The mineral I am referring to is located at the center of both photos. The photos correspond to a thin section viewed in non-polarized (to the left) and polarized light (to the right). The fault-rock is from northern Norway and kinematic indicator point towards extensional tectonics. Does anyone have any idea? Any input would be very appreciated! Thanks in advance! :)
It looks like a slightly altered lenticular biotite with undulatory extinction due to the strong deformation.
Dear Mr. Koehl:
The mineral aggregates have a birefringence of between 0.030 and 0.100 (it is difficult to give a better assessment based upon the images), its color and pleochroism attest to an Fe-bearing phyllosilicate. Phyllosilicates which fulfill these optical properties belong to the
-biotite-group
-nontronite-group (dioctahedral Fe-bearing smectite)
-stilpnomelane-group
Intermediate optical properties may be attained in case of an intimate intergrowth of biotite (< 0.100) and vermiculite (approx. 0.02).
As it is a postmagmatic process and a typical fault gauge I suspect of a mineral of the nontronite- or biotite-vermiculite alteration.
Any further classification on my part would be speculative.
I suggest a separation of the material either for XRD and/or CEC analysis.
Best regards
H.G.Dill
I endorse Harald Dill's views and suggestions. The best course is to have XRD or EMPA done.
Dear all,
first I would like to thank you very much for your very nice answers and suggestions! Today I got the opportunity to use our SEM and get the attached graph for the mineral I am interested in! Does this point towards fossil? Stilpnomelane? Biotite? Smectite? other? Thanks again for your time and help! :)
All the best,
JB
Dear Mr. Koehl,
I do not see any indication for fossils in your thin section. I have had problems to open the images properly; could you please provide some more information on the host rock and the geological setting. A microchemical analysis may be very difficult to achieve in this fine-grained material so as to allow for the calculation of a mineral composition matching one or the other potential candidates.
Best regards
Harald G. Dill
Based on the thin-section, it seems that biotite is strongly deformed in a clay gouge layer. In such a situation, the kaolinization and chloritization of bitoite is common which may accompany with berthierine minerals.
From your attached EDS spectrum, have you tried same quick mineral calculation? These might give you a first indication. Try this website: http://serc.carleton.edu/research_education/equilibria/mineralformulaerecalculation.html
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