Hi all,
I'm dealing with ultrapotassic rocks from the Roman Magmatic Province (Central Italy), and I stumbled on such a phenocryst (the euhedral grey one). Zonations are clearly visible, but interference colours and habit confuses me a bit..may anyone help me out?
Is it a cpx or what? if yes, diopside?
Thanks!
Francesco
Ok, for this to be a CPX, it would need to be a near basal section to have such low interference colours, and even then you would probably expect pale yellows rather than greys. Moreover, the prismatic section does not suggest a basal section, so CPX just doesn't sit right with me. It that case, I would suggest that this is an alkali feldspar. The interference colours are good, and you appear to have dissolution and regrowth features which are quite common in feldspar phenocrysts. There may also be signs of exsolution, but I can't be sure.
One section does not provide a clear picture of a mineral under the petrographic microscope. Taking the angle of extinction in the range 40° t0 45° ( Z vs. c) would point to pigeonite or augite. The cleavage is rather poorly expressed and is o.k. in comparison with amphiboles. This is also true for the yellow section at almost right angle to c where you might expect a rectangular pattern of cleavage planes what is not the case. The outline ,however, fits very well with a face combination labelled according to the Miller´s indices as follows: (100)-(110)-(010). Also try and determine the optical character of extinction ( positive or negative). In this set of section, the optical and crystallographic features are in favor of CPx by approx. 75 %
HGD
I agree with Dr. Preston, this doesn't look like pyroxene but is more likely to be a feldspar. Try to analyze the mineral in SEM or EPMA to exactly know its composition.
Dear Dr. Nayak,
how can you explain the high interference colors of the Xat its corner on the left which is quite the same shade as that of the yelloy one. No alkaline feldspar can achieve this. This is especially valid as the X is thining at its margin.
HGD
It is likely a pyroxene. This rock looks like a leucitite, being full of leucite and cpx, without (or with very rare) plagioclase. No leucitites have such large plagioclase phenocrysts. Also the shape and the overall relief points to a classical augite (not diopside). The grey interference colour (assuming that it is the highest colour) could be related to the fact that it has been cut close to its circular section.
I would concur with Michele Lustrino that it is a pyroxene, the shapes are good and show rare orthogonal fractures. The low birefringence color of the pyroxenes crystals can be due to an overcut in the thickness of the thin section. A thinner section (below the usual 30 microns thickness) can explain these lower birefringence colors even for the pyroxenes with their C axis nearly parrallel to the thin section that only reach the 1st order blue. It often happens when there is no good target minerals present (such as quartz and feldpars) unto which the technician can estimate that the thickness is near 30 microns. A Michel-Lévy interference color chart vs the thin section thickness will help in this case.
Why is there some speculation sparked on the type of pyroxene when the angle of extinction already points to augite-pigeonite ? Yes the C axis is parallel to the thin section plane (angle of extinction in the range 40° t0 45° ( Z vs. c))
The birefrigence is rather pronounced and changes regularly according to the sections cutting the indicatrix and the crystallographic system. There is only one question still not raised and answered. What is the optical character like, positive or negative? Feldspar shows twinning lamellae following different laws, e.g. albite law. Only in metamorphic rock such twins are missing quite often.
HGD
I hadn’t spotted that bright spot in the corner of the crystal, so would now concur that it can’t be a feldspar However, I have worked on ternary feldspars from Hebridean pitchstones that show no or very little twinning, which is why I suggested this might be feldspar in the first place.
Thank you all so much for your interest, I'll carry out SEM analyses in January/February, I'll keep you updated!
Cheers
Francesco
Dear Mr. Palozza,
"One swallow does not make spring yet"
There are several options on the table, each of which added up with some data and opinions.
It is now your turn as the man at the helm (looking at the thin section under the petrographic microscope) to collect the data, evaluate them and tell us which way the train goes. To be more precise. Are there CPx sections fulfilling these information obtained, have you got similar XX showing polysynthetic twinning like that of feldspar, does the optical character point to apatite?
It is not my duty to take a decision, it is yours.
We want to know what is your guess based upon all that. The remark, that you are going to conduct SEM is a step which you could have done without that post by yourself. It is a scientific discussion on all parts.
HGD
Cpx at 99.9%. It is very common in this kind of rocks and it is very similar to the other smaller phenocryst to the left.
There are not so large melilite in these Italian rocks.
Cheers
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