly calcite. How does calcite change to quartz, and is chert still sedimentary after undergoing the change?
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Post by Deleted on Oct 17, 2013 18:24:48 GMT -5
I had some of this down but I was still a dummy so I printed a hard copy so I have a quick reference. Thanks Mel. Jim
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Sabre52
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Post by Sabre52 on Oct 17, 2013 18:26:56 GMT -5
Scott, True flint nodules are supposedly only from chalk deposits, chert from limestone. As I understand it, the process for both is a void filling situation. Acidic waters cut through the limestone or chalk leaving channels or voids or even marine organism burrows or fossils eroding away can create voids in the sedimentary rock. Then an overlaying layer of silica based fossil sediment must be present for the silica source. Diatoms, radiolarians and sponge spicules are often present in huge numbers in marine sediments and are composed of silica. This silica source overlaying the voids, much like the layers of volcanic ash in the western agate beds, provides the silica which is carried by ground waters over time and penetrates and fills the voids creating flint or chert nodules or veins in the sedimentary rock. The process is very visible here on our ranch. The native limestone is all full of tunnels and voids and many of these are oddly shaped and filled with chert that exactly mirrors the weird voids still showing in the limestone. Much of the limestone doesn't quite make it to chert and is much harder and silica rich and shows silica infusion into the carbonate rock. This forms hard limestone nodules which look much like the chert nodules until broken open. Most all the chert nodules also contain fossils that apparently were loose and were included in the silica gel that filled the voids. We have those nodules in situ all over the ranch. Very cool to see all those strange and bizarre shapes laying about.
I read an interesting article about Mexican Crazy Lace Agate years ago that brought up an interesting point. The point being, that the crazy lace originates in limestone deposits. They did not explain it but it made me wonder if crazy lace is actually a fancy patterned chert. If the layered chalcedony filled in cracks in the limestone, it would explain the presence of all those stalk aggregates, dogtooth calcite replacements, sagenite inclusions etc as those begin as carbonate minerals. If silica infused the carbonate minerals and created silica pseudomorphs of those fancy crystal structures, that would explain how all those fancy patterns in crazy lace developed.....Mel
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Sabre52
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Post by Sabre52 on Oct 17, 2013 18:33:52 GMT -5
Rob: I have heard the name metasilicate ( a metamorphic "changed" siliceous rock) assigned to some rock types where silica has infused and "changed" a sedimentary rock into a chert, flint, picture jasper or agate. Takes more of an expert than me to know if that's correct or not, but it makes sense. If one exerts pressure to and infuses silica into an argillite type rock ( basically a fancy mudstone) and that rock becomes jasper hard and fine grained, I'd say it had undergone a metamorphic process so metasilicate might be a good term to use.
In the case of chert or flint that has filled cracks or voids it would be sedimentary. It has not been changed but rather just deposited as silica gel into the void. I've often wondered if that is the case, then are all amygdaloidal agates sedimentary too? They form in gas pockets in igneous rocks but are deposited as gel in the voids in a sedimentary fashion when silica percolates in from surrounding volcanic ash or rhyolite...Mel
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Post by Jugglerguy on Oct 17, 2013 19:01:37 GMT -5
Thanks Mel. You're stretching the limits of my mind, but I think I understand a little bit. I sure know more about rocks than I did a couple years ago. I need to borrow some of my wife's middle school earth science textbooks!
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gemfeller
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Post by gemfeller on Oct 17, 2013 19:37:50 GMT -5
Mel, I wish you'd read and comment on the new theories of Marco Campos-Venuti. He's an Italian volcanologist who's devoted a lot of study to agate and jasper formation. He's published a very interesting book: "Genesis and Classification of Agates and Jaspers: a New Theory." I think you'd find it very interesting whether you agree with it 100% or not.
He believes, for instance, that amygdaloidal agates and thundereggs depend on sedimentary processes for their quartz/agate content and has an interesting explanation of how it occurs. His chapter "Genesis of Jaspers" begins: "In the case of abyssal jaspers, the genesis of jasper (chert) and flint start from an organic precursor of amorphous silica called opal-A. The transition from opal to jasper is usually attributed to diagenesis, which is an extremely indefinite word that indicates all the processes that occur after the deposition of the loose sediment leading to its final structure in the form of hard rock."
Quoting more would probably violate his copyright protection but hopefully it will pique your curiosity and motivate you to read the entire book.
Rick
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Sabre52
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Post by Sabre52 on Oct 17, 2013 20:01:06 GMT -5
Rick, I'll keep my eye open for the book. My transition through college was many moons ago so my info on theory may be dated. I started out as a marine biology major, then got interested in geology and finally wildlife biology so my geology info is pretty old, though I think most is still applicable. I have read some stuff on the marine ooze theory of chert formation as it applies to poppy jasper formation which sounds a lot like some of the theory your Venuti has mentioned Problem I have with the marine ooze theories that involve the amorphous silica is some folks I've read seem to imply the process took place while the material was underwater and that would not seem to account for the obviously filled erosion created channels we see around here. I would think the voids and channels were created after the limestone was on dry land and was exposed to rainwater erosion. Our local nodules are in lower Cretaceous limestone that was obviously sea bottom at one time but, to be frank, I'd have to do some research as to exactly how the nodules formed and when. They do often show Liesgang lines though so sedimentary deposition of silica in a layered manner was definitely at work.....Mel
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Post by Hard Rock Cafe on Oct 17, 2013 20:31:51 GMT -5
Excellent description of the varieties of cryptocrystalline quartz!
I've often wondered about amygdaloidal agates, too, particularly with Lakers. Specifically, we know that many amygdaloids form in the gas pockets in basalt (think Lakers), so the basalt must be fairly solid at that point. Could it still be relatively hot? Not likely, because that would likely lead to large crystals, unless there was significant pressure, too, which would have to occur underground.
On Stillwell Ranch you can find _lots_ of nodules with a rough chalcedony exterior that lead you to believe they're agates. Cut into them, however, and you'll find chert (often pretty, but opaque grey, brown, black, and white). Is that what you have, too, Mel? I've never seen them anywhere else.
Chuck
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gemfeller
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Post by gemfeller on Oct 17, 2013 20:33:16 GMT -5
Mel, I think a lot of Venuti's ideas will be controversial. While I'm very interested in the subject and have a fair amount of geology background I don't feel in any way qualified to take a strong position on his theories. I tend to let information about this stuff build up in subconscious sedimentary layers until it finally make some kind of sense to me. I started cutting precious opal when I was much younger and after working with a lot of it and observing some of its very puzzling attributes I started wondering about the entire quartz/silica series. I think it's one of the most complex -- and least studied -- areas of mineralogy. I try to follow new studies as they emerge so I can add another layer of mental sediment. The "Venuti Layer" began to suggest some solutions to me but I'm interested in following criticisms as they emerge.
Rick
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Sabre52
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Post by Sabre52 on Oct 17, 2013 21:35:20 GMT -5
Chuck, I have found nodules like you're talking about in the George West area and in the Rio Grande gravels. We have three types of material here, a nodular and vein form of chert that is primarily solid gray to black that comes mainly from the Edwards Formation . All of them have a tan siliceous crust or cortex. Another white cortexed material with rootbeer colored interior with lots of liesgang lines that kind of resembles Polish Flint, and a material called locally Pedernales Chert which is quite pretty and in veins. Mostly earth tones but ofte wiith lacy effects or bands that gives it the appearance of petrified wood.
I think most agate nodule silica deposition is now thought to have occurred at fairly low temperatures about 125-600 degrees F and I think the gas pockets were supposed to have filled after the basalt cooled but I'd have to go back and check the literature to be sure *L*....Mel
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Post by Deleted on Oct 19, 2013 20:12:43 GMT -5
Scott, True flint nodules are supposedly only from chalk deposits, chert from limestone. As I understand it, the process for both is a void filling situation. Acidic waters cut through the limestone or chalk leaving channels or voids or even marine organism burrows or fossils eroding away can create voids in the sedimentary rock. Then an overlaying layer of silica based fossil sediment must be present for the silica source. Diatoms, radiolarians and sponge spicules are often present in huge numbers in marine sediments and are composed of silica. This silica source overlaying the voids, much like the layers of volcanic ash in the western agate beds, provides the silica which is carried by ground waters over time and penetrates and fills the voids creating flint or chert nodules or veins in the sedimentary rock. The process is very visible here on our ranch. The native limestone is all full of tunnels and voids and many of these are oddly shaped and filled with chert that exactly mirrors the weird voids still showing in the limestone. Much of the limestone doesn't quite make it to chert and is much harder and silica rich and shows silica infusion into the carbonate rock. This forms hard limestone nodules which look much like the chert nodules until broken open. Most all the chert nodules also contain fossils that apparently were loose and were included in the silica gel that filled the voids. We have those nodules in situ all over the ranch. Very cool to see all those strange and bizarre shapes laying about. I read an interesting article about Mexican Crazy Lace Agate years ago that brought up an interesting point. The point being, that the crazy lace originates in limestone deposits. They did not explain it but it made me wonder if crazy lace is actually a fancy patterned chert. If the layered chalcedony filled in cracks in the limestone, it would explain the presence of all those stalk aggregates, dogtooth calcite replacements, sagenite inclusions etc as those begin as carbonate minerals. If silica infused the carbonate minerals and created silica pseudomorphs of those fancy crystal structures, that would explain how all those fancy patterns in crazy lace developed.....Mel This explains georgetown flint quite nicely. Not that chert/flint replace but instead are formed within chalk/. That makes sense. The reason I asked was the origin issue I have with jamesp's corals. His corals are certainly "silicated" in some form or another. They are in replacement of calcium life forms butnow the calcium seems to be gone. Certainly there is no reaction to pool acid. I'll always wonder how the limestone animal is now gone and the silica has replaced it even showing anatomy.
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jamesp
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Post by jamesp on Oct 19, 2013 21:19:59 GMT -5
I know the acid water in the river ate away a gorge or river channel thru the limestone. The steep walls of limestone are staring at you as you float on the river when it is low. On top of those walls is the coral. But the coral that was sitting over the river on the limestone is all sitting in the bottom of the river since the limestone was dissolved out from under it. Also sitting on the top of the limestone walls is dense clays that the coral is imbedded in. If the limestone /cherty coating on the coral i sent you Scott was sitting in the acid water for a long period of time is it possible that it is neutral in ph ? Or even acidic...
Petrified wood and coral replacements puzzle me too. I see so many partial silicifications of the coral. And hollow botyroidal/drusy cavities is another story. But the other partial silicifications are in many strange forms in the coral. Typically breaking into tubes with hex cross sections. You called them spaghetti Scott. I never see healed silicifications in the coral-never. Like shrinkwood and breccias. And many of the silicifications are glass like. And the glassier they are the less anatomy preserved... The more incomplete the silicification the more anatomy that was preserved, to a point.
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