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Almandine or Spessartine ???

Posted by José Zendrera  
avatar Almandine or Spessartine ???
March 15, 2012 11:37PM
Hola,

Some time ago I bough a small lot of amazing garnets from Haramosh Mts., Pakistan, with icositetrahedral form but with rhombododecahedral faces (¿?). They have some small windows showing his inner structure, something like a fenster skeletal garnet. Unlike most almandines that were born included in mica-schist, these seems grown in hydrothermal conditions into a pocket, so have such bright faces without schist remains but some remains of embedded cleavelandite.

The question is that garnets came labeled as almandine but I think they could be spessartine. Here in Mindat gallery can see similar garnets from this locality, some identified as almandine and some others as spessartine, for example:
[www.mindat.org]
[www.mindat.org]

They have very good luster, they don't have the typical dark outer layer of almandines, they are associated to albite... to me, all these garnets look as spessartine.
What you think about? Thanks for your comment.

3,4 x 3 x 2,2 cm.



2,6 x 2,5 x 2,4 cm.


Josele
Re: Almandine or Spessartine ???
March 16, 2012 01:15AM
    
I think you'll need an analysis to find out for sure. All the opinions in the world won't settle it otherwise. Many garnets have spessartine-richer compositions at their cores and more almandine toward the rims, so it could be both.
avatar Re: Almandine or Spessartine ???
March 16, 2012 01:49AM
Thanks Keith, I had not considered this possibility. I understand that you mean can be two spices in one crystal, where both Mn and Fe are present.

I have two questions:
There is implanted hidrothermal born almandines?
There is included magmatic born spessartines?
I appreciate your comments.

Josele
avatar Re: Almandine or Spessartine ???
March 17, 2012 05:33PM
Jose,

May I came at your questions sideways smiling smiley

Pyrope, Almandine and Spessartine form an isomorphous solid solution series with the formula (A)Al2(SiO4)3, in which the (A) site is filled by either Mg3, Fe32 or Mn32. The percentage of each of these atoms present in a single crystal lattice can vary for each from 0% to 100%. Accordingly, whereas one may have a pure Pyrope, Almandine or Spessartine crystal, many if not most Pyralspite Garnets have two or more of the isomorphous replacement elements present in some part.

So, 'you pays you money and makes your choice' when assigning a name to a particular stone without benefit of analysis at the atomic level. However, basic gemmological analysis helps up to a point. With RI, absorption bands in the visible spectrum and even SG giving sometimes helpful pointers to the composition of a particular stone.

As to method of formation:

- I know of no record of hydrothermal formation of Almandine. If such exists, there seems no reason why it should be restricted to the formation of pure Almandine only and not extend right across the solid solution series.

- Yes, volcanic occurrence of Almandine is in the literature.

Chap 11 of 'Gems' 6th edn (Butterworth Press) was a complete re-write by Brian Jackson for that edition from the earlier writings of Robert Webster. This new chapter is not only a masterly concise exposition on the Garnet group but makes plain the mess that Garnet nomenclature is in.



Edited 1 time(s). Last edit at 03/19/2012 12:02AM by Owen Lewis (2).
avatar Re: Almandine or Spessartine ???
March 17, 2012 09:38PM
    
Hi,

if anyone is interested, I have a pdf version of 'Gems' 6th edn (Butterworth Press).
Let me know at vikvanrusselt@hotmail.com.

Vik
avatar Re: Almandine or Spessartine ???
March 18, 2012 01:44AM
Owen, Vik, thanks very much for your help.
Last night I was rereading the garnet chapter in my old mineralogy textbook (Klockmann - Ramdohr, 1955). Solid solution in garnets is well know from long time ago, I should have reread this text before starting this thread... After all, my first question did not make much sense. My specimen is probably both almandine and spessartine, I will label it as "almandine - spessartine series".
Greetings,



Josele
Re: Almandine or Spessartine ???
March 18, 2012 10:00PM
Is spessartine.
avatar Re: Almandine or Spessartine ???
March 19, 2012 12:26AM
Caro Salvatore, this is the answer I wanted to hear. Could you argue it a little? Thank you very much.

Josele
avatar Re: Almandine or Spessartine ???
March 19, 2012 01:46AM
I'd agree that the colour suggests Spessartine. But since Jose has at least a couple of clean samples (internally as well as externally?) some testing might be informative. If it was me, I'd check the spectra using transmitted light and, with a good balance, work out the SG of two or three specimens.

There's good news and bad news. The good news is that Pyrope is not found in Pakistan, so we can put it on the back burner. The bad news is that, in addition to Almandine and Spessartine, Grossular is found in Pakistan and Grossular molecules are sometimes mixed in with Almandine-Spessartine.

Pure Spessartine with a perfect crystal (no adherents or inclusions) has a calculated SG of 4.179. For Almandine, the theoretical SG is 4.313 - higher than the SG for Spessartine. Grossular (from the Swat Valley) has been reported as having an SG of 3.64.

Soo... If the SG of two or three clean specimens of this stuff shows up In the range 4.17 - 4.31, There's Almandine present in combination with other things (probably but not necessarily Spessartine).If the SG is very close to 4.31, then the Almandine must be pretty pure. It has been shown that when the percentage of Almandine present drops to 50% (with Grossular in the mix?) the SG of the sample can drop to 3.95. However, if the mix is pure Almandine - Spessartine then (clean stones only) the SG must lie in the range 4.17 to 4.31.. In short, SG testing alone may provide some definite answers but, taken in conjunction with the spectrum absorption patterns of Spessartine and Almandine (and with a bit of luck), one might get close to the makeup of the samples.

Here, courtesy of Gemology Tools Professional software database, are the 'ideal' and diagnostic spectra for Almandine and Spessartine. Neither the Pyrope nor the Grossular spectra will show anything of use use for this analysis.:




Edited 2 time(s). Last edit at 03/19/2012 02:16AM by Owen Lewis (2).
avatar Re: Almandine or Spessartine ???
March 20, 2012 01:08AM
After read garnet chapter of 'Gems' 6th edn (Butterworth Press) by courtesy of Vic and after Owen suggestion, and do not have as a spectrometer, I checked SG of this 4 garnet from same location in Haramosh Mts. I've done with a decimal scale and the Archimedes method for volume, using distilled water, here the results:
32,0 g / 7,6 cm3 = 4,21
28,1 g / 6,8 cm3 = 4,13
25,2 g / 6,0 cm3 = 4,20
21,5 g / 5,2 cm3 = 4,13
Should be borne in mind that garnets have some inclusions of albite, so that the density should be slightly higher than these results. If we apply a correction of 0.5%, the results would be: 4,23 - 4,15 - 4,22 - 4,15.
Theorical spessartine SG is 4,18. Real spessartine SG varies between 4,15 and 4,22. Almandine calculated SG is 4,31 but according to the text quoted above you can find garnets with 50% almandine from >3,95...
After all, at the moment all I can say is that these garnets are mostly spessartine with some almandine mixture.
Thank you for your interest.

These are the checked garnets:


Josele



Edited 2 time(s). Last edit at 03/20/2012 01:13AM by Jose Zendrera.
avatar Re: Almandine or Spessartine ???
March 20, 2012 12:40PM
Jose,

Much as expected then smiling smiley Thanks for satisfying (my) curiousity.

It's curious too the way one's interests develop. Mine started with cut gems but didn't stay just there very long. To understand much about cut gems one has to learn a lot about the the stones in their natural crystalline state. In turn, if one is to understand that, then one must learn about how, where and under what conditions the natural crystals form. And, finally, that leads to a need for understanding of the why's and wherefore's of other non-gem minerals, that are found both within gem crystals and also as host to the crystals themselves. By which time one is quite some little way into mineralogy. And that's been my journey to this point.

Garnet's are one stone for the ID of which, a hand-held spectroscope and a good strong (incandescent) light are really useful things to have about. Sadly though, few people can just pick one up and use it effectively, most needing time and practice with the instrument to get good results from it. This is a disincentive to those who have only very occasional need, compounded by the fact that, with many transparent mineral species, the spectroscope will not help with a positive identification at all sad smiley Almandine and Spessartine however generally are willingly responsive to this type of examination, even in unpracticed hands. Perhaps you have a friend local to you who has one and might make possible such an examination? If you have no handy friend and would like to PM me, maybe something else can be worked out.

From your SG determinations - and as you say - the stones seem to be mainly Spessartine with some Almandine present to drag the SG up. Any substantial mixing with Grossular (which would drag it down) is unlikely. The variance between the spectra yielded by your specimens and 'ideal' results, given by Gemology Tools Pro and other good reference sources of reference, should give a good steer as to how close to pure Spessartine your stones are (probably a better guide than the SG's because of the extraneous material on/in the specimens tested). Some stones really do yield spectra which are 'diagnostic' being so close to the reference standard. As we know, pure Spessartine specimens (or pure Almandine) are quite uncommon but there's a Spessartine on Mindat auction at the moment that shows the 'diagnostic' Spessartine spectrum.



Edited 1 time(s). Last edit at 03/20/2012 08:16PM by Owen Lewis (2).
avatar Re: Almandine or Spessartine ???
March 20, 2012 11:42PM
Thanks very much for your interest in this thread.

My calculations are just aproximated, measurements are not taken with scientists instruments and the applied corrections are arbitrary. That's why I don't give a percentage proportion of mixture, I only said "spessartine with some almandine". Is a pity that I have not a DGS to check my garnets. After your comments, perhaps I will find one and learn to use it.

After all, for me most important is to check that the theory that almandines grow included and spessartines grow implanted remains in force

Thanks again and greetings.

Josele
avatar Re: Almandine or Spessartine ???
March 21, 2012 12:52AM
Spessartine can grow within a rock, as well as in vughs, if that is what you mean? I cannot recall seeing verified almandine crystals
Lining vughs, but it can be hydrothermal, so is possible.

Regards,
Ralph
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