An interesting Article on North American Coti's.

wdwrx

Well-Known Member
Strictly for your reading pleasure, I'd like to share with you all an interesting article regarding a simlar species of rock in North America. A quick skim through quickly disabused me of any fantasies of being located next to my own private coti mine:D but it might shed a glimmer of light on the general nature of coti's, especially for those of you more learned than I.
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Direct link:
http://pubs.nrc-cnrc.gc.ca/mineral/mineral39/tcm-102139-4.pdf


Cheers,
-Chris
edit: this one's a little more transparent:
http://www.minsocam.org/ammin/AM50/AM50_1477.pdf
 

wdwrx

Well-Known Member
Anybody read it?

I think it shows nicely how lucky we are that we have these rocks. The conditions that lead to the creation of coti's aren't unique, but to have them occur with exactly the right particle size, in layers large enough to be practical seems to have only occurred in the one location in all the world.
 

Tcensor

Well-Known Member
I did not understand exactly, the extent to which the areas discussed in the article where surveyed. It appears to me that there may very well be locations in the discussed areas (which are somewhat large) that would provide a nice source of coticules on a personal scale. Of course, this does not make them economically viable, rather a novelty, a conversation piece if you will - and yes, we are very lucky to have had the rare occurrence in Belgium. But I am not convinced that more extensive surface level survey of the discussed areas in N. America would not yield surprising results - not to mention deeper strata.

An interesting read either way. Thanks for posting. :)
 

wdwrx

Well-Known Member
Well, since there will be a test that follows.... kidding. Since there will be NO test that follows, I haven't been taking notes.
I'm trying to parse my way through it, and get a faint glimmer of understanding of the origin of coti's in general but I'm in danger of extrapolating too far with my limited understanding of geochemistry.
Near as I can tell, they are formed in open ocean of sedimentary deposits, generally in a rift valley.... (?... think mid-atlantic trench?). The presence of Manganese has some relation to the garnet. Then through a process I don't understand, during metamorphasis, the garnets crytalize out. ?
400 million years later, we've got Coti's!

Any further enlightenment would be appreciated.

WTH's Woodash when we need him?:D (IIRC he's a geologist?)
 

Tcensor

Well-Known Member
I looked around a little bit. Found this
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. It may be of interest. Most of it is over my head, but from what I understand, formation processes differ from region to region - and even within a region, hence the relevance of "different veins" we often see discussed here on the site. Moreover, it would appear that the N. American coticule's crystal is bell shaped and not a rhomboid like it's Belgian cousin - and this may very well affect the way it would function as a whetstone or hone. Here is the abstract:


Crystal size distribution (CSD) in rocks and the kinetics and dynamics of crystallization

Katharine V. Cashman and John M. Ferry

Crystal size distributions (CSDs) measured in metamorphic rocks yield quantitative information about crystal nucleation and growth rates, growth times, and the degree of overstepping (DeltaT) of reactions during metamorphism. CSDs are described through use of a population density function n=dN/dL, where N is the cumulative number of crystals per unit volume and L is a linear crystal size. Plots of ln (n) vs. L for olivine+pyroxene and magnetite in high-temperature (1000° C) basalt hornfelses from the Isle of Skye define linear arrays, indicating continuous nucleation and growth of crystals during metamorphism. Using the slope and intercept of these linear plots in conjunction with growth rate estimates we infer minimum mineral growth times of less than 100 years at DeltaT<10° c,="" and="" nucleation="" rates="" between="">–4 and 10–1/cm3/s. Garnet and magnetite in regionally metamorphosed pelitic schists from south-central Maine have CSDs which are bell-shaped. We interpret this form to be the result of two processes: 1) initial continuous nucleation and growth of crystals, and 2) later loss of small crystals due to annealing. The large crystals in regional metamorphic rocks retain the original size frequency distribution and may be used to obtain quantitative information on the original conditions of crystal nucleation and growth. The extent of annealing increases with increasing metamorphic grade and could be used to estimate the duration of annealing conditions if the value of a rate constant were known. Finally, the different forms of crystal size distributions directly reflect differences in the thermal histories of regional vs. contact metamorphosed rocks: because contact metamorphism involves high temperatures for short durations, resulting CSDs are linear and unaffected by annealing, similar to those produced by crystallization from a melt; because regional metamorphism involves prolonged cooling from high temperatures, primary linear CSDs are later modified by annealing to bell shapes.
 

wdwrx

Well-Known Member
Thank you Tcensor.
I love digging around in the internet. It's like having access to the word's biggest library.
A compulsive reader like me can get lost for days.

I also found that article (well, the others too) a little over my head as well. Though I did gather a bit about what makes the other "coti" type rock not as suitable as out favorite Begium stones.

Here's a little remedial reading for us: http://en.wikipedia.org/wiki/Metamorphic_rock
(I love Wikipedia! I can follow those links for hours)
 

Tcensor

Well-Known Member
wdwrx said:
Thank you Tcensor.
It's like having access to the word's biggest library.
Indeed. It's the best thing that happened to mankind since penicillin. :)

See: http://en.wikipedia.org/wiki/Penicillin
 
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