FMF - Friends of Minerals Forum, discussion and message board

[marco campos-venuti]

A quartz crystal from Arkansas with a chlorite phantom has been sitting in a drawer of my collection for many years as an oddity, a strange shape found only once. A few days ago I discovered in a FB group a second crystal with the same shape, from Newfoundland, Canada belonging to Josh March. I don't think that chance can act twice in exactly the same way and so I tried to find a reason for that shape.
The termination of the crystal has only two large faces belonging to two distinct rhombohedrons. The vertex is constituted by an edge formed by these two large faces. The crystal is perfectly symmetrical on the two sides of this edge. The faces of the rhombohedrons show a mosaic pattern typical of penetration twins.
I suggest it could be a Brazil twin of two individuals with Dauphiné habit. I would like to know what do you think.

[James Catmur]

I need to go through my quartz specimens to see if I have anything similar

[Bob Morgan]

There is nothing like this in my collection. I have several 'tabular' crystals with opposing wide prism faces but none with so small rhombohedral faces to either side. You have a good eye to discover them!
I doubt that c-axis twinning accounts for this morphology. More rapid growth has occurred to both sides accounting for the narrower pairs of prism faces in comparison to the wider ones between the pairs.
That's not so rare. Although the natural etching on the prominent rhombohedral faces indicates Dauphine twinning, such twinning does not lead to faster growth of crystals in specific directions.
What is rare are the pairs of very small to non existent rhombohedral faces. On my more 'tabular' crystals those faces tend to be larger being elongated by extension of the growth of the narrower adjoining prism faces. On yours, the rhombohedral faces have rapidly grown almost out of existence. I conjecture there may be some spiral growth involved on those faces.
There have been some studies of quartz crystals using cathodoluminescence, which indicate stages of growth. Such an examination of your crystals might shed some light of how they got to be that way.
Whatever the mystery of how and why they are so rare remains. They are marveolus.