FMF - Friends of Minerals Forum, discussion and message board

[stef4412]

All these samples seem to be ... unbelievable
Thans for these photos

[Michael Shaw]

It has been a while since this thread was active. The photos below show a recently acquired calcite specimen from the marble quarries at Estremoz, Portugal. This location is known to produce complex calcite twins and I would like to learn more about this particular twin type. The larger crystal (shown upright in the first photo and horizontal in the second photo) has a couple of interesting faces which form a "w" shape on the edge of the rhomb. The crystal on the right side of the photo has a prominent shallow notch running along the edge which appears to be where two flattened rhombs come together perpendicular to the c-axis. At the top there is another intersecting rhombic form. Any explanation about this specimen is welcome.

[Heimo Hellwig]

I would like to add some clear Calcite specimens from Tsumeb to this thread.

[Tobi]

A yellow hemisphere:

[am mizunaka]

[Turbo]

This is new material from Fletcher Mine, Centerville, Viburnum Trend District, Reynolds Co., Missouri, USA. It is being found in pillar pockets this year. Glittering pyrite matrices on many of them.

[Tobi]

A few more:

[Tobi]

And some more that are (unfortunately) not in my own collection:

[Collection Terra Mineralia, Freiberg (my photos)]

[Gerhard Brandstetter]

As an absolute lover of Calcites I must love this thread! Now here are my 50 cents:

[Roger Warin]

Hi Gerhard,
I had fun with your calcites, especially the Príbram one. Very nice specimens and stunning photos.
Roger.

[Turbo]

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.

[Pete Richards]

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.

[Turbo]

[Pete Richards]

[quote="Turbo"]