Maniraptoran Posted October 24, 2011 Share Posted October 24, 2011 what is a "nodule" and how do i know if i have one? Link to comment Share on other sites More sharing options...
Wrangellian Posted November 2, 2011 Share Posted November 2, 2011 Good question.. Looks like no one has an answer judging by when you posted this. I would guess it's kind of a catchall term describing something's form rather than its origin. I don't know how it differs from a concretion (and maybe to some it's the same thing), except that a concretion is specifically sedimentary whereas a nodule can be made of anything. Link to comment Share on other sites More sharing options...
Coco Posted November 2, 2011 Share Posted November 2, 2011 Hi, I'll try to explain simply. A nodule is a generally oval or rounded off shape, of rather homogeneous material. If it is "full", we call it nodule ; if it has a cavity with crystals, we call it geode. A concretion is the meeting of various chemical and physical bodies which solidify together. The stalactites of caves form by concretion. The water, loaded with carbon dioxide, dissolves the limestone of the rocks which it crosses and by arriving at the contact of the air (more warm) cavities, it deposits the transported calcite. This one accumulates in stalactites in the places where the water gets loose from the ceiling and in stalagmites in the places where drops fall on the ground. Hope that will help you to understand. Coco ---------------------- OUTIL POUR MESURER VOS FOSSILES : ici Ma bibliothèque PDF 1 (Poissons et sélaciens récents & fossiles) : ici Ma bibliothèque PDF 2 (Animaux vivants - sans poissons ni sélaciens) : ici Mâchoires sélaciennes récentes : ici Hétérodontiques et sélaciens : ici Oeufs sélaciens récents : ici Otolithes de poissons récents ! ici Un Greg... Link to comment Share on other sites More sharing options...
Wrangellian Posted November 4, 2011 Share Posted November 4, 2011 So I was half right... the concretion is something sedimentary which forms like concrete, while a nodule is any sort of rock that happens to have formed in a roundish shape? Link to comment Share on other sites More sharing options...
trout hunter Posted November 4, 2011 Share Posted November 4, 2011 blast it with a black light and you might see something cool! i like to keep one with me at all times when hunting as they seam to lead me to others... Link to comment Share on other sites More sharing options...
tracer Posted November 4, 2011 Share Posted November 4, 2011 So I was half right... the concretion is something sedimentary which forms like concrete, while a nodule is any sort of rock that happens to have formed in a roundish shape? um, not exactly. the things are not particularly simple so it might be best to read a few articles online about them to try to get even a basic understanding of the differences. but from a fossil-collecting standpoint, concretions are the thing. but i've met some nodules that were interesting too. Link to comment Share on other sites More sharing options...
Wrangellian Posted November 9, 2011 Share Posted November 9, 2011 (edited) I didnt mean that any round rock was a nodule, I know it's more complicated, but are there nodules that are found in sedimentary rock, that are not concretions? I guess so but every such thing we find around here is called a concretion - maybe things are different elsewhere. Edited November 9, 2011 by Wrangellian Link to comment Share on other sites More sharing options...
Vordigern Posted November 9, 2011 Share Posted November 9, 2011 where Im hunting in South Jersey the nodules tend to be rare but when I do find them they are typically matte black and if you look closely at the surface you may see a spot or two where there is something shiny and black peeking through the surface, also the ones I find tend to have a surface texture that I can only describe as a black rice ball look. Picture black pieces of rice formed into a nodule , they crisscross eachother. Now I also find a LOT of concretions where I hunt and they come in all kinds of crazy shapes and tend to be either grayish tan or black. The black ones are the only ones Ive ever found anything in and they were all found with the fossil already wearing out of the concretion, I havent been able to figure out how to get any intact concretions to open so who knows what may or may not be inside Link to comment Share on other sites More sharing options...
Lloyd Posted November 10, 2011 Share Posted November 10, 2011 The distinction made between "nodule" and "geode" is right on... they look the same on the outside, you only know which you have when you open them as any cavity <no matter how small> makes it a geode, while a nodule is solid. The distinction between "nodule" and "concretion" is, I think, slightly different, I think nodule implies there will be some sort of mineral or rock inside while concretion means there will be a fossil inside. I don't know how concretions form but nodules/geodes can be either volcanic or sedimentary... either way, they form in "bubbles" in the volcanic flow or sediment. I have some Crab concretions I traded for some years ago but I have never even attempted to open them. What I've heard is that you can break them open with a hammer to give you an idea of the location and position of the fossil inside... then glue them back together and use whatever it is you use to take away the material down to expose the fossil.... I also know a guy who had some small concretions who would put them in a can of water and freeze them in his freezer, thaw and repeat until the action of the ice broke them open. I never saw the result but his contention was that the concretion would break open at the fossil. I have no idea if it worked or not. Link to comment Share on other sites More sharing options...
jkfoam Posted November 10, 2011 Share Posted November 10, 2011 There is always a lot of discussion about concretions and it is an interesting topic of particular importance to the fossil collector as many fossils may be found associated with concretions. I think we would all benefit by taking a few minutes to Google "Concretions" and reading a few of the articles referenced for concretions. It will answer many of our questions, I know it answered several of mine. For example, I have always wondered why some concretions are found with crab claws sticking out of them. Well, one web site defined a concretion as "A compact mass of mineral matter, usually spherical or disk shaped, embedded in a host rock of a "different composition". Further, Concretions occur when a considerable amount of cementing material precipitates locally around a nucleus, often organic matter such as a leaf, tooth, piece of shell or fossil" (Extracted from www.desertusa.com). It seems that in some circumstances in geologic times when a crab died and was buried in sediment and because the crab shell and body contains a lot of phosphorus, decomposition releases the phosphorous materials to be converted to phosphate which subsequently reacted with the calcium in the water/mud sediments to form a cement rich in calcium phosphate. (Whew, what a sentence.) Anyway it wound up a concretion of sediment glued together with calcium phosphate with what ever part of the crab shell that remained as a fossil for us to find. All along the eastern US coastline from Florida to N. Carolina there are phosphate mines. They generally mine small phosphate rich concretions. The source of the phosphate that made the concretion is marine life that lived in the seas when the sediments were laid down. Shark teeth, bone, scales, etc., would have been rich sources of phosphorous to form the concretions. Now calcium phosphate is only one type of cement to glue concretions together. There are many others. Calcium carbonate, siderite, silica, iron oxides, etc., just to name a few. Concretions can vary in size. I have seen them about a 1/4 inch in diameter up to about 2feet in diameter. I think I'm going to Google concretions again and see what the size of the largest one is. Also note that a concretion does not necessarily have a fossil contained in it. If the organism contained in the center of the concretion when it was being formed completely decomposes or had no hard parts (shell, etc) then there will be no fossil for us to find when we break open the concretion. Jim Note- this post is very similiar to one I made on another board on this subject about 4 years ago. JKFoam The Eocene is my favorite Link to comment Share on other sites More sharing options...
painshill Posted November 11, 2011 Share Posted November 11, 2011 (edited) Could I just offer that the conventional geological distinction between a nodule and a concretion is that concretions are formed around a nucleus by precipitation of minerals, whereas a nodule is a replacement body. Typically, concretions form in sedimentary rocks or deposits when a mineral ‘cement’ fills the porous gaps around the sediment grains and then hardens. This is what makes the concretion more resistant to erosion than the sediment that it forms in, such that it may weather out of its host rock as a discrete lump. They are normally formed early on in the history of a stratum, after sedimentation has occurred. The word itself comes from the Latin words “con” and “crescere”… meaning “growing together”. Depending on prevailing conditions, concretions can be “pervasive” (simultaneous filling through the entire area where there is porosity and then gradual hardening of the entire mass) or “concentric” (successive deposition layers precipitating and hardening in a gradual manner). Frequently they are round or ovoid, but any shape is possible – even highly irregular shapes. The nucleus is frequently organic… a leaf, a shell, a sponge or whatever… and so concretions can contain fossils. In England, concretions of siderite have been found with WWII military ordnance inside them… so it doesn’t necessarily take a huge amount of time for them to form. There was also an infamous metal “artefact” found in the States within what was believed to be an ancient concretion. After cleaning and a more thorough inspection, it turned out to be a modern spark-plug. By contrast, a nodule is a secondary structure – typically a solid replacement body of chert or iron oxides formed during diagenesis (relatively low temperature and pressure modification) of a sedimentary rock. Chert (in limestone) and flint (in chalk) nodules form by recrystallisation of amorphous silica from dead marine organisms such as sponges and diatoms. They can also form in volcanic flows (from cooling and recrystallisation) or in ash deposits. Diagenesis may well alter the original mineralogy of a nodule and the rock that it forms in, but it’s a less severe modification than metamorphosis. Nodules are frequently round or rounded and may also be hollow, like geodes or vugs and may also have a crystal lining. They also frequently exhibit shrinkage patterns, as in septarian nodules. The truth is that these terms are used interchangeably without proper geological distinction and the term “nodule”, in particular, is widely used by non-geologists to describe anything rounded or knobbly in shape. Roger Edited November 11, 2011 by painshill Roger I keep six honest serving-men (they taught me all I knew);Their names are What and Why and When and How and Where and Who [Rudyard Kipling] Link to comment Share on other sites More sharing options...
Auspex Posted November 11, 2011 Share Posted November 11, 2011 ^^^ Good piece, painshill; thanks! "There has been an alarming increase in the number of things I know nothing about." - Ashleigh Ellwood Brilliant “Try to learn something about everything and everything about something.” - Thomas Henry Huxley >Paleontology is an evolving science. >May your wonders never cease! Link to comment Share on other sites More sharing options...
Scylla Posted November 11, 2011 Share Posted November 11, 2011 I read somewher that ammonia can trigger the precipitation of a crab concretion. Link to comment Share on other sites More sharing options...
xonenine Posted November 12, 2011 Share Posted November 12, 2011 well put Painshill, thanks. "Your serpent of Egypt is bred now of your mud by the operation of your sun; so is your crocodile." Lepidus Link to comment Share on other sites More sharing options...
Vordigern Posted November 12, 2011 Share Posted November 12, 2011 (edited) thank you jkfoam and painshill, that clears up a lot of the incorrect information and suppositions , think I'll print that info so I'll have it for future concretion/nodule questions Edited November 12, 2011 by Vordigern Link to comment Share on other sites More sharing options...
Wrangellian Posted November 17, 2011 Share Posted November 17, 2011 I second that, thanks for the essays. Link to comment Share on other sites More sharing options...
Maniraptoran Posted November 18, 2011 Author Share Posted November 18, 2011 wow, thats a lot of replies anyway heres why im curious about nodules. last summer i found a lot of these odd conretions while combing the beaches in Petoskey (i kept four or five but i left a lot more behind). i dissolved one in vinegar and found a shell piece in the center. would these count as nodules? 1. 2. Link to comment Share on other sites More sharing options...
painshill Posted November 18, 2011 Share Posted November 18, 2011 (edited) wow, thats a lot of replies anyway heres why im curious about nodules. last summer i found a lot of these odd conretions while combing the beaches in Petoskey (i kept four or five but i left a lot more behind). i dissolved one in vinegar and found a shell piece in the center. would these count as nodules? 1. 2. Well, I'd call 'em concretions. The presence of a piece of shell acting as a nucleus is highly diagnostic. Those look to be calcareous, bordering on limestone. There's a lot of coralline (and other oceanic) limestone in the area you're looking. The one on the extreme right may just have suffered erosion of softer mineral accumulations but, I believe it may be what is known as "stink stone".... a concretion out of limestone within which bubbles of hydrogen sulphide have been trapped. If you give it a bit of a scratch you will probably get the characteristic smell of rotten eggs! Scylla commented earlier in this thread that he had heard ammonia released by organic decomposition will trigger concretion around crab fossils. That's correct... any reactive gas from rotting organic material will do this. Ammonia, carbon dioxide, sulphur dioxide and most frequently hydrogen sulphide. Roger Edited November 18, 2011 by painshill Roger I keep six honest serving-men (they taught me all I knew);Their names are What and Why and When and How and Where and Who [Rudyard Kipling] Link to comment Share on other sites More sharing options...
lordpiney Posted November 19, 2011 Share Posted November 19, 2011 I havent been able to figure out how to get any intact concretions to open so who knows what may or may not be inside Dynamite works great! Link to comment Share on other sites More sharing options...
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