FMF - Friends of Minerals Forum, discussion and message board

[Josele]

I've been trying to determine Miller indices of a calcite specimen from Iraí, Brazil. I think it could be a combination of very steep rhombohedron, something like {0.16.1}, finished by the flat rhombohedron {012}. Big crystals seem be modified by an acute scalenohedron, maybe {541}. (Indices in accord to old calcite unit cell).

I will appreciate comments about this approach.

[Bob Carnein]

Although I have no idea what the actual Miller indices are, I agree with your assignment of faces to those forms.

[Pete Richards]

I think those forms are pretty reasonable guesses. {012} for sure. {0 16 1} is possible. {541} is about in the right place.

Attached is a smorf drawing I made of a crystal with these forms. Anyone who wants to can try out a crystal concept for himself at www.smorf.nl. It's easy and free!

[Josele]

Thanks for your comments.

A calcite dedicated page that for sure you know well was a great help for determining most common steep scalenohedron forms: http://nsminerals.atspace.com/calcite.html

Relating very steep rhombohedron, is hard distinguish {0.16.1} from {0.17.1} or even {0.20.1} without having very good crystals, a precise goniometer and the ability and training to do it.

Calcite is a generous mineral with hundreds if not thousands of crystal forms and enough common as to be affordable.
Anybody knows how many calcite forms are listed in Goldschmidt Atlas der Krystallformen?
There is any book exclusively dedicated to calcite crystals as it exists for fluorite?

[Pete Richards]

[Bob Morgan]

I've become more of a sceptic about so many forms. Tokody listed 459 for pyrite. Most are doubtful. Kostov wrote Calcite forms exceeded 620 (Crystal Habits of Minerals) I think I saw an article in German purporting to list them all. It was written in the 1930's.
More helpful is an article by Kenneth Brock "The Crystal Forms of Calcite" in the mineralogical Record, 24:Nov-Dec, 1993. The web site in the preceding entry is good too.

One test I use in looking at a face is to go into a dark room and flash a laser pointer on the surface and see what kind of reflection appears on a nearby white surface. If it is a relatively well defined point of reflection or a defined line or intersecting lines, there is a face. But in other instances there are large, spread out, weak blobs of light that I take to be indefinite surfaces made up of small curved surfaces but not faces. Yes, they have a relatively consistent overall angle and may be considered a face because of relatively overall flatness.

But often they resulted from some dissolution effects. An analogy would be like being sanded by a sanding block as the etching works consistently on a different angle of the crystal structure than growth, and also leaves pitting as well.

Calcite etches relatively easy. High Miller Index surfaces seem to be particularly prone to such irregularities.

Scalenahedral faces are often striated. Are these faces or stepped alternating faces? Their reflection will make a line perpendicular to the striations, hopefully with two nodes representing each of two alternating forms.

There is a lot of literature about the effect of impurities and impurity layers that help make less likely faces, but this is getting beyond my 'pay grade'.

Back to your original question: If you get a good reflection off the faces in question, a laser pointer can give accurate angle reads if set up right. It requires the reflection to be distinct over 60+ cm from the crystal to dampen out any slight irregularities of measurement.

If you decide to try this, I'll send some suggestions for set up.

What a topic!

[Josele]

Thanks for these impressive calcite data. I suppose calcite is the specie with the largest number of documented combinations of basic forms. Is that true? More than fluorite?

This is another remarkable feature to point in calcite wonders list along with its optical properties, perfect cleavage, solubility inverse to temperature and so on.

In his priceless book "Krystallformen von Fluorit" Eddy Van Der Meersche tells that it took 25 years to gather and document the 85 combinations of basic forms presented in his book. If calcite is more prolific than fluorite, a comparable book for calcite seems unaffordable, it would take longer than a lifetime!