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Post by holajonathan on Jan 22, 2021 13:50:41 GMT -5
I've been tumbling lots of agates: Mexican crazy lace, Mexican moss, all sorts of west Texas varieties, baby Brazilians and big boy Brazilians, Bahia, Moroccan seam, Montana, South Dakota Prairie, Lake Superior, Botswana, Indian yellow skin and black skin, to name a few.
One thing has become clear: when it comes to hardness, not all agates are created equally.
On the one hand, Mexican crazy lace shapes fairly quickly for an agate.
On the other hand, I have some Brazilian agates that have been rolling 6 weeks in a big barrel, 46 grit, 5 day clean outs, and I still haven't worn through the smooth exterior layer -- what I think of as the shell -- that is underneath the matrix but before the colored or banded part.
The hardest agates I've run across seem to be: Brazilian, Montana, and the Indian black skin. Lace agates and seam agates seems to be less hard, generally speaking, than nodular agates. Although some of the west Texas plume agates are incredibly hard also.
I am no mohs scale expert, but my tumbling experience tells me that there is a significant range of hardness among the different types of agates. Some of this has to do with how much quartz is in the rock. Larger quartz crystals wear faster than the microscopic agate crystals, even though they supposedly have the same hardness. (I think tumbling creates tiny surface fractures in the quartz, causing it to wear off in sand size pieces, whereas the agate wears down in dust size pieces.) But putting aside the issue of quartz crystals in agates, they are still not all the same hardness.
What is this all about? Am I imagining this? Shouldn't all agate be the same hardness if it's chemical makeup and structure is the same? If some agates, like Mexican crazy lace, have minerals other than micro-crystalline silica mixed in, shouldn't those areas undercut when tumbling? In other words, how can it be less hard, but uniformly less hard?
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Post by stardiamond on Jan 22, 2021 14:08:41 GMT -5
"The main difference between Chalcedony, Agate and Jasper are crystal size, composition and transparency (otherwise known as diaphaneity), with Jasper being opaque and Agates being more translucent. If you were unable to tell the difference you could just call it Chalcedony and not be wrong."
Chalcedony is quartz based.
I don't tumble but recognize how hard a particular slab is when working on it. It is not the material but a particular piece of material and there can be a lot of variation from piece to piece. I start grinding and see how fast the material cuts. Montana is usually the hardest material I work with.
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Post by Peruano on Jan 22, 2021 14:53:50 GMT -5
My apologies that have heard me recommend this page before but you can't beat if for info on agates, and all quartz relatives. The quartz page www.quartzpage.de/agate.html#varietiesThe reason why Mexican crazy lace is so pretty is that it is quite heterogeneous with soft areas, and hard areas. The reason agate is so interesting is that it is so variable in the expression of contaminants and form. Go figure. |
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RWA3006
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Post by RWA3006 on Jan 22, 2021 20:02:22 GMT -5
My apologies that have heard me recommend this page before but you can't beat if for info on agates, and all quartz relatives. The quartz page www.quartzpage.de/agate.html#varietiesThe reason why Mexican crazy lace is so pretty is that it is quite heterogeneous with soft areas, and hard areas. The reason agate is so interesting is that it is so variable in the expression of contaminants and form. Go figure. That's a great link. |
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NRG
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Post by NRG on Jan 22, 2021 22:04:59 GMT -5
All previous answers are solid knowledge. What I am about to present ALSO applies. This is not exclusive to any other inputs.
The agate sol-gel polymerizes at acid (<7) pH levels. The lower the faster polymerization time. And also the harder the resulting agate.
The reason this is not exclusive is because the sol-gel polymerizes in other conditions as well. For instance, temps above 200°F will also cause the reaction to happen.
So, as you have found, there are differences between each formation event. This is useful knowledge. Please keep reporting your findings. We all win.
Thank you for sharing your findings!
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Post by holajonathan on Jan 22, 2021 22:31:28 GMT -5
"The main difference between Chalcedony, Agate and Jasper are crystal size, composition and transparency (otherwise known as diaphaneity), with Jasper being opaque and Agates being more translucent. If you were unable to tell the difference you could just call it Chalcedony and not be wrong." Chalcedony is quartz based. I don't tumble but recognize how hard a particular slab is when working on it. It is not the material but a particular piece of material and there can be a lot of variation from piece to piece. I start grinding and see how fast the material cuts. Montana is usually the hardest material I work with. Your point about the variance from piece to piece of the same material is a good point, and suggests that the crystal structure of the pieces are different. Thanks for the info. |
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Post by holajonathan on Jan 22, 2021 22:36:03 GMT -5
My apologies that have heard me recommend this page before but you can't beat if for info on agates, and all quartz relatives. The quartz page www.quartzpage.de/agate.html#varietiesThe reason why Mexican crazy lace is so pretty is that it is quite heterogeneous with soft areas, and hard areas. The reason agate is so interesting is that it is so variable in the expression of contaminants and form. Go figure. I have not seen that website before. It is good. I will likely waste the next hour reading it and enjoy every minute of it. Thank you. |
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Post by holajonathan on Jan 22, 2021 23:01:21 GMT -5
All previous answers are solid knowledge. What I am about to present ALSO applies. This is not exclusive to any other inputs. The agate sol-gel polymerizes at acid (<7) pH levels. The lower the faster polymerization time. And also the harder the resulting agate. The reason this is not exclusive is because the sol-gel polymerizes in other conditions as well. For instance, temps above 200°F will also cause the reaction to happen. So, as you have found, there are differences between each formation event. This is useful knowledge. Please keep reporting your findings. We all win. Thank you for sharing your findings! This is interesting and goes directly to what I want to understand. In other words although all agates are micro-crystalline, there is variation in how those microscopic crystals are bonded together, depending on variables present during formation. Do I have that right? So some agates that seem to be softer are simply not bonded as strongly, even though the individual micro-crystals of silica are just as hard as any other quartz. Am I making sense still? |
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quartzilla
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Post by quartzilla on Jan 22, 2021 23:39:25 GMT -5
Well isn’t it also not so much hardness as toughness? For instance why jade is so tough; the way the crystal structure is jack strawed together. Maybe some of the micro crystalline varieties we love and tumble have similar composition with the microscopic crystals every which way. They are all 7 hardness but some are structurally tougher than others.
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Post by stephan on Jan 23, 2021 1:03:45 GMT -5
Well isn’t it also not so much hardness as toughness? For instance why jade is so tough; the way the crystal structure is jack strawed together. Maybe some of the micro crystalline varieties we love and tumble have similar composition with the microscopic crystals every which way. They are all 7 hardness but some are structurally tougher than others. Precisely! Toughness (resistance to fracture) is a huge factor. If there are (relatively) large crystals, the toughness will be different than microscopic crystals. The individual crystals will also separate from each other more easily than they will break. This is a big factor in many lace “agates.” Remember, too, Mexican crazy lace agates are not really agates, but jasperoids. Rather than forming by deposition of silica, they are pseudomorphs — replacing limestone with silica, making them structurally different than, say, Brazilian or Montana agates, and uneven in terms of both hardness and toughness. |
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NRG
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Post by NRG on Jan 23, 2021 9:50:49 GMT -5
Well isn’t it also not so much hardness as toughness? For instance why jade is so tough; the way the crystal structure is jack strawed together. Maybe some of the micro crystalline varieties we love and tumble have similar composition with the microscopic crystals every which way. They are all 7 hardness but some are structurally tougher than others. Quartz will scratch agate. Give it a try. Agate is a 6.5 |
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Post by stephan on Jan 25, 2021 22:08:47 GMT -5
Well isn’t it also not so much hardness as toughness? For instance why jade is so tough; the way the crystal structure is jack strawed together. Maybe some of the micro crystalline varieties we love and tumble have similar composition with the microscopic crystals every which way. They are all 7 hardness but some are structurally tougher than others. Quartz will scratch agate. Give it a try. Agate is a 6.5 Everything I’ve read says that agate is 7-7.5. Keeping in mind that the Mohs scale is ordinal, not absolute, this is pretty squishy. It makes sense that how it is formed influences the hardness. Actual silica content probably plays a role. Jasper is listed with the same hardness as well, but then there is a plethora “jasper” used as trade name, but geologically inaccurate. |
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Post by pauls on Jan 26, 2021 0:07:33 GMT -5
Agate is strange stuff for sure. Talking Aussie Agate Creek Agates here, Oranges are quite soft and easily fractured, so much so that it's difficult to get a decent cuttable piece, Blues and purple are hard but saw and cut easily, The hardest is yellows and yellow greens, these are so tough that it's difficult to cut a full slab in one go, half way through the saw stops cutting and needs to be sharpened. Most pinks are quite soft, I believe they are chemically altered which makes them softer. By altered I mean weathered or maybe some chemical magic happening while forming, I know I have inadvertently made pink Agate by bathing other colours in hot highly caustic solutions attempting to fill cracks.
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Post by holajonathan on Jan 26, 2021 0:36:43 GMT -5
Well isn’t it also not so much hardness as toughness? For instance why jade is so tough; the way the crystal structure is jack strawed together. Maybe some of the micro crystalline varieties we love and tumble have similar composition with the microscopic crystals every which way. They are all 7 hardness but some are structurally tougher than others. I think you nailed it. Anyone who has tumbled agates with a lot of quartz in them has noticed that the quarts wears faster than the agate, even though they should both have the same hardness. In that case, the explanation is obvious -- you can see the big quartz crystals which tend to be heavily fractured and easily broken. It is less obvious -- although I think you must be right still -- when talking about materials that appear to be very solid. Translucent white chalcedony is about the hardest material in the quartz family that I have come across. When I put it to a diamond grinding wheel, the wheel screams for help and removes the material very, very slowly. Botswana agates are also very hard. But there are other equally solid looking agates, without a bunch of quartz, that grind / tumble must faster than the white chalcedony or Botswana agates. I guess there are at least 4 variables, and probably many more: the mineral (silica), the crystal size (macro-crystalline / microcrystalline / cryptocrystalline), how the crystals are bonded (don't know the name for this?), and what other minerals are mixed with the silica (all colors, for example, comes from something other than silica). With silica as a given for the agate family, I suppose that differences in the other 4 factors would all affect toughness. |
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Post by stephan on Jan 26, 2021 13:32:05 GMT -5
Well isn’t it also not so much hardness as toughness? For instance why jade is so tough; the way the crystal structure is jack strawed together. Maybe some of the micro crystalline varieties we love and tumble have similar composition with the microscopic crystals every which way. They are all 7 hardness but some are structurally tougher than others. I think you nailed it. Anyone who has tumbled agates with a lot of quartz in them has noticed that the quarts wears faster than the agate, even though they should both have the same hardness. In that case, the explanation is obvious -- you can see the big quartz crystals which tend to be heavily fractured and easily broken. It is less obvious -- although I think you must be right still -- when talking about materials that appear to be very solid. Translucent white chalcedony is about the hardest material in the quartz family that I have come across. When I put it to a diamond grinding wheel, the wheel screams for help and removes the material very, very slowly. Botswana agates are also very hard. But there are other equally solid looking agates, without a bunch of quartz, that grind / tumble must faster than the white chalcedony or Botswana agates. I guess there are at least 4 variables, and probably many more: the mineral (silica), the crystal size (macro-crystalline / microcrystalline / cryptocrystalline), how the crystals are bonded (don't know the name for this?), and what other minerals are mixed with the silica (all colors, for example, comes from something other than silica). With silica as a given for the agate family, I suppose that differences in the other 4 factors would all affect toughness. Remember that toughness is very different than hardness. Toughness is resistance to shattering, and the nephrite variety of jade is tougher than diamond. The macro vs micro-crystalline (crypto- is the same as micro-crystalline) might affect the toughness at the junctions of the large crystals. They might be bonded to each other, or they may just be adjacent. In the latter case they could just fall apart when exposed to stress. When you talk about bonding, I'm not sure if you are talking about inter- or intra-crystalline. For the former, it kind of gets at what I was talking about above, for intra-crystalline, the color-imparting impurities are generally metal salts, which would interfere with the silica lattice. Remember that, although the chemical formula is SiO 2, the lattice is SiO 4: O O O | | | O=SiO vs. O-Si-O-Si-O-Si-O | | | O O O |
O-Si-O | O Now, picture it going on and on, with each Si bonded to 4 Os. and each O bonded to 2 Sis. Also, it would be three-dimensional, rather than flat, with each SiO4 unit forming a tetrahedron (or a four-sided die, for us D&D nerds), with each O-Si-O bond at a 109.5° angle. You easily can see where metal salts like FeO, FeOH, CuO..., might case interruptions or distortions in the chain, causing weaker bonding. Distortions would be more likely, so small (individual molecules) these ionic salts would often be found in the spaces between two adjacent Os that are not bonded to each other. Larger crystals would cause more distortion as they physically wedge themselves into the lattice. This distortion-induced strain would potentially affect both hardness and toughness. Stopping now, before I write a dissertation.  |
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NRG
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Post by NRG on Jan 26, 2021 20:14:21 GMT -5
Quartz will scratch agate. Give it a try. Agate is a 6.5 Everything I’ve read says that agate is 7-7.5. Keeping in mind that the Mohs scale is ordinal, not absolute, this is pretty squishy. It makes sense that how it is formed influences the hardness. Actual silica content probably plays a role. Jasper is listed with the same hardness as well, but then there is a plethora “jasper” used as trade name, but geologically inaccurate. Mohs scale certainly isn't even close to linear. Diamonds something like 50 times harder than sapphire for instance. Yes, jasper has such a high clay/iron content it simply must be a bit softer. Of course ignoring misrepresented materials. It's my experience that clear agate like brazilian will scratch feldspars, but not quartz. Quartz will scratch brazilian agate, certainly one of the hardest agates. |
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Post by stephan on Jan 26, 2021 23:20:03 GMT -5
Everything I’ve read says that agate is 7-7.5. Keeping in mind that the Mohs scale is ordinal, not absolute, this is pretty squishy. It makes sense that how it is formed influences the hardness. Actual silica content probably plays a role. Jasper is listed with the same hardness as well, but then there is a plethora “jasper” used as trade name, but geologically inaccurate. Mohs scale certainly isn't even close to linear. Diamonds something like 50 times harder than sapphire for instance. Yes, jasper has such a high clay/iron content it simply must be a bit softer. Of course ignoring misrepresented materials. It's my experience that clear agate like brazilian will scratch feldspars, but not quartz. Quartz will scratch brazilian agate, certainly one of the hardest agates. Funny thing is, there actually is an absolute hardness scale, but I rarely see it used. Being a science guy, I would actually prefer that (it takes x psi. [or kg/cm 3] to scratch mineral y, for instance) over an ordinal scale. Something I can fully visualize. |
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