Dear RG members,
I have produced a thin section of a rock in our rock collection, used for students enrolled in the second year of the bachelor of Geology at Sapienza University of Rome. Unfortunately I do not know the sampling locality.
The rock has a quite simple mineralogy. It is made up of ~40-50% euhedral to subhedral plagioclase laths, ~10-15% columnar cpx with greenish rim, ~10-15% acicular biotite and ~10-15% orange, partially devitrified, glass (see attached picture; sorry for the poor quality of the images).
The rock is certainly igneous and should be classified as basalt, but my curiosity goes to the abundant and acicular biotite laths without any evidence of iso-orientation. Interesting is also the presence of prisms of cpx with greenish (likely Fe-rich) rims.
Any idea how it could have formed? I saw many kinds of basaltic rocks, but none with this characteristics.
Cheers,
michele
In my opinion this is a sill or dyke, the specimen of which belongs to the lamprophyres subtype kersantite where plagioclase is greater than K feldspar as in minette.
HGD
Thank you Harald G. Dill . The problem is that the amount of mica is relatively minor (likely less than 5%). I do not see any reason to define this as lamprophyre. A volcanic rock rich in plagioclase, with, in order of abundance, cpx and biotite is and remains a basalt. I was wondering how the absolutely random and acicular biotite could have formed.
Dear Dr. Lustrino,
I think better than providing thin section images which in this case pretend a higher mica and CPx contents than what you mentioned now is giving a quantitative analasis accompanied by an image like the present ones. It trims down the field of speculation.
HGD
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