There is, for me, a more significant problem with the spherical crystals in a granulite facies rock (prismatine rock from Waldheim). Spherical crystals in larger spherical crystals and bulk rock are not rare – I think they are very characteristic features of this unusual rock. According to geologists, this rock is usually formed by exhumation and involving melting due to decompression. And that is the problem! The formation of spherical crystals in a static state is nearly impossible. Necessary is a separation of the equilibrium melt from the corroded spherical crystals. If not such separation happens, the spherical crystals will obtain her equilibrium habitus. I assume at least a two-stage evolution. Crystallization of the bulk rock, maybe from the decompression melt and input of a fast-rising very low viscous melt from a greater deep carrying the spherical crystals (high quartz with zircon, zircon, garnets, feldspars, corundum, and many others) and more exotic phases like coesite and stishovite. The two-stage evolution is characterized by a normal metamorphic crystallization and an input of a more pegmatite-forming melt from great deep, like the Starkoč abyssal pegmatite in the Gföhl unit (characterized by HT/HP rocks) in the Bohemian Massif (Jan Cempirek and Milan Novák, 2010). In this pegmatite, I have also found a large number of spherical crystals. Is there any other explanation? Karl Hermann Scheumann: Das Kornerupingestein von Waldheim in seinem genetischen Zusammenhang (1960) discussed already the meaning of a pegmatitic phase for the formation of the prismatine rock (however, in a bit of old-fashioned manner).
Dear Rainer,
thank you for posting this interesting technical question on RG. It seems to me that you already know everything about spherical crystals, so what could be added? In my personal experience, it often pays off to directly search the "Publications" section of RG for relevant literature references. For example, please have a look at the following potentially useful articles:
A proposed mechanism for the formation of spherical vivianite crystal aggregates in sediments
Article A proposed mechanism for the formation of spherical vivianit...
Unfortunately this article is not freely available as pubic full text, and none of the authors has an RG profile.
Also please see:
Formation Mechanism of CaCO3 Spherulites in the Myostracum Layer of Limpet Shells
Article Formation Mechanism of CaCO3 Spherulites in the Myostracum L...
This paper has been posted as public full text on RG. In this context you might also want to check out the following patent literature:
Forming spherical crystal habit
Patent Forming spherical crystal habit
Good luck with your work!
Dear Frank,
Many thank for your contributions to my problem. I will answer after reading that.
Best regards,
Rainer
I think we have here an analogon to the rapid migration of kimberlite to the surface, presented by Arndt et al. (2010) - however, not with a classic silicate melt, but a supercritical melt.
Arndt, N.T., Guitreau, M., Boullier, A.-M., Le Roex, A., Tommasi, A., Cordier, P., Sobolev, A. (2010) Olivine, and the Origin of Kimberlite. Journal of Petrology, Vol. 51, 3, 573-602
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