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Calcite Forms - (4)
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Turbo




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PostPosted: Mar 12, 2017 17:19    Post subject: Re: Calcite Forms - (4)  

So I am working on my identification skills for the calcite twin laws. As I was looking through my Tucson photos, I came across some Indiana specimens that were less obvious to me. I've read Pete Richards' paper on the calcite twin laws, and I want to say the one on the left is 018 and the one on the right is 104?

What throws me off with the one on the right is that is seems to have both diagnostic cleavage planes for 018 as well as the diagnostic cleavage planes for 104 parallel to the twin plane. So which is it? I'd love to hear your takes.



IMG_4152.JPG
 Mineral: Calcite
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Irving Materials Quarry, Anderson, Madison County, Indiana, USA
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PostPosted: Mar 13, 2017 13:05    Post subject: Re: Calcite Forms - (4)  

My eye says these are both twins on {018}. If you can estimate the orientations of the c axes, they make an angle of about 120°, not 90° as would be characteristic of a {104} twin. The cleavages that we most often use to evaluate calcite twins are those that lie at the bottom of the twin where it came off the matrix, and which are perpendicular to the twin plane. These faces are not visible on these specimens except as traces of the cleavage on the surface of the twin, at least in the orientation photographed.

{018} twins can be tricky because one set of cleavages intersects the front and back of the twin in vertical lines, which suggests the vertical cleavages of the {104} twin. However, these cleavages are not parallel to the twin plane, as they must be in the {104} twin. See my paper's figure 8,reproduced below, for this distinction.

Also see a marked up version of your photo for the location of the diagnostic cleavages and axes on your specimens. The right one shows these features more plainly than the left one. Also, to the extent that the specimens are not viewed with the twin plane perpendicular to the photo and the c-axes in the plane of the photo, these angles will be somewhat distorted. The orientations of these particular specimens are actually pretty good.



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Turbo




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PostPosted: Mar 13, 2017 18:59    Post subject: Re: Calcite Forms - (4)  

Pete Richards wrote:
My eye says these are both twins on {018}. If you can estimate the orientations of the c axes, they make an angle of about 120°, not 90° as would be characteristic of a {104} twin. The cleavages that we most often use to evaluate calcite twins are those that lie at the bottom of the twin where it came off the matrix, and which are perpendicular to the twin plane. These faces are not visible on these specimens except as traces of the cleavage on the surface of the twin, at least in the orientation photographed.

{018} twins can be tricky because one set of cleavages intersects the front and back of the twin in vertical lines, which suggests the vertical cleavages of the {104} twin. However, these cleavages are not parallel to the twin plane, as they must be in the {104} twin. See my paper's figure 8,reproduced below, for this distinction.

Also see a marked up version of your photo for the location of the diagnostic cleavages and axes on your specimens. The right one shows these features more plainly than the left one. Also, to the extent that the specimens are not viewed with the twin plane perpendicular to the photo and the c-axes in the plane of the photo, these angles will be somewhat distorted. The orientations of these particular specimens are actually pretty good.


Thanks for the great explanation, Pete. That figure makes it more obvious. Even though I cannot see the right hand crystal from above, I can now see that the "vertical" cleavage planes are not being seen end-on even from the side view, but diving a bit inward. In contrast, the Brazilian butterfly twin on page 8 looks like a {104} for sure.

Can I share this marked up version of my photo and your figure to my facebook group "Calcite Collectors"?
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PostPosted: Mar 13, 2017 21:01    Post subject: Re: Calcite Forms - (4)  

[quote="Turbo"]
Pete Richards wrote:


(snip)
Can I share this marked up version of my photo and your figure to my facebook group "Calcite Collectors"?


Since the question was asked publicly, I'll answer publicly so as not to leave the question dangling in the air.... Sure, I see no problem with sharing these images in that manner.

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