Dear Mr. Udagedara,

there are several minerals which come close to this chemical composition. We must take into consideration the presence of OH and B which during these analyses are not determined. With this in mind, it would be helpful to have more information about the minerals associated with the unknown silicate and the lithology / metamorphic grade. Unless these data are  available, it will be too speculative to make up my mind . This is not a full-blown chemical composition but a "spotlight". We have no idea about the valence state of Fe either.

Best regards

H.G.Dill

Dear Tharanga,

It is possible to have Fe2O3 in sillimanite, but not to the extent reported above by you. If Al+3 in sillimanite is replaced by Fe3+, the Fe2O3 content is commonly reported to be

Thank you Prof Herald and LRK Sir!

yes, its possible i observed it 

Dear Tharanga,

what kind of metamorphic rock are you looking at? Don't you have optical features or is the sillimanite (?) so small to be seeon only under SEM (wht SEI and not BSE?). Al2SiO5 polymophs can contain Fe2O3 (se review in Kerrick 1990) but 5 wt% seems really too much. On the other hand mullite is not common at all in a metamorphic rocks and requires extreme conditions.

regards,

BC

Did you look under cossed polars on your silimanite? Did you notice red internal reflections? It is worthy to check this because in some metamorphic minerals Fe exsolves in the form of micron to- submicron nebular inclusions due to secondary exsolution of  very fine hematite 

Is it possible to have Fe2O3 in Sillimanite yes (because we have replace Al+3 and Fe+3.

Secondary hematite micro powder is quite common in some rocks when even mild oxidation is takin place. You do not have to have high hematite content because red staining gives very very strong effect. Obviously Al can folllow Fe and vice versa. This may not be taking place in your sample but it is worthy to check measurinf Fe content. Fe Ka spectral line is very efficient and you can get detection limit around 0.01wt%.

Cheers, HK

Silimanite and Fe oxides are common to see together however, they are not paragenesis. you may see Fe oxide and silimanite in both synthetics and natural samples. I recommend share SEM and/or thin section image please then we can be more sure about it. What is your sample? outcrop? core? industrial specimen?

Regards

Thank you all! Thanks for your insights and references.

Dr. Ehsan, it is an outcrop and sillimanite-garnet gneiss. But this nomenclature is vague. Sure, Ill share SEM images 

Thank you all! Thanks for your insights and references.

Dr. Ehsan, it is an outcrop and sillimanite-garnet gneiss. But this nomenclature is vague. Sure, Ill share SEM images 

Hi again look at the minerals you list, these are sublimates from life origins. The silimanite is fascia that has fossilized, see my fossil lung. The DNA tests prove the lung was alive at one time. https://www.youtube.com/watch?v=AROw8rK0WT8

It is a long story but very interesting. Silicon, iron and fascia is the preservative in mud fossils. Here is the wet "mud" fossil process.

1..Wet soils create natural electric currents.

2..Molecular silicates are Tiny, platy and polar, some 2 atoms in size.

3..Creatures buried in mudslides can be sealed anaerobically in cold deep platy polar silicates (mud and clay) influenced by these wet natural electric earth currents.

4...Silicon is 50x higher in skin and connective tissue than other flesh. Si is not found by itself, it is almost always SiO2 and SiO4 which is sand, amourphous quartz and feldspar.

5...Natural electroplating of the creature occurs as Si in the skin bonds and becomes SiO2 and SiO4 (amourphous quartz and feldspars), This occurs on the surface but the insides remain preserved under extreme pressures due to deep mud on top.

6...Leachates from above percolate and interact with the buried creatures exterior surfaces. Chemicals in leachates determine fossil properties. An example is Opal which is a mineralized fossil.

7...Normal oxygenated decomposing bodies literally explode from the gases as they expand. Sometimes skin is "degloved" and Fluids are expelled from weak areas and orifices but very little is expelled in mud fossils due to compaction.

8...As plating continues the surface becomes more and more restricted until OH- becomes the primary invader. OH- is extremely reactive and destroys calcium and phosphorous in bone. This is why bone is hard to recognize in mud fossils.

9...Eventually glacial tilling disassembles the creature along fascial planes and body parts appear.

In such a rocks, sillimanite could have 2 origin: First from polymorphic reactions from Adnalusite. This kind could be prismatic and mainly as pseudo morph. Second generation is from Biotite and some other minerals. In this case, they are niddle like (fibrolite). In fibrolites, you can have more Fe replacing Al and lowering sillimanite stability temperature to Andalusite field. Also. since they are very thin, in large Beams, it is possible that one Fe bearing mineral complicated with sillimanite. So, if you have fibrolites, you must use very small beam (say 1 nm) for analysis.   

I believe rocks are the mineral bodies of sublimated creatures. https://www.youtube.com/watch?v=GVWSopgKzMU