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 Post subject: Diamond interaction with minerals
PostPosted: Mon Mar 05, 2018 8:38 am 
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This is a spin-off from the thread 'Having difficulty with polishing sapphire' https://www.gemologyonline.com/Forum/phpBB2/viewtopic.php?f=8&t=24292. If you are interested in more details about the interaction of diamond with various minerals you can download a pdf of my engineering PhD thesis: Miller, D.E. 1986. Rock drilling with impregnated diamond microbits:1-277. Ph.D. thesis: University of Cape Town. http://open.uct.ac.za/handle/11427/4962. Although it is about rock drilling it deals with diamonds moving over mineral surfaces under load, so has some relevance to faceting and diamond sawing, and it has lots of SEM images. There are two shorter papers summarising this work, but they are behind the publisher's paywall and only readily available to people with institutional library access. If you really want scanned copies, you can send me a personal message.
Miller, D.E. & Ball, A. 1990. Rock drilling with impregnated diamond microbits - an experimental study. International Journal of Rock Mechanics and Mining Sciences 27:363-371.
Miller, D.E. & Ball, A. 1991. The wear of diamonds in impregnated diamond bit drilling. Wear 141:311-319.


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 Post subject: Re: Diamond interaction with minerals
PostPosted: Mon Mar 05, 2018 7:34 pm 
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Thanks for posting that, but the download did not work for me. Could the url be malformed?

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 Post subject: Re: Diamond interaction with minerals
PostPosted: Mon Mar 05, 2018 7:39 pm 
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We published recently a work dealing with polishing here:
Saldi, G.D., Voltolini, M. and Knauss, K.G., 2017. Effects of surface orientation, fluid chemistry and mechanical polishing on the variability of dolomite dissolution rates. Geochimica et Cosmochimica Acta, 206, pp.94-111.

Polishing was not our main interest (therefore only its practical effects are discussed in the paper), but we found that polished surfaces have a different chemical reactivity than the equivalent cleaved ones.
The most interesting result, faceting-wise, is that a surface that looks (actually "is") polished (we checked it with interferometry and AFM: I did a good job :D on the dolomite cleavage plane as well), after etching can reveal hidden scratches*, those act as etch-pit nucleation points, screwing up the very first part of the research, at some conditions.
We are talking about few microns tops, tho.

* Those scratches were caused mostly by tiny bits of dolomite detaching from the edges and running under the facet during cutting/pre-polish, and were obliterated by the final polishing, but some inherited damage was still present under the very surface.

And we are talking about dolomite. Not your typical faceted gemstone. ;)

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 Post subject: Re: Diamond interaction with minerals
PostPosted: Tue Mar 06, 2018 1:13 am 
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thomas.adamas wrote:
Thanks for posting that, but the download did not work for me. Could the url be malformed?

Hi Thomas - I have just clicked on the link in my message and the download worked fine for me. (The link opens to the abstract. Then you have to request the free pdf download.) Please do try again. Duncan

Funny, I have just tried clicking it again and this time it didn't work. I have also tried copy and paste to a new tab without success. The URL is correct. Evidently the UCT server is misbehaving. Please try again later.

Couple of minutes later, it's working again.


Last edited by Duncan Miller on Tue Mar 06, 2018 2:35 am, edited 3 times in total.

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 Post subject: Re: Diamond interaction with minerals
PostPosted: Tue Mar 06, 2018 1:32 am 
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maialetto wrote:
The most interesting result, faceting-wise, is that a surface that looks (actually "is") polished (we checked it with interferometry and AFM: I did a good job :D on the dolomite cleavage plane as well), after etching can reveal hidden scratches*, those act as etch-pit nucleation points, screwing up the very first part of the research, at some conditions.
We are talking about few microns tops, tho.
Thanks Marco - This is very interesting and is reminiscent of claims for a 'Beilby layer', in which acid etching of a polished surface apparently revealed prior scratches. I suspect that polishing produces a layer of plastic deformation in some materials that can mask underlying damage. I have read that some traditional emerald cutters claim they can 'heal' fine cracks by polishing. I don't know if this is true, but it also would explain the apparently greater hardness of the polished surface of some materials, and the messy, rough surface exposed below when you grind through this layer if reworking a polished facet. You have to grind away the messy layer completely before repolishing. Here is a photograph of a partially reworked tourmaline table showing this phenomenon.

Added: I should have included the link to this earlier discussion of repolishing a facet, and mention the 'Kerez effect', in which polished tourmaline surfaces can display multiple refractive indices. It complicates the physical puzzle. https://www.gemologyonline.com/Forum/phpBB2/viewtopic.php?f=8&t=23752&hilit=kerez+effect


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 Post subject: Re: Diamond interaction with minerals
PostPosted: Tue Mar 06, 2018 2:54 am 
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Duncan Miller wrote:
Thanks Marco - This is very interesting and is reminiscent of claims for a 'Beilby layer', in which acid etching of a polished surface apparently revealed prior scratches. I suspect that polishing produces a layer of plastic deformation in some materials that can mask underlying damage. I have read that some traditional emerald cutters claim they can 'heal' fine cracks by polishing. I don't know if this is true, but it also would explain the apparently greater hardness of the polished surface of some materials, and the messy, rough surface exposed below when you grind through this layer if reworking a polished facet. You have to grind away the messy layer completely before repolishing. Here is a photograph of a partially reworked tourmaline table showing this phenomenon.


Accurate observation, Duncan and I agree with your conclusion about the plastic deformation. I have encountered that phenomenon on quite a few recuts. Yes, you can deform the surface over a scratch on a number of materials and Beryls exhibit this property notably. Of course, Beilby Layer applies only to metals, but plastic deformation has been well documented on a number of oxide and silicate materials.

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 Post subject: Re: Diamond interaction with minerals
PostPosted: Tue Mar 06, 2018 3:11 am 
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thomas.adamas wrote:
Beilby Layer applies only to metals, but plastic deformation has been well documented on a number of oxide and silicate materials.
Apparently the term is used not only for metals, e.g. Suratwala, T., Steele, W., Wong, L., Feit, M. D., Miller, P. E., Dylla-Spears, R., Shen, N. and Desjardin, R. (2015), Chemistry and Formation of the Beilby Layer During Polishing of Fused Silica Glass. J. Am. Ceram. Soc., 98: 2395–2402. doi:10.1111/jace.13659 (The Miller in this reference is not me. By the way, the link to my thesis seems to be working again.)


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 Post subject: Re: Diamond interaction with minerals
PostPosted: Tue Mar 06, 2018 3:18 am 
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Duncan Miller wrote:
thomas.adamas wrote:
Beilby Layer applies only to metals, but plastic deformation has been well documented on a number of oxide and silicate materials.
Apparently the terms is used not only for metals, e.g. Suratwala, T., Steele, W., Wong, L., Feit, M. D., Miller, P. E., Dylla-Spears, R., Shen, N. and Desjardin, R. (2015), Chemistry and Formation of the Beilby Layer During Polishing of Fused Silica Glass. J. Am. Ceram. Soc., 98: 2395–2402. doi:10.1111/jace.13659 (The Miller in this reference is not me. By the way, the link to my thesis seems to be working again.)

Yes, thanks! I downloaded it and will schedule a time to read it.
So, these researchers have used the term Beilby Layer for amorphous silica with a 2015 publication year. I guess we can use the term accordingly now. Note that for the record.

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