Dear Amjad: phengite is one of the so called white micas, the one you show here is a mafic mica, it has far more FeO than MgO, and much lore K than Na, and quite a bit of TiO2, so it could be annite (Fe-Ti-rich biotite), the background mineral seems to be garnet, if I’m not wrong, it shows higher relief than the mica...

Regards, Sebastián Grande

Dear Amjad, the explanation from Prof.Sebastian Grande is great and annite can be a potential phase but in that case the SiO2 content is too higher for annite ( a sub-type of biotite) which should have SiO2 in the range of 36~40wt%. The dark color is, as pointed out in above comment, could be due to Fe content.

I am not sure, but it seems the K2O decrease and SiO2 and FeO increase could be due to hydrothermal alteration as you mentioned the rocks are hydrothermally altered. Another option is annite but the silica increase is an issue then.

Raman analysis is the best option to confirm.

Dear Amjad,

I would suggest to plot your data on compositional classifcation and discrimination diagrams: (a) Fe/ (Fe + Mg) versus IVAl (after Deer et al., 1992); (b) Si (apfu) versus Fe/ (Fe + Mg) (after Makutu et al., 2004); you will get your answer. As Suggested by Prof. @Sebastián Grande and @Prof. Hafiz U. Rehman your data may fall in annitic field if IVAl apfu ranges < 2.5 which it seems due to high SiO2 content, Si apfu >5.5 and Fe/Fe+Mg>0.5. I think Fe number will decide weather it is annite or phlogophite.

Regards

It's likely a clay mineral. Too low K2O and too much SiO2 to be a mica.

Cheers.

Dear Amjad, to shed light on the nature of this phase I would also pay attention to the following features of the EMPA analyses (which has been also mentioned by the other people that replied your question):

- too high SiO2;

- too low alkali;

These points have to be considered in the frame of the fairly too low H2O (the total oxides amounts of your analysis are quite high).

Now, in my view, your problem with those analyses might be due to a loss of volatiles during the EMPA, which is a quite common issue when dealing with phyllosilicates. This occurs mainly when a too small size of the analysed spot is set, which ends with an overheating of the phases.

This would explain the low content of H2O and Na, and the relative increase of the remaining elements.

Best

Francesco

Hi Amjad. I think you are dealing with an altered phase. Likely an altered biotite. I often see this when you analyze a biotite that is partially altered to chlorite or other phyllosilicates. Indeed, it looks quite unhappy in the PPL image you show!

I agree that this could be altered mineral. The optical image looks somewhat like altered biotite but chemical analyses like phengite with low potassium. Probably the biotite is altered to phengite Muscovite and chlorite. Should be easily confirmed in crossed polarizedlight or backscatter image

Dear all: the picture was obviously taken without crossed polars, but the problem is that biotite, and all micas, have maximum absortion colors when parallel to one of the reticle wires. In this image biotite cleavage is parallel to the E-W direction, and so it looks quite dark. Better images should be gotten turning the stage first 90° without crossed polars, and then 45° under crossed polars. Only then one can make sure what the mineral is, and if it is altered or not. Regards, Sebastian.