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Posted: Jan 02, 2010 06:11 Post subject: Scepters - (1) |
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When I posted this photo in the thread about my collection it occurred to me some of you may wonder about the the two types of scepters shown here, one a normal scepter and the one on its right a so-called "reverse" scepter. How do these two very different types end up growing side by side? I am not sure that anyone knows, but it is a very common phenomenon, especially in highly silicic volcanic rocks. My own theory is that the larger crystal, because of its superior height, was better able to attract nutrients (i.e. more silica) from the environment in which they both were growing, at the expense of the smaller crystal, so the former one was able to develop a large termination while the smaller one could only manage to produce a very small tip. Once the disparity in growth begins, it likely accelerates as the larger termination becomes bigger and better able to attract nearly all of the available silica is this relatively small enclosed system in which the amount of silica is probably very limited.
Perhaps this introduction to this subject will bring a response from someone who really knows what he or she is talking about. I am only speculating.
I have a painting of another example from Dugway, Utah, and will try to obtain a photo of it to post here as well.
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Normal scepter on left and reverse scepter on right. |
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Jean Sendero
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Posted: Jan 02, 2010 10:41 Post subject: Re: Scepters - (1) |
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I will not try to answer, agree or disagree by presenting an alternative explanation. To the contrary, I have an abundance of other questions when it comes to the formation of these quartz filled cavities from Liliana.
First, I would like to get some basic geological background, what is the host (more than highly silicic volcanic), are we talking intrusive or truly extrusive. What is the age?
Secondly, do we know the actual orientation of the crystals within the cavity or does it matter? Are they growing in all direction or have a tendency to grow from top to bottom, or bottom to top or else, sideways, or any of the above with no preferential orientations? Does gravity play a role here?
Thirdly, are these cavities filled with clay when first broken open? Do we know which clays? What does this cavity looks like when it is still "hot", prior to having crystallized.
I love these little cuties from Liliana which are, unfortunately, seriously, under-represented in my collection.
Cheers
Jean
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John S. White
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Posted: Jan 02, 2010 11:17 Post subject: Re: Scepters - (1) |
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Jean:
I am guessing that Peter Megaw will be able to answer many of your qestions. I do know that the rock is a rhyolite, but I know nothing about the orientation of the crystals when discovered, whether or not a cavities and/or seams are clay-filled, but I will try to ask the man who recovered these when I see him in Tucson in February and will file a report on what he says.
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Posted: Jan 02, 2010 11:43 Post subject: Re: Scepters - (1) |
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Here is a photo of the painting that I mentioned before, a quartz scepter from Dugway, Utah. The painting was done by Saul Krotki, it measures about 3 ft 4 in by 2 ft 6 in and is done in egg tempura and the piece was illuminated by Laser Light. Note the juxtaposition of a normal scepter next to a reverse scepter.
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Posted: Jan 02, 2010 16:31 Post subject: Re: Scepters - (1) |
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That ia a very good theory, John and very plausible. It would be similar to two plants growing next to each other and the larger one sucking up more of the smaller ones nutrients and stunting the growth or in space in a bianary system a larger star attracting and swallowing more matter than the smaller one and growing bigger..although two totally different examples it's the same idea.
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Peter Megaw
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Posted: Jan 02, 2010 18:18 Post subject: Re: Scepters - (1) |
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Clearly supply and ability to "consume" is fundamental to all crystal growth, but that alone probably will not explain the tapering versus sceptered terminations. I'm not sure what does, but it is pretty certain that this type of growth...Dugway, Liliana, Amado, etc. is a product of vapor phase growth. Vapors driven from a degassing and devitirfying rhyolitic ash-flow piles vent into open gas pockets and form these crystals. This may suggest that the reverse sceptres formed first as "normal crystals" and were later cannibalized by the sceptres as somwhat undersaturated vapor picks material off the one and immediately deposits it on the other. This would require a late stage in-fill to glaze over the crystal faces. The Lilianas are clearly multi-stage growth with the parasitic late crystals forming on the sides of the larger earlier ones...in nicely ordered fashion. I have dug similar crystals in several places...all silicic volcanic piles and rhyolite flow domes and they do not show any prefereable orientation other than in towards the center. For the last several years I have been getting large lots of chalcedony geodes from Chihuahua that show similar sceptres and tubular worms and stacks that I think reflect vapor vents. In some cases the sceptres occur across the geode from the vents, in other cases on them in other cases you can;t identify the vents. It has been fun culling a few hundred beautiful examples from the thousands that have come through. Interestingly although chalcedony seems more common or abundant in more intermediate compositions...dacite and andesite, I am not familiar with good sceptres from those compositions which may explain some of this.,,however, this is heading into the transition to the kinds of fluids and crystal growth that results in zeolite pockets in basaltic traps, where sceptres do occur
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geode with prominent tubular "vents" appear to have been built up like black smokers from venting vapors |
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![chalcedony geode gas vents.jpg](files/chalcedony_geode_gas_vents_214.jpg)
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geode with vents, spires and a druze of tiny crystals covering everything |
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![chalcedony geode gas vents and xals.jpg](files/chalcedony_geode_gas_vents_and_xals_122.jpg)
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geode with group of sceptres across from the inferred vent...I can almost see the vapors shooting out of the vent and depositing silica as crystals on the opposite wall |
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![chalcedony geode oriented.jpg](files/chalcedony_geode_oriented_737.jpg)
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geode filled with worms, elongate crystals ans sceptres...no visible vent |
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![chalcedony geode no orientation.jpg](files/chalcedony_geode_no_orientation_174.jpg)
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geode composed dominantly of ropy chalcedony.,,with crystal garden growing on inferred vent |
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![chalcedony geode xal cup.jpg](files/chalcedony_geode_xal_cup_116.jpg)
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geode completely lined with limpid sceptres, hopper crystals, "normal" crystals and reverse sceptres...even geode itself is crystallized |
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![chalcedony geode all xals.jpg](files/chalcedony_geode_all_xals_212.jpg)
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Posted: Jan 03, 2010 06:26 Post subject: Re: Scepters - (1) |
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I have often felt that the reverse scepters appear cannibalized because they seldom, if ever, are bounded by sharp faces, but appear etched or rough surfaced instead. However I have always found it difficult to comprehend how one crystal can experience growth while its neighbor is being dissolved. I still have difficulty with this so here we have a classic enigma. The appearance strongly suggests that this has occurred, but reason tells us that it makes no sense.
By the way Peter I am guessing that your chalcedony photos are yellow because you are using incandescent lighting.
While on the subject of scepters, here is one of my favorites, from Tafelkop, Gobosebberge,Namibia, 4.5 cm across.
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Posted: Jan 03, 2010 07:45 Post subject: Re: Scepters - (1) |
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Here is one of the chalcedony geodes from Chihuahua that I obtained from Peter Megaw back in 2004. It is very difficult to get a close-up on the crystals inside.
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Amir Akhavan
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Posted: Jan 03, 2010 08:35 Post subject: Re: Scepters - (1) |
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As far as I know, there's no common theory on scepter formation.
All I've read is speculative and not based on experiments (almost worthless).
So I'm not the guy who knows what he's talking about that John is looking for. Maybe when I finished the Sunagawa crystal books, I become enlightened, but so far I'm still confused.
The basic question is: why does additional material get preferentially added on some part of the crystal to finally form a scepter, instead of being evenly deposited on already present crystal faces?
What we seem to know:
- Scepters are a growth form that is typical for quartz (Can anyone name another mineral with similar growth forms? There must be some)
- Scepters are frequenty associated with skeletal growth forms
- Scepters always show normal habit
- Scepters don't seem to grow on certain habits and growth forms, like needle quartz and split growth forms (artichoke etc.)
- The surface structure of the faces in skeletal and scepter quartzes often differs from the usual. The prisms on many scepters have no or a very weak striation (not to be confused with curved steps as seen in the Goboboseb specimen seen above)
- Scepters are probably low temperature formations
- The striation on prism faces is typical for quartz. In that sense, prism faces are special.
- The striation on prism faces is *not* caused by environmentally driven oscillations in growth speed. Industrial quartz plates grown under steady state conditions do or do not develop striation depending on the composition of the growth medium (NaOH - no striation, NaCl - striation) and possibly the growth speed. There are other arguments against the "environmental theory of striations"
- Hence, the mode of growth on prism faces is different from that on rhombohedral faces.
What we do not seem not know:
- If there is only one type of scepter. Are, for example, scepters found in alpine-type fissures the same as those associated with chalcedony roses? We do not know.
Reverse scepters are apparently absent in alpine-type fissures, common in volcanic gas cavities. Both are commonly associated with the development of amethyst color centers, though.
- Are all scepters lamellar in structure as sometimes is claimed? (I'm not a fan of the Bambauer/Friedlander structural schism that is sometimes proposed)
Provocative speculative statements:
- A "second generation quartz crystals explanation" is not an explanation
- An interruption of growth is not an explanation
- Non-quartz material accumulated on the surface is not a good general explanation: there are all sorts of phantoms that do not interfere with the habit of the crystals and it would not explain multiple scepters
- Scepters represent one or two modes of growth
- At least one mode of growth allows for multiple scepters to form (see Goboboseb scpecimen).
- Scepters may occur because there's a environmentally triggered switch in the mode of growth of a single crystal (typical example: alpine-type fissure scepters - amethyst normal habit on smoky tessin habit)
- Reverse scepters and normal scepters in one pocket are the same thing.
- Scepters occur in quartz because the growth mechanism differs in some way between the prism and the rhombohedral faces. Under certain conditions this makes it difficult to evenly accumulate material on an "old" prism face while it is still possible to deposit material on the rhombohedral faces and newly formed prism faces.
Slightly off-topic
- Ostwald ripening does not account for size differences of crystals larger than a few millimeters (some would even say micrometers). So, in my opinion, having one crystal feeding on another is not a good theory once they are larger than a certain size. This explanation is sometimes given for the growth of crystals in alpine-type fissures, but that is a completely different discussion....
- I agree on the significance of the tubes in the chalcedony roses (that are also seen in chalcedony roses from Brazil, for example). I guess it's watery solutions and not vapor, though: 1. I do not know how much silica will be transported in a gas phase (I guess very little) 2. Slow fluid dynamics would probably better account for the strange rounded shapes around the tubes, vapor would appear to fast and chaotic for the development of such shapes on such a small scale. Pure speculation.
To show what is my point, I've attached an images of something I also call a scepter. So I stress the suspected mode of growth differences. I consider the scepter shape just as a consequence of these (assumed but very likely) differences.
A small double terminated amethyst clinging to the scarred prism face of a large old smoky quartz of Tessin habit. It is from an alpine-type fissure near Dasoto village, Kato Nevrokopi Basin, Greece, that just happened to grow from a single spot on the prism and not on the tip, but it did so parallel to the crystallographic axes of the older crystal, like all scepters.
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Posted: Jan 03, 2010 11:05 Post subject: Re: Scepters - (1) |
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I am delighted that Amir has entered the discussion. He is one of the most knowledgeable people when it comes to quartz. While he has few answers, he does raise some interesting questions and observations.
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Posted: Jan 03, 2010 11:22 Post subject: Re: Scepters - (1) |
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In response to Amir's question about other minerals showing scepters, I am certain that there are many examples of calcite scepters. In addition, I have this little gem in my collection, a beryl from Teofilo Otoni, Minas Gerais, Brazil.
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Posted: Jan 03, 2010 11:28 Post subject: Re: Scepters - (1) |
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"Reverse scepters" very distinct in some Brazil anatases.
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Posted: Jan 03, 2010 12:58 Post subject: Re: Scepters - (1) |
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aqua scepter
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Peter Megaw
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Posted: Jan 03, 2010 13:06 Post subject: Re: Scepters - (1) |
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I have seen "sceptres" like this before that look like the crystal expanded and grew laterally once it grew past adjoining crystals that bound it in rather than a second stage or unrestricted but differential growth..
Surely someone can chime in with a tourmaline sceptre/// Jesse???
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Posted: Jan 03, 2010 13:12 Post subject: Re: Scepters - (1) |
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I have dug through and sold maybe 50-100 kilos of all different kinds of grades or tourmalines in the past year mostly afghan and pak stones and I don't recall every seeing a scepter..I have found tons of cool rehealed ones..I actually had a small collection going of bent ones, broken then re-healed ones(fav are the 90 degree broken rehealed ones), curved ones..I sold most of them in a lot once i compiled them..might see if i can dig up a pic or two. Never really saw a scepter..found some that had the center portion stick up farther..some a good deal farther but I always attributed that to the "rind" breaking off and leaving the stronger central core
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Amir Akhavan
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Posted: Jan 03, 2010 13:29 Post subject: Re: Scepters - (1) |
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I've read that tourmaline scepters occurred on Brazil
(in an old article on scepters by G. Niedermayr).
No idea how that would look like, though.
Anatase is a crazy mineral, so many forms and habits... I wonder what scepters look like.
I agree with Peter, many things go as scepters that one would rather not call so. Scepters from Arkansas, for example, are usually crystals with a thin stem caused by growth inhibition by calcite that got dissolved naturally or during the cleaning process.
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Posted: Jan 03, 2010 13:38 Post subject: Re: Scepters - (1) |
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Peter Megaw wrote: | Surely someone can chime in with a tourmaline sceptre |
Here I am with these "not true" scepters
( acording https://www.mineral-forum.com/message-board/viewtopic.php?p=8961#8961 & https://www.mineral-forum.com/message-board/viewtopic.php?p=8986#8986 )
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Elbaite "not true" sceptre Barra de Salinas, Coronel Murta, Minas Gerais, Brazil Mined in 1994 Specimen size: 4 × 0.6 × 0.6 cm. Photo: Reference Specimens -> http://www.fabreminerals.com/specimens/SHQ-reference-fine-minerals.php#TP66H1 |
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![Elbaite Barra de Salinas.jpg](files/elbaite_barra_de_salinas_106.jpg)
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Sceptered (not true sceptre) cuprian Elbaite (Paraíba) São José da Batalha, Paraíba, Brazil Mined about 1992 Specimen size: 3.2 × 0.8 × 0.8 cm. Photo: Reference Specimens -> http://www.fabreminerals.com/specimens/SHQ-reference-fine-minerals.php#TD46L1 |
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Posted: Jan 03, 2010 13:43 Post subject: Re: Scepters - (1) |
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And I agree with Amir that there are scepter-like forms that are not true scepters. The best examples are the quartzes from China in which the stems were growing at the same rate as the the dolomite which enclosed them. When the dolomite ceased to grow, the quartz could then develop without restrictions and so you have normal-appearing "heads" with sharp faces on etched-appearing stems. The locality is Shangbao, Leiyang, Hunan. Some say the dolomite has been dissolved by the dealers to expose the stems.
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A poor photo of the quartz from China that resembles scepter quartz but is not. This piece is in the Erika Pohl collection at Schloss Freudenstein, Freiberg, Germany. |
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John S. White
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Posted: Jan 03, 2010 13:48 Post subject: Re: Scepters - (1) |
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I suspect that Jordi's "scepters" are not true scepters. The first appears to be one of those tourmalines where a thni skin of tourmaline on the lower part has broken away. The Paraiba crystal appears to be one where the stem may have grown in feldspar which was subsequently eroded away. I would not consider these scepters.
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Amir Akhavan
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Posted: Jan 03, 2010 13:49 Post subject: Re: Scepters - (1) |
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Very pretty!
:-)
So do we know anything about them. Did they grow freely?
It's strange that the color transitions look as if they were not really related to the scepter formation...
The most amazing thing about the Erika Pohl Collection specimen is that they did not break it (so far) ;-)
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