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B-Cristobalite Needles

Last Updated: 18th Mar 2020

By Dave Crosby

Cristobalite Needles

~ Fifty years ago I started studying "Thundereggs" and observed the casts of long needle structures that enclosed their agate cores. Almost no one talks about them.

On my own I concluded they had to be Beta-Cristobalite needles, that had to begin forming in magma chambers to remain intact and grow as large as I found them in the Table Mountain Welded Tuff layers, and as huge as they got at Las Piedras Bolas of Ahualulco Mexico and Silver Cliff Colorado.

Nilia Dunbar made a study of them available at:
http://geoinfo.nmt.edu/staff/dunbar/publications/abstracts/Dunbar_gem_and_mineral_symposium_abstract-2002.pdf



As I said, there isn't much talk about them.

This morning after three hours researching on line I finally found:

https://www.sciencedirect.com/science/article/pii/0272884295909093?via%3Dihub
by Ling-YiWangMin and HsiungHon.
the evidence shows that the cristobalite seed could induce a compressive stress due to the abrupt volume increase as cristobalite transformed from α phase to β phase at elevated temperature. From the kinetic analysis, the time exponent, n, decreases with increasing cristobalite seed additions, suggesting that the transformation kinetics is controlled by a long-range diffusion mechanism from a two-dimensional plate-like growth to a one dimensional needle-like growth.


Later I found:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/37/5/435/29927/Formation-of-cristobalite-nanofibers-during?redirectedFrom=fulltext
by Martin Reich, Alejandro Zúñiga, Álvaro Amigo, Gabriel Vargas, Diego Morata, Carlos Palacios, Miguel Ángel Parada, René D. Garreaud.
The nanofibers are identified as the high-temperature (>240 °C), beta form of cristobalite, with average lengths of hundreds of nanometers and widths on the order of tens of nanometers. We propose that the beta-cristobalite nanofibers are formed during explosive eruptions by the reduction of amorphous silica by carbon monoxide to its reactive suboxide SiO, which is later oxidized to form one-dimensional crystalline silica nanostructures. Nucleation and growth of the nanofibers are enhanced by the high surface area of the micrometer- to nanometer-sized fragments of silica glass in the volcanic column.


Spheroids of b-cristobalite fibers have been known since Charles Darwin's day.
03973520015845348364914.jpg
1878 Charles Darwin Dwg #6
where he stated
One of the commonest transitional rocks deserves in several respects a further description. It is of a very complicated nature, and consists of numerous thin, slightly tortuous, layers of a pale-coloured feldspathic stone, often passing into an imperfect pitchstone, alternating with layers formed of numberless little globules of two varieties of obsidian, and of two kinds of sphærulties, embedded in a soft or in a hard pearly base.

[page] 67

The sphærulites are either white and translucent, or dark brown and opaque; the former are quite spherical, of small size, and distinctly radiated from their centre. The dark brown sphærulites are less perfectly round, and vary in diameter from the 1/20th to 1/30th of an inch; when broken they exhibit towards their centres, which are whitish, an obscure radiating structure; two of them when united, sometimes have only one central point of radiation; there is occasionally a trace of a hollow or crevice in their centres.


Their internal structure can be examined on the west side of Bailey Ridge (north of Bailey Mt.) and a half mile southeast of the Blundell Powerhouse - all located seven miles northeast of Milford, Beaver County Utah.





Casts of the needles are often visible on the cores of Thundereggs.




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