FMF - Friends of Minerals Forum, discussion and message board

[Elise]

Hi,
I am trying to find out more about this type of pink grossular; specifically I am looking for the high end of RI range, UV reaction and the spectrum (or any references someone can point me to). From different references including MinRec and the Garnet Lithographie issue, the Jeffrey Mine Canada and the Lake Jaco Chihuahua or Coahuila states in Mexico seem to be the sources and somewhere else in my notes, I have also the state of Morelos.

In the references above, it indicates that it is one of the few garnets which fluoresce ("scarlet red" under SW, due to the Mn). I would think that any iron would quench that effect, but it is indicated that Fe concentration is very low. Are there transparent crystals which do not have the high Ti and Fe andradite cores and are not so included? And do the transparent crystals, including those from Morelos, always fluoresce?

Best wishes,
Elise

[Peter Megaw]

Elise: Virgil Lueth at New Mexico Tech has done a fair amount of work on the garnets from northern Mexico...what you're calling Lake Jaco. The correct locality is Sierra de las Cruces in Coahuila...just east of Lake Jaco. It is a very extensive Mn-rich skarn system. I have seen a wide range of garnets from there, some with dark cores and some very pale grey or pink to the core. Virgil can probably help you out on those.

The Morelos locality you refer to is another skarn system near a village of Xalostoc...my Mexican company happens to own the locality. The garnets I have seen from there are pink to the core, but I have never lamped them.

[Antonio Alcaide]

Hi, Elise, reading your comments I cannot avoid posting my favourite piece from Sierra de las Cruces, Coahuila. I am always looking for something like that, but I haven't had luck. I picked it from Jordi Fabre's web long time ago, before belonging this message board. I have no the exact reference. Please, pay attention to the white grossular garnets. The combo with the vesuvianite xls. is perfect.

Pink garnets from there are usually damaged, I love them though.

Greetings

[Ed Huskinson]

Antonio Alcalde!!

Nice vesuvanite(s) specimen!!

I'm jealous. And you got it from Jordi too. So, double Woo Hoo for you.

Enjoy your specimen.

Ed

[Elise]

[Antonio Alcaide]

[Peter Megaw]

You're making me want to get out and dig some of the garnets in Morelos. At least locally they are abundant

[Antonio Alcaide]

Elise, I agree with you. Vesuvianite and white grossular garnets are amazing together.

Peter, do it for us. That thing: to look for garnets at Morelos makes me jealous. Maybe a day in the future I could go with you. Who knows?

best regards

[Peter Megaw]

Come on over...we're just getting this property going for gold exploration, which means we should be in and out over the next few years. I want to get a few big boulders of the garnet material carved out...the best zones I;ve seen run 35-50% garnet in white marble with some wollastonite.

[Jim]

[Peter Megaw]

Most of the crystals are over an inch across

[Jordi Fabre]

[Jim]

[Jason]

It's very rare to haver any garnets flour...since like you eluded to Iron tends to hamper that..what is unique is these mexican garnets you speak of..I have not had the pleasure of owning or having any..the grossulrs from Easdt Africa can and have been known to flour...they have a typical 400nm. which is cause for flour. in them..the gossulars from east africa tend and have the highest concentrations of calcium and a lack of anadradite there is in that locale..why..well east africa was a shallow sea for however many millions of years..seashells and calcium rich lifeforms flourished and died there..when you have lots of calcium rich things they tend to dilute iron which is universal in the earth crust..hecnee fine quality grossulars and flour. in them..maybe the same can be said about these mexican garnets...similar formation and environment..have you done a transmission spectrum on any yet, Elise?

[GneissWare]

Here is an example of one of the rarer colors of garnet from the Sierra de la Cruz, Mun. de Sierra Mojada, Coahuila, Mexico (Lake Jaco) locality. Graham Sutton and I found this piece and prepped it. It was later sold to the Pohl collection at the Freiburg Museum. The photo is a scan from a photographic print, so the quality is not so good.

By the way, a few years ago, George Rossman analyzed this color of garnet. I reproduce a portion of his note to me for those interested in the chemistry of these garnets.

"Results: The garnet is unambiguously a grossular

The approximate formula is (Ca 2.93 Mg 0.06) [Al 1.75 Fe 0.25] (SiO4)3

The chemical analysis indicates a little over 1% of the atoms are iron, but give no clue to its oxidation state.

However, the optical spectrum shows no trace of an almandine component which is easy to see in these spectra, so we conclude that the iron is present as Fe3+ which is much more difficult to see in the optical spectrum. Fully reasonable for a deposit that has manganese so oxidized.

So, we conclude that the garnet has a minor andradite component. This means that the iron replaces aluminum and would modify the color.

The magnesium would replace calcium for a minor pyrope component but not cause color.

What the quick analysis I ran did not show was any manganese. The conditions of the analysis would have shown Mn if it were present at more than about 0.1 wt%. A more careful (read more expensive) analysis would have picked it up, I am sure. The optical spectrum gives clear evidence for a trace of manganese in the garnet, and shows that much of it must be oxidized to the Mn3+ state (the dominant cause of the color of the garnet). This locality is known in the literature to produce the pink (raspberry) garnets that owe their color to Mn3+.

So, all said, we should call the garnet a grossular
It has a minor andradite component
It has a minor pyrope component

If you want to call components, this analysis of this particular spot would say:

Grossular 90
Andradite 8
pyrope 2"