FMF - Friends of Minerals Forum, discussion and message board

[Roger Warin]

Hello,
I suggest that the needle-like rutile included in topaz of Tepetape (Mexico) are in fact columbite-(Mn) that appears in the evolved pegmatites.
This is by analogy with needles included in Elba beryls I propose this hypothesis.
Pegmatite works as a distiller of alcohol. At the end of the process, impurities appear and crystallize. Also found in the terminal phases some rare lithophile elements such as Rare Earths Elements REE. That is why it seems probable to me that we should find these minerals in this area and abandon assumptions of rutile and goethite fibers within topaz.
In some pegmatites, these impurities are in too small quantities to form single crystals and can be found as inclusions.
Fine_Lame

[gemlover]

The initial composition of the pegmatite melt that intruded the rock and any materials assimilated from the country rock that was intruded will determine what crystals will be produced. All pegmatites do not have the same initial composition. Your hypothesis could work, however, without knowing the original composition, determining in a theoretical process what minerals could be formed does not determine what you actually have. Analysis of the sample would give the chemical composition, and thusly, the mineral. Your sample is quite beautiful whatever it is made of.

[Roger Warin]

Of course you're right. Here this is a small impurity.
Thank you for your comment.
Fine_Lame.

[Roger Warin]

Acicular Columbite-(Mn) inclusions within Beryls (Elba Island).

The source of REE is in magma chambers. Rocks penetrated by pegmatite (hangingwall) aren’t the root of RRE.
REE concentration increases gradually as the principal minerals crystallized (olivine, pyroxenes, amphiboles, feldspars, micas, quartz ...). It was then that crystallize beryl, tourmaline etc.
It is also for this reason that tourmaline compositions were so variable.
It is always nice to see these rare minerals of Elba.
Fine_Lame.

[alfredo]

The problem with this analogy is that Tepetate topaz comes from a volcanic tuff, not a pegmatite. In extrusive volcanic rocks, Ti is likely to be more common than Nb. Still, it would be nice to get an actual analysis of the needles to confirm it one way or the other.

Thanks for the photo, and for reminding us of this interesting locality!