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Elise · Joined: 22 Dec 2009 Posts: 243 Location: New York State

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?

Pete Richards

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.

marco campos-venuti · Joined: 09 Apr 2014 Posts: 227 Location: Sevilla

I found also a rhombohedral phantom in a calcite prism.

Elise · Joined: 22 Dec 2009 Posts: 243 Location: New York State

That's great Pete - you made my day! Hoping Conny will post more pictures.

Conny Forsberg · Joined: 15 Nov 2014 Posts: 4 Location: Sweden

Another image of the scalenohedral phantom. Close up through the Wild.

marco campos-venuti · Joined: 09 Apr 2014 Posts: 227 Location: Sevilla

A very strange double twin from Dalnegorsk.

marco campos-venuti · Joined: 09 Apr 2014 Posts: 227 Location: Sevilla

Another curious twin from China.

Turbo · Joined: 30 Sep 2009 Posts: 260 Location: Delaware

A few of my best.

Tobi · Posted: Nov 22, 2014 08:43 Post subject: Re: Calcite Forms - (4)

Very interesting samples of this mineral! I will contribute to this thread with some closer shots of some calcites from my collection - and with another small collage ;-)