FMF - Friends of Minerals Forum, discussion and message board

[Matt_Zukowski]

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.

[Ru Smith]

Much in common this old Dana drawing and the calcite crystal from Huqf, Oman.

[Ru Smith]

By the way, I really like the termination photo above (0087 Calcite Verchniy Mine, Dal'negorsk 8.jpg).

[Ru Smith]

Calcite twin with hematite inclusions, Gloucestershire.

[Ru Smith]

Two of several beautiful twins to 65 mm on 26 cm matrix.

[Ru Smith]

I particularly like the Meikle example.

[Roger Warin]

Hi Ru,
The first is the rarer of the two. This is the twin on {01.2}.
Roger.

[Ru Smith]

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.

[Ru Smith]

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.

[Jordi Fabre]

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"

[Pete Richards]

[Conny Forsberg]

I got this one during late eighties or early nineties, and guess it was mined in the eighties.

Conny

[marco campos-venuti]

I guess the phantom is just a scalenohedral crystal of black calcite, not really hematite.
Location can be Egremont, Cumbria.

[Conny Forsberg]

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.

[John Betts]

I sold a similar crystal years ago that was from the Stank Mine. I too guessed the inclusions were hematite.