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Matt_Zukowski
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Location: Alaska
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Posted: Jul 18, 2013 21:45 Post subject: Re: Calcite Forms - (4) |
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Here is a strange calcite specimen that i think of as something like a double twin. This Z shaped specimen (which unsurprisingly I call the Z, given my last name) seems to be some sort of a double twin. The top and bottom twin off the diagonal part of the specimen. For each face on the top part of the specimen there is a corresponding face on the bottom part, with the same striation patterns, so this specimen must be crystallographically connected throughout.
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Calcite Verchniy Mine, Dal'negorsk, Primorskiy Kray, Far-Eastern Region, Russia 10.2 x 6.8 x 5.0
Photo by James Elliot, Fine Minerals International |
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Calcite Verchniy Mine, Dal'negorsk, Primorskiy Kray, Far-Eastern Region, Russia 10.2 x 6.8 x 5.0
Photo by Joe Budd |
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Calcite Verchniy Mine, Dal'negorsk, Primorskiy Kray, Far-Eastern Region, Russia 10.2 x 6.8 x 5.0
Photo by Joe Budd |
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Calcite Verchniy Mine, Dal'negorsk, Primorskiy Kray, Far-Eastern Region, Russia 10.2 x 6.8 x 5.0
Photo by Joe Budd |
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Calcite Verchniy Mine, Dal'negorsk, Primorskiy Kray, Far-Eastern Region, Russia 10.2 x 6.8 x 5.0
Photo by Joe Budd |
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Calcite Verchniy Mine, Dal'negorsk, Primorskiy Kray, Far-Eastern Region, Russia 10.2 x 6.8 x 5.0
Photo by Joe Budd |
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Calcite Verchniy Mine, Dal'negorsk, Primorskiy Kray, Far-Eastern Region, Russia 10.2 x 6.8 x 5.0
Photo by Joe Budd |
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Calcite Verchniy Mine, Dal'negorsk, Primorskiy Kray, Far-Eastern Region, Russia 10.2 x 6.8 x 5.0
Photo by Joe Budd |
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Calcite Verchniy Mine, Dal'negorsk, Primorskiy Kray, Far-Eastern Region, Russia 10.2 x 6.8 x 5.0
Photo by Joe Budd |
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Ru Smith
Joined: 13 Oct 2012
Posts: 362
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Posted: Aug 05, 2013 22:24 Post subject: Re: Calcite Forms - (4) |
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Much in common this old Dana drawing and the calcite crystal from Huqf, Oman.
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Calcite. Huqf, Oman. 15 mm. |
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Calcite. Huqf, Oman. 15 mm. |
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Ru Smith
Joined: 13 Oct 2012
Posts: 362
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Posted: Aug 06, 2013 22:14 Post subject: Re: Calcite Forms - (4) |
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By the way, I really like the termination photo above (0087 Calcite Verchniy Mine, Dal'negorsk 8.jpg).
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Ru Smith
Joined: 13 Oct 2012
Posts: 362
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Posted: Aug 08, 2013 20:38 Post subject: Re: Calcite Forms - (4) |
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Calcite twin with hematite inclusions, Gloucestershire.
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Calcite twin. Cromhall Quartzite Quarry, Gloucestershire, England, UK. 39 mm long. |
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Calcite twin. Cromhall Quartzite Quarry, Gloucestershire, England, UK. 39 mm long. |
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Ru Smith
Joined: 13 Oct 2012
Posts: 362
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Posted: Sep 29, 2013 22:57 Post subject: Re: Calcite Forms - (4) |
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Two of several beautiful twins to 65 mm on 26 cm matrix.
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Calcite twins on matrix. Elmwood, Tennessee, USA. 60 and 65 mm on 26 mm matrix. |
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Ru Smith
Joined: 13 Oct 2012
Posts: 362
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Posted: Oct 03, 2013 21:33 Post subject: A couple of American Calcite Twins |
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I particularly like the Meikle example.
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Calcite twins on matrix. Ocotillo, Imperial County, California, USA. Twins to 14 mm on 3 cm matrix. |
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Gemmy lustrous calcite twin. 1450 Level, Meikle Mine, Bootstrap District, near Carlin, Elko County, Nevada, USA. 3 cm. |
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Roger Warin
Joined: 23 Jan 2013
Posts: 1177
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Posted: Oct 04, 2013 01:59 Post subject: Re: A couple of American Calcite Twins |
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Hi Ru,
The first is the rarer of the two. This is the twin on {01.2}.
Roger.
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Ru Smith
Joined: 13 Oct 2012
Posts: 362
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Posted: Oct 04, 2013 18:28 Post subject: Re: A couple of American Calcite Twins |
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Thanks Roger,
Here's a similar one from Dalnegorsk and also another view of the more common habit in the Meikle example, with twin rotated to show composition plane horizontal.
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Calcite twin. Dalnegorsk, Russia. 26 mm. |
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Calcite twin. 1450 Level, Meikle Mine, Bootstrap District, near Carlin, Elko County, Nevada, USA. 3 cm. |
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Ru Smith
Joined: 13 Oct 2012
Posts: 362
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Posted: Oct 05, 2013 13:19 Post subject: Re: A couple of American Calcite Twins |
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I think I'm starting to "get" the different classes of calcite twin.
Here's an old page of drawings from Dana and a selection of four twins to represent the four classes (if I've understood correctly). Twin planes are vertical in the bottom three twins and horizontal in the top one.
Most accessible to me is to think of the angles between c-axes of each member in the twins. I was confused about the labeling of some of the twin planes after taking a look at a paper by Bruno et al. (2010) until it dawned that there’s been a change in the unit cell definition since Dana’s day (old news to most of you!). So, {10.1} becomes {10.4} and so on.
So, we have class 1 with c-axes parallel (twin plane {00.1}): 29-31 in the Dana pictures and the Meikle twin (top) in the photo. Common according to Dana and I agree; I've seen lots of these.
In class 2, c-axes are inclined at a little over 127 degrees (twin plane {01.8}): 32-35 in Dana and the Wuzhou twin (right) in the photo. Very common according to Dana, but I've not seen that many of these.
In class 3, c-axes are inclined at a little less than 91 degrees (twin plane {10.4}): 36-37 in Dana and the Leiping twin in the photo (left). Not common according to Dana, but I've seen many examples of these.
In class 4, c-axes are inclined at a little less than 54 degrees (twin plane {01.2}): drawing 38 in Dana and the Torr works twin in the photo (bottom). Rare according to Dana and I agree.
Obvious to many of you I’m sure, but it took me a while get this more or less clear this morning.
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Page of calcite twins, Dana 6th edition. |
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Four calcite twins. Top, Meikle Mine, Nevada, USA; left, Leiping Mine, Hunan, China; right, Wuzhou Mine, Guangxi, China; bottom, Torr Works Quarry, Somerset, UK. Meikle twin at top is 3 cm tall. |
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Jordi Fabre
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Joined: 07 Aug 2006
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Location: Barcelona
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Posted: Oct 05, 2013 13:38 Post subject: "A couple of American Calcite Twins" merged in Calcite Forms - (4) |
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Considering the kind of development of this thread I merged it in this other thread: Calcite Forms - (4) where many similar questions were already discussed, so better coherence and visibility for "A couple of American Calcite Twins"
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Pete Richards
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Location: Northeast Ohio
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Posted: Oct 07, 2013 09:18 Post subject: Re: A couple of American Calcite Twins |
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Ru Smith wrote: | I think I'm starting to "get" the different classes of calcite twin. |
Yes, you've got it exactly right, although the twins on {10.4} and {01.2} are about equally uncommon in my experience.
Your comment about the change of unit cell bears emphasis, as the issues it raises are often overlooked, even by professional mineralogists. The Miller indices of a form must be expressed relative to a specific choice of unit cell. The unit cell establishes the coordinate system for the crystal geometry, and multiple unit cells can be chosen that can describe the same crystal morphology. In the pre x-ray days, there were conventions for choosing a unit cell, related to things like major faces, cleavages, etc., and these varied over time and from country to country. (Many of the drawings in textbooks are inaccurate because the text reports one unit cell and the drawing presumes a different one.) Now we can determine much more about the details of the atomic structure of a crystal, and there are rules that dictate the choice of unit cell; there should be no ambiguity about the "official" unit cell, even though other choices can describe the same geometry. In the case of calcite, the structural cell has a c-dimension that is four times as long relative to the a-dimension (c= 17.061Å, a= 4.9896Å, and c:a=3.4193) as is the case with the old morphological cell (for which c:a = 0.8543, and the absolute lengths along a and c could not be known). Thus a given translation is only 1/4 as many units along the structural c-axis as it is along the morphological c-axis. Because Miller indices are related to reciprocals of translations, this results in final indices that are four times as large, when expressed relative to the structural cell, as they are when expressed relative to the morphological cell.
What twin is common seems to vary from location to location. At some places, {10.4] is quite common and is apparently almost the only twin found. At others, only twins on {00.1} appear to be found. I've never found all four twins from one locality.
_________________ Collecting and studying crystals with interesting habits, twinning, and epitaxy |
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Conny Forsberg
Joined: 15 Nov 2014
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Location: Sweden
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Posted: Nov 15, 2014 10:48 Post subject: Calcite crystal with phantom of hematite (?) covered calcite in different crystal shape. |
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I got this one during late eighties or early nineties, and guess it was mined in the eighties.
Conny
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marco campos-venuti
Joined: 09 Apr 2014
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Location: Sevilla
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Posted: Nov 15, 2014 11:30 Post subject: Re: Calcite crystal with phantom of hematite (?) covered calcite in different crystal shap |
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I guess the phantom is just a scalenohedral crystal of black calcite, not really hematite.
Location can be Egremont, Cumbria.
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Conny Forsberg
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Posted: Nov 15, 2014 11:51 Post subject: Re: Calcite crystal with phantom of hematite (?) covered calcite in different crystal shap |
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Thanks Marco, possible locality noted!
Regarding the scalenohedral crystal one sees clearly under microscope tiny metallic looking crystals on most sides but one crystal face reveals white calcite. The tiny crystals has plate like as well as needle like forms. Scalenohedral phantom, for sure clad with other mineral. The strong double refraction unfortunately makes it very hard to shoot a good focused photomicrograph.
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John Betts
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Posted: Nov 15, 2014 12:14 Post subject: Re: Calcite crystal with phantom of hematite (?) covered calcite in different crystal shap |
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I sold a similar crystal years ago that was from the Stank Mine. I too guessed the inclusions were hematite.
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Elise
Joined: 22 Dec 2009
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Location: New York State
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Posted: Nov 21, 2014 12:35 Post subject: Re: Calcite crystal with phantom of hematite (?) covered calcite in different crystal shap |
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I hope Pete Richards will comment more on the crystallography of this really neat Calcite crystal: Calcite crystal with phantom of hematite (?) covered calcite in different crystal shape - Conny, perhaps you can add some more pictures to show different sides. I am unclear from Marco's comments if the host crystal is scalenohedral or if he is saying that just the phantom within is - if the latter, what happened during its growth?
Conny Forsberg wrote: | The strong double refraction unfortunately makes it very hard to shoot a good focused photomicrograph. |
If it is not scalenohedral and those are parallel prism faces as they seem to appear in your photo, then you should be able to image it perpendicular to the prism face and c-axis without double refraction as one ray is directly behind the other - or do the inclusions at different depths still just make a mess due to that? I have not thought about how inclusions would be manifested in that direction. If you look at figure 1 which is an image Dr. Mickey Gunter gave me to illustrate this page, you will see what I mean: https://www.nordskip.com/calciteoptics.html#1
(Link normalized by FMF)
Thanks so much for sharing this on FMF Conny!
Best wishes,
Elise
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Pete Richards
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Posted: Nov 21, 2014 14:38 Post subject: Re: Calcite crystal with phantom of hematite (?) covered calcite in different crystal shap |
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OK, Elise, you've called me out! I was thinking of commenting on that crystal, but let it slide by. I have no confident opinion about the locality.
As far as I can judge from one picture, the external form of this crystal is a prism terminated by (one end of) a hexagonal bipyramid. Working with known form for calcite, I think the drawing shows the right faces.
What happened to it to change it from scalenohedral to prismatic? I can't comment on how it happened, except to appeal to the standard ideas that something changed - temperature or chemistry, and that's a weak "explanation". I will say that I've seen the same change - except going to a steep hexagonal bipyramid rather than a prism - at several other localities.
Kostov and Kostov (Crystal Habits of Minerals) also show a sequence of calcite forms that includes scalenohedral going to prismatic. They also cite various studies that attribute changing habits to changing temperature, degree of supersaturation, pH, and CO2 content. In both of the examples I am familiar with, the crystals grew in cavities in limestone that was never metamorphosed, so changes in temperature, pH, and CO2 concent should have been quite modest, though a change in supersaturation seems more possible. The growth conditions of Conny's crystal could have been entirely different, of course.
On another piece of this topic, you can get rid of double refraction by using a polarizing filter on your camera lens. Not sure I've actually tried it, but it should work. As you turn the filter, you can alternately block one and then the other image. This should work in any orientation.
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_________________ Collecting and studying crystals with interesting habits, twinning, and epitaxy |
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marco campos-venuti
Joined: 09 Apr 2014
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Location: Sevilla
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Posted: Nov 21, 2014 14:59 Post subject: Re: Calcite crystal with phantom of hematite (?) covered calcite in different crystal shap |
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I found also a rhombohedral phantom in a calcite prism.
Mineral: | Calcite prismatic crystal with romboedral phantom, |
Locality: | Verkhnii Mine, Dalnegorsk, Dalnegorsk Urban District, Primorsky Krai, Russia | |
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Dimensions: | 35 mm |
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Elise
Joined: 22 Dec 2009
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Location: New York State
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Posted: Nov 21, 2014 15:01 Post subject: Re: Calcite crystal with phantom of hematite (?) covered calcite in different crystal shap |
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That's great Pete - you made my day! Hoping Conny will post more pictures.
Pete Richards wrote: | On another piece of this topic, you can get rid of double refraction by using a polarizing filter on your camera lens. Not sure I've actually tried it, but it should work. As you turn the filter, you can alternately block one and then the other image. This should work in any orientation. |
We do use a polarizing filter to cut strong doubling when observing inclusions under the microscope. I also use it in photomicrography, most notably with rhodochrosite in order to image its inclusions. I think I was just curious about viewing the inclusions in that one orientation perpendicular to the c axis, whether there is any effect given parallel path of the 2 rays - now that we assume it is a prism face.
Cheers!
Elise
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Conny Forsberg
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Posted: Nov 21, 2014 15:01 Post subject: Re: Calcite Forms - (4) |
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Another image of the scalenohedral phantom. Close up through the Wild.
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