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Ru Smith
Joined: 13 Oct 2012
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Posted: Mar 24, 2013 11:54 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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So that's intriguing. What favoured this growth habit at Blackdene and Boltsburn and not at the other mines in the district (bar rare exceptions)? Enjoy the sun!
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Ru Smith
Joined: 13 Oct 2012
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Posted: Mar 24, 2013 12:15 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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Pete Richards wrote: | Our messages may have crossed... but those oscillations are typical, and I presume they represent small changes in the relative growth rates of fluorite and pyrite. |
Yes, we posted simultaneously. The shared "microstratigraphy" of the pyrite columns confirms your balanced growth interpretation. I had never seen this before and yesterday was the first time I looked so closely at that particular crystal. Are there any "brackets" on possible growth rates available from lab work (or any other source)?
I thought to take a look at growth zoning patterns in the elongate crystals to look for further clues on growth patterns. I can see nothing in the Boltsburn crystal, but the Blackdene one is interesting. Two of the long faces grew much faster (one faster than the other) than the other two, such that the early core of the crystal is "left behind" against one long edge. Perhaps this is the explanation for Jesse's yellow corner zone in his Rogerley crystal over on the Fluorite growth zoning thread.
Dissection of such a zoned elongate crystal would be revealing.
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Fluorite Blackdene Mine, Ireshopeburn, Weardale, Co Durham, UK. 95 mm specimen with 45 mm elongate fluorite (L/W = 5). |
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Steve Maslansky
Joined: 01 Mar 2013
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Location: Prescott, Arizona
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Posted: Mar 24, 2013 12:25 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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Here is a example from the Denton Mine of Illinois
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Fluorite on sphalerite Denton Mine, Hardin County, Illinois, USA 4 x 4 inches |
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Jordi Fabre
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Posted: Mar 24, 2013 12:54 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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Double-dealing: elongated and etched! ;-)
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Fluorite with Calcite Annabel Lee Mine, Hardin County, Illinois, USA Specimen size: 7.8 × 6.2 × 3.1 cm Mined on 05/17/1985 Former collection of James Catmur Fluorescent long & short UV
Photo: Reference Specimens |
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Jesse Fisher
Joined: 18 Mar 2009
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Posted: Mar 24, 2013 13:56 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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Ru - the Rogerley "yellow corners" are always on an outside corner of the crystal, never at the base or core. Also, they often occur on crystals that are otherwise fairly equant in shape, and show no suggestion of preferential growth in a particular direction.
It occurred to me that, while twinning is common in North Pennines fluorite, the examples of highly elongate crystals I've seen are usually untwinned. Looking through the collection, I found a few examples of twinned crystals that show a tabular habit, but nothing as extreme as the Boltsburn and Blackdene crystals previously posted. Below is one from the Boltsburn Mine. I also have a few, though much smaller examples from the Rogerley.
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Fluorite Boltsburn Mine, Rookhope, Weardale, England 11x8x8 cm overall size penetration-twinned crystal showing elongate habit |
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Mark Ost
Joined: 18 Mar 2013
Posts: 516
Location: Virginia Beach
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Posted: Mar 24, 2013 14:32 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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Spectacular. My Goodness.
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Malcolm Southwood
Joined: 04 Feb 2011
Posts: 15
Location: Melbourne
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Posted: Mar 24, 2013 15:29 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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Jordi, thanks for the correction, (thereby sparing future blushes!)
Pete,
I'd like to revisit, for a moment, the screw dislocation idea for these elongated crystals. Radovan (2004, Rocks and Minerals 79 (6) pp 415-417) attributes the development of vicinal faces and growth hillocks in fluorite to spiral growth as a result of such dislocations. Surely this would suggest that the mechanism might not be limited to whisker crystals?
Cheers
mal
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Jordi Fabre
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Pete Richards
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Posted: Mar 24, 2013 18:50 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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Malcolm Southwood wrote: |
I'd like to revisit, for a moment, the screw dislocation idea for these elongated crystals. Radovan (2004, Rocks and Minerals 79 (6) pp 415-417) attributes the development of vicinal faces and growth hillocks in fluorite to spiral growth as a result of such dislocations. Surely this would suggest that the mechanism might not be limited to whisker crystals?
Cheers
mal |
You are right that screw dislocations lead to spiral growth on macroscopic crystals, and in fact they are probably the main promoter of growth in many environments where the level of supersaturation is low, because spiral growth eliminates the need to nucleate new layers of atoms, which is the growth rate limiting step in classical layered growth mechanism (in the absence of screw defects and their resulting "spiral staircase" of a single atomic layer).
The problem is that screw defects are abundant - a few to millions per square centimeter. And they should be expected to occur in roughly the same density intersecting all faces of the same form (e.g. a cube). They may promote growth overall, but unless they are selectively concentrated perpendicular to one cube face, they will not promote differential growth. I'm not aware of anyone suggesting a mechanism by which they are selectively present in one direction in macroscopic crystals.
In whisker crystals, it does seem to be the case that whiskers are promoted by a single screw defect.
_________________ Collecting and studying crystals with interesting habits, twinning, and epitaxy |
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Malcolm Southwood
Joined: 04 Feb 2011
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Location: Melbourne
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Posted: Mar 24, 2013 20:04 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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Thanks Pete,
That's helpful.
mal
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John Medici
Joined: 02 Mar 2011
Posts: 124
Location: Ohio
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Posted: Mar 25, 2013 13:18 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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The midwest USA has produced many elongated fluorites and pyrites, and a few other elongated crystals that are usually not elongated. Below are a couple of examples, maybe a few duplicates of ones posted in USA-Ohio. Halls Gap, Kentucky has pyrite needles (and rings) that are often found in small quartz geodes. I have also found an elongated cube (only one, of around 500 collected over the years) at Fort Wayne, Indiana, which is similar to the Weardale ones shown; it is a yellow one, clear with sharp edges like the other ones found there.
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Fluorite (needles) Suever Quarry, Delphos, Ohio, USA 5 mm long needles |
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Fluorite Suever Quarry, Delphos, Ohio, USA 1 cm high sheaf of elongated needles, with cube on top |
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Fluorite needles on head of pin Suever Quarry, Delphos, Ohio, USA around 5 mm long Most Delphos fluorites of this type were found in petroleum-filled pockets, sometimes associated with pyrite. |
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Fluorite needle, bifurcated Suever Quarry, Delphos, Ohio, USA around 5 mm long George Robinson SEM photo |
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Ru Smith
Joined: 13 Oct 2012
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Posted: Mar 26, 2013 18:50 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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Fascinating specimens, John.
Do you know of any published research that attempts to explain them?
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Duncan Miller
Joined: 25 Apr 2009
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Location: South Africa
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Posted: Mar 26, 2013 23:12 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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With rapid crystallization many minerals form dendritic, acicular or skeletal crystals. In slags for example, where crystal growth is stopped by rapid solidification, one commonly finds skeletal olivines and dendritic magnetite. Ice crystals in snow are a more familiar example. Micromounters see habits that are not frequently encountered in hand specimen. For instance, on page 15 of LAPIS 4/2010 there is a photograph of a clearly dendritic fluorite crystal 1,5 mm long. With more prolonged slow growth these dendritic, acicular and skeletal crystals may fill out and assume more equant shapes. It is not hard to imagine the fluorite needles illustrated above being the rapid-growth precursors to the fatter elongated examples, preserving the same aspect ratio. This would point to fast initial dendritic growth followed by a slower more prolonged equant growth. (This is speculative because I am not an expert on crystal growth, although I have studied a lot of slags petrographically).
Duncan Miller
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Pierre Joubert
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bugrock
Joined: 24 Nov 2008
Posts: 137
Location: Michigan
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Posted: Mar 27, 2013 09:33 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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The image of the fluorite on the head of a pin from Ohio is not perfectly sharp but the pattern of growth suggests a series of small stacked cubes, perhaps some elongate and more rectangular than cubic.
Have seen elongate micro cuprite with small steps suggesting a series of stacked cubic to rectangular microcrystals. Without seeing fine detail such groups can look like tapering filaments when the diameter of the cubic crystals changes very gradually along the axis of stacking.
So for the smaller specimens of fluorite one could imagine that the component crystals are conventional cubic form. Might this also happen in the larger specimens? If so two or more cubes would have to grow together with the same diameter and with perfect fusion between the components. Seems unlikely but another possibility to consider?
George
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ian jones
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Posted: Apr 02, 2013 10:57 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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How could I possibly forget Rogerley, the place that I spend my summers drinking beer and digging rocks.
Although atypical, this rather grubby purple fluorite 6x3x3 cm on the top of the specimen came from the vein as we drove forward in 2012 rather than the flats to the side of the vein where the green fluorite occurs. For some reason the vein fluorites are purple and the flats green.
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Fluorite Rogerley Mine, Rogerley Quarry, Frosterley, Weardale, Co. Durham, England, UK 8x7cm |
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ian jones
Joined: 29 Jul 2009
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Location: london
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Posted: Apr 03, 2013 06:50 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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I'm starting to think that elongated fluorite crystals from the northern Pennines are perhaps a bit more common than previously thought.
In my collection I have a second Blackdene and also a Boltsburn piece, and, not mentioned as a locations elsewhere, also one from Coldberry Mine, Middleton in Teesdale and an elongated and a tabular crystal from the Cement Quarry, Eastgate.
Additionally, the frontispiece in the current issue of the UK Journal of Mines & Minerals shows an elongated crystal from Frazer's Hush Mine.
However, the extremely distorted crystals like the British Museum's Boltsburn piece shown above remain exceptionally rare.
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Jesse Fisher
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Posted: Apr 03, 2013 13:49 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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Another unusual specimen from the Blackdene Mine. A cluster of uniformly twinned crystals with one larger tabular, untwinned crystal perched on the face. The tabular crystal is 2.5 cm on edge. I haven't seen many that show both habits on the same specimen.
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Fluorite with minor chalcopyrite Blackdene Mine, Weardale, England 11x7x4 cm overall size. A cluster of twinned crystals with one larger, untwinned, tabular crystal on the face. |
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John Medici
Joined: 02 Mar 2011
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Location: Ohio
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Posted: Apr 08, 2013 19:28 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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Ru Smith wrote: | Fascinating specimens, John.
Do you know of any published research that attempts to explain them? |
Ru, I don't think anyone has done much studying of the Delphos fluorites. My sons and I have collected most of the existing needle type fluorites and the single needles, including a few bifurcated ones, were mainly from one pocket which was oil-filled, around 3+ cm diameter, and contained no pyrites. (We were lucky to notice the micro fluorites in the oil, which we eliminated with a gasoline wash. The only person who has looked at these is George Robinson, and he took the SEM photo of the bifurcated one (which seems to be more like a single crystal than stacked cubes, although that is debatable I guess). I also gave him a piece of another pocket around 15 cm at longest dimension which contained fluorite "sheaves" and some micro pyrites, with a few cubes, as shown in the one picture (and George also has some of the petroleum from that pocket in case it has any relevant value).
Sorry for the late response; I'm often away from the computer for a week or more.
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Jordi Fabre
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Posted: Apr 09, 2013 02:04 Post subject: Re: Elongate and tabular fluorite crystals - (7) |
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Considering its quality and interest I numbered and moved this topic to the Featured Columns of FMF section.
For more info about how the Featured Columns of FMF section works, please use this link
BTW, Ru Smith the author of the first post in this thread and therefore creator of it is already mentioned in the THANK-YOUs for the reference thread
Jordi
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