Zircon experts -- please help. Most zircon grains display concentric growth zones that almost never cross-cut each other. These attached images show thin-sectioned zircons with sharply-defined, parallel lamellae (yellow line; some are triplets) that cross-cut the typical growth lamellae (red lines). Grains are about 250 um long from S. America. C-axis is indicated. In your opinion, what is the most likely method of formation for the yellow-marked lamellae? Tectonics? Volcanism? Impact shock? Something else?
I have 55 years of preparation experience, especially with the polishing of tiny mineral grains (apatite, zircon, etc.). Concerning the parallel lines (yellow), I see no other interpretation than relic grooves from the polish on the surface of the zircon grains. It strongly depends on the polishing method you used. The criss-crossing scratches are also an indication of poor polish. Since the sub-parallel tracks cannot be traced over the entire surface, these are probably surface effects. Focusing into the depths of the crystals will bring clarity.
Best regards,
Guenter Grundmann
Thank you, Guenter. Some of the lamellae appear to go deep into the grain. For zircons, have you ever seen cross-cutting lamellae that were not polishing marks? If so, what caused them?
Dear Allen,
I think I have found a very interesting publication to solve your question:
(see Google)
Atlas of Zircon Textures
by
Fernando Corfu,
John M. Hanchar,
Paul W.O. Hoskin,
Peter Kinny
Please see Figure 12 in this publication.
Figure 12. Appearance and texture of zircon in meteorite impact structures.
Probably, the solution could be found here.
I wish you success,
Best regards,
Guenter
Thank you very much, Guenter. Fig. 12 is very helpful. By the way, these grains came from a known impact site, but I did not want to mention it so as not to bias a response.
There is one way to distinguish between polishing scratches and PDFs (or any in-mineral linear feature). Go at the highest objective and using the focus of the microscope follow the lines that go deeper in the thickness of the mineral. If they are going in the zircon at an angle (most of the times) you will see them moving sideway as you slowly change your focus. It will not be the case with scratches which are only at the surface of the mineral.
Thanks, Francine. I changed focus as you suggested, and both the lamellae and growth zones clearly penetrate through the thin section at angles to the surface. So, they are not scratches.
Dear Allen West
Zircon precipitated entirely from fluid or fluid-saturated melt may exhibit coherent internal structures with oscillatory or sector zoning – features that are also typical of magmatic zircon. hydrothermal origin of a zircon grain is its occurrence in hydrothermal quartz or carbonate, together with other hydrothermal minerals and/or fluid inclusions, and the presence of hydrothermal mineral inclusions. Morphology is not diagnostic (Geisler et al 2007). Your zircons have fluid inclusions and mineral inclusions.
Hydrothermally overprinted zircon may show complex internal textures in which secondary domains cut across primary growth zones. In hydrothermal origin, zircon crystals are polyfacetted and may form intergranular films (Geisler et al 2007).
I think you can find the answer to your question in these references:
https://www.researchgate.net/publication/277093078_Mineralogical_studies_on_zircon
DOI: 10.2113/gselements.3.1.43
DOI: 10.3390/min10010083
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