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Complex calcite
  
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Josele




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PostPosted: Jun 23, 2020 19:18    Post subject: Complex calcite  

A few weeks ago I received a calcite crystal on dolomite from La Florida mining area (Spain). The crystal shows faces of a polyhedron combination: flat rhombohedron, scalenohedron, prism, “normal” rhombohedron and maybe any one more.

It's one of those specimens that you never get tired of turning around and around, identifying the faces of every polyhedron and discovering new details.

Besides evident large faces there are some small faces that I'm not able to recognize.

Can you help please?



C_LF_0.jpeg
 Mineral: Calcite on dolomite
 Locality:
La Florida Mining Area, Herrería-Valdáliga-Rionansa, Comarca Costa Occidental/Saja-Nansa, Cantabria, Spain
 Dimensions: 7.7 x 5.3 x 5.5 cm
 Description:
Photo: Joaquim Callén
 Viewed:  1615 Time(s)

C_LF_0.jpeg



C_LF_1_Miller.jpg
 Mineral: Calcite on dolomite
 Locality:
La Florida Mining Area, Herrería-Valdáliga-Rionansa, Comarca Costa Occidental/Saja-Nansa, Cantabria, Spain
 Dimensions: Crystal size: 4.5 cm
 Description:
Complex calcite single crystal composed by a combination of faces belonging to flat rhombohedron {012} (Miller indices in white), scalenohedron {211} (black), prism {100},{001} (yellow) and rhombohedron {101} (orange) perched on a dolomite aggregate.

Scalenohedral faces (black) are somewhat distorted due to rhombohedral modifications, creating striation.

Indices assuming c-axis positive in this side.

Photo: Joaquim Callén
 Viewed:  1617 Time(s)

C_LF_1_Miller.jpg



C_LF_2_Miller.jpeg
 Mineral: Calcite
 Locality:
La Florida Mining Area, Herrería-Valdáliga-Rionansa, Comarca Costa Occidental/Saja-Nansa, Cantabria, Spain
 Dimensions: Crystal size: 4.5 cm
 Description:
Backside.

White Miller indices: flat rhombohedron {012}
Black: scalenohedron {211}
Yellow: prism {100}
Orange: “normal” rhombohedron {101}


Photo: Joaquim Callén
 Viewed:  1615 Time(s)

C_LF_2_Miller.jpeg



C_LF_3.jpg
 Mineral: Calcite
 Locality:
La Florida Mining Area, Herrería-Valdáliga-Rionansa, Comarca Costa Occidental/Saja-Nansa, Cantabria, Spain
 Description:
Prism face at reflex with a moustache shaped step conformed by two scalenohedral faces.

Over the top of the prism face can see three small faces.
 Viewed:  1616 Time(s)

C_LF_3.jpg



C_LF_4.jpg
 Locality:
La Florida Mining Area, Herrería-Valdáliga-Rionansa, Comarca Costa Occidental/Saja-Nansa, Cantabria, Spain
 Description:
I think the one above could be a negative "normal" rhombohedral face. The edge between it and the upper flat rhombohedral face is perpendicular to the maximum slope line (assuming c-axis in vertical position).

The two trapezoid faces are my doubt.
Maybe of the acute rhombohedron {021}?
Maybe of an acute pyramid with high Miller indices?

This pair of small trapezoid faces are also present in others vertex of the crystal in the same relative position.
 Viewed:  1612 Time(s)

C_LF_4.jpg



C_LF_5.jpg
 Mineral: Calcite
 Locality:
La Florida Mining Area, Herrería-Valdáliga-Rionansa, Comarca Costa Occidental/Saja-Nansa, Cantabria, Spain
 Dimensions: Face is 5 mm long
 Description:
Supposed "normal" rhombohedral face at reflex:
 Viewed:  1612 Time(s)

C_LF_5.jpg



C_LF_6.jpg
 Mineral: Calcite
 Locality:
La Florida Mining Area, Herrería-Valdáliga-Rionansa, Comarca Costa Occidental/Saja-Nansa, Cantabria, Spain
 Dimensions: Face is about 2 mm wide
 Description:
One of the not recognized trapezoid faces at reflex:
 Viewed:  1614 Time(s)

C_LF_6.jpg



C_LF_7.jpg
 Mineral: Calcite
 Locality:
La Florida Mining Area, Herrería-Valdáliga-Rionansa, Comarca Costa Occidental/Saja-Nansa, Cantabria, Spain
 Dimensions: Face is about 2 mm wide
 Description:
The other one:
 Viewed:  1617 Time(s)

C_LF_7.jpg


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Bob Morgan




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PostPosted: Jun 23, 2020 22:01    Post subject: Re: Complex calcite  

For the thin face below the {012}, I suggest the {011}. The bowtie 2 faces below that are a scalenohedral pair perhaps the {321}. If you could draw the crystal using a crystal drawing program, you might be able to get matching angles. Peter Richards could confirm the faces much better than I. I hope he'll pick up on this.
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Josele




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PostPosted: Jun 24, 2020 07:14    Post subject: Re: Complex calcite  

Thank you Bob, you are always ready to help.

At the moment I have not a program to deal with crystals. It's long time ago that I miss this kind of application.



C_LF_5.2.jpg
 Description:
Angle (1) between flat rhombohedral face and the small elongated face is about 10 - 12 degrees according to my protractor.

Angle (2) between the two trapezoid faces seems even smaller.

Angle (3) between elongated face and any of the trapezoid faces looks somewhat bigger
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C_LF_5.2.jpg


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jose luis jara jara




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PostPosted: Jun 24, 2020 07:51    Post subject: Re: Complex calcite  

Go precious Calcite crystal, congratulations
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Pete Richards
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PostPosted: Jun 24, 2020 13:20    Post subject: Re: Complex calcite  

A crystal like this presents an interesting challenge! Josele, your measurements are helpful, even if they may not be very precise, being made with a protractor. The assignment of the major forms to {012}, {211}, and {100} seems very likely to be right. Since the thin form lies between (012) and (010), it must be a negative rhombohedron. {011}, suggested by Bob Morgan, is a very uncommon form for calcite, unlike its positive equivalent {101}, the cleavage rhombohedron, though it would be in about the right place. Two common negative rhombohedra that are nearby (in terms of angles) are {045} and {054}, and {045} is in fact inclined to the c-axis 12°2' more than {012}, so this is most probably the identity of the thin face, entirely consistent with Josele's measurement.

The bow-tie shaped pair of faces are more difficult. Given their position on the crystal, they must belong to a negative scalenohedron, one with indices of the form {hkl}, with h<k. There are many of these, and here there are two angular degrees of freedom, not just one. Some experimentation with SHAPE produced a number of candidates, of which I chose {162} because it produced a bow-tie most nearly with the right shape to correspond to Josele's photos. It also fits with Josele's qualitative assessments of the angle between this form and {045}, and the angle between the two faces of the bow-tie.

So {162} is a reasonable guess, but it may well be wrong. A more certain solution would require goniometric study, which could not be done on this crystal.

The left drawing shows a crystal with these forms in the standard clinographic projection. The right drawing is oriented so that the face (045) is in the plane of the drawing. The red triangle indicates the position of the positive end of the three-fold c-axis.



Jos.jpeg
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Jos.jpeg



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Collecting and studying crystals with interesting habits, twinning, and epitaxy
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Josele




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PostPosted: Jun 25, 2020 05:09    Post subject: Re: Complex calcite  

Pete, thank you very much for your help. That's more than an elaborate response, it is a master class lesson! It's amazing your ability to draw conclusions from a few simple and imprecise data. A pleasure learning with teachers like you.

Drawings are very much like what I can see having the piece in hand. What a nice tool when one has the knowledge to run it properly! I have neither the one nor the other. That’s why I often use Smorf crystal database (thanks Mark Holtkamp!) to identify and/or check crystal features. It has a quite large collection of calcite forms.

Also thanks for the spell corrections of my very poor English.



C_forms.jpg
 Description:
With Smorf sketches and a protractor one can estimate some angles of crystals.

The difference in inclination between (012) and (045) faces relative to c-axis is effectively about 12º

Crystal sketches © Smorf (Mark Holtkamp)
 Viewed:  1357 Time(s)

C_forms.jpg


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fena kuiperi




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PostPosted: Jun 25, 2020 06:35    Post subject: Re: Complex calcite  

I totaly agree with Josele: Pete thanks for such a great explanation about how to understand these crystal forms!
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Bob Morgan




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PostPosted: Jun 25, 2020 08:13    Post subject: Re: Complex calcite  

Thanks Pete. I knew you were the one to get this done.
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