Garnets and Pyroxenes from Huanggang Mine, Inner Mongolia

Zhenhua Zhou, Hongwei Liu, Guoxiong Chang, Linsu Li, Tao Li, Yongjun Yang, Ruijun Zhang, and Xianhe Ji (2011): Mineralogical characteristics of skarns in the Huanggang Sn-Fe deposit of Inner Mongolia and their metallogenic indicating significance. Acta Petrologica et Mineralogica 30(1), 97-112 (in Chinese with English abstract).

Free pdf (note that this server is very slow and may be unresponsive during daytime in China):
[www.yskw.ac.cn]

From analyses of skarn mineral compositions in a large number of samples (analytical results included), this paper comes to the following results:

Garnets:
Composition varies from andradite (Adr 28.69-96.44) to grossular (Grs 2.00-67.38). Garnets formed at an early stage are andradites of nearly end-member composition, while garnets formed in the major ore-forming stage are grossulars. Grossular is also dominant in retrograde skarn assemblages.

Pyroxenes:
Composition varies from diopside (Di 11.80-94.12) to hedenbergite (Hd 4.08-81.28). Pyroxenes formed at an early stage are diopsides, while pyroxenes formed in the major ore-forming stage are hedenbergites.

Conclusions:
- It is very unlikely that all garnet specimens traded from Huanggang Mine are andradites. I assume that they are notoriously labelled as such without having ever been analyzed.
- Combinations of andradite with diopside or grossular with hedenbergite are common, while those of andradite with hedenbergite or grossular with diopside are very unlikely to occur. Again, the mindat gallery of Huanggang Mine grossly contradicts any scientific evidence here.

Measures:
(1) I will sent messages.
(2) All specimens of supposed andradite and/or hedenbergite should be listed as garnet and/or pyroxene until analyses can be provided that confirm their true identity.

I have a very zoned garnet cluster from Huanggang Mine. It came labeled as Andradite and the specimen is slightly magnetic, then I upload it to my Mindat gallery just as Andradite.
Yesterday I received a message from a Mindat supervisor warning me that the species could be wrong and sent a link to this post.
After reading this and the abstract of the link, and take a look to my piece, I am convinced that will be better to change the label to Andradite-Grossular series. As said, my specimen is slightly magnetic, wich suggest some Fe in the composition, also have associated diopside as generally exists in Huanggang Andradites, but as I can not know if Andradite is majority it is better to be cautious.
Peter, thanks for the information and for helping to keep Mindat a reliable site.

Andradite-Grossular series garnet cluster with associated diopside.
Huanggang Mine, Hexigten Banner (Keshiketeng Co.), Ulanhad League (Chifeng Prefecture), Inner Mongolia Autonomous Region, China
Overall size: 5,2 x 4 x 3,5 cm


Josele

Magnetism is probably caused by magnetite inclusions. There is another paper by Wang et al. (see reference list on the Huanggang Mine page) that explains that the main ore minerals (magnetite and cassiterite) predominantly occur as fine-grained disseminations in the skarn minerals (chiefly garnets and pyroxenes). They are both rarely found in visible crystals.

The composition change from andradite to grossular (and diopside to hedenbergite) from early to late mineralization stages suggests that the zoned garnets have a higher andradite component in the core and grade outwards to higher grossular components. Specimens with a second generation of greenish/orangish yellow crystals overgrowing earlier dark brown crystals are likely grossular on andradite (but this still needs to be confirmed by analysis).

I do not know if magnetism of Huanggang Mine garnets is caused by magnetite inclusions or is due to Fe normal composition of Andradite. In fact, all garnets in my collection containing some Almandine or Andradite molecules are more or less magnetics, as you can see here:
[www.foro-minerales.com]
These neodimium magnets are very strong!

Almandine-Spessartine series garnet
Shigar Valley, Skardu District, Baltistan, Gilgit-Baltistan (Northern Areas), Pakistan
5 x 4,5 x 3 cm. 118 g.


Josele

Bob Downs of the U of A has analyzed what I thought were hedenbergite specimens associated with ilvaite and arsenopyrite and it turned out to be an amphabole rather than hedenbergite.

Rock Currier
Crystals not pistols.

Some of the Huanggang skarns contain a considerable fraction of amphiboles. Actinolite, ferrohornblende, ferro-edenite and ferrotschermakite have been reported (and confirmed by analysis, see the paper by Zhao and Li).

EDIT:
I have added a comment to the locality page. Otherwise, I only wonder why people apparently don't learn their lessons and still strongly believe in labels. Remember the many Yaogangxian specimens that have been proven to be mislabelled ...



Edited 1 time(s). Last edit at 08/17/2012 10:42AM by Peter Haas.

Jose,

In my experience, the Huanggang Mine garnets are not themselves magnetic; I believe that the matrix on some specimens is rich in magnetite. I acquired a large lot of these at the Tucson show and I detect no magnetism in the garnet crystals themselves, but a strong magnetism on pieces with matrix. In some cases I could get my neodymium magnet to adhere loosely to a garnet crystal, but only if there was a significant amount of matrix just beneath it.

I'd be curious to know if that is also the case with your specimens.

Nik Nikiforou

"Sometimes being smart is just dumb luck"