Monday, 29 August 2011

Calcite and aragonite ~

Calcite and aragonite share an identical chemical forumla: CaCO3, which represents calcium carbonate.  How is it then, that they have such very different appearances?  First let's look at an example of each.  All specimens displayed in this article are shown considerably enlarged so that detail is visible.

Here is a typically shaped and marked piece of calcite:  Note that calcite comes in a wide range of soft colours.

And here is a piece of aragonite:

The answer is that they are different forms (termed polymorphs) of the same thing.  Polymorphism is when a substance can take one form or another, depending on environmental factors such as heat or the presence of other minerals.  In this case calcite is the more stable of the two crystal structures. 

For those interested in pursuing this theme further I provide the links here to Wikipedia's companion articles on Calcite seas and Aragonite seas.

In my own collection I have continuing uncertainty about a string beads which was sold to me as aragonite.  I have never been able to reconcile the look of them with the appearance of the aragonite 'star' above.  Here they are:

They look and feel much more like calcite to me, right down to the rather waxy texture of the beads.  However, I have read that aragonite can look like calcite that has banding in it, which these do, but even so I remain unconvinced!

Here are my other pieces of calcite:
This piece has been machine-finished to the egg shape:

In this piece you can see the rectangular / rhomboid structure that is so typical of calcite:

Same piece - different angle:

A very rough-hewn piece:

This yellow globe has no visible cracks and delineations within it - an unusually sound specimen, and a beautiful one:

This completely opaque piece has been identified for me as calcite, but as I haven't seen others like it I don't feel certain.

Calcite comes in other colours.  I used to have a piece which was a lovely apple green, which has since gone to another home.

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