Hi Vincent.

What about pyroxene? looks like augite.

I wouldn't expect any silica polymorphs in such a mafic rock, and I would guess the vescicle in images 4 and 5 has been filled later by hidrothermal fluids?

Regards!

Dear Vincent,

The fine-grained crystals in the matrix show the optical characteristics of clinopyroxene. The filling compound in the small cavity is probably from a narrow edge of seladonite and in the core of a mineral of the zeolite group. A more detailed analysis is only possible with the help of X-ray diffractometry.

Best regards,

Guenter

I also vote for clinopyroxene. The larger laths show the expected interference colors for augite; the smaller grains have lower interference colors, consistent with thicknesses less than 30 microns. The relief and inclined extinction are also consistent with cpx.  -- Jane

Many thanks to all for your useful comments ! Unfortunately, the goal here is to work only on a thin section, so the correct identification of some minerals (zeolites for example) may be a little tricky...

Dear Vincent,

Totally agree with above comments. I'd interpret the matrix as cpx, probably augitic for interference and color. 

You observations are a bit hampered by the thickness of your section, which in principle should be 'much smaller' than the mineral grain size, to minimize grain overlap. In practice, an experienced operator may be able to lap  down to ~10micron, but even 20mu would already make a big difference. Double-sided polishing will also help (but adds an extra preparation challenge), to remove surface relief that becomes more important with such small mineral grains.

Cheers, Maarten

Yes, it is clinopyroxene the prismatic high relief mineral forming small crystals in the groundmass. At first, I was expecting it to be melilite, due to the abbundance of olivine phenocrysts and the lack of plagioclase in the rock, but the anomalous blue interference color of melilite is not present, moreover the mineral has an evident inclined extinction, and melilite doesn't, because is a tetragonal mineral with straight extinction. The rest of the groundmass is composed of partially devitrified mafic glass (somewhat chloritized), opaques and a colorless, anhedral, negative relief, isotropic mineral, which could only be ANALCITE, which is considered as a feldespathoid, even being a zeolite. The amygdules (not vesicles) are filled with some hydrothermal secondary zeolite mineral, celadonite is good possibility, but there are dozens of fibbrous-radial low birefringence zeolite minerals, so this only could be ascertained with XRD or microprobe analysis. I guess the rock itself could be called olivine-ANKARATRITE or analcite-FOIDITE, because of the analcite and cpx in the groundmass, and the olivine phenocrists. This kinds of voclanic rock is quite typical of intra-plate magmatism, continental or oceanic, related to hot spots. Since your city, Douai, is located in NE-France, I assume it could be a sample from the famous and nearby Alkaline Intraplate Volcanic Province of the Eiffel Region, in western Germany, famous for its lherzolite mantle nodules and maars. Also, this volcanic rock is almost unaltered, doesn't show any metamorphism whatsoever, the olivine and cpx are fresh, and its mafic glass is not totally devitrified, therefore we can assume it is a quite young lava, not nore than Miocene in age, probably coming from one of the Europeran Tertiary volcanic provinces.

Many thanks Sebastián for your answer. Unfortunately I do not have any precision concerning the origin of the sample (this is a completely blind testing) but your ideas are great. I will go further into the optical analysis.

Dear Vincent,

Yes, it is definitely early crystals of clinopyroxene (augite), the prismatic high relief mineral forming small crystals in the groundmass with phenocrysts of olivine.

Thank you Ali !

Dear all, at last I think that I have more or less identified what I was looking for. The mineral with the negative relief is most probably leucite and/or analcime (according to my petrographical handbooks the distinction between the 2 is difficult) - I have identified the complex twinning which is characteristic of leucite. I was not satisfied by the "glassy" background which is not glassy (1st order greys under crossed polars, not isotropic)  which is in optical continuity with the leucite/analcime. So this is most probably, as suggested by Sabastian, a foidite; considering that I expect only french rocks it should come from the French Massif Central. It is not so much different from this one : http://www.alexstrekeisen.it/immagini/vulc/foiditeleucitevulsini(8).jpg although my leucites are less well formed, and there are numerous vesicles with zeolites. But anyway many thanks to all. This is quite different from my usual metamorphic rocks !

Yes, leucite and/or analcime (analcite) are quite possible in a rock with so many fresh olivine phenocrysts. I also thought about a provenance from the Massif Central of France, also an large Tertiary intra-plate alkaline magmatic province, but being Douai so close to the Eiffel, it would have been a good possibility. In the five images you sent along I didn't see the typical very low birrefringence and sector twinning of leucite, also the reflief of the colorless mineral in the groudmass is very low and it is too "colorless" and anhedral to be leucite, which usually forms perfect trapezohedra with oriented inclusions of pyroxene or melanite, or it is often altered to clay minerals, as it happens in the Roman Potassic Alkaline Province, of Central Italy. But if you have an image of the leucite you found in the sample, please send it to this blog.

Yes I agree, it is quite too anhedral to be leucite - but what about the twinning pattern? The negative relief is not obvious on the micrograph but I have checked on the microscope, no doubt about it.

Unless Vincent Thiéry shows us a photomicrograph with the sector twinned leucite there's nothing else to say. All leucites I have seen in thin section collections, usually from the EiffelField and the Roman Province, are completely idiomorphic, perfectly shaped trapozohedra. Actually the very low, possibly negative relief is quite visible in the center of the second photo, taken with uncrossed polars The cpx prisms seem to float in the colorless analcime... which is totally isotropic in the third photo.

Can the complex twinning from my last message (picture entitled M_0011) be considered as leucite twinning? It's not so much different fom that : http://www.science.smith.edu/geosciences/petrology/petrography/leucite/leucite8.jpg&sa=X&ved=0CD0Q9QEwA2oVChMIq922xJfBxwIVgn4aCh0vZgZm

volcanic glass, sometimes devitrified, somewhere fine-grained ground-mass consisting of pyroxene and plagioclase.

Yes, Vincent, that really looks like leucite sector twinning, so the rock contains leucite, analcime, olivine , cpx, and ore, it is indeed a foditie, with NO plagioclase, so it can't be called "basalt" at all. Perhaps a name like olviine-ankaratrite could apply to it.  

Sometimes it is useful to know the major element chemistry of the rock. Do you have any analyses for this rock? This might take a lot of guesswork out of the equation.

Surely a rock with so many unsaturated minerals (olivine, leucite, analcime) will have a %SiO2 quite low, in the order of 44% or even less. But in the original question of Vincent Thiery there's no mention of any chemical analysis, only of a "round robin test" on petrography.

 this texture is typical for undersaturated basalt with leucite (poly synthetitc twinning, low burefrengence|) and typical clinpyroxene laths, magnetite and olivine