FMF - Friends of Minerals Forum, discussion and message board

[John Jaszczak]

I have been looking at some of the interesting surfaces of some topaz crystals in my collection from San Luis Potosi, Mexico. Unfortunately,
no more detailed locality information was supplied by the dealer. The samples have pinacoid terminated orange crystals associated with rutile needles
and some specimens have hyalite opal, while others seem to have quartz. Similar specimens are shown on Mindat from Villa Garcia, Zacatecas, Mexico
as well as Tepetate, Mun. de Villa de Arriaga, San Luis Potosí, Mexico and Lourdes, Mun. de Villa de Arriaga, San Luis Potosí, Mexico. I am wondering if any of the Mexico mineral experts can speak to the entries on these localities- are they really distinct localities with remarkably similar mineralization?

[Dennis Beals]

Hi John,
All the different localities are correct and many more are probably out there. This is a huge area of topaz hosted in rhyolites.
For many years a lot of work was done in Tepetates by Antonio Luna and others.. In the early 90's I was getting topaz from Villa Zaragoza, San Luis Potosi. The last one to have mined for topaz was Mike New at Villa Zaragoza, Zacatecas.

[Peter Megaw]

John, Dennis is right...tin-topaz rhyolite systems are widely distributed in a swath from Guanajuato through Queretaro, San Luis Potosi, Zacatecas and Durango..at least. Since these states have sinuous jig-saw borders it is possible to go from one to the other repeatedly over short distances and town names repeat so it can get complicated.

Add to that the fact that many dealers/collectors have heard of Tepetate so that is often their default labeling regardless of where something really comes from. Dennis spends more time in this area than I do, but I have not seen decent material from Tepetate proper since the 1980s...the recent bulk has been from Villa Garcia

Rutile needles included in attractive hyalite is a very common association at the Villa Garcia locality (aka Mina Veronica, named for Ruben Avila's daughter) which produced prodigious amounts of specimens during the 1990s through mid 2000s.

In most of these localities if the hyalite is not attractive it is typically burned off with HF (also true of most from Veronica). That said, there are morphological luster and color differences that can help narrow things down.

A picture of the entire crystal would help a lot

[John Jaszczak]

Thank you Dennis and Peter for your helpful information.
Unfortunately, my specimens were only labeled as from "San Luis Potosi, Mexico".
I obtained them in 1982 and 1985. Unfortunately it seems a lot of my specimens from
Mexico lack detailed locality information. Both specimens have similar surface features on
the topaz and a lot of acicular rutile inclusions. One has hyalite opal and the other (used for the photo above) did not. The one that does have opal has a lot and it is attractive- the main reason I purchased it, in fact, since the opal follows the rutile. An image is posted below. Sorry its quality isn't the best, but our photo studio isn't yet recreated in the new museum.

Thanks
John

[Peter Megaw]

Dennis may chime in here too...but the habit, termination, color, luster, hayaite and rutile look like Villa Garcia material to me. Dennis are there Villa Garcias on both sides of the SLP/ZAC border...or is this a single town?

[GneissWare]

This is another example of a Topaz -- This was bought in 1985 and was marked Chihuahua, Mexico....which seems unlikely.

[Peter Megaw]

Looks like Villa Garcia to me...

There is topaz in Chihuahua...but nothing showy I know about

[GneissWare]

[Peter Megaw]

Yes, this is a place that has produced a long time....just intermittently. Actually most of the topaz localities owe their specimen/gem mining interest to the fact that they have associated tin mineralization. Wood Tin has long been aggressively placered from the streams leading outward from the clusters of these rhyolite domes and crystalliine cassiterite locally mined where the cassiterite veins are well developed. Rarer species like durangite and ordonezite are locally present as well.

What gets me excited is that tin-topaz rhyolite domes occur in the proximal parts of major silver districts like Guanajuato, Pachuca, Fresnillo and Bolanos...suggesting a genetic link to the processes of the Bolivian Tin-Silver belt..and creating an interesting prospecting tool.

[John Jaszczak]

John Rakovan emailed me that his specimen shows the ends of the topaz crystals fluoresce cream colored in shortwave UV. His specimen only has general locality information as well. My specimen may show this, but the opal fluoresces green so brightly it is hard to see if the topaz really fluoresces or not.

[alfredo]

"Wood Tin has long been aggressively placered from the streams..." (Mexico)

This surprises me. Perhaps mexican wood tin has a higher tin content than bolivian wood tin? I will do some specific gravity measurements to find out.

Placering wood tin was rarely (ever?) economically successful in Bolivia. If I understand the material correctly, it is composed of cassiterite and silica ("chalcedony"?) fibres in parallel growth in varying proportion (hence the variable specific gravity). Smelters didn't like it, because of the variable tin content and difficulty in concentrating it further.

Topaz seems, unfortunately, to be much less abundant in Bolivia's wood tin deposits than in Mexico's, although there are some other interesting things, like the type locality roseveltite.

[Peter Megaw]

Years ago I looked at thin sections of Mexican wood tin and saw very little silica in them. Perhaps there is an inherent bias though since the ones I looked at and the ones I have in my collection were recovered by placering so they clearly have a high enough tin content to have responded to gravity separation.

Last I knew, a substantial part of the world's tin comes from Malaysian placer tin deposits, a lot of which is wood tin apparently. There is (was) a smelter in San Luis Potosi Mexico that treated these...in the 1990s I saw huge bags of wood tin there.

The Taylor Creek area in the Black Range of New Mexico was also placered for wood tin into the 1960s.

If the Bolivian tin deposits have little topaz, are they rich in some other fluorine species?

[vic rzonca]

Peter, I'd like to see an image of a choice wood tin specimen, if possible.

[Enrique Llorens]

This topaz is also home town of San Luis Potosi, according to its label. It is possible to more precision in their place of origin?

[Peter Megaw]

Vic...I'm at the Denver Show and will attend to this on my return later in the week.