FMF - Friends of Minerals Forum, discussion and message board

[Roger Warin]

Tourmaline (liddicoatite) grew in a negative imprint of a crystal disappeared. The substrate is a feldspar crystal. Crystals are terminated.

Roger.

[Roger Warin]

Rare nesosilicate beryllium. Two habits are possible:
or equant, flattened,
or as here in under the aspect of prisms drawn according to the tri-fold axis. These hexagonal prisms {11.0} are streaked by elongation.
The fan-shaped dispersion of crystals is random. This is not a twin.
Roger.

[GneissWare]

[Roger Warin]

Hi,
Crystals of corundum from Madagascar are famous. They can even be sapphires and rubies.
Two specimens of polychromatic corundum.
Roger.

[Roger Warin]

Hi,
Here a flashy small specimen. It’s a liddicoatite crystal (a variety of the big tourmalines group).
The upper part of a tourmaline is different from the lower part (because the low symmetry of tourmaline). There is no center of symmetry in the crystal.
Roger.

[Roger Warin]

Hi,
Almandine « Star Garnet ».
Fanotaniana, Alaotra Lake, Madagascar.
65 mm.
Cut into sphere, this garnet gives a nice asterism.
Roger.

[Mike Wood]

A very fine and large garnet crystal Roger, beautiful surface structure.

Forgive me but this is a serious question: does almandine garnet occur in pegmatites? I thought garnets from pegmatites were usually spessartine. I have some spessartine garnets from Pakistani pegmatites with very similar surface structure, that are deep red (not that I've had them tested personally, just taking them from the dealers at face value). I know it depends on the Mg/Fe ratio in the garnet, it just seems a bit unusual to be almandine. But I suppose exotic pegmatites such as occur in Madagascar can produce many unusual minerals.

Mike

[Jesse Fisher]

Fe-rich garnets of the spessartine/almandine series are usually a deeper red than the Mn-rich ones, which are orange. A number of years back, when I had access to an SEM with an EDS detector I analyzed a number of pegmatite garnets, including the reddish ones from Pakistan pegmatites and some orange ones for the Little Three pegmatite in San Diego. The Little Three ones had very little Fe where as the Pakiststani ones were pretty much in the middle of the series, having Mn:Fe ratios around 1:1. I suspect that these should be referred to as "spessartine-almandine series" in the absence of analytical data as they could be on either side of the divide.

[Roger Warin]

Hello Mike , Jesse ,
Thank you for your comments. I share the Jesse’s ideas.
Well as exotic , this geological province has been well studied in the past.
The limits of solid-solutions are rarely achieved, but the abundance of iron does not seem exceptional.
Fine crystal morphology depends mainly on growth rates although the middle retains its influence.
I do not think the nature of the cation is considered. Rather metamorphic rocks form occur as { 101 } rhombic dodecahedra while pegmatites ones occur preferentially as trapezohedra { 211} .
It also seems inconsistent that metamorphism gives crystals as large.
This crystal has both forms though the faces { 211} resulting from a coarser oscillating growth.
This combination of the two forms is typical of that of grossular, but there is no calcium here.
But there may be some Mn, thus tending to spessartine. This garnet from Alaotra is often accompanied by black tourmaline.
The color of large individuals is dark red.
Roger .

[Mike Wood]

Thank you Roger and Jesse for your informative and enlightening answers.
Don't know why I said Mg/Fe ratio - sorry, of course it's the Mn/Fe ratio in the spessartine/almandine series! (Thanks Jesse).
Interesting to learn that garnets from metamorphic environments produce (101) dominant crystal forms, and garnets from pegmatites produce (211) dominant crystal forms. (Thanks Roger).
I really must re-read some of my books sometime.
Mike -_-

[Roger Warin]

Many thanks Mike,
Like you I have ever read but in the end, it is a coherence between the mineralogy and geochemistry. I also would have liked (but this is off topic on this forum) to show you how the mineralogy of asteroids is similar to that of Earth, but with differences.
The study of meteorites is an input to geochemistry which I think is an easier door because the celestial metamorphic rocks are simpler than terrestrial ones subjected to oxygen and water. Thus there is no amphibole in meteorites (except in in some Martian).
Mineralogy is a part of my happiness.
Roger.

[Roger Warin]

Hi,
Another specimen of almandine star garnet . It is similar to the previous, but he suffered a deformed growth. Thus see its interior. Rhombic dodecahedron faces show that was not broken (subconchoidal fracture of garnets).
Internal variations in color can be seen.
Roger.

[Roger Warin]

Hi,
Other Star Garnet showing asterism.
Roger.

[Roger Warin]

Hi,
Madagascar, the country of tourmalines ... This is modest compared to specimens mined in the 1960s and before.
But it is pretty well associated with feldspar (orthoclase I presume). Its origin is Mount Ibity.
Roger.

[Roger Warin]

Hi,
Monazite crystal, from Ampitilina (Betsiboka Valley) Madagascar.
I do not know what rare earth element is predominant in this specimen
Roger.