I am currently studying thin sections of a wyomingite (leucite lamproite) from the Leucite Hills, Wyoming. The illustrated structures are common and make me think of spherulites, at least in XPL view (especially the last example), but in PPL, there's no visible acicular, needle-like crystal, only a slightly mottled texture with round darker rims. It looks as if devitrification had not occurred yet (this rock is about 0.9 My old); however the color in PPL (very realistic on the 400x and 600 px photos) is much yellower than the unfrequent glass found elsewhere on the slides; but since the groundmass of the lamproite consists of leucite and yellow-brown K-richterite ("magnophorite"), it could be some sort of "K-richteritic" glass? In XPL, especially in the LHG-13 sample, the vivid colors could be due to the interaction of water with the lava during supercooling? What do you think? Your answers will be appreciated. Fields of view: 40x-2.85mm, 400x-0.285mm, 600x-0.19mm
I agree with you that these are spherulite texture due to devitrification. The radiated needle like crystals (secondary chalcedony or zeolite) are typical for this strucutre
Thanks for your answer, Sobhi. I think you're right. What surprises me is the absence of visible texture or crystals at high magnification in PPL.
Chalcedony and/or zeolites do have a low relief under PPL and so not surprising not see any structure under PPL observation
Yes, it makes sense. Actually, when looking back at them I was able to discern a couple of very faint radiating lines in PPL. Thanks again.
Radiating needles are visible. Interesting to find sphurilitic structures in undersaturated leucocratic volacnic rocks.
Well, it looks like perlites after devitrification, not very well / fully devolped yet.
Thanks to you all for your answers. Jordi, I am not convinced they are perlites,rather spherulites. Sunil, yes, needles are visible in XPL and 600x PPL. It's true it's unusual in undersaturated rocks but silica must have concentrated in glass before devitrification?
By the way, again in the same slides, I found the following structures (photos attached). First I thought they would be some sorts of altered phenocrysts, because of their crystal-like shapes. But here, common (micro-) phenocrysts are phlogopite, olivine, K-richterite (magnophorite), richterite, priderite and leucite. Diopside occurs only in elongated laths. So, in my opinion, there's no match. I think rather of amygdules filled with zeolites (very pleochroic, maybe thomsonite) along with some tiny unresolved minerals. What do you think?
Both are very small. Field of view of first photo is 0.19 mm, of 2nd photo, 0.28 mm.
Jennifer, thanks your answer and your nice photo. Unfortunately, my spherulites are not as perfect as yours and don't make a full circle, but the cross is here all the same. As I stated before, the radiating texture is slightly noticeable at higher magnification (600x)
These, at least the last two, are spherulitic. I do not remember to have seen spherules of such type in undersaturated rockss. However, I attach a picture of a highly altered rock from the Salt Range of Pakistan which essentially comprises phrnocrysts and spherules of talc-like secondary minerals (probabdly after enstatite) in a ground mass of K-feldspar.
I support jenifer
Spherulites donot have visible acircular needles. A dark extinction "is simply a step to aid in easier identification of the spherulites
I worked with a lot of rock with spherulitic textures many years ago. I agree that-given the glassy nature of the rock that is consistent with rapid cooling- this seems a reasonable conclusion. One way to check might be to etch a small sample with HF acid vapor (erodes the glass) and look with SEM to see if you can see evidence of the underlying (residual) structure.
Thanks for your answers, Betty and David. As for David's suggestions, unfortunately I am a retired geographer without access to a lab...
What do you see under high magnification with reflected light (or hand lens)? Can you see any surface texture?
I will not discuss the first part of the question, as much has been said. The latter two photos seems to represent epidote, probably an early product of alteration.
The latter two photos are similar to radial structure of zeolite minerals (philipsite? or chapasite?). Are you identified some zeolite minerals through XRD or by thin section. Check it.
David,
Thanks for your suggestion. The problem is most of these structures are very small: 0.2mm or less (except the first sample I photographed). Attached is an image with the stereo microscope of what COULD be one of these structures. I tried with a EPI metallurgical microscope but can't have a clear picture.
Mirka,
Your answer makes sense; the shape looks like a crystal and it suggests a zoned epidote. The problem is that nobody in the literature about the Leucite Hills ever mentioned epidote, even as a xenolith.
Khalid,
Thanks to your suggestion. I also thought about a cognate xenolith (autolith). But your suggestion about zeolite is good. I re-attach the XPL photo of one of the samples and the PPL photo of the same sample. In PPL, it suggests a growth from the center, maybe with some inclusions.
Within the last three samples the crystallinity seems to increase. Therefore the radiating arrangement of the crystals in the "aggregates"is barely , but is recognizable. In the last two sample in particular in the last, the spherulitic nature is obviuos.
Arno,
Thanks for your opinion. I agree. What do you think of the two last photos I attached on page 2 of this topic (PPL and XPL views of what could be a small vesicle filled with zeolites)?
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