FMF - Friends of Minerals Forum, discussion and message board

[Roger Warin]

Pezzottaite
Pezzottaite is rhombohedral and it is not a Cs-rich beryl but a new mineral species closely related to beryl, with 15% Cs.
Morganite (a variety of beryl) tolerates a maximum of 3% Cs. The Cs content of pezzottaite is 15%. The crystal lattice of beryl does not support it.
This tiny crystal was presented to me as a pezzottaite.
I doubt it! The elongation along the c axis is marked. The color is colorless.
It is a beryl that elongates a little.
But at a certain point the arrival of Cs+ cations from the alteration of the host granite is incorporated into the beryl.
This results in a break in the elongation of the crystal and the termination in pezzottaite, with an hourglass growth.
I don't know if my words are correct, you will tell me.
What seems certain is that after an hourglass growth, the growth stops. Because the presence of Cs blocks the elongation of the crystal.
In summary, do we have an epitaxy of pezzottaite on beryl?
Thank you for your help.

[Pete Richards]

It appears that the orange color was deposited by growth on the (0001) end faces of the crystal, and perhaps was not deposited at all in the earliest period of growth. The question then becomes, what is the composition of the colorless part of the crystal and what is the composition of the colored part. The interesting possibility is that this crystal contains two mineral species in its different parts, corresponding to the two colors. However, it often takes only a very small amount of an impurity to cause a color, and it might not be enough to represent a change in the species involved.