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[lluis]

"The fact that both styles of rhomb-stacking occur on the same specimen suggests that something changed and caused the spiraling to begin after the straight growth stage....chemical or simply physical???
Peter Megaw
PostPosted: Jul 08, 2018 16:00 Post subject: Re: Stalactites?"

Hard to say..... As a physics teacher said one time to a friend of mine that studied Physics, "Chemistry is only a cancer of Physics. Pity is that tumor is bigger than body!"
At those levels, just a question of chemical-physics, I guess...

With best wishes

Lluís

[Roger Warin]

Growth of a crystal.

The growth mechanisms of the crystal seed are varied. They are correlated with the growth rates of the faces, that is with the kinetics of crystallization: either the growth is slow and two-dimensional and one obtains plane faces, or it appears screw dislocations, or even spirals according to the speed of growth.
The geometry of the atomic site varies according to surface irregularities. Some external bonds remain vacant, occupied on the surface by ligands of the hydrothermal solution which must be displaced by the deposition of a new atom of the molecule which crystallizes.
The presence of natural catalytic effects is possible by the presence of impurities in the hydrothermal phase.
Thus the crystal takes its habit.
In the case of spiral crystals, the elongation is maximal.
There is another type of spiral growth for chiral species, such as quartz. In this case, it is subparallel growths that introduce the spiral deformation as for twisted quartz (or Gwindels) with a double possibility.
At least that's what I think, and you?

[lluis]

Hi, Roger

I think I said same as you: it is chemical-physics.... (for a physician, physical-chemistry... No need to explain that chemist and physicists do not go alongs very well..... :-))

With best wishes

Lluís

[Bob Harman]

Somewhat similar to Tracy's 2008 pictured quartz examples are these 2 examples from Indiana. The first is a "stalactite" type grouping of tiny individual terminated and doubly terminated quartz crystals. The second example is more of a "free form" growth of similar tiny terminated and doubly terminated quartz crystals. I have others as well. How these form has always intrigued me.

[lluis]

Hi, Bob

Here, to me, it is less stunning than the stalactite brought to us by Peter Megaw...
In this case, it is just a quick crystallization, growing in all places it can. Some would be in a shape, some in another... maybe just a difference in the flow of components....
Again, chemical-physics. But to my eyes, far less spectacular...

With best wishes

Lluís